The Circulatory System
The Circulatory SystemIs the min transport system in which substances are carried from one part of the body to another in a variable size tubes called vessels. We have: Blood circulatory system: in which O2, CO2, nutrients, cells of immune system, hormones,...etc. are transported to various parts of the body. Lymphatic circulatory system: which drain the extra-cellular fluid from the tissue returning it to the blood circulatory system after their passage through lymph nodes.
The Blood Circulatory System is formed of the following structures:
Heart: to pump the blood. Arteries: series of vessels that carry blood from the heart to the tissues and they become smaller and smaller as they branch until they end in the capillaries. Capillaries: diffuse network of thin tubules that anastomose with each others & through their wall the exchange of O2 & nutrients between blood & tissues occur. Veins: series of vessels that will carry blood from the tissues to the heartIn general the Blood Circulatory System is divided into: Macrocirculation: vessels more then 0.1 mm. in diameter. Microcirculation: vessels only visible by microscope. The vessels of the microvasculature are particularly important because of its participation in the process of exchange of materials between blood & tissues.
General structure of blood vessels
The walls of all the blood vessels are composed of three identifiable layers named: tunica intima, tunica media, tunica adventitia. (tunica = coat (طبقة أو غلاف.Tunics of the blood vessel wall
Tunica Intima: consist of a layer of highly specialized cells called endothelial cells that line the vessel’s interior surface, These cells rest on a basal lamina. Beneath these cells is the sub endothelial layer which consist of loose connective tissue that may contain some smooth muscle cells.
Functions of endothelial cells:Have a role in the transport of substances from and to blood vessels.They are able to sense any change in blood pressure, O2 tension and blood flow by unknown mechanisms & they respond to these changes by secretion of substances that have a powerful effect on the tone of the vascular smooth muscle (endothiline, nitric oxide, prostaglandins…..) these substances will cause relaxation of the vascular smooth muscles leading to vasodilatation & so increase blood flow.
These cells are important for the control of blood coagulation process. Under normal conditions intact endothelial surface prevent blood clotting by secretion of certain factors that prevent formation of blood clot, however any damage to the endothelial surface will induce the aggregation of platelets & initiate a cascade of events leading to formation of fibrin from blood fibrinogen (called thrombus). This thrombus may enlarge in size until full obstruction of the vessel occur leading to death of the tissues. Endothelial cells can be activated after any form of tissue injury or tissue damage, they become cuboidal & secrete substances that facilitate adhesion of lymphocytes & nutrophiles to the endothelial surface to help in their migration to the damaged tissues.
In arteries the T. intima is separated from the T. media by the internal elastic lamina (membrane) which is composed of elastic fibers & has fenestrae (gabs) that allow substances to diffuse and nourish the deeper cells in the blood vessel wall, this membrane is very obvious in the muscular arteries and arterioles. The intima often appears wrinkled in section due to contraction of the smooth muscle cells present in the subendothelial layer (particularly true in arteries).
Tunica Media: Is the middle layer in the blood vessel wall. Composed mainly of smooth muscle cells reinforced with layers of elastic & reticular fibers that are found between the muscle cells. In larger arteries a thin external elastic lamina often separates the media from the adventitia. In capillaries & post capillary venules the media is replaced by cells called Pericytes.
The media is: particularly prominent in arteries relatively indistinct in veins non existent in very small vessels.
Tunica Adventitia: The outer layer of the blood vessel. Composed mainly of collagen and elastic fibers & few smooth muscle cells. Smooth muscle cells are particularly prominent in the adventitia of veins.
The adventitia is the most prominent layer in the wall of the veins. The adventitia will gradually become continuous with the connective tissues surrounding the organ through which the vessel pass.
Within the T. adventitia we can notice: Vasa Vasorum: (vessel of a vessel) these are very small blood vessels found in the tunica adventitia, they branch profusely here & they also send branches to the outer parts of the T. media to supply them with blood. Vasa vasorum are found in the adventitia of larger blood vessels because their wall is too thick & cannot be nourished by diffusion from the wall only. Vasa vasorum are more frequent in veins then in arteries which could be due to lack of O2 & nutrient materials in the venous blood. Vasa vasorum can arise as branches from the same artery they supply or from neighboring artery.
Specific structure of blood vessels:
The histological classification of blood vessels is made according to the size of the vessel & to the tissue composition so we have: Large size (elastic artery), Medium size (muscular artery), Arteriole, Capillary, Venule, Medium size vein, Large vein.Elastic arteries: The largest arteries, they receive the main output of the left ventricle. They include the Aorta & its main branches such as the carotid, subclavian & ranal arteries. Macroscopically these arteries have a yellowish color due to accumulation of elastin in their wall
Tunica intima is thicker then the intima of the muscular artery with thick sub-endothelial layer. Internal elastic lamina is present but it is difficult to be distinguished because it is very similar to the elastic laminae found in the media.
T. media is thick and highly developed contain many elastic laminae, in adult they reach up to 70 in no. & their no. increase with age.
Muscular arteries: (medium size arteries)
Muscular arteries varies in diameter from about 1 cm. close to their origin to about 0.5 mm., in the large muscular artery there might be 30 or more layers of smooth muscle cells whereas in the smallest muscular artery there are only 3 muscle layersMuscular artery has the following features: T. intima thinner then that of the elastic artery, consist of endothelial cells & very thin subendothelial layer. The internal elastic lamina is prominent. T. media consist mainly of smooth muscle cells which make these vessels highly contractile (the diameter of the lumen is controlled by the autonomic nervous system). Few elastic fibers can be seen between the smooth muscle cells.
Arterioles:
Less then 0.5 mm in diameter with narrow lumen T. intima consist of endothelial cells with very thin subendothelial layer internal elastic lamina only found in the larger arterioles. T. media is muscular & consist of one or 2 layers of smooth muscle cells. As the arteriole get smaller the smooth muscle cells become discontinuous (called met-arteriole) which will branch later on to form a network of capillaries. No external elastic lamina. T. adventitia is thin and insignificant.Capillaries
Capillary wall is formed of single layer of endothelial cells that are rolled in the form of a tube, these cells rest on a basement membrane. Occasional scattered contractile cells called pericytes are found external to the endothelial cellsTypes of capillaries: Depending on the structure of endothelial cell & the presence or absence of basal lamina, capillaries can be grouped into 3 groups:
Continuous (somatic) capillary: characterized by the absence of fenestrations in it’s wall, it is found in all types of muscle tissues, connective tissues, exocrine glands & nervous tissues. The cytoplasm of the endothelial cells of the continuous capillary contain many large pinocytotic vesicles, these vesicles are important for the transport of macromolecules in both directions across the endothelial cell.
fenestrated (visceral) capillary: characterized by the presence of large fenestrae in the wall of the endothelial cells, these are closed by a diaphragm that is thinner then the cell membrane, this type of capillaries have a continuous basal lamina, it is mainly found in tissues where rapid exchange of substances occur as in kidney, intestine & endocrine glands.
discontinuous sinusoidal capillary:
These are direct communications between arteriolar and venous circulations. At its arteriolar end, the arteriovenous anastomosis has a thick wall mainly due to abundant smooth muscle coat in the wall of the arteriole. Contraction of this smooth muscle coat lead to closure of the lumen of the anastomosis so the blood will be diverted more to the capillary network.Arteriovenous anastomosis:
Veins:
Will return blood to the heart. As with arteries the veins are classified into venules, Small veins medium veins large size veins.MEDIAN SIZE VEIN
MEDIAN SIZE ARTLARGE VEIN
Vena cava
aorta