human anatomy handout 2

8/10/2019 Human Anatomy Handout 2

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Urinary system

The kidneys perform a chemical balancing act that

would be tricky even for the best chemical engineer.

They maintain the bodys internal environment by:

Regulating the total volume of water in the body

and the total concentration of solutes in that water(osmolality).

Regulating the concentrations of the various ions in

the extracellular fluids. (Even relatively small

changes in some ion concentrations such as K+ can

be fatal.)

Ensuring long-term acid-base balance.

Excreting metabolic wastes and foreign substances

such as drugs or toxins.

Producing erythropoietin and renin, important

molecules for regulating red blood cell production

and blood pressure, respectively.

Converting vitamin D to its active form.

Carrying out gluconeogenesis during prolonged

fasting

The urine-forming kidneys are crucial components of

the urinary system. The urinary system also

includes:

Ureterspaired tubes that transport urine from the

kidneys to the urinary bladder

Urinary bladdera temporary storage reservoir

for urine

Urethraa tube that carries urine from the

bladder to the body exterior

The kidneys

The kidneys are bean shaped organs. In fresh

state the kidneys are reddish brown in color. They lie

on the posterior abdominal wall. In the abdomen, the

right kidney is slightly lower than the left. It is

because of the presence of liver superior to it. The

kidneys are surrounded by adipose tissue. Each

kidney is about 11 cm in length, 6cm in breadth and

3cm in anteroposterior dimensions.

The inner margin of each kidney has a small

depression called the hilum. The renal artery and

nerves enter and the renal vein and the ureter exit at

this region. The hilum opens into a cavity called the

renal sinus. Each kidney is enclosed by a fibrous

connective tissue layer, called the renal capsule.Internally the kidney is divided into an outer cortex

and an inner medulla. The medulla consists of

several cone-shaped renal pyramids. Extensions of

the pyramids called the medullary rays, project from

the pyramids into the cortex. Extension of the cortex

called renal columns, project between the pyramids.

The tips of the pyramids are called the renal

papillae. They are pointed toward the renal sinus.

The renal papillae are surrounded by funnel shaped

structures called the minor calyces. The minor

calyces of several pyramids join together to form

larger funnels called major calyces. There are 8-20

minor calyces and 2 or 3 major calyces per kidney.The major calyces converge to form an enlarged

channel called the renal pelvis. The renal pelvis then

narrows to form the ureter. The ureter leaves the

kidney and gets connected to the urinary bladder.

NephronThe basic functional unit of each kidney is the

nephron. There are approximately 1.3 million

nephrons in each kidney. Atleast 450,000 of them

must remain functional to ensure survival. Each

nephron consists of an enlarged terminal end called

the renal corpuscle, a proximal tubule, a loop of

Henle and a distal tubule. The distal tubule opens

into a collecting duct. The renal corpuscle, proximaltubule and distal tubules are in the renal cortex. The

collecting tubules and parts of the loops of Henle

enter the renal medulla.


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Most nephrons measure 50-55 mm in length.

15% of the nephrons are larger and they remain near

the medulla. These are called the juxtamedullary

nephrons. They have larger loops of Henle. The

renal corpuscle of the nephron consists of a

Bowmans capsule and a bunch of capillaries called

the glomerulus.

In the Bowmans capsule the outer and inner

layers are called parietal and visceral layersrespectively. The outer parietal layer is composed of

simple squamous epithelium. The inner visceral layer

surrounds the glomerulus. It consists of specialized

cells called podocytes. The walls of the glomerular

capillaries are lined with endothelial cells. There is a

basement membrane between the endothelial cells of

the glomerular capillaries and the podocytes of

Bowmans capsule. The capillary endothelium, the

basement membrane and the podocytes of Bowmans

capsule make up the filtration membrane.

The glomerulus is supplied with blood by an

afferent arteriole. It is drained by an efferent arteriole.

The cavity of Bowmans capsule opens into the

proximal tubule. The proximal tubule is also called

the proximal convoluted tubule. It is approximately14mm long and 60 m in diameter.

Posteriorly the proximal tubule continues as the

loop of Henle. Each loop has a descending limb and

an ascending limb. The first part of the descending

limb is similar in structure to the proximal tubule.

The loops of Henle that extend into the medulla

become very thin near the end of the loop. The first

part of the ascending limb is also very thin and it

consists of simple squamous epithelium, but it soon

becoms thick. The distal tubules, also called the distal

convoluted tubules are not as long as the proximal

tubules.

Ureters and Urinary bladderThe ureters extend inferiorly from the renal

pelvis. They arise medially at the renal hilum to reach

the urinary bladder. The bladder is meant for

temporarily storing the urine. The urinary bladder is a

hollow muscular bag. It lies in the pelvic cavity. The

size of the bladder depends on the presence or

absence of urine. The bladder capacity varies from

120-320ml. Filling upto 500ml is tolerated.

Micturition will occur at 280ml. The ureters enter the

bladder inferiorly on its posterolateral surface. The

urethra exits the bladder inferiorly and anteriorly. At

the junction of the urethra with the urinary bladder

smooth muscles of the bladder form the internal

urinary sphincter. Around the urethra there is

another external urinary sphincter. The sphincters

control the flow of urine through the urethra.

In the male the urethra extends to the end of the

penis where it opens to the outside. In male the

urethra is 18-20cm long. In the female the urethra is

shorter. It is about 4 cm long and 6 mm in diameter.


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The Circulatory System

The multicellular organisation in animal world

has resulted in the origin and evolution of circulatory

system in animals. This arrangement facilitates

internal transport of various substances to all organsand organ systems. Among majority of multicellular

animals this system remains as a closed type. It has

blood running inside closed blood vessels, the blood

being pumped by heart.

Functions of the Circulatory SystemThe functions of the circulatory system can be

divided into three broad areas: transportation,

regulation, and protection.

1. Transportation. All of the substances essential

for cellular metabolism are transported by the

circulatory system. These substances can be

categorized as follows:a.Respiratory. Red blood cells, or erythrocytes,

transport oxygen to the cells. In the lungs,

oxygen from the inhaled air attaches to

hemoglobin molecules within the erythrocytes

and is transported to the cells for aerobic

respiration. Carbon dioxide produced by cell

respiration is carried by the blood to the lungs

for elimination in the exhaled air.

b.Nutritive. The digestive system is responsible for

the mechanical and chemical breakdown of food

so that it can be absorbed through the intestinal

wall into the blood and lymphatic vessels. The

blood then carries these absorbed products of

digestion through the liver and to the cells of thebody.

c.Excretory. Metabolic wastes (such as urea),

excess water and ions, and other molecules not

needed by the body are carried by the blood to

the kidneys and excreted in the urine.

2. Regulation. The circulatory system contributes to

both hormonal and temperature regulation.

a.Hormonal. The blood carries hormones from

their site of origin to distant target tissues, where

they perform a variety of regulatory functions.

b.Temperature. Temperature regulation is aided

by the diversion of blood from deeper to more

superficial cutaneous vessels or vice versa. Whenthe ambient temperature is high, diversion of

blood from deep to superficial vessels helps to

cool the body, and when the ambient temperature

is low, the diversion of blood from superficial to

deeper vessels helps to keep the body warm.

3. Protection. The circulatory system protects

against blood loss from injury and against foreign

microbes or toxins introduced into the body.

a.Clotting. The clotting mechanism protects

against blood loss when vessels are damaged.

b.Immune. The immune function of the blood is

performed by the leukocytes (white blood cells)that protect against many disease-causing agents

(pathogens).

Major Components of the Circulatory SystemThe circulatory system consists of two

subdivisions: the cardiovascular system and the

lymphatic system. The cardiovascular system

consists of the heart and blood vessels, and the

lymphatic system consists of lymphatic vessels and

lymphoid tissues within the spleen, thymus, tonsils,

and lymph nodes.

Composition of the BloodBlood consists of formed elements that are

suspended and carried in a fluid called plasma. The

formed elementserythrocytes, leukocytes, and

plateletsfunction, respectively, in oxygen transport,

immune defense, and blood clotting. Plasma contains

different types of proteins and many water-soluble

molecules.

The total blood volume in the average-sized

adult is about 5 liters, constituting about 8% of the

total body weight. Blood leaving the heart is referred

to as arterial blood. Arterial blood, with the

exception of that going to the lungs, is bright red

because of a high concentration of oxyhemoglobin

(the combination of oxygen and hemoglobin) in thered blood cells. Venous blood is blood returning to

the heart. Except for the venous blood from the lungs,

it contains less oxygen, and is therefore a darker red

than the oxygen-rich arterial blood.

Blood is composed of a cellular portion, called

formed elements, and a fluid portion, called plasma.

When a blood sample is centrifuged, the heavier

formed elements are packed into the bottom of the

tube, leaving plasma at the top. The formed elements

constitute approximately 45% of the total blood

volume (a measurement called the hematocrit), and

the plasma accounts for the remaining 55%.

PlasmaPlasma is a straw-colored liquid consisting of

water and dissolved solutes. The major solute of the

plasma in terms of its concentration is Na+. In

addition to Na+, plasma contains many other ions, as

well as organic molecules such as metabolites,

hormones, enzymes, antibodies, and other proteins.


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Plasma ProteinsPlasma proteins constitute 7% to 9% of the

plasma. The three types of proteins are albumins,

globulins, and fibrinogen. Albumins account for

most (60% to 80%) of the plasma proteins and are the

smallest in size. They are produced by the liver and

provide the osmotic pressure needed to draw water

from the surrounding tissue fluid into the capillaries.

This action is needed to maintain blood volume and

pressure. Globulins are grouped into three subtypes:

alpha globulins, beta globulins, and gamma

globulins. The alpha and beta globulins are produced

by the liver and function in transporting lipids and

fat-soluble vitamins. Gamma globulins are antibodies

produced by lymphocytes (one of the formed

elements found in blood and lymphoid tissues) and

function in immunity. Fibrinogen, which accounts

for only about 4% of the total plasma proteins, is an

important clotting factor produced by the liver.

During the process of clot formation, fibrinogen is

converted into insoluble threads of fibrin. Thus, thefluid from clotted blood, called serum, does not

contain fibrinogen, but it is otherwise identical to

plasma.

Plasma VolumeA number of regulatory mechanisms in the body

maintain homeostasis of the plasma volume. If the

body should lose water, the remaining plasma

becomes excessively concentratedits osmolality

increases. This is detected by osmo-receptors in the

hypothalamus, resulting in a sensation of thirst and

the release of antidiuretic hormone (ADH) from the

posterior pituitary. This hormone promotes waterretention by the kidneys, whichtogether with

increased intake of fluids helps to compensate for

the dehydration and lowered blood volume.

The Formed Elements of BloodThe formed elementsof blood include two types

of blood cells: erythrocytes, or red blood cells, and

leukocytes, or white blood cells. Erythrocytes are by

far the more numerous of the two. A cubic millimeter

of blood contains 5.1 million to 5.8 million

erythrocytes in males and 4.3 million to 5.2 million

erythrocytes in females. The same volume of blood,

by contrast, contains only 5,000 to 9,000 leukocytes.

ErythrocytesErythrocytes are flattened, biconcave discs,

about 7 m in diameter and 2.2 m thick. Their

unique shape relates to their function of transporting

oxygen; it provides an increased surface area through

which gas can diffuse. Erythrocytes lack nuclei and

mitochondria (they obtain energy through anaerobic

respiration). Partly because of these deficiencies,

erythrocytes have a relatively short circulating life

span of only about 120 days. Older erythrocytes are

removed from the circulation by phagocytic cells in

the liver, spleen, and bone marrow.

Each erythrocyte contains approximately 280

million hemoglobin molecules, which give blood its

red color. Each hemoglobin molecule consists of four

protein chains calledglobins, each of which is bound

to one heme, a red-pigmented molecule that contains

iron. The iron group of heme is able to combine with

oxygen in the lungs and release oxygen in the tissues.

LeukocytesLeukocytes differ from erythrocytes in several

respects. Leukocytes contain nuclei and mitochondria

and can move in an amoeboid fashion. Because of

their amoeboid ability, leukocytes can squeeze

through pores in capillary walls and move to a site of

infection, whereas erythrocytes usually remain

confined within blood vessels. The movement of

leukocytes through capillary walls is referred to asdiapedesis or extravasation.

White blood cells are almost invisible under the

microscope unless they are stained; therefore, they

are classified according to their staining properties.

Those leukocytes that have granules in their

cytoplasm are called granular leukocytes; those

without clearly visible granules are called agranular

(or nongranular) leukocytes.

The stain used to identify white blood cells is

usually a mixture of a pink-to-red stain called eosin

and a blue-to-purple stain called a basic stain.

Granular leukocytes with pink staining granules are

therefore called eosinophils, and those with blue-staining granules are called basophils. Those with

granules that have little affinity for either stain are

neutrophils. Neutrophils are the most abundant type

of leukocyte, accounting for 50% to 70% of the

leukocytes in the blood. Immature neutrophils have

sausage-shaped nuclei and are called band cells. As

the band cells mature, their nuclei become lobulated,

with two to five lobes connected by thin strands. At

this stage, the neutrophils are also known as

polymorphonuclear leukocytes (PMNs).There are two types of agranular leukocytes:

lymphocytes and monocytes. Lymphocytes are

usually the second most numerous type of leukocyte;they are small cells with round nuclei and little

cytoplasm. Monocytes,by contrast, are the largest of

the leukocytes and generally have kidney- or

horseshoe-shaped nuclei. In addition to these two cell

types, there are smaller numbers of plasma cells,

which are derived from lymphocytes. Plasma cells

produce and secrete large amounts of antibodies.

Platelets


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Platelets, or thrombocytes, are the smallest of

the formed elements and are actually fragments of

large cells called megakaryocytes,which are found in

bone marrow. (This is why the termformed elements

is used instead of blood cells to describe erythrocytes,

leukocytes, and platelets.) The fragments that enter

the circulation as platelets lack nuclei but, like

leukocytes, are capable of amoeboid movement. The

platelet count per cubic millimeter of blood ranges

from 130,000 to 400,000, but this count can vary

greatly under different physiological conditions.

Platelets survive for about 5 to 9 days before being

destroyed by the spleen and liver.

Platelets play an important role in blood clotting.

They constitute most of the mass of the clot, and

phospholipids in their cell membranes activate the

clotting factors in plasma that result in threads of

fibrin, which reinforce the platelet plug. Platelets that

attach together in a blood clot release serotonin, a

chemical that stimulates constriction of the blood

vessels, thus reducing the flow of blood to the injuredarea. Platelets also secrete growth factors, which are

important in maintaining the integrity of blood

vessels. These regulators also may be involved in the

development of atherosclerosis, as described in a

later section.

HematopoiesisBlood cells are constantly formed through a

process called hematopoiesis (also called

hemopoiesis). The hematopoietic stem cellsthose

that give rise to blood cellsoriginate in the yolk sac

of the human embryo and then migrate to the liver.

Hematopoiesis thus occurs in the liver of thefetus. The stem cells then migrate to the bone

marrow, and shortly after birth the liver ceases to be a

source of blood cell production. The term

erythropoiesis refers to the formation of

erythrocytes, and leukopoiesis to the formation of

leukocytes. These processes occur in two classes of

tissues after birth, myeloid and lymphoid. Myeloid

tissue is the red bone marrow of the long bones, ribs,

sternum, pelvis, bodies of the vertebrae, and portions

of the skull. Lymphoid tissue includes the lymph

nodes, tonsils, spleen, and thymus. The bone marrow

produces all of the different types of blood cells; the

lymphoid tissue produces lymphocytes derived fromcells that originated in the bone marrow.

Hematopoiesis begins the same way in both

myeloid and lymphoid tissue. A population of

undifferentiated (unspecialized) cells gradually

differentiate (specialize) to become stem cells, which

give rise to the blood cells. At each step along the

way the stem cells can duplicate themselves by

mitosis, thus ensuring that the parent population will

never become depleted. As the cells become

differentiated, they develop membrane receptors for

chemical signals that cause further development

along particular lines. The earliest cells that can be

distinguished under a microscope are the

erythroblasts (which become erythrocytes),

myeloblasts (which become granular leukocytes),

lymphoblasts (which form lymphocytes), and

monoblasts (which form monocytes).

Erythropoiesis is an extremely active process. It

is estimated that about 2.5 million erythrocytes are

produced every second in order to replace those that

are continuously destroyed by the spleen and liver.

The life span of an erythrocyte is approximately 120

days. Agranular leukocytes remain functional for 100

to 300 days under normal conditions. Granular

leukocytes, by contrast, have an extremely short life

span of 12 hours to 3 days.

The production of different subtypes of

leukocytes is stimulated by chemicals called

cytokines. These are autocrine regulators secreted by

various cells of the immune system. The particularcytokines involved in leukopoiesis are discussed

below. The production of red blood cells is

stimulated by the hormone erythropoietin, which is

secreted by the kidneys. The gene for erythropoietin

has been commercially cloned, so that this hormone

is now available for the treatment of the anemia that

results from kidney disease in patients undergoing

dialysis.

Scientists have identified a specific cytokine that

stimulates proliferation of megakaryocytes and their

maturation into platelets. By analogy with

erythropoietin, they named this regulatory molecule

thrombopoietin. The gene that codes forthrombopoietin also has been cloned, so that

recombinant thrombopoietin is now available for

medical research and applications. In clinical trials,

thrombopoietin has been used to treat the

thrombocytopenia (low platelet count) that occurs as

a result of bone marrow depletion in patients

undergoing chemotherapy for cancer.


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Blood circulationIn man, as in all mammals there is a double

circulation of blood. The primary circulation through

pumping action of heart, supplies blood to all regions

of the body. The blood later returns to the heart. It is

called the systemic circulation or body circulation.A similar circulation carries blood to lungs for

oxygenation and returns it back to the heart. It is

called the pulmonary circulation.

Systemic and Pulmonary circulationsThe most important component of this system is

the heart. It is a large, muscular, valved structure

having four chambers. The chambers are the rightatrium, left atrium, right ventricle and left

ventricle. Each atrium opens into a corresponding

ventricle. The right and left chambers are separated

by septa.

Systemic circulation: - The left atrium receives

oxygenated blood from the lungs, through the

pulmonary vein. When the atria contract, blood from

the left atrium is forced into the left ventricle. Later

by a contraction of the ventricle, the blood leaves the

heart through the aorta. The aorta is the single

systemic artery emerging from the heart. By

successive branching, the aorta gives rise to hundreds

of arteries taking blood to all regions of the body. As

the branching happen, the arteries divide into

numerous (4 106) arterioles. In the target organs

they produce four times as many capillaries. A

similar number of venules converge into each other

forming veins of increasingly larger size. Finally,

only two veins, the superior and inferior vena cavae

return the blood to the right atrium. Thus the course

of blood from left ventricles through the body organs

and back to the atrium forms the systemic circulation.

Pulmonary circulation: - The venous blood from

right atrium is conducted to the right ventricle. The

ventricle expels the blood via the pulmonary trunk tothe lungs. The oxygenated blood later returns by the

pulmonary veins to the left atrium. This circulation

from right ventricle to the left atrium via the lungs is

termed the pulmonary circulation.

Portal circulation: - In the systemic circulation the

venous blood passing through spleen, pancreas,

stomach and intestine is not carried back directly to


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the heart. It passes through the hepatic portal vein to

the liver. This vein begins as capillaries from the

visceral organs and ends in the liver again as

capillaries. These capillaries converge to form the

hepatic vein which joins the inferior vena cava,

conveying blood to right atrium. This route is the

portal circulation.

Components of Circulatory systemBlood vessels

The blood vessels carrying blood away from the

heart are the arteries. The Veins carry blood towards

the heart. The arteries and veins are named and

classified according to their anatomical position.

They can also be classified according to their size and

wall structure. Functionally, arteries are subdivided

into conducting, distributing and resistance vessels.

1. Conducting vessels: - These are large arteries

from the heart and their main branches. The walls

of these vessels are elastic in nature.

2. Distributing vessels: -These are smaller arteries

reaching individual organs. They branch into the

organs. They have muscular walls.

3. Resistance vessels: -These are mostly arterioles.

While these vessels are smaller, their walls are

highly muscular. Hence these vessels can reduce

pressure of blood due to peripheral resistance.

4. Exchange vessels: -These are the capillaries. The

walls of these vessels allow exchanges between

blood and the tissue fluid surrounding the cells.

The substances commonly exchanged are oxygen,

carbon-di-oxide, nutrients, water, inorganic ions,

vitamins, hormones, metabolic products and

antibodies.

5. Capacitance or reservoir vessels: -These are the

larger vessels and veins. These are of varying

sizes. They collect and convey blood back to the

heart. The higher capacitance of these vessels isdue to their distensibility. Hence their blood

content is more, even at low pressure. The number

of such veins is also enormous. Thus the veins are

called as the blood reservoirs

Structure of blood vesselsThe blood vessels show a vast range of structural

modifications. However a few basic patterns can be

studied.

A blood vessel consists of a wall and a lumen or

cavity. The wall of the blood vessels is made up of 3

distinct layers or tunica. They are the tunica intima,

tunica media and tunica externa or tunicaadventitia.

The tunica intima is formed of an endothelium, a

delicate connective tissue and elastic fibers. The

tunica media contains smooth muscle cells. It causes

vasoconstriction and vasodilation. The tunica

externa is composed of connective tissue. The

composition and thickness of layers varies with the

diameter of the blood vessels and the type.

Types of blood vessels1. Large elastic arteries: - The walls of these

arteries contain elastic fibers. The smooth wall

measures about 1micron in thickness. It gets

stretched under the effect of pulse and recoils

elastically.


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2. Muscular arteries: -There are larger and smaller

muscular arteries. The larger muscular arteries are

inelastic and they have thick walls. The wall has

30-40microns in diameter in the layers of smooth

muscles. Since they regulate blood supply, they

are called distributing arteries. The small

muscular arteries are capable of vasodilation and

vasoconstriction.

3. Arterioles: - They conduct blood from the

arteries to the capillary bed. These are small

vessels capable of vasodilation and

vasoconstriction.

4. Capillaries: - These are fine vessels found

between arterioles and venules. They measure 5-

8micron in diameter.

5. Venules: - These are tubes of flat, oval or

polygonal endothelial cells. Each venule is formed

by the convergence of two or more capillaries. Its

diameter ranges up to 30micron.

6. Veins: - Veins seen in anatomy are medium

veins. They run in between venules and largeveins. Large veins transport blood to the heart.

Veins with diameter above 2 mm have valves.

They are of semilunar type. They allow

movement of blood towards the heart. There are

several valves in the medium veins.

Branching of blood vessels: - When an artery

divides into two equal branches, the original artery

ceases to exist. Hence the branches are called

terminal branches. The smaller branching vessels

formed on the sides are called the collateral

branches. When arteries are joined to each other it is

named as anastomosis.

Blood supply to blood vessels: - As any other

region, the cells and tissue on the wall of the blood

vessel require nourishment. Some amount can diffuse

from blood in the lumen. For vessels having diameter

greater than 1 mm, diffusion of nutrients may not be

possible. Such vessels have very minute vessels

called vasa vasorum spread over them. They

penetrate into the wall of the blood vessels.

Innervations of blood vessels: - The walls of the

blood vessels are innervated by sympathetic nerve

fibers. They regulate the contraction of themusculature. They affect vasoconstriction.

The Heart

The heart is a hollow, fibro muscular organ. It is

somewhat conical or pyramidal in form. It is roughly

the size of a closed fist. An average heart measures

12 cm from base to the apex. Transverse diameter at

its broadest region is 8-9 cm. It is 6 cm thick antero-

posteriorly. The thoracic organs such as heart, trachea

and esophagus form a midline partition called the

mediastinum. The heart lies obliquely in the

mediastinum. The heart is surrounded by a double

layered membrane called the pericardium. The outer

layer is called the fibrous pericardium. The inner

membrane is called the serous pericardium. In

between heart and pericardium, there is a pericardial

space. This space is filled with a fluid called the

pericardial fluid. The wall of the heart is made up of

three tissue layers. They are the epicardium,

myocardium and endocardium. The epicardium

forms the smooth outer surface of the heart. The

middle myocardium is composed of cardiac muscle.

This layer plays an important role in the functioning

of the heart. The endocardium forms the smoothinner surface. It is formed of squamous epithelium.

Lymphatic systemLymphatic circulation along with blood

circulation plays a key role in maintaining the fluidity

in all regions of the body. It helps to maintain fluid

balance in tissues and it absorbs fat from the

digestive tract. It also functions as bodys defense

system against micro organisms and other harmful

substances. This system includes lymph,

lymphocytes, lymphatic vessels, lymph nodules,

lymph nodes, tonsils, the spleen and the thymus

gland.

Lymphoid cells and tissues

Lymphatic organs contain lymphatic tissues.These tissues primarily consist of lymphocytes. They

also contain macrophages, dendritic cells and

reticular cells. Lymphocytes are a type of white

blood cells. They originate from red bone marrow

and are carried by blood to lymphatic organs and

other tissues. There are several classes of

lymphocytes. The B-lymphocytes or B cells

synthesize antibodies for recognizing and

neutralizing alien macromolecules. T- lymphocytes


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can recognize and selectively kill cells infected with

viruses. B and T lymphocytes are produced from

stem cells present in the bone marrow. The T

lymphocytes get matured only after entering into

Thymus, a lymphoid organ through circulation.

Maturation and differentiation of B cells will occur in

the bone marrow itself. Thus the thymus and bone

marrow are described as central or primary lymphoid

organs.

Thymus - It is a roughly triangular, bilobed gland. It

is located in the mediastinum (i.e., between the

lungs). It lies between the sternum and the

pericardium. Its size varies with age. It is largest in

the early part of life (up to 15 years). At birth it

weighs 10 - 15 g. After puberty it greatly decreases in

size. Each thymus lobe is surrounded by a thin

capsule made of the connective tissue. It has 2 layers.

The inner layer is the medulla, the outer layer is

cortex. The lymphocytes are found only in cortex

layer.

Lymph nodes - These are small round structures.

Their size ranges from 1-25 mm. They are distributed

throughout the course of the lymphatic vessels. These

nodes are found all over the body. However they are

concentrated as aggregations in 3 regions of the body.

These are the inguinal nodes in the groin, the

axillary nodes in the axillary region and the cervical

nodes of the neck.

The lymph enters the lymph nodes through

afferent lymphatic vessels and exits through efferent

vessels. The nodes contain open spaces called

sinuses. The sinuses are lined with phagocytic cells.

Spleen - It is roughly the size of a clenched fist. It is

located on the left side of the abdominal cavity. It has

a fibrous capsule. The spleen contains two types of

lymphatic tissues, namely the red pulp and the white

pulp.

Tonsils - These are the largest lymph nodules. They

provide protection against bacteria and other harmful

materials. In adults the tonsils decrease in size and

may disappear. There are 3 groups of tonsils in the

pharyngeal walls. Of the three, the palatine tonsils

are usually referred to as the tonsils. These are

larger lymphoid masses on each side of the junction

between the oral cavity and the pharynx. The

pharyngeal tonsil or adenoid is found near the

junction between the nasal cavity and the pharynx.

The lingual tonsil is a loosely associated collection

of lymph nodules on the posterior surface of the

tongue.

The lymphatic circulation - The lymph fluid from

the tissues is drained by lymphatic capillaries. These

capillaries though present in many tissues are absent

in epidermis, hairs, nails, cornea, cartilages, CNS and

bone marrow. The lymphatic capillaries join into

larger vessels. The larger vessels pass to local or

remote lymph nodes. These vessels and associated

lymph nodes are arranged in regional groups. Each

group has its region of drainage. Nodes within a

group are interconnected. Such regional groups with

nodes and vessels are organized in (1) Head and neck

(2) Upper limbs (3) Lower limbs (4) Abdomen andpelvis (5) thorax. The regional vessels return to the

venous blood circulation via the right and left

lympho venous portals. Nearly eight lymphatic

trunks converge at the site of the vertebral column

and open into the venous portals nearer to the neck

Endocrine system

Our body has two major regulatory systems.

They are the nervous and endocrine systems.

Together they regulate and co-ordinate the activities

of all other body structures. The endocrine system

sends information to the tissues it controls in the form

of chemical signals. These signals, in the form of

hormones are released into the circulatory system.

They are carried to all parts of the body. Body cells


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are able to recognize the chemical signals and

respond to them. The hormones of the endocrine

glands regulate and control the functioning of several

organs in the body. Thus, this system in general helps

to maintain homeostasis. There are several endocrine

glands in our body. The major glands are the

pituitary, thyroid, parathyroids, pancreas,

adrenals, testes and ovaries.

Pituitary gland (or) HypophysisIt is an organ, which secretes eight major

hormones. These hormones regulate numerous body

functions and control the secretory activities of

several other endocrine glands. The hypothalamus

of the brain is connected to the pituitary. The

posterior pituitary is an extension of the

hypothalamus.

Structure of pituitary glandThis gland is approximately 1 cm in diameter. It

weighs 0.5-1g. It is placed in a region called the sella

turcica of the sphenoid bone in the floor of the skull.It is placed inferior to the hypothalamus. It is

connected to it by a stalk of tissue called the

infundibulum. Based on origin and function the

pituitary is divided into two parts. They are the

posterior pituitary or neurohypophysis and

anterior pituitary or adenohypophysis.

Posterior pituitary or Neurohypophysis

The posterior pituitary is continuous with the

brain. Hence it is called the neurohypophysis.

During embryonic development, it is formed as an

outgrowth of the inferior part of the brain in the area

of the hypothalamus. The outgrowth of the brain

forms the infundibulum. The distal end of the

infundibulum enlarges to form the posterior

pituitary. Since this part of the pituitary is an

extension of the nervous system, its secretions are

known as neurohormones.

Anterior Pituitary or AdenohypophysisDuring embryonic development an out pocketing

of the roof of the oral cavity arises. It is called as the

Rathkes pouch. This pouch grows towards the

posterior pituitary. Later, the pouch loses its

connection with the oral cavity and becomes the

anterior pituitary. The anterior pituitary is subdivided

into three areas. They are, the pars tuberalis, pars

distalis and pars intermedia.

Relationship of the pituitary to the brainThere is a network of blood vessels on the

hypothalamus. It is called the primary capillary

network. A portal system called the

hypothalamohypophyseal portal system extends

from a part of the hypothalamus to the anterior

pituitary (a portal blood vessel begins and ends as

capillaries). The portal system in turn opens into the

secondary capillary network of the anterior pituitary.

The neurohormones produced by the hypothalamus

are collected by the primary capillary network.

Through the portal system they enter into the

secondary network of the anterior pituitary.The hormones of the anterior pituitary are,

1. Growth hormone (GH, or somatotropin). GH

promotes the movement of amino acids into cells

and the incorporation of these amino acids into

proteins, thus promoting overall tissue and organ

growth.

2. Thyroid-stimulating hormone (TSH, or

thyrotropin). TSH stimulates the thyroid gland to

produce and secrete thyroxine (tetraiodothyronine,

or T4) and triiodothyronine (T3).

3. Adrenocorticotropic hormone (ACTH, or

corticotropin). ACTH stimulates the adrenal

cortex to secrete the glucocorticoids, such ashydrocortisone (cortisol).

4. Follicle-stimulating hormone (FSH, or

folliculotropin). FSH stimulates the growth of

ovarian follicles in females and the production of

sperm cells in the testes of males.

5. Luteinizing hormone (LH, or luteotropin). This

hormone and FSH are collectively called

gonadotropic hormones. In females, LH

stimulates ovulation and the conversion of the


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ovulated ovarian follicle into an endocrine

structure called a corpus luteum. In males, LH is

sometimes called interstitial cell stimulating

hormone, or ICSH; it stimulates the secretion of

male sex hormones (mainly testosterone) from the

interstitial cells (Leydig cells) in the testes.

6. Prolactin (PRL). This hormone is secreted in

both males and females. Its best known function is

the stimulation of milk production by the

mammary glands of women after the birth of a

baby. Prolactin plays a supporting role in the

regulation of the male reproductive system by the

gonadotropins (FSH and LH) and acts on the

kidneys to help regulate water and electrolyte

balance.

The posterior pituitary, or pars nervosa, stores and

releases two hormones, both of which are produced

in the hypothalamus.

1. Antidiuretic hormone (ADH). ADH promotes

the retention of water by the kidneys so that lesswater is excreted in the urine and more water is

retained in the blood.

2. Oxytocin. In females, oxytocin stimulates

contractions of the uterus during labor and for this

reason is needed for parturition (childbirth).

Oxytocin also stimulates contractions of the

mammary gland alveoli and ducts, which result in

the milk-ejection reflex in a lactating woman. In

men, a rise in oxytocin secretion at the time of

ejaculation has been measured, but the

physiological significance of this hormone in

males remains to be demonstrated.

Thyroid glandThe thyroid gland is composed of two lobes.

They are placed on the lateral sides of the upper

portion of the trachea. These lobes are connected by a

narrow band of thyroid tissue called the isthmus. The

isthmus extends across the anterior aspect of the

trachea. The thyroid is one of the largest endocrine

glands. It weighs approximately 20g. It is richly

supplied with blood capillaries. It is redder than its

neighboring tissues.

The gland is composed of numerous follicles.

They are small spheres. Their walls are made up of

cuboidal epithelial cells. The central cavity or lumenof each follicle is filled with a protein called the

thyroglobulin. It stores large amount of thyroid

hormone. The thyroid secretes thyroxine and

calcitonin.

Thyroid hormone (TH) is actually two iodine-

containing amine hormones, thyroxine or T4 and

triiodothyronine or T3. T4 is the major hormonesecreted by the thyroid follicles. The principal

difference between them is that T4 has four bound

iodine atoms, and T3 has three (thus, T4 and T3). TH

affects virtually every cell in the body. Like steroids,

TH enters a target cell, binds to intracellular receptors

within the cells nucleus, and initiates transcription of

mRNA for protein synthesis. Effects of thyroid

hormone include:

Increasing basal metabolic rate and body heat

production, by turning on transcription of genes

concerned with glucose oxidation. This is the

hormones calorigenic effect (calorigenic = heat

producing).Regulating tissue growth and development. TH is

critical for normal skeletal and nervous system

development and maturation and for reproductive

capabilities.

Maintaining blood pressure by increasing the

number of adrenergic receptors in blood vessels.

Calcitonin, a polypeptide hormone released by the

parafollicular, or C, cells of the thyroid gland in

response to a rise in blood Ca2+ levels. Calcitonin

targets the skeleton, where it (1) inhibits osteoclast

activity, inhibiting bone resorption and release of

Ca21 from the bony matrix, and (2) stimulates Ca21

uptake and incorporation into bone matrix. In other

animals, calcitonin rapidly reduces blood Ca2+levels.

Parathyroid glandsThe parathyroid glands are found in association

with the thyroid glands. They are found embedded in

the posterior part of each lobe of the thyroid gland.

There are four parathyroid glands. Inside the glands

the cells are organized in densely packed masses. The

cells of the glands secrete parathyroid hormone.


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Parathyroid hormone (PTH) is the only hormone

secreted by the parathyroid glands. PTH, however, is

the single most important hormone in the control of

the calcium levels of the blood. It promotes a rise in

blood calcium levels by acting on the bones, kidneys,

and intestine.

Adrenal glands or Suprarenal glands

These glands are found near the superior pole ofeach kidney. They are surrounded by adipose tissue.

The glands are enclosed by a connective tissue

capsule. The adrenal glands are composed of an inner

medulla and outer cortex. These regions are formed

from two separate embryonic tissues. The medulla

consists of closely packed polyhedral cells. They are

centrally located in the gland. The cortex is

composed of smaller cells. These cells form three

distinct layers, namely the zona glomerulosa, the

zona fasciculate and the zona reticularis. These

layers are structurally and functionally specialized.

The adrenal cortex secretes steroid hormones

called corticosteroids, or corticoids, for short. There

are three functional categories of corticosteroids: (1)

mineralocorticoids, which regulate Na+ and K+

balance; (2) glucocorticoids, which regulate the

metabolism of glucose and other organic molecules;

and (3) sex steroids, which are weak androgens

(including dehydroepiandrosterone, or DHEA) that

supplement the sex steroids secreted by the gonads.

Aldosterone is the most potent

mineralocorticoid. The mineralocorticoids are

produced in the zona glomerulosa and stimulate the

kidneys to retain Na+and water while excreting K+in

the urine. These actions help to increase the blood

volume and pressure and to regulate blood electrolyte

balance.

The predominant glucocorticoid in humans is

cortisol (hydrocortisone), which is secreted by the

zona fasciculata and perhaps also by the zona

reticularis. The secretion of cortisol is stimulated by

ACTH from the anterior pituitary. Cortisol and other

glucocorticoids have many effects on metabolism;

they stimulate gluconeogenesis (production of

glucose from amino acids and lactic acid) and inhibit

glucose utilization, which help to raise the blood

glucose level; and they promote lipolysis (breakdown

of fat) and the consequent release of free fatty acids

into the blood.

The cells of the adrenal medulla secrete

epinephrine and norepinephrine in an approximateratio of 4 to 1, respectively. The effects of these

catecholamine hormones are similar to those caused

by stimulation of the sympathetic nervous system,

except that the hormonal effect lasts about ten times

longer. The hormones from the adrenal medulla

increase the cardiac output and heart rate, dilate

coronary blood vessels, increase mental alertness,

increase the respiratory rate, and elevate the

metabolic rate. The adrenal medulla is innervated by

preganglionic

sympathetic axons, and secretes its hormones

whenever the sympathetic nervous system is

activated during fight orflight. These sympatho-adrenal effects are supported

by the metabolic actions of epinephrine and

norepinephrine: a rise in blood glucose due to

stimulation of hepatic glycogenolysis (breakdown of

glycogen) and a rise in blood fatty acids due to

stimulation of lipolysis (breakdown of fat).


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PancreasThe pancreas lies between the greater curvature

of the stomach and the duodenum. It is an enlarged

structure. It is approximately 15 cm long. It weighs

85 -100g. The pancreas is both an exocrine and an

endocrine gland. The endocrine part consists of

pancreatic islets (islets of Langerhans). They are

approximalety 500,000 to 1,000,000 in number. The

islets are distributed in the pancreas. The islets are

composed of alpha () cells (20%) and beta () cells

(75%). While the cells secrete glucagon the cells

secrete insulin. A third type of cells called the delta

() cells (5%) has been identified. These cells secrete

somatostatin.

Alpha cells secrete glucagon in response to a fall

in blood glucose concentrations. Glucagon stimulates

the liver to hydrolyze glycogen to glucose

(glycogenolysis), which causes the blood glucose

level to rise. Glucagon also stimulates the hydrolysis

of stored fat (lipolysis) and the consequent release offree fatty acids into the blood. This effect helps to

provide energy substrates for the body during fasting,

when blood glucose levels decrease. Glucagon,

together with other hormones, also stimulates the

conversion of fatty acids to ketone bodies, which can

be secreted by the liver into the blood and used by

other organs as an energy source. Glucagon is thus a

hormone that helps to maintain homeostasis during

times of fasting, when the bodys energy reserves

must be utilized.

Beta cells secrete insulin in response to a rise in

blood glucose concentrations. Insulin promotes the

entry of glucose into tissue cells, and the conversionof this glucose into energy storage molecules of

glycogen and fat. Insulin also aids the entry of amino

acids into cells and the production of cellular protein.

Thus, insulin promotes the deposition of energy

storage molecules (primarily glycogen and fat)

following meals, when the blood glucose

concentration rises. This action is antagonistic to that

of glucagon, and the secretion of glucagon is

normally decreased when insulin secretion increases.

During times of fasting, conversely, the secretion of

insulin is decreased while the secretion of glucagon is

increased.

Pineal GlandThe tiny, pine coneshaped pineal gland hangs

from the roof of the third ventricle in the

diencephalon. Its secretory cells, called pinealocytes,

are arranged in compact cords and clusters. Lying

between pinealocytes in adults are dense particles

containing calcium salts (brain-sand or pineal

sand). These salts are radiopaque, making the pineal

gland a handy landmark for determining brain

orientation in X rays.

The endocrine function of the pineal gland is still

somewhat of a mystery. Although many peptides and

amines have been isolated from this minute gland, its

only major secretory product is melatonin, an amine

hormone derived from serotonin. Melatonin

concentrations in the blood rise and fall in a diurnal

(daily) cycle. Peak levels occur during the night and

make us drowsy, and lowest levels occur around

noon. Recent evidence suggests that melatonin also

controls the production of protective antioxidant and

detoxification molecules within cells.

The pineal gland indirectly receives input from

the visual pathways concerning the intensity and

duration of daylight. The suprachiasmatic nucleus

of the hypothalamus, an area referred to as our

biological clock, is richly supplied with melatonin

receptors, and exposure to bright light (known to

suppress melatonin secretion) can reset the clock

timing. As a result, changing melatonin levels mayinfluence rhythmic variations in physiological

processes such as body temperature, sleep, and

appetite.

Hormone Secretion by Other OrgansOther hormone-producing cells occur in various

organs including the heart, gastrointestinal tract,

kidneys, skin, adipose tissue, skeleton, and thymus

1. Adipose TissueAdipose cells release leptin, which serves to tell

your body how much stored energy (as fat) you

have. The more fat you have, the more leptin therewill be in your blood. It also appears to stimulate

increased energy expenditure. Two other

hormones released by adipose cells both affect the

sensitivity of cells to insulin.Resistin is an insulin

antagonist, while adiponectin enhances sensitivity

to insulin.

2. Gastrointestinal TractEnteroendocrine cells are hormone secreting cells

sprinkled in the mucosa of the gastrointestinal

(GI) tract. These scattered cells release several

peptide hormones that help regulate a wide variety

of digestive functions. (1) Gastrin; (2) Ghrelin; (3)Secretin; (4) Cholecystokinin (CCK); (5) Incretins

[glucose-dependent insulinotropic peptide (GIP)

and glucagon-like peptide 1(GLP-1)].

Enteroendocrine cells also release amines such as

serotonin, which act as paracrines, diffusing to

and influencing nearby target cells without first

entering the bloodstream.

3. Heart


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The atria contain specialized cardiac muscle cells

that secrete atrial natriuretic peptide (ANP).

ANP decreases the amount of sodium in the

extracellular fluid, thereby reducing blood volume

and blood pressure.

4. KidneysInterstitial cells in the kidneys secrete

erythropoietin, a glycoprotein hormone that

signals the bone marrow to increase production of

red blood cells. The kidneys also release renin,

which acts as an enzyme to initiate the renin-

angiotensin-aldosterone mechanism of

aldosterone release.

5. SkeletonOsteoblasts in bone secrete osteocalcin, a

hormone that prods pancreatic beta cells to divide

and secrete more insulin. It also restricts fat

storage by adipocytes, and triggers the release of

adiponectin. This improves glucose handling andreduces body fat. Interestingly, insulin promotes

the conversion of inactive osteocalcin to active

osteocalcin in bone, forming a two-way

conversation between bone and the pancreas.

Osteocalcin levels are low in type 2 diabetes, and

increasing its level may offer a new treatment

approach.

6. SkinThe skin produces cholecalciferol, an inactive

form of vitamin D3, when modified cholesterol

molecules in epidermal cells are exposed to

ultraviolet radiation. This compound then entersthe blood via the dermal capillaries, is modified in

the liver, and becomes fully activated in the

kidneys. The active form of vitamin D3,

calcitriol, is an essential regulator of the carrier

system that intestinal cells use to absorb Ca21

from food. Without this vitamin, bones become

soft and weak.

7. ThymusLocated deep to the sternum in the thorax is the

lobulated thymus. Large and conspicuous in

infants and children, the thymus shrinks

throughout adulthood. By old age, it is composed

largely of adipose and fibrous connective tissues.

Thymic epithelial cells secrete several different

families of peptide hormones, including

thymulin, thymopoietins, and thymosins. These

hormones are thought to be involved in the

normal development of T lymphocytes and the

immune response, but their roles are not well

understood. Although still called hormones, they

mainly act locally as paracrines.

8. Gonads and PlacentaThe gonads (testes and ovaries) secrete sex

steroids. These include male sex hormones, or

androgens, and female sex hormonesestrogens

and progesterone. The androgens and estrogens

are families of hormones. The principal androgen

secreted by the testes is testosterone, and the

principal estrogen secreted by the ovaries is

estradiol-17. The principal estrogen during

pregnancy, however, is a weaker estrogen called

estriol, secreted by the placenta. After menopause,the principal estrogen is estrone, produced

primarily by fat cells.

The testes consist of two compartments:

seminiferous tubules, which produce sperm cells,

and interstitial tissuebetween the convolutions of

the tubules. Within the interstitial tissue are

Leydig cells, which secrete testosterone.

Testosterone is needed for the development and

maintenance of the male genitalia (penis and

scrotum) and the male accessory sex organs

(prostate, seminal vesicles, epididymides, and vas

deferens), as well as for the development of male

secondary sex characteristics.The placentathe organ responsible for nutrient

and waste exchange between the fetus and

motheris also an endocrine gland that secretes

large amounts of estrogens and progesterone. In

addition, it secretes a number of polypeptide and

protein hormones that are similar to some

hormones secreted by the anterior pituitary. These

hormones include human chorionic gonadotropin

(hCG), which is similar to LH, and

somatomammotropin, which is similar in action to

both growth hormone and prolactin.

Nervous system

A complete understanding of the human nervous

system remains a challenge. Several billion cells

remain associated with this system. The varying

functions of these cells and the nervous system are


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responsible for human behavior and activities. Hence,

scientists from different fields collectively are

interested in understanding the functioning of this

system.

Functions and Divisions of the Nervous SystemThe nervous system has three overlapping functions,

1. Sensory input. The nervous system uses its

millions of sensory receptors to monitor changes

occurring both inside and outside the body. The

gathered information is called sensory input.

2. Integration. The nervous system processes and

interprets sensory input and decides what should be

done at each momenta process called integration.

3. Motor output. The nervous system activates

effector organsthe muscles and glandsto cause a

response, called motor output.

We have only one highly integrated nervous system.

For convenience, it is divided into two principal

parts, central andperipheral.

The central nervous system (CNS) consists of

the brain and spinal cord, which occupy thedorsal body cavity. The CNS is the integrating

and control center of the nervous system. It

interprets sensory input and dictates motor

output based on reflexes, current conditions, and

past experience.

The peripheral nervous system (PNS) is the

part of the nervous system outside the CNS. The

PNS consists mainly of the nerves (bundles of

axons) that extend from the brain and spinal

cord. Spinal nerves carry impulses to and from

the spinal cord, and cranial nerves carry

impulses to and from the brain. These peripheral

nerves serve as communication lines that link all

parts of the body to the CNS. The PNS has two

functional sub divisions;

The sensory, or afferent, division consists of

nerve fibers (axons) that convey impulses to the

central nervous system from sensory receptors

located throughout the body. The sensory division

keeps the CNS constantly informed of events going

on both inside and outside the body.

Somatic sensory fibers convey impulses from

the skin, skeletal muscles, and joints

Visceral sensory fibers transmit impulses from

the visceral organs (organs within the ventral

body cavity)

The motor, or efferent, division of the PNS

transmits impulses fromthe CNS to effector organs,

which are the muscles and glands. These impulses

activate muscles to contract and glands to secrete. In

other words, they effect (bring about) a motor

response. The motor division also has two main

parts:

The somatic nervous system is composed of

somatic motor nerve fibers that conduct

impulses from the CNS to skeletal muscles. It is

often referred to as the voluntary nervous

system because it allows us to consciously

control our skeletal muscles.

The autonomic nervous system (ANS)

consists of visceral motor nerve fibers that

regulate the activity of smooth muscles, cardiac

muscles, and glands. Autonomicmeans a law

unto itself, and because we generally cannotcontrol such activities as the pumping of our

heart or the movement of food through our

digestive tract, the ANS is also called the

involuntary nervous system.

The ANS has two functional subdivisions, the

sympathetic division and the parasympathetic

division. Typically these divisions work in

opposition to each otherwhatever one stimulates,

the other inhibits.


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Nervous TissueThe nervous system consists mostly of nervous

tissue, which is highly cellular. For example, less

than 20% of the CNS is extracellular space, which

means that the cells are densely packed and tightly

intertwined. Although it is very complex, nervous

tissue is made up of just two principal types of cells:

Supporting cells called neuroglia, small cells

that surround and wrap the more delicate

neurons

Neurons, nerve cells that are excitable(responsive to stimuli) and transmit electrical

signals

NeurogliaNeurons associate closely with much smaller

cells called neuroglia (nerve glue) or glial cells.

There are six types of neurogliafour in the CNS

and two in the PNS. Supporting cells aid the

functions of neurons and are about five times more

abundant than neurons. Unlike neurons, which do not

divide mitotically, glial cells are able to divide by

mitosis. This helps to explain why brain tumors in

adults are usually composed of glial cells rather than

of neurons. There are two types of supporting cells in

the peripheral nervous system:

Schwann cells, which form myelin sheaths

around peripheral axons

Satellite cells, which support neuron cells

bodies within the ganglia of the PNS.

There are four types of supporting cells, called

neuroglial, in the central nervous system:


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Oligodendrocytes, which form myelin sheaths

around axons of the CNS

Microglia, which migrate through the CNS and

phagocytose foreign and degenerated material

Astrocytes, which help to regulate the external

environment of neurons in the CNS

Ependymal cells, which line the ventricles(cavities) of the brain and the central canal of

the spinal cord.

NeuronsAlthough neurons vary considerably in size and

shape, they generally have three principal regions: (1)

a cell body, (2) dendrites, and (3) an axon. Dendrites

and axons can be referred to generically as processes

or extensions from the cell body. The cell body is the

enlarged portion of the neuron that contains the

nucleus. It is the nutritional center of the neuron

where macromolecules are produced. The cell body

also contains densely staining areas of rough

endoplasmic reticulum known asNissl bodies that are

not found in the dendrites or axon. The cell bodies

within the CNS are frequently clustered into groups

called nuclei (not to be confused with the nucleus of

a cell). Cell bodies in the PNS usually occur in

clusters calledganglia.

Dendrites (dendron = tree branch) are thin,

branched processes that extend from the cytoplasm of

the cell body. Dendrites provide a receptive area that

transmits electrical impulses to the cell body. The

axon is a longer process that conducts impulses away

from the cell body. Axons vary in length from only a

millimeter long to up to a meter or more (for those

that extend from the CNS to the foot). The origin of

the axon near the cell body is an expanded region

called the axon hillock; it is here that nerve impulses

originate. Side branches called axon collaterals may

extend from the axon.

Proteins and other molecules are transportedthrough the axon at faster rates than could be

achieved by simple diffusion. This rapid movement is

produced by two different mechanisms: axoplasmic

flow and axonal transport. Axoplasmic flow, the

slower of the two, results from rhythmic waves of

contraction that push the cytoplasm from the axon

hillock to the nerve endings. Axonal transport,

which employs microtubules and is more rapid and

more selective, may occur in a reverse (retrograde)

direction as well as in a forward (orthograde)

direction. Indeed, retrograde transport may be

responsible for the movement of herpes virus, rabies

virus, and tetanus toxin from the nerve terminals intocell bodies.

Classification of Neurons

Structural Classification:Neurons are groupedstructurally according to the number of processes

extending from their cell body. Three major neuron

groups make up this classification: multipolar,

bipolar, and unipolar neurons. Multipolar neurons

have three or more processesone axon and the rest

dendrites. They are the most common neuron type in

humans, with more than 99% of neurons belonging to

this class. Multipolar neurons are the major neuron

type in the CNS.


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Bipolar neurons have two processesan axon and adendritethat extend from opposite sides of the cell

body. These rare neurons are found in some of the

special sense organs. Examples include some neurons

in the retina of the eye and in the olfactory mucosa.

Unipolar neurons have a single short process that

emerges from the cell body and divides T-like into

proximal and distal branches.

Functional Classification: The functional

classification is based on the direction in which they

conduct impulses. Sensory, or afferent, neurons

conduct impulses from sensory receptors into the

CNS. Motor, or efferent, neurons conduct impulses

out of the CNS to effector organs (muscles and

glands). Association neurons, or interneurons, are

located entirely within the CNS and serve theassociative, or integrative, functions of the nervous

system. There are two types of motor neurons:

somatic and autonomic. Somatic motor neurons are

responsible for both reflex and voluntary control of

skeletal muscles. Autonomic motor neuronsinnervate (send axons to) the involuntary effectors

smooth muscle, cardiac muscle, and glands. The cell

bodies of the autonomic neurons that innervate these

organs are located outside the CNS in autonomic

ganglia. There are two subdivisions of autonomic

neurons: sympathetic and parasympathetic.

Autonomic motor neurons, together with their central

control centers, constitute the autonomic nervoussystem.

BrainThe brain is safely kept inside the cranial vault.

Inside the skull the brain is surrounded by three

protective coverings. They may be grouped under

two divisions.

Pachymenix -It includes the duramater.

Leptomeninges - In includes the arachnoid

mater and pia mater.

The duramater is the outermost membrane. It is thick

and inelastic in nature. The arachnoid mater is the

middle covering over the brain. In between arachnoid

and piamater there is a space called the subarachnoid

space. It contains cerebro-spinal fluid and blood

vessels. The piamater is a delicate membrane closely

applied to the brain. This membrane contains blood

capillaries supplying blood to the brain cells.

The human brain weighs about 1.3 Kg. It

contains more than a billion neurons. Based on

embryological development the brain can be divided

as follows.

1.Prosencephalon (Fore brain) - It consists of the

cerebrum and the diencephalon.

The cerebrum is the largest part of the brain. It

is divided into right and left hemispheres by a

longitudinal fissure. However, at the base the

two hemispheres are connected by a sheet of

nerve fibres called the corpus callossum. Theouter surface of the cerebrum is called the

cortex or grey mater. It is 2 to 4 mm thick.

The inner content of the cerebrum is the white

mater. The surface of the cerebrum has several

folds called the gyri. They greatly increase the

surface area of the cortex. The shallow grooves

in between the gyri are called the sulci. A

central sulcus runs in the lateral surface of the

cerebrum from superior to inferior region. Each

cerebral hemisphere is divided into four lobes.

They are the frontal at the front, the parietal

towards the top of the head, the temporal on

the side and the occipital at the rear.

The diencephalon contains the thalamus andhypothalamus. This region is found between

the cerebrum and the brain stem. The thalamus

has a cluster of nuclei which act as the relays

for particular sensory pathways. Just beneath

the thalamus, the hypothalamus is present. It

contains reflex centres linked to the autonomic

system. A funnel shaped stalk called the

infundibulum extends from its floor. It is


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connected to the neurohypophysis of the

pituitary gland.

2. Mesencephalon (mid brain) - It is the smallest

region of the brainstem. On its dorsal surface

there are four rounded bodies called the carpora

quadrigemina.

3. Rhombencephalon (hind brain) - The three

main regions of the rhombencephalon are the

medulla oblongata, the pons varoli and the

cerebellum.

The cerebellum consists of two hemispheres.

Its surface has many ridges called folia. The

cerebellum consists of three parts. They are the

small anterior flocconodular lobe, a narrow

central vermis and two large lateral

hemispheres.

The pons is just superior to the medulla

oblongata. It contains ascending and

descending nerve tracts.

The medulla oblongata is about 3 cm long. It

is continuous with the spinal cord. It remainsas a bridge between the brain and the spinal

cord.

Brain stem - The medulla oblongata, pons and

mid brain form the brain stem. It connects the

spinal cord to the brain. Ten of the twelve

cranial nerves enter or exit the brain through

the brain stem.

Spinal cordThe spinal cord extends from the foramen

magnum to the level of the second lumbar vertebra. It

is considerably shorter than the vertebral column.

There are two enlargements in the spinal cord. Theyare the cervical and lumbar enlargements. Below the

lumbar enlargement the spinal cord tapers to form a

cone like region called the conus medullaris. A

connective tissue filament called the filum terminaleextends inferiorly from conus medullaris to the

coccyx. The conus medullaris and the nerves

extending below resemble a horses tail. Hence it is

called cauda equine. A cross section of the spinal

cord reveals a central grey portion and a peripheral

white portion. The white matter consists of nerve

tracts and the grey matter consists of neuron cell

bodies and dendrites.

The dorsal and ventral sides have long fissures.

There are 31 pairs of spinal nerves arising from the

spinal cord. Each nerve has a dorsal root and a

ventral root from the spinal cord. The dorsal roots

have dorsal root ganglia.

VentriclesThe entire CNS remains as a hollow tube. The

tube inside the adult brain forms ventricles. Each

cerebral hemisphere contains a large cavity called the

lateral ventricle. It corresponds to the hypothetical

first and second ventricles. The two lateral ventricles

communicate with the third ventricle located in the

centre of the diencephalon. This connection is made

through two interventricular foramina (foramen of

Monro). The third ventricle in turn opens into the

fourth ventricle found inside the medulla oblongata.

This communication happens through a narrow canal

called the cerebral aqueduct (aqueduct of sylvius).

The fourth ventricle is continuous with the central

canal of the spinal cord. The central canal extends

nearly to the full length of the cord.

Cerebro-spinal fluid (CSF)This fluid fills the ventricles of the brain and the

central canal of the spinal cord. About 80-90 % of

CSF is produced by specialized cells called

ependymal cells within the lateral ventricles.

Remaining 10-12 % is produced by similar cells in

the 3rd and 4th ventricles. These ependymal cells,

their supportive tissue and the associated blood

vessels together are called choroid plexuses. The

plexuses are formed by invagination of the vascular

piamater into the ventricles.

Nerves and Associated Ganglia

Structure and ClassificationA nerve is a cordlike organ that is part of the

peripheral nervous system. Nerves vary in size, but

every nerve consists of parallel bundles of peripheral

axons (some myelinated and some not) enclosed by

successive wrappings of connective tissue:

Each axon is surrounded by endoneurium, a

delicate layer of loose connective tissue that

also encloses the fibers associated Schwann

cells.

A coarser connective tissue wrapping, the

perineurium, binds groups of fibers into

bundles called fascicles.

A tough fibrous sheath, the epineurium,encloses all the fascicles to form the nerve.

Axons constitute only a small fraction of a nerves

bulk. The balance consists chiefly of myelin, the

protective connective tissue wrappings, blood

vessels, and lymphatic vessels. Nerves are also

classified according to the direction in which they

transmit impulses:


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Mixed nerves contain both sensory and motor

fibers and transmit impulses both to and from the

central nervous system.

Sensory (afferent) nerves carry impulses only

toward the CNS.

Motor (efferent) nerves carry impulses only

away from the CNS.

Most nerves are mixed. Pure sensory or motor nerves

are rare. Because mixed nerves often carry both

somatic and autonomic (visceral) nervous system

fibers, the fibers in them may be classified according

to the region they innervate as somatic afferent,

somatic efferent, visceral afferent, and visceral

efferent. For convenience, the peripheral nerves are

classified as cranial or spinal depending on whether

they arise from the brain or spinal cord. Ganglia are

collections of neuron cell bodies associated with

nerves in the PNS, whereas nuclei are collections of

neuron cell bodies in the CNS. Ganglia associated

with afferent nerve fibers contain cell bodies of

sensory neurons.

The Integumentary System

The word integument means covering. The

integumentary system covers the outside of the body.

It protects internal structures, prevents the entry of

infectious agents, reduces water loss, regulates body

temperature, produces vitamin D and detects stimuli

such as touch, pain and temperature. Since the

integument performs several functions, it iscommonly referred to asJack of all trades.

Functions of the Integumentary System1.Protection. The skin protects by chemical barriers

(the antibacterial nature of sebum, defensins,

cathelicidins, the acid mantle, and the UV shield of

melanin), physical barriers (the hardened

keratinized and lipid-rich surface), and biological

barriers (dendritic cells, macrophages, and DNA).

2.Body temperature regulation. The skin

vasculature and sweat glands, regulated by the

nervous system, play an important role in

maintaining body temperature homeostasis.3.Cutaneous sensation. Cutaneous sensory receptors

respond to temperature, touch, pressure, and pain

stimuli.

4.Metabolic functions. A vitamin D precursor is

synthesized from cholesterol by epidermal cells.

Skin cells also play a role in some chemical

conversions.

5.Blood reservoir. The extensive vascular supply of

the dermis allows the skin to act as a blood

reservoir.

6.Excretion. Sweat contains small amounts of

nitrogenous wastes and plays a minor role in

excretion.

The skin or integument rests on layers of cells called

hypodermis. The hypodermis attaches the skin to

underlying bones and muscles. It supplies blood

vessels and nerves to the skin.

The skin is composed of two major tissues, namely

dermis and epidermis. The dermis is mostly formed

of connective tissue having fibroblasts, adipose cells

and macrophages. It provides the structural strength

to the skin. The dermis accommodates nerve endings,

hair follicles, smooth muscles and glands.


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It is divided into two layers, namely the

superficial papillary layer and deeper reticular

layer. The papillary layer has projections called

papillae. The reticular layer is the major layer of the

dermis. It is dense in nature. It is continuous with the

hypodermis.

Epidermis :- The epidermis is made up of stratified

squamous epithelium. It is separated from the dermis

by a basement membrane. It contains mel-anocytes

giving colour to the skin. Many of the cells of the

epidermis produce a protein substance called keratin.

Hence they are called as keratinocytes.

The deepest layers of the epidermis produce nerve

cells by mitosis. As new cells are formed, the older

cells are pushed to the surface. The surface cells will

protect the inner new cells. Gradually the shape and

chemical nature of the surface cells will get altered.

Slowly they get filled with keratin. This process is

called keratinization. During this process the

epidermis gets divided into five distinct regions or

strata. They are the stratum basale, stratum

spinosum, stratum granulosum, stratum lucidum

and stratum corneum.

Stratum basale is in the deeper region of theepidermis. It consists of one layer of columnar cells.

Keratinization of cells begins in this region. Above

this layer stratum spinosum is seen. It has 8-10

layers of polygonal cells. The stratum granulosum

is the next upper layer. It has 3-5 layers of flattened

cells. Above this layer stratum lucidum occurs. It is

a thin zone having several layers of dead cells. The

top most layer is called the stratum corneum. It

consists of more than 20 layers of dead cells. These

cells get filled with keratin. They are said to be

cornified. The cornified cells are surrounded by a

hard protective envelope.

The skin can be either thick or thin. All fiveepithelial layers are seen in the thick skin. However

stratum corneum contains more number of cells.

Thick skin is formed in the soles of the feet, the

palms of hands and tips of fingers. The general body

surface has thin skin. In the thin skin each epithelial

layer inturn has few layers of cells. There are only

one or two layers of cells in stratum granulosum.

Callus :- The regions of skin subjected to constant

friction or pressure are thickened to form the callus.

The callus has several layers of cells in the stratum

corneum.

Skin colour :- The colour of the skin is due to

pigments in the skin. The thickness of the stratum

corneum and blood circulation can also cause skin

colour. Normally the colour is caused by the pigment

melanin. It provides colour to skin, hair and eye. It

protects the body from suns ultraviolet rays. Melanin

is produced by melanocytes. Melanin production is

genetically determined. However, hormones and

exposure to light can also alter the colour.

Skin dervatives

Hair :- The hairs are integumentary structures. A hair

has a root and a shaft. While the shaft projects above

the skin, the root remains well below the surface. The

base of the root has a hair bulb. It is an expanded

region. The shaft and most of the root of the hair areformed of dead keratinized epithelial cells. These are

arranged in three concentric layers called the

medulla, the cortex and the cuticle. The central axis

of the hair is formed of the medulla. Major part of

the hair is formed of a single layer of cells.

According to the amount and types of melanin, the

hair color may vary. The color of the hair is

genetically determined. During old age the amount of

melanin decreases causing white hair. Grey hair has

a mixture of faded, unfaded and white hairs.

The hair growth is due to addition of cells at the

base of the hair root. The growth stops at specific

stages. After a resting period, new hair replaces old

hair. The hairs on the head grow for a period of three

years and rest for 1-2 years.

The muscle cells found associated with hair

follicles are called the arrector pili. Contractions of

these muscles cause goose flesh making the hairs to

stand on end. The skin has sebaceous glands andthe sweat glands. The sebaceous glands are located

in the dermis. They produce an oily substance called

the sebum. These glands are connected by a duct to

the upper part of the hair follicles. The mammary

glands are the modified sweat glands.

The most common type of sweat gland on the

skin are the merocrine glands. They are simple

coiled tubular glands. They open directly on to the


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skin through sweat pores. The gland has two parts.

They are the deep coiled portion and the duct which

passes to the surface of the skin. The number of

sweat glands is more in the palms of the hands and

soles of the feet.

Nails :- Each nail is made up of two parts. They are

the nail root and the nail body. The nail body is the

visible part. The nail root is covered by the skin. The

proximal and lateral edges of the nail are covered by

nail fold.

The stratum corneum of the nail fold grows onto the

nail body as the eponchium. The free edge of the nail

body is the hyponchium. The nail is found placed on

the nail matrix and nail bed. A small white region

seen at the base of the nail is the lunula. It contains

the nail matrix. The nails grow at an average rate of

0.5-1.2 mm per day.

The Sensory Organs

Living organism respond to several stimuli such as

light, heat, sound, chemicals, pressure, touch, stretch

and orientation. These stimuli are felt by specific

receptors. The receptors convert the stimuli into

impulses in the nervous systems.

The touch receptors in the skin are the simplest

receptors. Such receptors are single nerve cells

responding directly to the stimulus. Other receptors

are complex sense organs. On these organs the

stimulus is channeled into a receptive region of the

organ. Among the several organs, the most important

are the eyes and ears.

The eyeThe eye is formed of 3 coats or tunics.

Coats or tunic Regions

1. Outer or fibrous - sclera & cornea2. Middle or vascular - choroid, ciliary body & iris

3. Inner or nervous retina

The sclera is the white outer layer of the eye. Itcovers posterior five-sixths of the eye. This firm layer

provides shape and protects the internal structures. A

small region of the sclera can be seen as the white of

the eye. In the front, the outer layer forms a

transparent region called the cornea. It permits entry

of light. The cornea is made up of a connective tissue

having collagen, elastic fibres and proteoglycans.

The middle tunic of the eyeball is the vascular

tunic. It contains most of the blood vessels. The

vascular tunic contains melanin containing pigment

cells. It appears black in colour. A major part of the

vascular tunic is found in association with the sclera

and called the choroid. Anteriorly this layer forms

the ciliary body and iris. The ciliary body consists

of smooth muscles called the ciliary muscles.

Contraction of the ciliary muscles can change the

shape of the lens.

Front of the EyeThe iris is the coloured part of the eye. It may be

black, brown or blue. It is a contractile structure

surrounding an opening called the pupil. Light enters

the eye through the pupil. The iris regulates such

entry by controlling the size of the pupil.

The inner most tunic of the eye is the retina. It

consists of an outer pigmented retina and an innersensory retina. The sensory retina is light sensitive.

It contains nearly 120 million photoreceptor cells

called rods and another 7 million cones.

Compartments of the eye : The eye has 2 major

compartments. There is a smaller compartment

anterior to the lens. Behind the lens there is a larger

compartment.


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The anterior compartment is divided into two

chambers. There is an anterior chamber found

between the cornea and iris. A smaller posterior

chamber lies between the iris and lens. These two

chambers are filled with a substance called the

aqueous humor. It helps to maintain intraocular

pressure.

The posterior compartment of the eye is much

larger and it contains a transparent jellylike substance

called vitreous humor. The eye lens is an unique

biological structure. It is transparent and biconvex. It

is made up of long columnar epithelial cells called

lens fibres. These fibres have an accumulation of

proteins called crystallines. The lens is placed

between the two eye compartments by suspensory

ligaments. The functioning of the eye is aided byaccessory structures. They include the eyebrows,

eyelids, conjunctiva and lacrimal apparatus.

The eyebrows prevent the sweat during

perspiration from running down into the eye. They

help to shade the eyes from direct sunlight. The

eyelids and associated lashes protect the eyes from

foreign objects. The medial region where the eyelids

join has a small reddish-pink mound called the

carun

human anatomy handout 2

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