L26 Organizational Structure of Circulation PDF

Organizational structure of circulation L26 ILOs    Differentiate between arteries & veins on a histological basis. Interpret structural adaptation in walls of large & medium sized arteries in relevance to function. Interpret structural specialization of arterioles & capillaries in regulation of...

Organizational structure of circulation L26 ILOs    Differentiate between arteries & veins on a histological basis. Interpret structural adaptation in walls of large & medium sized arteries in relevance to function. Interpret structural specialization of arterioles & capillaries in regulation of arterial blood pressure. General Histological Features of the wall of the blood vessels Histologically, the wall of the blood vessels, except capillaries, is composed of three layers: the tunica intima, the tunica media, and the tunica adventitia (Figure 1). A. Tunica intima It is the innermost layer and is composed of simple squamous endothelial cells, which form a tube lining the lumen of the vessel, resting on a basement membrane. The cells are joined together by occluding junctions. This endothelium provides a physical barrier between the blood and the rest of the blood vessel wall. Disruption of this barrier is an important step in development of atheroma. The underlying subendothelial connective tissue is composed of loose connective tissue and a few scattered smooth muscle cells. Beneath the subendothelial layer, in many arteries, is an internal elastic lamina which is formed of elastin fenestrated sheet that permits the diffusion of substances into the deeper regions of the arterial wall to nourish the cells there. B. Tunica media It consists mainly of concentric layers of spirally-oriented smooth muscle fibers. The smooth muscle provides mechanical strength to the wall and controls the luminal diameter of the blood vessels. These fibers are separated by varying amounts of collagen, elastic fibers and extracellular matrix of proteoglycans, all of which are produced by the vascular smooth muscle cells. Again in many arteries, elastic fibers may form the thin external elastic lamina separating the media from the adventitia. C. Tunica adventitia It is the outermost layer of the blood vessel wall, blends into the surrounding connective tissue. It is composed mostly of fibroblasts, type I collagen fibers, and longitudinally oriented elastic fibers. The adventitia also contains small blood vessels called vasa vasorum (vessels of the vessel), which provide nutrition to the vessel wall. HISTOLOGICAL STRUCTURE OF ARTERIES Arteries are classified into three major types based on their relative size and morphological characteristics. From largest to smallest, they are as follows: Large elastic (conducting) arteries Medium sized muscular (distributing) arteries Arterioles Large elastic (conducting) arteries These include aorta and its large branches. These arteries have the general histological structure of a blood vessel, together with the following characteristics (figure 2a): TUNICA INTIMA:  It is composed of an endothelium that is supported by a subendothelial layer. TUNICA MEDIA: It is the thickest coat, constituting 70% of the wall thickness. It contains concentric layers of fenestrated elastic laminae (membranes) that are separated by few smooth muscle fibers. The elastic lamina provide elasticity to the wall of the large arteries enable them to get inflated by the entry of additional blood during systole. The fenestrations facilitate the diffusion of nutrients through the arterial wall. The internal and external elastic laminae are present but not distinguished from the elastic membranes in the tunica media. TUNICA ADVENTITIA: It is about 20% of the wall thickness that merges into the surrounding connective tissue. Numerous vasa vasorum are present, which provide nutrition to the adventitia and outer part of the media since the wall is too thick to be nourished only by diffusion from the lumen. Medium sized muscular (distributing) arteries These include most of the named arteries. These arteries have the previously-mentioned general histological structure of a blood vessel, together with the following characteristics (figure 2b): TUNICA INTIMA:  It is a relatively thin layer.  Thick, prominent internal elastic lamina is present. It is thrown into folds due to post mortem contraction of the smooth muscle fibers in the tunica media. TUNICA MEDIA:  It is about 50% of the wall thickness.  It is composed of several concentric layers of smooth muscle fibers with little elastic and collagen fibers in between.  The external elastic lamina is present, particularly in larger muscular arteries. TUNICA ADVENTITIA:  It is as thick as the media (about 50%).  It contains few vasa vasorum.  Arterioles    Arterioles are responsible for the peripheral resistance. Histologically, they are characterized by: They have a relatively thick wall compared to their narrow lumen. The media is formed of one to two layers of smooth muscle fibers. There is slight thickening of the smooth muscle at the origin of a capillary bed from an arteriole is called the precapillary sphincter. Closure or opening of these sphincters control blood flow into the capillary beds. For instance, 90% of these sphincters of the muscular tissue are shut at any time but will open during exercise. These sphincters have no nerve supply, but are regulated by local metabolite concentrations. HISTOLOGICAL STRUCTURE OF VEINS Veins are described as having the same three layers (tunicae intima, media, and adventitia) as arteries. In histological section, veins have collapsed lumen that may contain RBCs and not circularly-opened like that of corresponding arteries. Their caliber is larger than the arteries with their walls are much thinner. Veins are classified into three groups on the basis of their diameter and wall thickness: large veins, medium-sized veins and venules. Large veins They are paired with the elastic arteries and these include the venae cavae, subclavian and portal veins. They have relatively wide lumen and they are characterized by the following (Figure 3): TUNICA INTIMA: thin layer with no internal elastic lamina. TUNICA MEDIA:  It is about 30% of the wall thickness.  No external elastic lamina. TUNICA ADVENTITIA:  The thickest coat, 70% of the wall.  It is composed of dense fibro-elastic connective tissue which strengthens the wall and prevents distention of the vessel together with circular and longitudinal arrangement of smooth muscle fibers which provides a peristaltic pumping of the blood to the heart. Medium-sized veins: These include most named veins that accompany their medium-sized arteries in the neurovascular bundle. They are characterized by the following (figure 4): TUNICA INTIMA:  Intimal folds are projecting into the lumen as VALVES to prevent backflow of venous blood except in one direction, towards the heart.They are abundant in veins of the extremities (lower and upper limbs). TUNICA MEDIA:  It is about 30%of the wall thickness.  It is composed of concentric layers of smooth muscle fibers alternating with little collagen and elastic fibers. TUNICA ADVENTITIA:  The thickest coat, 70% of the wall.  Medium-sized veins contain more vasa vasorum in their tunica adventitia than arteries as luminal blood has low oxygen content and less nutrients. Venules: They have a relatively thin wall compared to their wide caliber. They are further classified into: A. Post-capillary venules:  These are just coming out from the capillary bed.  Their wall consists of a single layer of endothelial cells with its basal lamina and the surrounding pericytes (similar to those of capillaries).  They are the site of trans-migration of WBCs during inflammation. B. Muscular venules:  The media is formed of one to two layers of smooth muscle fibers.  The adventitia is thin. PERIPHERAL CIRCULATION It is the connection between arteries and veins. It includes capillaries and arteriovenous shunts. HISTOLOGICAL STRUCTURE OF CAPILLARIES Capillaries are the smallest of the vascular channels that conduct blood from terminal arterioles to the post-capillary venules. Capillaries are formed by a single layer of squamous endothelial cells rolled into a tube, giving the lumen a diameter that ranges from 8 to 10 mm. The external surfaces of the endothelial cells are surrounded by a basal lamina. The large number of pinocytotic vesicles associated with the entire plasmalemma is an identifying characteristic of capillaries. THE PERICYTES:  These are perivascular cells with branching cytoplasmic processes.  They partially surround the endothelial cells of capillaries and post- capillary venules along their course.  Pericytes are enclosed in the basal lamina of the endothelial cells. Functions of pericytes: 1. Support of capillaries and post- capillary venules. 2. These cells contain well developed cytoskeleton, which is related to the contractile process that regulates blood flow through the capillaries. 3. Stem cells; after tissue injury pericytes may undergo differentiation to become smooth muscle cells and endothelial cells in the walls of arterioles and venules. Classification of Capillaries (figure 5) Capillaries are of three types: (1) continuous, (2) fenestrated, and (3) sinusoidal A. Continuous capillaries:      The wall of these capillaries has no fenestrations with continuous basal lamina. The endothelial cells are joined together by occluding junctions. At these cellular junctions, the endothelial cells tend to overlap, forming a marginal fold that projects into the lumen. The cytoplasm is filled with numerous pinocytotic vesicles that function in trans-endothelial transport of molecules. Sites: in muscular tissue, lung, skin and nervous tissue. Function: provides well-regulated metabolic exchange and thus share in the formation of different blood barriers e.g. blood-brain barrier and blood-thymus barrier. B. Fenestrated capillaries:     The endothelial cells have numerous fenestrations (circular openings), with continuous basal lamina which covers the fenestrations. These fenestrations are closed by a thin non-membranous diaphragm (resemble a cartwheel-like structure with wedgeshaped gaps) derived from the glycocalyx of the cell membrane. Sites: endocrine glands, glomerulus of the kidney and intestinal villi. Function: allows more molecular exchange between tissues and blood. C. Sinusoids (sinusoidal capillaries):       These are irregular vascular channels with a relatively large caliber. The endothelial cells have large openings in their cytoplasm without diaphragms and are separated by wide intercellular gaps. The endothelial cells rest on a discontinuous basal lamina and are supported by delicate reticular fibers. Macrophages, not pericytes, are usually associated either within or around the sinusoids. Sites: bone marrow, liver, spleen. Function: allows movement of cells and maximal exchange between tissues and blood as in ARTERIOVENOUS ANASTOMOSES (A-V SHUNTS)      These are direct routes between the arterioles and venules without passing in the capillary bed. Sites: skin of palm & sole, nose, lip and erectile tissue of penis. Structure: it arises as a side branch from terminal arteriole and runs directly to small venule. The arteriole of the A-V shunt has a relatively thick smooth muscle layer and is richly innervated. (Figure 6) Contrary to the ordinary precapillary sphincter, contraction of the arteriole smooth muscle of the AV shunt sends blood to a capillary bed; relaxation of the smooth muscle sends blood to a venule, bypassing the capillary bed. Function: AV shunts serve in thermoregulation at the body surface. Closing an AV shunt in the skin causes blood to flow though the capillary bed, enhancing heat loss. Opening an AV shunt in the skin reduces the blood flow to the skin capillaries, conserving body heat.