Characteristics Of Circulatory System Tissue Contributing To Function Biology Essay


The circulatory system is one of the important systems of the body which provide a medium gaseous exchange, nutrients and waste removal besides helping in regulating normal physiological balance. It is composed of two separate but related components; the cardiovascular system and lymphatic vascular system. It integrates three basic functional parts, or organs; a pump (the heart) that circulates a liquid (the blood) through set of containers (the vessels). This integrated system is able to adapt to the changing circumstances of normal life. Demands on this system are enormous and its capability to withstand the dynamic changes is due to its structure and the ability of endothelium to produce various substances for maintenance of normal vascular tone. It would be unfair not to discuss each functional part separately in order to assess the uniqueness(Stevens and Lowe, 2008) of this system and their impact on circulation.

The heart is a specialised structure which is responsible for pumping of blood. It's a highly modified muscular structure composed of two atrial and ventricular chambers; each chamber is separated by heart valves which prevent backflow of blood. It is a type of involuntary striated muscle consists of three layers of tissues each contributing to its specialised function in circulation. The outer layer is covered with flat mesothelial cells to produce smooth outer surface; middle layer which makes up most of the bulk of heart is composed cardiac muscle which is a specialised muscle responsible for the pumping action of the heart and lastly; endocardium is a thin layer of smooth lining covered by endothelial cells which are in direct contact with circulating blood.

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The epicardium is a protective layer of flat mesothelial cells, lying on a stroma of fibrocollagenous support tissue, which contains elastic fibres covered by epithelium. This layer includes the blood capillaries, lymph capillaries, and nerve fibres.

The myocardium on other hand is relatively thick consisting largely of cardiac muscle responsible for forcing blood out of the heart chambers; this layer is mainly primarily collection of a specialised muscle cells called cardiomyocytes.

Cardiac myocytes have five major components:-

cell membrane (sarcolema) and T-tubules, for impulse conduction

sarcoplasmic reticulum, a calcium reservoir needed for contraction

contractile elements



One of the important features of cardiac muscle is the organisation of parallel myofilaments with a spindle shaped central nucleus. The functional intracellular contractile unit of cardiac muscle is the sarcomere, an orderly arrangement of thick filaments composed principally of myosin, and the thin filaments contain actin. These sacromere also contain regulatory proteins troponin and tropomyosin. These cells contain more mitochondria between myofibrils then usual skeletal muscles this reflects complete dependence of cardiac muscle on aerobic metabolism. Myocites comprises only approximately 25% of the total number of cells in the heart, since cardiac myocites are larger than intervening cells make them to account for 90% of volume of the myocardium.

Lastly, endocardium is a tissue layer which is mostly associated with the rich myocardial capillary network, and fibroblasts. It consists of three layers; outer most is irregular containing collagen fibres which merge with surrounding adjacent cardiac muscle fibres. Purkinje fibres part of conducting system is contained in this layer of endocardium. Middle layer is more regular with variable number of elastic fibres. The inner layer is primarily made of endothelial cells, which are continuous with the endothelial layer emerging and entering the heart.

The endocardial layer displays an immense contact surface between the circulating blood and subjacent myocardium. There is considerable difference between the atrial and ventricular counterparts. This is due the storage of atrial natriuretic peptide (ANP), a polypeptide secreted into the blood under conditions of atrial distension. This a very characteristic of endothelial tissue of heart, this peptide produces variety of physiological effects which are in the form of vasodilatation, natriuresis, and diuresis, these actions are important in regulating the phathophysiological effects in hypertension and congestive heart failure. The other natriuretic peptide which is released by the ventricular endothelium in response to elevation in ventricular pressure is B-Type. Brain Natriuretic peptide (BNP) is seen to activate the membrane bound guanylyl cyclise-A (GC-A) receptor, which results in the accumulation of intracellular cGMP in target tissues which causes vascular smooth muscle relaxant effect; in addition, it relaxes the arteries and veins procontarcted with phenylephine (Zellner et al., 1999).

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The heart has to contract in a rhythmic fashion in-order to empty the atrial blood to ventricles and from there to aorta. This systemic circulation helps in protecting the natural physiology of circulation in tact. The ability of heart muscle is a special characteristic of cardiac muscle, the presence of junctional complexes called intercalated discs. This characterises is regulated by the cell membranes of two adjacent cardiac muscles which are extensively intertwined and bound together by gap junctions and desmoseomes. These connections help stabilize the relevant positions of adjacent cells and maintain the three dimensional figure. The gap junctions allow ions of small molecules to move from one cell to another. This characteristic is important in creating an electrical pathway between the two muscle cells; which keeps the intrinsic ability to contract spontaneously and rhythmically and in a coordinated fashion. Its been found that abnormalities in the spatial distribution of gap junctions and their respective proteins due to myocardial ischemia are the contributors to electrochemical dysfunction of heart resulting in arrhythmias. On top of this heart contains specialised excitatory and conducting cells that regulate the cardiac conduction from sinoatrial node to right and left bundle branches.

Another feature which is the regulation of heart contraction is due to the innovation of autonomic nervous system and is contrast to skeletal muscle tissue.

The vascular wall is composed of three basic structural constituents, the endothelium, the muscular tissue, and the connective tissue. The arrangement and the amount of these tissues in vessels are influenced by metabolic factors, mechanical factors and primary blood pressure reflecting local needs of tissues.


The endothelium is a specialised type of epithelial tissue generally of squamous morphology; usually elongated in the direction of blood flow. This layer is recognised to be a dynamic organ; important in several house keeping function. The endothelial cells (EC) form 0.2-4 micrometer thick monolayer that lines the lumen of the entire surface of blood vascular tree. It is often considered to be body's largest organ with 6 trillion cells covering an area of 100,000 km and weighs about 1kg, representing 1% of body mass. Endothelial functions include angiotension-converting enzyme; von Willebrand factor; vascular endothelial growth factor receptor (VEGFR) - 1and -2; the vascular endothelial (VE) -cadherin, platelet-EC adhesion molecule-1 (PECAM-1; CD31); p-selecting; the mucin-like molecule CD34; and E-selectin.

Endothelium features a compact monolayer characterised by scant intercellular spaces, thus forming an active barrier between blood and underlying tissues; several elements including intercellular junctions, cell-surface-binding proteins, and basement membrane composition regulate endothelial integrity and permeability. Three major intracellular junctions occur in endothelial cells. Tight junctions allow very close contacts between adjacent cells and their number varies accordingly; brain has got the highest number of tight junctions while as post capillaries have few or none. Brian and large arteries contain many tight junctions, while post capillary endothelial cells contain few or none that enable blood cells to move in and out of the capillary easily. Gap junctions are another intracellular junction that promotes direct exchange of ions and second messengers between the cells; these connections facilitate communication and contraction between endothelial cells and other cell types. Third type of junction is adherens junction which is formed from a transmembrane protein termed as cadherins; their role remains controversial however during neovascularisation there increased presence may be key to disorganised endothelial cell-to-cell interactions within neovessels and may represent a significant event in artherogenesis. Also an altered rate of macromolecular diffusion or transport through endothelial junctions provides the major mechanism for increased endothelial permeability in response to selected inflammatory mediators such as thrombin or histamine. These substances can increase endothelial permeability within minutes by modulating phosphorylation of proteins involved in organisation of endothelial junctions, followed by actin-myosin contractions, and increasing the inter-endothelial gaps. This helps to balance metabolic demands of the tissue during inflammatory process.

Also in arthrosclerosis monocyte infiltration and foam cell formation ensue, followed by further endothelial dysfunction and damage which precipitates platelet adherence and proliferation of vascular smooth muscle. These key processes in atherogenesis are opposed by nitric oxide. NO suppresses the expression and signaling of adhesion molecules involved in monocyte adhesion to the vessel wall, and inhibits platelet adherence and vascular smooth muscle cell proliferation. The NO synthase pathway is perturbed by hypercholesterolemia and other metabolic disorders that predispose to atherosclerosis.

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Endothelial Regulation

As described earlier endothelium secretes mediators that influence vascular haemodynamics and contribute to the regulation of blood pressure and blood flow by releasing vasodialators such as Nitric Oxide (NO) and prostocylin (PGI²) as well as vasoconstrictors, including endothelin (ET) and Platelet-activating factor(PAF). These chemically active compounds usually do not reside performed in intracellular granules; their major biological effects depend on their rapid synthesis. NO is main vaso-relaxing factor produced by endothelial cells continuously by enzyme NO synthase, which converts L-arginine to NO. NO although labile, rapidly diffuses out of the endothelial cells to cause smooth muscle relaxation or inhibit platelet aggregation in the blood. These reactions are a result of increased cyclic guanosine monophosphate (cGMP) formation which results in relaxation of smooth muscles in blood vessels leading to vasodilatation through second messenger system. This effect is usually observed during specific agonist (acetylcholine or bradykinin) binding to endothelial cells, increases shearing forces action on the endothelial surfaces and cytokines such as tumour necrosis factor and interleukin release during inflammation or infection by leukocytes. Loss of endothelium dependent NO-mediated vasodilatation occurs during endothelial dysfunction due to numerous vascular diseases, including atherosclerosis and NO bioavailability decreases in patients with coronary artery disease. Maintenance of vascular cell wall by inhibition of inflammation, thrombosis, and proliferation of platelets and limiting mitochondrial oxidative phsophorylation are effects of endothelial cell derived nitric oxide. Also another factor which plays an important role in smooth muscle vasodilatation is endothelium derived-hyperpolarising factor (EDHF) which acts by activating ion channels between adjacent cells or by contact-mediated mechanism bestows hyperpolarisation that ultimately results in vasodilatation. The changes in (EDHF) are of critical importance for the regulation of organ blood flow, peripheral vascular resistance, and blood pressure.

In addition endothelial cells synthesize endothelin-I, a powerful vasoconstrictor. This synthesis is stimulated by angiotension- II, vasopressin, thrombin, cytokines, and shearing forces and it is inhibited by NO and PGI². Endothelin-I leave the endothelial cell binds to receptors on vascular smooth muscles, which cause calcium mobilization and ultimately smooth muscle contraction. PGI² is a product of acachidonic acid metabolism within endothelial cells which also causes smooth muscle relaxation and platelet aggregation through formation of cyclic adenosine monophosphate (cAMP).

Regulation of blood cell behaviour is a major endothelial function that ensures blood fluidity. The endothelium actively supports the fluid state of flowing blood and prevents activation of circulating cells. This control of homeostasis is due to endothelium derived NO which inhibits platelet adhesion, activation, secretion and aggregation through cGMP dependant mechanism. During damage by atherosclerotic lesions the uncovered sub-endothelial connective tissue induces aggregation of blood platelets which with the cascade of events leads to thrombus formation and obstruction of distant blood vessel. This process can be potentially life threatening; thus the integrity of the endothelial layer, which prevents contact between platelets and the sub-endothelial connective tissue, is an important anti-thrombogenic mechanism.

Vascular smooth muscle structure:

Vascular smooth muscle cells are typically 5-10 µm in diameter and vary from 50-200 µmin length. It is invaginated by numerous small (caveolae) in the cell membrane increasing the surface area of the cell. Vascular smooth vessel sarcoplasmic reticulum is poorly developed with actin and myosin filaments joined together and anchored by dense bodies within the cell or dense bands on the inner surface of the sarcolemma, which functions like Z-line. Each myosin filament is surrounded by several actin filaments and is connected electrically by gap junctions. These low-resistance intercellular connections allow propagated response along the length of the blood vessels.

Vascular Connective tissue

Connective tissue components are present in blood vessel wall in amounts and proportions that vary based on local functional requirements. Collagen fibres, omnipresent elements in vascular system wall are found between muscle cells, in adventia and in some subendothelial layers. Another form of fibres elastic fibres guarantees the resilient shrinkage of the expanded vascular wall. These fibres predominate in large arteries where they are organised in parallel lamellae regularly distributed between the muscle cells throughout the entire media. Ground substances form a heterogeneous gel in the extracellular spaces of the vessel wall. These substances contribute to physical properties of the vessel wall and show their effect on diffusion and permeability across the vessel wall. As in aging process the extracellular matrix is disordered by increase secretion of collagen and gylcosaminoglycans. With the changes in the molecular conformation of elastin and other gylcoprotiens facilitation of lipoproteins and calcium ions in tissues occur with subsequent calcification. This modification of components with associated more complex factors lead to formation of atherosclerotic plaque.

Most of blood vessels have several features that are structurally similar; however dissimilarities exist due to the role they fulfil and their structural position in circulatory system. Also the distinction between different types of vessels is not absolute; it is not often clear cut because the transition from one type to another type is gradual. In principle blood vessel are usually composed of three layers namely tunica intima, media and adventia. These three separate concentric layers of tissue make up a typical wall of a blood vessel.

Tunics Intima

Tunica intima is composed of a simple squamous endothelial connective tissue. These endothelial cells lining the lumen of the blood vessel rest on a basal lamina. In large vessels several to manly individual endothelial cells line the circumference of the lumen. As discussed above the endothelial cells not only provide an exceptionally smooth surface but also function in secreting several potent substances which are important in various physiological processes. They also possess membrane-bound enzymes, such as angiotensin-converting enzyme (ACE), as well as enzymes that inactivate bradykinin, serotonin, prostaglandins, thrombin, and norepinephrine; lastly they also bind lipoprotein lipase to degenerate lipoprotein which is important process in reduction of arthrosclerosis. Beneath endothelial cell layer is a subendothelial layer, composed of loose connective tissue and few scattered smooth muscle cells. These layers rest on another layer called an internal elastic lamina, which is well developed in muscular arteries and separates tunica intima form media. Internal elastic lamina is composed of elastin and has numerous fenestrations that permit diffusion of substances into deeper regions of arterial wall to nourish cell. Another form of cells which form a part of tunica intima is myointimal cell; it is responsible for deposit organisation on vessel wall. With increasing age these cells accumulate lipid and intima progressively thickens leading to artherosclerosis.

Tunica media

This is the thickest layer of blood vessel wall made up of concentric cell layers consisting of smooth muscle cells reinforced by organised layers of elastin, collagen and proteoglycans. The tunica media is often prominent in arteries, being relatively indistinct in veins and virtually non-existent in very small vessels. Smooth muscle cells in tunica media are elongated and have dark-pink centrally placed nuclei when stained. These cells are connected by gap junctions and also contain mitochondria, other organelles and contractile proteins for movement. Smooth muscles cells also secrete extra-cellular matrix and stabilize endothelial cells by activating transforming growth factor beta (TGF-β). These smooth muscle cells also actively regulate the contraction and relaxation of blood vessels. Elastin found in media is a polypeptide chain cross linked together, it provides ability to uncoil blood vessel into a more extended conformation when the fibre is stretched and will recoil spontaneously as soon as stretching force is relaxed. Collagen-I is also present in media; it has enormous tensile strength; that it can be stretched without being broken. Elastic arteries help to stabilize the blood flow; they have a thick, highly developed media of which elastic fibres are the main component. As the vessel ages it loses its elasticity due to loss of collagen and elastin making vessel wall weak. These vessels are prone to aneurysms and other disorders as they have lost elastic ability. Smooth muscle cells in vessel walls are also supplied with profuse network of unmyelinated sympathetic nerve fibres, whose neurotransmitter is norepinephrine which is responsible for vasoconstriction of the vessel wall. On other hand arteries in skeletal vessels have acetylcholine which is released by these vasodilator nerves acts on the endothelium to produce nitric oxide, which diffuses into the smooth muscle cells, activating a cGMP system of intracellular messengers. The muscle cells then relax, and the vessel lumen is dilated.

Tunica Adventitia

Outer layer of blood vessel is tunica adventitia, it is composed largely of collagen, but smooth muscles may also be present, particularly in veins. Other prominent structures which are present in adventitia are external lamina, connective tissue, fibroblasts, nerves and macrophages. Collagen and external lamina have similar functions as in media layer and also contain autonomic nerves which innervate smooth muscles of media and some lymphatic vessels that drain off excess interstitial fluid. Fibroblasts maintain the structural integrity of connective tissue by secreting extracellular matrix.

The vascular system comprises of extensively high pressure system and low pressure system. High pressure system is called arterial system which is structurally evolved to sustain extensively high pressures. In the systemic circulation blood flows through channels of six principle types; elastic arteries; muscular arteries; arterioles; capillaries; venules and veins. Each type of channels has specific properties due to their structure position in circulatory system. These channels are made up of three identifiable layers (tunics) referred as intima, media and adventitia. Two major arteries of the circulation are aorta and pulmonary artery; each differs in the structure due to their function. Aorta has an intima layer which is the innermost layer of endothelial cells; it's separated from the inner layer by a narrow layer of connective tissue which anchors to the cell wall.

the intima, or intermost layer, consists of a layer of endothelial cells separated from the inner layer by a narrow layer of connective tissues which anchors the cells to the arterial wall.

A large layer of elastic fibers forming the elastica interna layer.

Below this layer are concentric waves of smooth muscle cells intermixed with elastic fibers.

Elastic lamellae and smooth muscle cells are imbedded in a ground substance rich in proteoglycans. Proteoglycans are formed of disaccharides bound to protein and serve as binding or "cement" material in the interstitial spaces. The outer layer of the media is penetrated by branches of the vasa vasorum.

Between the smooth muscle layer and the adventitia, there is again another layer of elastic fibers, the elastica externa. Layers 2, 3 and 4 form the media.

The outer layer or adventitia is formed of irregularly arranged collagen bundles, scattered fibroblasts, a few elastic fibers and blood vessels which, because of their location, are called vasa vasorum or vessels of the vessels

Characteristics of aorta is its elasticity and extensive distensibility, besides being the main vessel to distribute blood from the heart to the arterial system, it dampens the pulsatile pressure that results from the intermittent ejection of blood from the left ventricle. The dampening is a function of aortic compliance, which is the ability of aorta to expand and contract passively with changes in pressure. Aorta also acts as a reservoir by storing blood during ventricular systole and the recoiling property with which blood is forced to towards the arterial tree. The close association of elastin, collagen and smooth muscle in the aortic media results in visoelastic properties that account for many of its static and dynamic mechanical features.

Large vessels like aorta usually have vasa vasorum ("vessels of the vessel"), which are arterioles, capillaries, and venules that branch profusely in the adventitia and the outer part of the media. The vasa vasorum provide metabolites to the adventitia and the media, since in larger vessels the layers are too thick to be nourished solely by diffusion from the blood in the lumen. Vasa vasorum are more frequent in veins than in arteries. In arteries of intermediate and large diameter, the intima and the most internal region of the media are devoid of vasa vasorum. These layers receive oxygen and nutrition by diffusion from the blood that circulates into the lumen of the vessel.

On the other hand pulmonary arterial and vascular system is a low pressure system with the mean pressure in the pulmonary artery between 25mmHg-30mmHg. It is structurally different to aortic tissue as it has to sever a different purpose. The pulmonary artery branches are elastic arteries with three main components. Intima is narrow and composed of a single layer of endothelium lying on a narrow layer of scanty collagen fibres and myofibroblasts. A media composed of many layers of elastic fibres which are main pulmonary arteries but more regular and intact in the peripheral branches; there is some collagen between the elastic and smooth muscles. Lastly elastic laminae composed of longitudinally running fibres that form flat, interlinked strands of varying breadths; which are particularly due to the adaptation to counteract the stretching forces during lung expansion (Stevens & Lowe 2009).

As the blood flows from aorta into distal arteries, characteristic changes take place in the shape of the pressure wave contour that is due to the decreased compliance of distal arteries and due to their structure and elastic ability. Depending on the size of the vessel, there may be several layers of smooth muscle cells, some arranged circumferentially and others arranged helically along longitudinal axis of the vessel which determine the reduction in vessel diameter on contraction.

As the blood travels away from heart towards the tissues characteristic changes in structure have to occur in order for ideal organ perfusion pressure. Large elastic arteries gradually merge to form muscular arteries by losing most of their medial elastic sheets. Muscular arteries vary in size about 1cm to 0.5mm in diameter. There intima has a very thin sub- endothelial layer and the internal elastic lamina. The tunica media may contain up to 40 layers of more prominent smooth muscle cells which are intermingled with a variable number of elastic lamellae. This feature makes them highly contractile and their contractility and relaxation is also controlled by autonomic innervations and vaso-active substances released by endothelial cells.

Smallest branches of the arterial tree are arterioles which are continuous with muscular arteries. Varying in diameter ranging from 0.3mm to 0.4mm and are composed of endothelial layer with one or two layer of smooth muscles. As the arteriole gets smaller, continuous layers of smooth muscle becomes progressively discontinuous with insignificant adventitia layer. They are richly innervated by sympathetic adrenergic fibres and are highly responsible to sympathetic vasoconstriction. As the structure explains the major resistance to blood flow is offered by arterioles due to their small radius. This phenomenon is important as the resistance varies inversely to the fourth power of radius of the vessel, thus if radius is halved, resistance in increased 16-fold enabling substantial blood flow changes to be effected by relatively small adjustments to the radius of an arteriole.

One other important structure which is important to mention is a specialised area at the junction between the terminal arteriole and capillary is known as pre capillary sphincters, this consists of a few smooth muscle cells arranged circularly. There effects on increasing the surface area greatly influence the diffusion of nutrients and gas exchange during need for increased organ demand. In active muscle, for instance, many more capillaries are patent due to relaxation of the sphincters and thus blood flow is increased; this has the effect of greatly increasing the surface area available for exchange of substances and at the same time reduces the distance across which substances have to diffuse to reach the cells.

The microvasculature starts at the level of the arterioles and their smaller branches; it is composed of small diameter blood vessels with partly permeable thin walls that permit the transfer of blood components to the tissues and vice versa. Exchange of nutrients and gases between blood and tissues occur in the extensive capillary network, the smallest arterioles empty into the capillary system. The ability of blood and plasma to deliver the gases and nutrients to tissues the vessels of the circulatory system have to be able to perform this without any problem; so the smallest vessels of the diameter are given this ability. Capillaries are vessels 5 to 10 micrometers in diameter and form the interlinking network. They have the thinnest walls of blood vessels and are mostly composed of single layer of endothelial cells, a basement membrane and occasionally pericytes called contractile cells. With the average length of an individual capillary being not more than 50 micrometer and altogether comprise 90% of all blood vessels their thin walls and slow flow capillaries are ideal places for exchange of water, solutes and macromolecules between blood and tissues.

Since every organ has specific requirements with respect to ideal organ perfusion. Capillaries have been differentiated on the basis of these requirements as they permit different metabolic exchanges between blood and surrounding tissues. The capillaries are often referred to as exchange vessels, because it is at these sites that oxygen, carbon dioxide, substrates, and metabolites are transferred from blood to the tissues and from the tissues to blood. They depend on the kind of molecule and also on the structural characteristics and arrangement of endothelial cells in each type of capillary. The continuous or tight capillary allows regulated exchange of material and is characterised by distinct continuity of the endothelial cells in its wall. This is the most common type of capillary and is found in all kinds of muscle tissue, connective tissue and exocrine glands as they have numerous transport vesicles; however capillaries in nervous system have fewer vesicles and are selective in exchange of material, this constitutes the blood brain barrier function. The fenestrated or visceral capillaries are characterised by the presence of several circular transcellular openings in the endothelium opening called fenestrae. Each fenestrae is usually covered by very thin diaphragm containing heparan proteoglycans but no lipid bilayer. These capillaries are found at places where more extensive molecular exchange across the endothelium is warranted like gastrointestinal mucosa, endocrine glands and renal glomeruli. Sinusoidal capillaries or discontinuous capillary permits maximal exchange of macromolecules as well as cells between tissues and blood, they have tortuous path greatly enlarged diameter which slows circulation, the endothelial cells form a discontinuous layer and are separated from one another by wide spaces. Sinusoids are highly specialised vascular channels irregularly in shape with diameters as large as 30-40 µm much greater than those of other capillaries. They are endothelium-lined channels with discontinuous or absent basement membrane without diaphragms. Due to their structure they allow macromolecules or cells to pass through which is why sinusoidal capillaries are found in the liver, spleen, some endocrine organs, and bone marrow. The interchange between blood and tissues is greatly facilitated by the structure of the capillary wall.

Special types of cells are found along capillaries and post capillary venules; they are of mesenchymal origin and partly surround the endothelial cells. Due to presence of actin, myosin and tropomyosin suggest that they have contractile function. It has been found after tissue injury these cells proliferate and differentiate to form new blood vessels and connective tissue cells. In some tissue there are arteriovenous anastmoses which enables blood to bypass the capillary beds. These vessels are abundant in skeletal and in the skin of hands and feet. There contraction causes blood to flow through capillaries however when they are relaxed most of the blood flows directly to a vein instead of circulating in the capillaries.

Circulation in capillaries is regulated by neural and hormonal stimulation. The metabolic activity of tissue determines the richness of capillary network organ possess. It is apparent that tissues in brain, heart, kidney and liver like organs have high metabolic activity therefore have abundant capillary networks however opposite is true for tissues with low metabolic rate like smooth muscles and dense connective tissue.

Capillaries join together to form post capillary venules which have a diameter ranging from 0.1 to 0.5 mm. They have similar features like capillaries like participation in inflammatory process and exchange of cells and molecules between blood and tissues. They are composed of collagen fibres, endothelial cells, pericytes but lack smooth muscles. Their endothelial junctions are of the loosest form which makes them the primary cite at which white blood cells leave circulation at sites of infection or tissue damage.

Post capillary venules converge into large collecting venules; in which pericyte layer becomes continuous and surrounding collagen fibres appear. As the collecting venules become larger the pericytes are replaced by smooth muscle cells called muscular venules. A characteristics feature of all venules is the large diameter of about 50 to 100 micrometer of lumen compared to thinness of the wall. Form venules blood is collected in veins of increased size, arbitrarily classified as small medium and large, over all they are thin walled vessels with large diameter ranging from 1mm to 4cms. There is a considerable variation in vein wall structure according to location. Veins are also composed of three layers however as compared to arteries their tunics are less demarcated and are often difficult to identify where one layer ends and another begins. Small veins are continuation of the muscular venules and have a similar structure, with more clearly defined muscle cell and outer fibrocollagenous layers. While as medium sized veins have inner layer endothelial cells on a basement membrane, separated by a narrow zone of collagen fibres. Inner layer is consistent in structure however outer layers still called media and adventitia are often arbitrarily and with little justification. Large veins on the other hand have more collagen and elastic fibres between the endothelial basement membrane and elastic lamina with a layer of smooth muscle embedded in collagen. Veins contain bicuspid valves in their interior consisting of two semi-lunar folds of tunica intima that project into the lumen; directing the venous blood towards heart and preventing back flow. The veins act as a collecting system, returning blood from capillary networks to the heart passively down pressure gradient and also with the help of skeletal muscle compression. This offers a low resistance to blood flow, however if these valves are damages or over stretched by high venous pressure for long periods of time like in pregnancy or in people who stand for prolong periods; high venous pressure causes valves to become incompetent and lose their function resulting in varicous veins, oedema and varicose ulcers. Venous thrombosis may also occur due to the damages to endothelial layer or hypercoagulopathy syndromes. Also major part of the blood volume lies within the venous system approximately 60% and due to this reason venous system is sometime referred as capacity vessel. The capacity of the venous system can be modified by altering the lumen size of the muscular venules and veins; this change can be mediated by altering the venomotor tone, that's the degree of contraction of smooth muscle in tunica media.

Another system which is a part of circulatory system is lymphatic system; it is a system of thin walled endothelial channel that collect excess interstitial fluid from the tissue spaces and return it to blood. Their general structure is similar to veins except they have more valves and thinner walls. There is an inner lining of single flattened cells composed of endothelial cells resting on a discontinuous basement membrane. The next layer is similar to smooth muscle of veins arranged circularly around endothelium contributing in contraction and relaxation of lymphatic vessels and cause peristalsis. The outermost layer is made of fibrous tissue called adventia; which is not present in smaller lymphatic vessels. Smaller lymphatic vessels also lack smooth muscles making them highly permeable lymph capillaries which allow passage of whole cell and macromolecules into lymphatic system.

The lymphatic circulation begins superficial lymph capillaries, formed of endothelial cells which allow fluid to pass through them due to changes in interstitial pressure and structure of lymphatic capillaries. These converge into large lymphatic vessels which are similar to veins and more valves which prevents the back flow of lymph along the lumen vessel, due to rhythmic contractions by smooth muscles in the vessel walls lymphatic fluid is transported to into two main lymphatic vessels; the thoracic duct which empties lymph into venous system at the junction of the left internal jugular and left subclavian veins and a more variable lymphatic vessel which empties on the right side of the body. Another structure which is encapsulated bean-bean shaped structures of lymphatic tissue located in many regions of the body. They lie along the course of lymphatic vessels vary in size from few millimetres to more then a centimetre in length. These nodes contain macrophages, lymphocytes, and other immune components which assist the immune defence. Further more all lymphatic capillaries from a particular organ drain into lymph nodes serving that area are particularly important in the spread of cancers. This happens when a cancer cell enters lymphatic capillaries it gets carried to other sites where they can multiply and produce secondary tumours.


Large vessels like aorta usually have vasa vasorum ("vessels of the vessel"), which are arterioles, capillaries, and venules that branch profusely in the adventitia and the outer part of the media. The vasa vasorum provide metabolites to the adventitia and the media, since in larger vessels the layers are too thick to be nourished solely by diffusion from the blood in the lumen. Vasa vasorum are more frequent in veins than in arteries. In arteries of intermediate and large diameter, the intima and the most internal region of the media are devoid of vasa vasorum. These layers receive oxygen and nutrition by diffusion from the blood that circulates into the lumen of the vessel.