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VASCULAR SYSTEM • The cardiovascular system is concerned with the transport of blood and

lymph through the body. • It may be divided into four major components: 1- the heart, 2- the macrocirculation,3- the microcirculation 4- and the lymph vascular system.• Essentially, the macrocirculation comprises all vessels, both arteries and

veins, that would be visible to the eye.• The vessels of the macrocirculation supply and drain a network of fine

vessels interposed between them, the capillaries. • This network is also called the capillary bed. • Water and other components of the blood plasma which exude from the

blood vessels form the interstitial fluid, which is returned to the circulation by the lymph vascular system.

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MAMMALIAN HEART

pulmonaryartery

aorta

leftatrium

cardiac

vein

left

ventricleright

ventricle

inferior vena cava

rightcoronary

artery

right atrium

superior vena cava

bicuspidvalve

Purkinje

system

pulmonarysemilunar

valve

tricuspidvalve

sinoatrialnode

(pacemaker)

atrioventricular

node

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HEART WALL

mesothelium

coronary vessel fat cells

cardiac muscle

subendocardial layer

subendothelial layer endothelium

heart interior

pericardium

pericardialspace

epicardium

myocardium

endocardium

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General Structure of Blood Vessels• a common structural pattern that can be seen in all blood vessels with the

exception of capillaries,• i.e. the division of the walls of the blood vessels into three layers or tunics.

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General Structure of Blood Vessels

The division of the walls of the blood vessels into three layers or tunics. • The tunica intima

– delimits the vessel wall towards the lumen of the vessel and comprises its endothelial lining (typically simple, squamous) and associated connective tissue. Beneath the connective tissue, we find the internal elastic lamina, which delimits the tunica intima from

• the tunica media. – The tunica media is formed by a layer of circumferential smooth

muscle and variable amounts of connective tissue. A second layer of elastic fibers, the external elastic lamina, is located beneath the smooth muscle. It delimits the tunica media from

• the tunica adventitia, – which consist mainly of connective tissue fibres. The tunica adventitia

blends with the connective tissue surrounding the vessel. The definition of the outer limit of the tunica adventitia is therefore somewhat arbitrary.

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VASCULAR TUNICS

TUNICA MEDIATUNICA

ADVENTITIA

(endothelium)

(smooth muscle) (connective tissue)

TUNICA INTIMA

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Variations of Vessel Wall Structure Arteries• All arterial vessels originate with either the

pulmonary trunk (from the right ventricle) or the aorta (from the left ventricle).

• Specialisations of the walls of arteries relate mainly to two factors:

1- the pressure pulses generated during contractions of the heart (systole)

2- and the regulation of blood supply to the target tissues of the arteries.

The tunica media is the main site of histological specialisations in the walls of arteries.

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Vessels close to the heart (aorta, pulmonary trunk and the larger arteries that originate from them) are

Elastic arteries – The tunica intima of elastic arteries is thicker than in other

arteries. – A layer of loose connective tissue beneath the endothelium

(subendothelial connective tissue) allows the tunica intima to move independently from other layers as the elastic arteries distend with the increase in systolic blood pressure.

– Distension of the walls is facilitated by concentric fenestrated lamellae of elastic fibres in a thick tunica media. In adult humans, about 50 elastic lamellae are found in the tunica media of the aorta.

– The energy stored in the elastic fibres of the tunica media allows elastic arteries to function as a "pressure reservoir" which forwards blood during ventricular relaxation (diastole).

– Smooth muscle cells and collagen fibres are present between the layers of elastic fibres.

– Both fibre types are produced by the smooth muscle cells.

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– Each elastic lamella forms together with interlamellar fibres and cells a lamellar unit.

– The external elastic lamina is difficult to discern from other layers of elastic fibres in the tunica media.

– The tunica adventitia appears thinner than the tunica media and contains collagen fibres and the cell types typically present in connective tissue.

– The walls of these large arteries are so thick that their peripheral parts cannot derive enough oxygen and nutrients from the blood of the vessel that they form.

– Larger vessels are therefore accompanied by smaller blood vessels which supply the tunica adventitia and, in the largest vessels, the outer part of the tunica media of the vessel wall.

– The vessels are called vasa vasorum. – In macroscopic preparations vasa vasorum are visible as

fine dark lines on the surface of the larger arteries.

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Aorta, human - H&E , elastin & van Gieson

• The thin endothelial lining of the aorta corresponds to that of other vessels.

• The flattened cells are easily damaged during preparation and it may be difficult to identify the endothelium.

• The subendothelial layer of connective tissue is characterised by a lower density of cells, i.e. fewer nuclei, a fibrous appearance of the tissue and the absence of well-defined elastic layers.

• Elastic lamellae become visible in the tunica media. • The majority of cells in the tunica media are smooth muscle

cells.• Smooth muscle cells and collagen fibres are found between

the layers of elastic fibres.• If you scan the periphery of the aorta you may find small

blood vessels, the vasa vasorum, in the tunica adventitia and penetrating into the outer part of the tunica media.

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1-Conducting or Elastic Arteries.

Aorta, human - H&E

elastin

van Gieson

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• The diameter of individual arteries decreases as we follow them further into the periphery.

• However, their total diameter increases, which leads to a fall in blood pressure.

• Also, the properties of the elastic arteries have to some extent evened out differences in diastolic and systolic blood pressure.

• The amount of elastic fibres in the tunica media decreases with these physiological changes.

• We now find a type of arteries which are termed

Muscular arteries

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Distributing or Muscular Arteries. • The tunica intima is thinner than in elastic arteries.• Subendothelial connective tissue other than the internal

elastic lamina is often difficult to discern. • The internal elastic lamina forms a well defined layer. • The tunica media is dominated by numerous concentric layers

of smooth muscle cells.• Fine elastic fibres and a few collagen fibres are also present.• The external elastic lamina can be clearly distinguished

although it may be incomplete in places. • The thickness and appearance of the tunica adventitia is

variable.• The basic structure of the walls of arteries does not change

much as we come to the next type of arterial vessels. • Size is used to differentiate them from muscular arteries.

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Arterioles

• are arterial vessels with a diameter below 0.1 - 0.5 mm (different values in different textbooks).

• Endothelial cells are smaller than in larger arteries, and the nucleus and surrounding cytoplasm may 'bulge' slightly into the lumen of the arteriole.

• The endothelium still rests on a internal elastic lamina, which may be incomplete and which is not always well-defined in histological sections.

• The tunica media consists of 1-3 concentric layers of smooth muscle cells.• It is difficult to identify an external elastic lamina or to distinguish the

tunica adventitia from the connective tissue surrounding the vessel.• The smooth muscle of arterioles and, to some extent, the smooth muscle of

small muscular arteries regulate the blood flow to their target tissues.• Arterioles receive both sympathetic and parasympathetic innervation. • The final branching of the arterioles finally gives rise to the capillary

network (microcirculation).

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Metarterioles

• These are small vessels that are on the border between arterioles and the capillary bed.

• They can act as sphincters and cut off the flow of blood into the capillary bed.

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Artery - H&E and elastin & eosin • There are two easily recognizable features which distinguish these

arteries from veins. • If two vessels have a similarly sized lumen, the walls of arteries will be

much thicker and more compact than the wall of veins.

• At high magnification, the internal elastic lamina forms a pink streak immediately below the endothelial cell lining in arteries and even arterioles, while it is difficult to identify in veins.

• The layer of subendothelial connective tissue is very thin, and the endothelium seems to rest on the internal elastic lamina.

• Smooth muscle cell nuclei are frequent in the tunica media. • The external elastic lamina stains similar to the internal elastic lamina, but

it is thicker and appears fibrous instead of forming a continuous band. • Collagen fibres and a few connective tissue cell nuclei are visible in the

tunica adventitia.

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Smooth muscle in arteries

• The smooth muscle in arteries is important in maintaining the vessel diameter during blood flow and also plays a role in blood pressure levels.

• The vascular smooth muscle is in a state of tonus (partial contraction).

• The degree of tonus of the smooth muscle cells in the wall of arteries and arterioles is controlled by the autonomic nervous system and also by endocrine secretions.

• In hypertension (high blood pressure), often associated with stress or aging, the peripheral arterial vessels show increased tonus.

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Endothelial cells

• The endothelial cells are derived from embryonic mesenchyme and should not be regarded as epithelial, but as connective tissue cells.

• Endothelial cells line the lumina of all the vessels of the blood vascular and lymphatic vascular systems.

• Endothelial cells lining the blood vessels are very flattened, elongated cells, with elongated nuclei that protrude into the lumina.

• Endothelial cells are known to produce a variety of local factors that are important in the functioning of the cardiovascular system.

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Capillaries• The sum of the diameters of all capillaries is significantly

larger than that of the aorta , which results in decreases in blood pressure and flow rate.

• capillaries are very small vessels.• Their diameter ranges from 4-15 µm. • The wall of a segment of capillary may be formed by a single

endothelial cell.• This results in a very large surface to volume ratio. • The low rate of blood flow and large surface area facilitate

the functions of capillaries in 1) providing nutrients and oxygen to the surrounding tissue, in 2) the absorption of nutrients, waste products and carbon

dioxide, and in 3) the excretion of waste products from the body.

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• These functions are also facilitated by a very simple organisation of the wall of capillaries.

• Only the tunica intima is present, which typically only consists of the endothelium, its basal lamina and an incomplete layer of cells surrounding the capillary, the pericytes.

• Pericytes have contractile properties and can regulate blood flow in capillaries.

• In the course of vascular remodelling and repair, they can also differentiate into endothelial and smooth muscle cells.

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Pericytes (Perivascular cells)

• Many capillaries have inconspicuous, elongated cells, similar in appearance to embryonic mesenchymal cells, associated with them.

• These cells, known as pericytes, or perivascular cells, are quite difficult to see in most histological preparations.

• These pericytes appear to have important roles in repair of blood vessels and connective tissue after injury.

• They have the potential to develop into fibroblasts, smooth muscle cells and may even be phagocytic.

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Three types of capillaries can be distinguished based on features of the endothelium.

• Continuous capillaries

• Fenestrated capillaries

• Discontinuous capillaries (Sinusoids)

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Continuous capillaries

• are formed by "continuous" endothelial cells and basal lamina.

• The endothelial cell and the basal lamina do not form openings, which would allow substances to pass the capillary wall without passing through both the endothelial cell and the basal lamina.

• Both endothelial cells and the basal lamina can act as selective filters in continuous capillaries.

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• In cross sections they are seen to be composed of 2-3

endothelial cells, connected by tight junctions.• In the region of the contact between the ends of two

endothelial cells there is a marginal fold, a small area in which the edge of one of the cells protrudes into the lumen.

• Continuous capillaries are characterized by abundant small invaginations of the cell surfaces (caveolae) and numerous micropinocytotic vesicles in the cytoplasm.

• All the materials crossing the cell (transcellular transport), in both directions do so via these micropinocytotic vesicles.

• Continuous capillaries are found in those organs that need strict control on access of the substances from the blood.

• These include all the organs with a "blood-barrier" such as the "blood-brain-barrier" of the Central Nervous System or the "blood-thymus barrier".

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Continuous capillaries

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Fenestrated capillaries • The endothelial cell body forms small openings called “pores”

“fenestrations”, which allow components of the blood and interstitial fluid to bypass the endothelial cells on their way to or from the tissue surrounding the capillary.

• The fenestrations may represent or arise from pinocytotic vesicles which open onto both the luminal and basal surfaces of the cell.

• The extent of the fenestration may depend on the physiological state of the surrounding tissue, i.e. fenestration may increase or decrease as a function of the need to absorb or secrete.

• The endothelial cells are surrounded by a continuous basal lamina, which can act as a selective filter.

• Fenestrated capillaries are common in most of the endocrine glands.

• One prominent site for fenestrated capillaries is in the renal glomeruli.

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Fenestrated capillaries

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Discontinuous capillaries • are formed by fenestrated endothelial cells, which may not

even form a complete layer of cells. • The basal lamina is also incomplete. • Discontinuous capillaries form large irregularly shaped

vessels, sinusoids or sinusoid capillaries. • Sinusoids (Discontinuous capillaries) are irregular vessels

with large diameters . • In most cases the sinusoids are not cylindrical.• They are found where a very free exchange of substances or

even cells between bloodstream and organ is advantageous (e.g. in the liver, spleen, and red bone marrow).

• And in those organs which need a very rich blood supply including most of the endocrine glands (hypophysis, suprarenal cortex, pancreas).

• Phagocytes are commonly associated with the walls of the sinusoids.

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Discontinuous capillaries

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CAPILLARY TYPES

ContinuousCapillary

TypicalLocations

fatmusclenervoussystem

Fenestrated

Capillary

TypicalLocations

intestinal villiendocrine glandskidney glomeruli

DiscontinuousCapillary

TypicalLocations

liverbone marrow

spleen

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WALLS OF CAPILLARY TYPES

discontinuity

Discontinuous Fenestrated

fenestra

withdiaphragm

Continuous

caveolus

vesicle     

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Cardiac Muscle, sheep - Whipf's polychrome • Large numbers of capillaries are

present in almost all tissues.• At least a few dozen cross sections are

present in every sqr. mm of section of poorly vascularised tissues.

• There may be thousands in highly vascularised tissues.

• However, a "good" capillary is not that easy to find because of their small size and because the capillary walls are very thin, which often leads to the collapse of the capillary during tissue preparation.

• Cardiac muscle is highly vascularised. • Each muscle cell is surrounded by one

or more capillaries. • The capillaries roughly follow the

course of the muscle cells.

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• To find capillaries in transverse and longitudinal sections it is easiest to first find areas in which the muscle cells have been cut in these planes.

• Only one or two red blood cells fit side by side in the capillary.

• A single endothelial cell forms the wall around the entire circumference of a segment of the capillary.

• Endothelial cell nuclei are therefore not always visible, and some red blood cells are only surrounded by a fine line representing the capillary wall.

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• The exchange of materials through capillary walls can be:

transcellular via :1-micropinocytotic vesicles in the endothelium

(as in continuous capillaries)2-fenestrations (as in fenestrated endothelium or

sinusoids) or intercellular via:1-gap junctions2-spaces between endothelial cells (as in

sinusoids of spleen, liver).

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ENDOTHELIAL PARACELLULAR PATHWAY

Paracellular Pathway (in between cells)

Tightjunctions

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ENDOTHELIAL TRANSCELLULAR PATHWAY

caveolus TRANSCELLULAR PATHWAY (across cells)

stoma (pits)

caveolus

raceme

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Arteriovenous anastomoses

• These represent direct connections between arterioles and venules.

• When there is no need for blood flow in the capillary bed these permit direct blood passage (arterial-venous-shunt).

• Arteriovenous anastomoses are very common in the dermis of the skin.

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Anatomical and Functional End Arteries• Anatomical end arteries are vessels whose terminal branches

do not anastomose. In the event that these vessels become blocked (atherosclerosis, blood clot) the tissues will be deprived of oxygen and an "infarct" develops (e.g. coronary arteries, kidneys, brain).

• Functional end arteries have anastomoses and in the event of blockage, alternative routes for blood and oxygen are available.

• Arterial pathology has major clinical importance in medicine. Common arterial disorders include: atherosclerosis (fatty deposits and occlusion), arteriosclerosis (hardening of the arteries), hypertension, aneurysms (ballooning of the vessel). Cardiac infarct and cerebral infarct due to occlusion of the lumen of arteries are two of the major causes of morbidity.

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Veins• The walls of veins are thinner than the walls of arteries, while

their diameter is larger. • In contrast to arteries, the layering in the wall of veins is not

very distinct.• The tunica intima is very thin. • Only the largest veins contain an appreciable amount of

subendothelial connective tissue.• Internal and external elastic laminae are absent or very thin.• The tunica media appears thinner than the tunica adventitia,

and the two layers tend to blend into each other.• The appearance of the wall of veins also depends on their

location. • The walls of veins in the lower parts of the body are typically

thicker than those of the upper parts of the body, and the walls of veins which are embedded in tissues that may provide some structural support are thinner than the walls of unsupported veins.

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Venous vessels originate from the capillary network which coalesce into the smallest venous vessels the

• Venules.

• Small to medium-sized veins

• The largest veins of the abdomen and thorax

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Venules.

• They are larger than capillaries.

• Small venules are surrounded by pericytes.

• A few smooth muscle cells may surround larger venules.

• The venules merge to form

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Small to medium-sized veins• which contain bands of smooth muscle in the

tunica media.• The tunica adventitia is well developed.• Aside from most veins in the head and neck,

small to medium-sized veins are also characterised by the presence of valves.

• The valves are formed by loose, pocket-shaped folds of the tunica intima, which extend into the lumen of the vein.

• The opening of the pocket will point into the direction of blood flow towards the heart.

• One to three (usually two) pockets form the valve.• Blood flowing towards heart will pass the

pockets.• If the flow reverses, blood will fill the pockets

which will occlude the lumen of the vein and prevent the return of blood into the part of the vein preceding the valve.

• The ability of the valves to prevent backflow depends to some extent on the state of contraction (tone) of the smooth muscle in the wall of the vein.

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The largest veins of the abdomen and thorax

• do contain some subendothelial connective tissue in the tunica intima, but both it and the tunica media are still comparatively thin.

• Collagen and elastic fibres are present in the tunica media.• The tunica adventitia is very wide, and it usually contains

bundles of longitudinal smooth muscle. • The transition from the tunica adventitia to the surrounding

connective tissue is gradual. • Valves are absent.

Vasa vasorum are more frequent in the walls of large veins than in that of the corresponding arteries - probably because of the lower oxygen tension in the blood contained within them.

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Vein, human - H&E1- The tunica intima is very narrow and the

internal elastic lamina is difficult to identify - even in elastin stained sections.

* A few elastic fibres below the endothelium form only a very thin and incomplete internal elastic lamina.

* Smooth muscle is present in the tunica media, but it is organised less regular than in the artery.

2- The tunica media is, again as compared to the artery, very thin and there is no sharp border between the tunica media and the tunica adventitia.

3- The tunica adventitia of the largest veins contains coarse collagen fibres, elastic fibres and longitudinal bundles of smooth muscle.

* Small and medium sized veins will not contain smooth muscle in the tunica adventitia.

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sections containing small to medium sized veins - H&E, van Gieson or trichrome

• skin slides, in which found quite a few nice valves in the veins located at the border between dermis and hypodermis.

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Vein Valve - H&E

• Valves are only found in small to medium-sized veins.

• one or two bands of tissue in the lumen of the vein.

• Each band is formed by two apposing layers of tunica intima.

• The bands may share their origin from the inner aspect of the wall of the vein or they may have separate origins.

• Folding of the tissue bands forming the valves is variable.

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Blood portal systems• In typical configurations an artery or arteriole carrying

oxygenated blood enters the capillary bed, where there is exchange of oxygen and metabolites, and the vessel exiting the capillary bed is a venule or vein with deoxygenated blood.

• Portal systems describe situations where the blood vessel leaving the capillary bed is of the same category as the blood vessel entering the capillary bed. (vein - capillary bed - vein or artery - capillary bed - artery).

• In a venous portal system (such as in the liver) a vein (hepatic portal vein) enters the capillary bed and a vein (hepatic vein) exits the capillary bed. A similar portal system is found in the hypothalmus -hypophysis.

• An example of an arterial portal system is found in the renal cortex. Afferent arterioles break up into the capillary bed of the glomerular tufts of the renal corpuscle and the blood exits in efferent arterioles.

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Additional Specialisations of Vessels• Small arteries and veins often form anastomosing networks, which

provides routes for alternative blood supply and drainage if one of the vessels should become occluded because of pathological or normal physiological circumstances.

• Some arteries are however the only supply of blood to their target tissues. • These arteries are call end arteries.• Tissues which are supplied by end arteries die if the arteries become

occluded.• The segments of the kidney and the heads of the gastrocnemius muscle are

examples of tissues supplied by end arteries.• Arteries and veins may also form arteriovenous shunts, which can shunt

the blood flow that otherwise would enter the capillary network between the vessels.

• These shunts usually contain specialisations of the smooth muscle in the region of the shunt.

• Arteriovenous shunts are frequently seen in the blood supply of distal parts of the limbs and the nose (thermoregulation) and in the blood supply of endocrine organs.

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Lymphatic Vessels

• Parts of the blood plasma will exude from the blood vessels into the surrounding tissues because of transport across the endothelium or because of blood pressure and the fenestration of some capillaries (this process is partly counteracted by the higher osmotic pressure of the blood).

• The fluid entering tissues from capillaries adds to the interstitial fluid normally found in the tissue.

• The surplus of liquid needs to be returned to the circulation. • Lymph vessels are dedicated to this unidirectional flow of

liquid, the lymph. • Three types of lymph vessels can be distinguished based on

their size and morphology.

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LYMPHATIC SYSTEM

artery

vein

lymphaticsinusoid

lymphatic capillary

bloodcapillariesvenule

arteriole

LYMPHATIC CAPILLARY

LYMPHATIC SINUSOID

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Lymph capillaries

• are somewhat larger than blood capillaries and very irregularly shaped.

• They begin as blind-ending tubes in connective tissue.• The basal lamina is almost completely absent and the

endothelial cells do not form tight junctions, which facilitates the entry of liquids into the lymph capillary.

• Temporary openings in the endothelial lining of the lymph capillaries also allow the entry of larger particles into the lymph capillaries (lipid droplets, which are absorbed from the lumen of the gut do not enter blood capillaries, but enter the circulation via lymph vessels which are found in the villi of the ileum and jejunum). Lymph capillaries merge to form

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LYMPHATIC CAPILLARIES

ARTERIOLE VENULE

LYMPHATIC

SMALL VEIN

LYMPHATIC

LYMPHATIC

SMALL ARTERY

BLOOD

CAPILLARY

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Lymph collecting vessels

• which are larger and form valves but otherwise appear similar to lymph capillaries.

• The lymph is moved by the compression of the lymph vessels by surrounding tissues.

• The direction of lymph flow is determined by the valves.• Lymph vessels empty intermittently into lymph nodes from which the

lymph continues in efferent lymph vessels.• Only very little lymph is returned from the limbs if they are immobilized,

which illustrates the importance of muscular action in lymph transport. • The effect can also be observed after long intercontinental flight when you

may feel that your shoes and socks are just about one number too small.• Finally, impeded lymph drainage is one of the problems associated with

surgery which requires the removal of lymph nodes and which thereby interrupts the lymph collecting vessels.

• Eventually the lymph collecting vessels merge to form

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Lymph ducts

• which contain one or two layers of smooth muscle cells in their wall . • They also form valves.• The walls of the lymph ducts are less flexible in the region of the

attachment of the valves to the wall of the duct, which may give a beaded appearance to the lymph ducts.

• Peristaltic contractions of the smooth muscle contribute to the movement of lymph towards the heart in addition to the compression of the ducts by surrounding tissues.

• The largest lymph duct of the body, the thoracic duct, drains lymph from the lower half and upper left quadrant of the body and empties the lymph into the circulation by merging with the vascular system close to the junction of the left internal jugular and subclavian veins.

• That it is the largest lymph duct does not mean that it is a large vessel when compared to the large arteries and veins.

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Sections of small intestine - H&E Jejunum, baboon - H&E

• At low magnification you will see villi extending into the lumen of the jejunum.

• In some of the villi you will see fairly large open spaces, which are surrounded by a layer of flattened endothelial cells.

• They should not contain any red blood cells - if they do you are looking at a capillary, which also should be somewhat smaller.

• These openings represent the blind end of lymph capillaries which originate in the villi.

• Their name, lacteals, is derived from the milky appearance of the lymph.

• This appearance is caused by suspended lipid droplets which enter these lymph capillaries.

• The jejunum slide is also good for revision. You should be able to find columnar epithelium, goblet cells, smooth muscle, small nerves, a few ganglion cells and, of course, lots of loose connective tissue and blood vessels.

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Differences between a medium sized -artery & a medium-sized vein

Medium –sized artery Medium sized Vein 1) The lumen appears rounded 1) The lumen appears collapsed.

2) The lumen contains no blood after death. 2) It is lumen usually contains blood.

3) It has a thick wall but narrow lumen 3) It has a thin wall but wide lumen.

4) There are no valves. 4) They often have valves.

5) The intima is thick , folded & has a well – developed internal elastic lamina

5) The intima is thin , not folded & has no elastic lamina.

6) The media is thick & contains elastic fibers.

6) The media is thin with very few elastic fibers.

7) External elastic lamina may be present between the media & adventitia.

7) No external elastic lamina

8) The adventitia is thin & contains some elastic fibers.

8) The adeventitia is thick & is very rich in collagenous fibers.

9) No lymphatic capillaries in it is wall . 9) Lymphatic capillaries may be present in it is adeventitia.

10) It has a rapid flow of arterial blood. 10) It is a slow flow of venous blood.

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Differences between capillaries & sinusoids

Capillaries Sinusoids

1) Present allover the body 1) Present in liver , spleen, bone marrow.

2) Having narrow regular lumens. 2) Having wider irregular lumens.

3) Their walls are formed of simple squamous cells & are surrounded by continuous basement membrane.

3) Their walls are lined by fenestrated cells & are surrounded by reticular C.T.

4) Their walls have no pores except in kidney & endocrine glands.

4) Their walls contain pores.

5) Undifferentiated pericyte cells are present in their walls.

5) Phagocytic macrophage littoral cells are present outsides their walls.

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Differences between blood capillaries & lymphatic capillaries

Blood Capillaries Lymphatic capillaries1) Present superficial in position under the skin & mucous membrane.

1) Present more deep in position than blood capillaries

2)They are the branching vessels of arterioles & are connected with venules at the other side.

2)They start as blind – ended channels & are connected to lymph vessels at one side only.

3)They have uniform diameters 3)Their diameters are irregular

4)The endothelium is fenestrated 4) Non-fenestrated endothelium

5)They are surrounded with basement membranes & pericytes.

5)They have no basement membrane & no pericytes.

6)The lumen is usually patent 6)The lumen may be collapsed.

7)No anchoring collagenous fibers outside their wall

7) Anchoring collagenous fibers connect their wall to surrounding C.T.

8)They carry blood. 8)They carry lymph.