EXCRETORY SYSTEM(REMOVES WASTES)

Kidneys Skin Lungs Digestive system Respiratory system

Metabolic processes give rise to chemical products which are of no use to the body.

In fact, some of them may prove detrimental to health, and accumulation of these waste products can even be fatal.

Hence waste products are removed from the body by the process of excretion.

The digestive and respiratory systems have been dealt with separately.

URINARY SYSTEM

URINARY SYSTEM

METABOLIC WASTES INCLUDE

1. EXCESS WATER AND SALTS

2. CARBON DIOXIDE FROM CELLULAR RESPIRATION

3. NITROGENOUS COMPOUNDS FROM THE

BREAKDOWN OF PROTEINS

4. UREA.

KIDNEY

The kidneys lie towards the back of the abdomen, one on each side of the spinal column.

A tube – the ureter – connects each kidney with the bladder, where urine collects to be passed through the urethra.

The 2 important functions are to remove waste products from the blood and to regulate the salt and liquid content of the body.

The kidneys also act as endocrine organs, secreting substances which have the following effects: Formation of red blood cells. Regulation of blood pressure. Involvement in the control of calcium

metabolism.

The main function of the kidneys is to filter waste products out of the blood passing through them.

Blood enters through the renal arteries, and passes back to the circulation through the renal veins.

The wastes, such as urea, and excess water, are secreted from the kidneys as urine.

The urine passes through the ureters to the urinary bladder, where it is stored and periodically released.

Each kidney contains about a million filtering units, called nephrons.

Filtration, selective resorption, and secretion processes occur in the nephrons.

The first part of the nephron is called the proximal tubule.

This part is 15 mm long, and begins with a cup like expansion (Bowman’s capsule).

A small bunch of renal capillaries (very narrow vessels, arising from the renal artery) is ‘pushed’ into this cup like capsule.

The blood in these capillaries is under pressure, hence fluid and some dissolved substances enter the nephron, while large molecules – such as proteins remain in the blood vessels.

This process is called filtration. Water and dissolved molecules pass from the

blood to the nephron. In a normal kidney this filtrate should not contain

protein or red blood cells. Hence, if protein is found in the urine it is a

sign of abnormal kidney function.

The other parts of the nephron are: Loop of Henle (descending limb) Loop of Henle (ascending limb) Distal tubule Collecting duct

The first three are approximately 12 mm in length.

The collecting duct is 5 mm in length.

Selective resorption occurs chiefly in the proximal tubule, but also occurs in the other segments, right up to the collecting duct.

The following substances are taken up from the tubule, back into the interstitial spaces or into the blood stream (i.e. resorbed), useful substances such as glucose, vitamins, and amino acids; and salts such as sodium, potassium, chloride.

A considerable amount of water is taken up once more as well.

In fact out of 180 liters/day, which pass through the kidney, only 1 to 1.5 liters is excreted as urine everyday.

Among other factors which influence the resorption of water in the collecting duct, the Anti Diuretic Hormone (ADH), which is stored in the posterior pituitary.

This ADH is released to conserve body fluid by decreasing the urine output.

This may be required when considerable fluid or blood has been lost.

Secretion of certain substances in the distal tubule and collecting duct, such as hydrogen ions and ammonia are added to the urine in the nephron.

Urine formed

Filtrate

(water & substances like urea)

Substances resorbed

(glucose, amino acids)

Substances secreted

(H+, NH4+)

=__

+

The collecting ducts open into large collecting ducts.

There are about 250 in each kidney, which collect urine from more than 4000 nephrons each.

These open into the ureters.

URETERS There are 2 muscular tubes, 25 to 30 cm in

length, with a very small inner diameter of about 3 mm.

They convey urine from the kidneys to the bladder.

URINARY BLADDER Pear shaped organ, which lies in the pelvis. Though the 2 ureters, the bladder receives waste

products, including urea from the kidneys. The average capacity of the bladder is up to 400

or 500 mL. The need to pass urine is usually felt when the

bladder is filled to about 280 mL. Sometimes up to 500 mL can be tolerated. Filling above 500 mL leads to pain.

EMPTYING THE BLADDER Emptying the bladder (via the urethra) takes

place by contraction of the smooth muscle – detrusor.

On either side of the urethra there are muscle bundles, called the internal urethral sphincter.

There is a significant difference between the 2 sphincter – the internal sphincter is not under voluntary control, i.e. we are unable to prevent it relaxing and allowing urine to pass through.

In contrast, the external sphincter is under voluntary control.

Thus it is possible to learn to empty the bladder, at appropriate times and places.

Occasionally, in adults there may be involuntary voiding of the bladder contents, when the intraabdominal pressure is raised – during coughing, sneezing, or laughing.

Sometimes this may also be brought on by emotional stress.

Parasympathetic nerves which arise from the sacral part of the spinal cord, send messages to empty the bladder, by causing contraction of the detrusor muscle, and relaxing the internal sphincter.

Sympathetic nerves also supply the internal sphincter.

It was believed that sympathetic nerves relax the detrusor, contract the internal sphincter, and prevent emptying.

The sympathetic nerves to the bladder are believed to be chiefly important – in males – in preventing semen from entering the bladder during ejaculation.

The voluntary (somatic) nerves regulate the external sphincter.

These nerves also arise from the sacral part of the spinal cord.

Some of the “higher” centers involved: The frontal and prefrontal cortices. Parts of the hypothalamus Brainstem (pons, midbrain)

Scientific studies have proved that the parts of the brain are involved in initiating and conducting

micturition

URETHRA Is a passage way through which urine is

discharged from the bladder. In men the urethra is longer than in females. It passes through the penis, and also has a

passage for the discharge of semen. In women the shorter urethra opens immediately

in front of the vagina. It serves solely for the discharge of urine.

MODIFYING THE FLOW An increase in the flow of urine can be stimulated by

substances called diuretics. These act in different ways.

Alcohol is a diuretic which stimulates both the blood circulation and the kidneys, directly.

Caffeine, which is present in both coffee and tea is also a diuretic which acts on the kidneys.

Temperature is another factor which influences the production of urine.

The lower the temperature the greater the volume of urine produced.

This is because less water is lost by evaporation through the skin and through sweat.

URINEThe average daily output of urine is 1.0 to 1.5

liters/day.Apart from excess water, urine contains

nitrogenous substances (chiefly urea) – approximately 32 g and mineral salts containing sodium, chloride, phosphorus, and potassium.

Urea and other nitrogen containing substances are derived from protein.

These compounds undergo further changes in the liver before passing to the kidneys to be excreted in the urine.

The urinary system thus regulates Fluid volume by excreting excess water or

conserving water when needed. Acid level (pH) by excreting appropriate

amounts of acid or alkali. This is very important as most of the enzymes,

and other processes in the body require the pH to be as close to the normal (i.e. 7.4 in arteries; slightly lower in veins) to function correctly.

Water, Electrolyte, & Acid-Base Balance A. To be in balance, the quantities of fluids and

electrolytes leaving the body should be equal to the amounts taken in.

B. Anything that alters the concentrations of electrolytes will also alter the concentration of water, and vice versa.

C. Distribution of Body Fluids A. A. Fluids occur in compartments in the body, and

movement of water and electrolytes between compartments is regulated.

B. Fluid Compartments

WATER BALANCE

A. Water balance exists when water intake equals water output.

B. Water Intake

C. Regulation of Water Intake

D. Water Output

E. Regulation of Water Output

ELECTROLYTE BALANCE

A. An electrolyte balance exists when the quantities of electrolytes gained equals the amount lost.

B. Electrolyte Intake

C. Regulation of Electrolyte Intake

D. Electrolyte Output

E. Regulation of Electrolyte Output

ACID-BASE BALANCE

A. Electrolytes that ionize in water and release hydrogen ions are acids; those that combine with hydrogen ions are bases.

B. Maintenance of homeostasis depends on the control of acids and bases in body fluids.

C. Sources of Hydrogen Ions.

D. Strengths of Acids and Bases.

E. Regulation of Hydrogen Ion Concentration.

THE SKIN (INTEGUMENT)The skin is the outer covering of the body. It is a major organ of the body, forming about

8% of the total mass.The average area (depending on the body build)

may range from 1.2 to 2.2 sqm.The total thickness is 1.4 to 4.0 mm.

SKIN STRUCTURE

The integument has a number of functions:

1. It is a barrier against germs.

2. It acts as a touch, resilient cushion – protection for the structures beneath.

3. It helps to regulate the body temperature. When it is hot, glands in the skin secrete perspiration on sweat. When sweat evaporates, cooling occurs. When it is cold, constriction of the blood vessels in the skin cuts down the flow of blood near the body’s surface. Hence it is decreased heat loss.

4. The skin is also a sense organ. Nerve ending (and other receptors) in the skin respond to pain, heat, cold, touch, and pressure.

5. The skin supplies much of the body’s needs for Vitamin D. a substance is produced in the skin, which changes into Vitamin D, when exposed to sunlight.

6. The skin gives rise to specialized structures such as hair and nails.

The skin has two distinct layers – the epidermis – outer covering and the inner dermis, or true skin.

The uppermost part consists of flat, old cells which are constantly being shed, or ‘sloughed off’.

The underlying part of the epidermis is made up of rapidly dividing cells.

These cells continuously push upward to replace dead cells.

Tiny blood vessels and nerve ending are densely woven into the flexible connective tissue which makes up the dermis.

Sweat and oil (sebaceous) glands are embedded in it.

There are also various types of receptors for the touch sensation, beneath the skin.

SWEAT GLANDS There are approximately 2 million sweat glands

all over the skin. They are tiny, coil shaped tubes. They extend from the deepest layer of the skin to

the surface. These glands help to regulate the body

temperature and also contribute to the excretion of water and salt from the body.

Sweat contains about 98-99% of water. Certain inorganic salts – specially sodium

chloride along with small make up the remaining 1-2%.

During exercise more blood moves through the vessels surrounding the sweat glands.

This increases the sweat secretion, helping to lose heat and to remove waste products.

In offering protection to the internal organs against germs and injuries, the skin itself is vulnerable to various insults.

In particular, the skin of certain persons is usually sensitive.

In extreme cases this hypersensitivity can lead to an allergic reaction.

This is observed in certain skin conditions, such as eczema.

SWEAT GLAND