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The Circulatory System
This system comprises both the blood and lymphatic vascular system. Blood vascular system is composed from; The heart : an organ whose function is to pump the blood. The arteries : efferent vessels that become smaller as they branch, and whose function is to carry the blood with its nutrition and O 2 to the tissues. The capillaries : are the smallest blood vessels that form a network of thin tubules that anastomose profusely, through whose wall the interchange between blood and tissue takes place. The veins : which result from convergence of capillaries they become larger as they approach the heart. The lymphatic vascular system begins in the lymphatic capillaries as close-ended tubules that anastomose to form vessels of steadily large in size, these vessels terminated in the blood vascular system. Innervation: Most blood vessels that contain smooth muscle in their walls are supplied with a profuse network of unmyelinated sympathetic nerve fibers (vasomotor nerves). Whose neurotransmitter is norepinephrene, discharge of norepinephrene from these nerves results in vasoconstriction. Arteries in skeletal muscles also receive a cholinergic vasodilator nerve supply. Afferent (sensory) nerve endings in arteries include the baroreceptors (stretch receptors) in the carotid sinus and the arch of the aorta as well as chemoreceptors of the carotid and aortic bodies. The circulatory system is divided into macrovasculature (vessels more than 0.1 mm in diameter) and microvasculature (vessels less than 0. mm in diameter can be seen only by light microscope. The microvasculature is particularly important because of its participation in
the interchange between the circulatory system and the surrounding tissues in both normal and inflammatory processes.
Tissue component of the vascular wall:
The vascular wall is composed of three basic tissues: the endothelium, the muscular tissue and the connective tissue. The amount and the arrangement of these tissues are influenced by the mechanical factors (represented by blood pressure) and the metabolic factors (which reflect the local needs of the tissues). The structural element of capillaries and postcapillary venules are the endothelium, basal lamina and pericytes. Endothelium: Is a semipermeable barrier which monitors the extensive bidirectional exchange of small molecules and restrict the transport of large molecules. Vascular smooth muscles: Smooth muscle tissue is present in all vessels except capillaries and post- capillary venules. Smooth muscle cells are arranged in helical layers in the tunica media of the blood vessels. Each smooth muscle is surrounded by basal lamina and they are connected by gap junction. Vascular connective tissue: The collagen fibers are found between smooth muscle fibers, in adventetia. The elastic fibers guarantee the resilient shrinkage of the expanded vessels. They are organized in parallel lamellae regularly distributed between muscle cells. The ground substance forms a heterogenous gel in the extracellular space of the vessel wall.
Figure 1. Drawing of a medium-sized muscular artery, showing its layers. Although the usual histologic preparations cause the layers to appear thicker than those shown here, the drawing is actually similar to the in vivo architecture of the vessel. At the moment of death, the artery experiences an intense contraction; consequently, the lumen is reduced, the internal elastic membrane undulates, and the muscular tunica thickens.
Large elastic arteries:
Elastic arteries include the aorta and its large branches. They have yellowish color due to accumulation of elastin. The intima is thicker than the corresponding tunic of muscular artery. An internal elastic lamina, although present may not be seen easily. The media consist of elastic fibers and series of helically arranged of elastic laminae whose number increase with age (40 in newborn and 70 in the adult), among these laminae are smooth muscle fibers, reticular fibers, proteoglycan and glycoprotein, T.adventitia is under developed.
Medium (muscular) sized artery:
These vessels control the affluence of blood to the organs by contracting and relaxing the smooth muscles of T.media. The intima has subendothelial layer which is thicker than that of arterioles, the internal elastic lamina is more prominent. The T. media may contain up to 40 layers of smooth muscle cells intermingled with elastic laminae, reticular fibers and proteoglycans. The external elastic
lamina which is the last component of the media is present only in large muscular arteries. Lastly the adventitia consists of connective tissue.
Figure 3. comparism between medium sized artery (Lt) and elastic artery (Rt)
Arterioles:
Arterioles are generally less than 0.5 mm in diameter and have narrow lumen. The subendothelial layer is very thin, the internal elastic lamina is absent in small arterioles, the media composed from 1-2 circularly arranged layers of smooth muscles, it show no external lamina. Above the arterioles are small arteries which have large lumen and more developed T.media, but in both these two vessel the T. adventitia is very thin.
Capillaries:
They are composed from single layer of endothelial cells. The diameter of capillaries ranged from 7-9 μm. The estimated total length of capillaries is 96000 km The endothelial cells rest on basal lamina (which is produced
2 - Fenestrated OR Visceral capillaries : They are characterized by the presence of several opening in the endothelium called fenestrae (80- 100nm in diameter). Each fenestrae is obliterated by a diaphragm that is thinner than the cell membrane, which do not have the tri-laminar structure of a unit membrane. The basal lamina of this type of capillaries is continuous. They are found in intestinal tract, kidney, gall bladder
Figure 4. Three-dimensional representation of the structure of a capillary with fenestrae in its wall. The transverse section shows that, in this example, the capillary wall is formed by 2 endothelial cells. Note the basal lamina surrounding endothelial cells.
3-Discontinuous sinusoidal capillaries : These capillaries have tortuous path and greatly enlarged diameter (30-40 μm), the endothelial cells form a discontinuous layer and are separated from one another by wide space. The cytoplasm has multiple fenestration without diaphragm, the basal lamina is discontinuous or absent. Sinusoidal capillaries are found in the liver and hematopoietic organs such as bone marrow and spleen. Capillaries anastomose freely to form a network that connects the arteries with veins. The arterioles branch into small vessels surrounded by a discontinuous layer of smooth muscle, the metarterioles , which branch into capillaries. Constriction of metarterioles helps to regulate the blood circulation in capillaries. In some tissues there are arteriovenous anastomosis that enable the arteriole to empty directly into the venules (was no capillaries), so when the vessels of AV anastomosis contract all blood pass into the capillary network. These interconnections are abundant in skeletal muscles, and in the skin of the hand (finger tips) and feet, lips, and nose. The capillary circulation is controlled by NEURAL and HORMONAL control. *The richness of capillary network is related to the metabolic activity of that organ. Because the capillaries have thin wall and slow blood flow 0.3 mm/sec therefore; they are the best place for the exchange of water, solutes, metabolites, etc… Functions of capillaries: Capillaries perform at least 3 important functions: 1- Permeability: capillaries and postcapillary venules are often referred to as exchange vessels since it is at these sites that O2,
Figure 2. Types of microcirculation formed by small blood vessels. ( 1 ) The usual sequence of arteriole —> metarteriole —> capillary —> venule and vein. ( 2 ) An arteriovenous anastomosis. ( 3 ) An arterial portal system, as is present in the kidney glomerulus. ( 4 ) A venous portal system, as is present in the liver.
Carotid bodies:
Are chemoreceptors sensitive to high CO2 and low O2 concentration in the blood located near the bifurcation of carotid artery. These structures are richly irrigated by fenestrated capillaries that surround type I and II, type II are supportive, type I contain numerous vesicles that store dopamine, serotonin and adrenaline. Most of the nerves of the carotid bodies are afferent nerve fibers.
Carotid sinuses:
Are slight dilatations of internal carotid artery, these sinuses are baroreceptors that detect changes in blood pressure and relay information to the CNS, the arterial media is thinner to allow it to respond to changes to blood pressure, the intima and adventitia are very rich in nerve endings. The afferent nerve impulses are processes in the brain to maintain normal blood pressure.
Veins:
When considered as a functional unit, the veins can be classified as capacitance vessels because more than 70 % of the total blood volume is in this portion at any time. The transition from capillaries to venules occurs gradually, the immediate postcapillary venule (pericyte venule) characterized by the presence of pericytes, these blood vessels have several features in common with capillaries e.g. inflammatory processes and exchange of cells and molecules between the blood and tissues. Traditionally, veins are divided into four types on the basis of size.
- Venules are further subclassified as postcapillary and muscular venules. They receive blood from capillaries and have a diameter as small as 0.1 mm. - Small veins are less than 1 mm in diameter and are continuous with muscular venules. - Medium veins represent most of the named veins in this category. They usually are accompanied by arteries and have a diameter of as much as 10 mm. The tunics are most evident. e.g: radial, popliteal veins - Large veins usually have a diameter greater than 10 mm. Examples of such veins include the superior and inferior vena cava and hepatic portal vein.
In large vein (close to the heart) the intima is well developed but the media is thinner and has few smooth muscle cells and abundant connective tissue, the adventitia is the thickest and best developed in large veins.
The myocardium: Is the thickest of the tunics of the heart and consist of cardiac muscle cells arranged in complex spiral way that most of them inserted into the fibrous skeleton. The heart is covered externally by simple squamous epithelium which constitutes the epicardium which is corresponded to the visceral layer of pericardium. Between the visceral and parietal layer of pericardium is a small amount of fluid that facilitate the heart movement. The cardiac valves consist of a central core of dense C.T.(have collagen and elastic fibers) lined on both sides by endothelial layers, the bases of valves are attached to the fibrous skeleton. The heart generate rhythmic stimulus that is spread to the entire myocardium by special system consist from the Sinoatrial node (S.A.node), atrioventricular node (A.V. node) and the atrioventricular bundles. S.A node is a mass of modified cardiac muscle cells that is fusiform and is smaller than atrial muscle cells. A.V node has cells which are similar to the cells of SA node. AV bundles are originated from AV node and branches to both ventricles. Distally the cells of AV bundles become larger than ordinary cardiac muscle and acquire special appearance, these so called (Purkinje cells) have one or two centrally located nuclei, their cytoplasm is rich in mitochondria and glycogen. The myofibrils are restricted to the periphery of the cytoplasm. Purkinje cells travel the subendocardial layer and penetrate the myocardium to become intramyocardic to allow the stimulus to go into the inner most layer of the ventricular musculature.
Both sympathetic and parasympathetic system innervate the heart, stimulation of parasympathetic system (vagus nerve) slows the heart beat, whereas stimulation of sympathetic system accelerate the rhythm of the pacemaker (SA node).
Lymphatic Vascular system:
Is endothelium lined thin walled channels that collects fluid from tissue spaces and returns it to the blood, this fluid is called Lymph. It circulates in one direction toward the heart. The lymphatic capillaries are close ended vessels that consist of a single layer of endothelium and an incomplete basal lamina. The thin lymphatic vessels gradually converge and become larger as they approach the heart. In the path of the lymphatic vessels are lymph nodes. The lymphatic vessels have a structure similar to that of veins except that they have thinner walls and lack clear cut separation between layers (intima, media and adventitia), they have also numerous valves.
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