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Histological characteristics of the different vascular systems are described below: 1. Arteries 2. Arterioles 3. Capillaries 4. Veins 5. Blood Vessels of Blood Vessels.
1. Arteries:
In the arteries, all the layers (Fig. 7.14) are present. The two outer layers are very thick, because it has to withstand considerable blood pressure. The tunica media consists chiefly of circularly arranged smooth muscle cells.
The tunica adventitia is composed chiefly of white fibrous connective tissue that runs parallel to the long axis of the blood vessels and a definite external elastic membrane is present very close to the media. This outer coat is comparatively non-elastic and limits the stretching of the artery and thus prevents undue distention and rupture.
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A special system of vessels—the vasa vasorum—passes into the arterial wall to supply blood to these layers. The tunica intima consists of a single layer of endothelium, set upon a basement membrane of elastic tissue— known as the internal elastic membrane (elastic lamina).
This lamina is thrown into folds to prevent injury to the endothelial lining as may occur owing to pulsation. Besides these, there is also an inner endothelial lining which is sub-endothelial layer of delicate fibro-elastic (areolar) connective tissue.
2. Arterioles:
The arterioles have got relatively thick walls and narrow lamina. These vessels are rich in vasomotor innervation and are capable of distributing blood to the different portions of the body by vasodilatation and vasoconstriction. Pressure and flow of the vascular systems are principally maintained through modifying the lumen of the arterioles.
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The arterioles also possess three layers (Fig. 7.15). The tunica intima consists of endothelial lining and internal elastic membrane. But it is devoid of sub-endothelial fibro-elastic connective tissue. The tunica media is composed of one to five layers of muscle cells and contains scattered elastic fibrils. The tunica adventitia is also present and is very thin. It is composed of loose connective tissue. There is no definite external elastic membrane.
3. Capillaries:
These make a connective link in between the arterioles and venules. They are linked by a single layer of flat endothelial cells which are the major component of the wall (Fig. 7.16). The capillary endothelium does not directly touch the elements of other tissues and is always separated from a supporting bed of the connective tissue by an intervening layer—the basal lamina. Each endothelial cell consists of curved thin plate having an ovoid or elongated nucleus.
The cells are generally stretched and tapered along the axis of the capillary. The capillaries are also surrounded by a thin layer of delicate collagenous and reticular fibres. These are also accompanied by fixed macrophages, fibroblasts, undifferentiated mesenchymal cells and a few cells belonged to accompanying nerve fibres.
Besides these, there are peculiar types of cells with long branching processes that surround the capillary walls. These cells are known as Rouget cells. These cells (Fig. 7.17) were known to function as contractile elements of the capillaries. But recent observations claim that the contractility of the capillaries is independent of Rouget cells.
The average diameter of the capillaries is 7-9 µ and just allowing blood corpuscles to pass through. In resting state the most of the capillaries remain closed and during functional state all of the capillaries mostly open up. According to Zweifach, the arteriovenous shunt is always present in a capillary bed. Recent electron microscopic studies have added a considerable knowledge on the structure of capillaries.
According to Fawcett and others, the capillaries can be grouped into:
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i. Muscular Type:
The muscular type. (Fig. 7.18) comprises the capillaries of the cardiac muscle, smooth muscle, lung central nervous system and also other tissues, which have got uninterrupted endothelial cell of approximately equal thickness with occasional luminal bulges due to location of cell nuclei.
ii. Fenestrated Type:
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The fenestrated type (Fig. 7.19) comprises the capillaries of many endocrine glands, intestinal mucosa, renal glomerulus and certain other organs. In this type the endothelial linings are not continuous and attenuated with numerous pores ranging from 300 to 500 A. The pores or circular fenestrations are not completely open but are covered by a diaphragm thinner than the cell membrane (Fig. 7.19).
The glomerular capillaries of the kidney have got large pores which are enveloped by the continuous basal lamina. These large pores are very important in production of glomerular filtrate. From electron microscopic studies, it reveals that all capillaries have got numerous small vesicles along the luminal and basal surfaces of the endothelial cells. These vesicles are known as pinocytic vesicles or caveolae intracellulars. These vesicles are presumably formed by the invagination of the cell membrane and are involved in the transport of fluid across the cell membrane.
Sinusoids:
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Sinusoids and sinusoidal capillaries are not true capillaries and they have got relatively large calibre (30 µ) with irregular and tortuous walls. Continuous endothelial lining is absent. There is also some incomplete lining of phagocytic cells. Due to absence of basal lamina, the blood gets direct contact with the tissue cells.
Sinousoids differ from the capillaries on the following points:
i. Capillaries have got complete endothelial lining whereas the sinusoids have got irregular linings of both endothelial and phagocytic cells. Basal lamina is absent and blood can have direct contact with the tissue cells within the sinus.
ii. The capillaries are surrounded by thin layer of connective tissue but the sinusoids are lined by a network of reticular fibres.
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iii. Capillaries always make a connecting link in between the arteries and veins whereas the sinusoids always make connecting link with the same type of vessels.
iv. Sinusoids are of comparatively large calibre (30 µ) and, the velocity of flow is very slow.
4. Veins:
In veins all the three layers are present but the intima and media are comparatively thinner than those of arteries only due to reduction of muscular and elastic components. Though the walls are very thin yet the vessels are very strong due to presence of connective tissue components.
In tunica intima, the endothelial cells are less elongated than those of the arteries. A little connective tissue along with a few fine elastic fibres is present. In tunica media, there is little elastic tissue and muscle. There is an also considerable collagen fibre. The tunica adventitia is much developed and much thicker than the tunica media due to presence of muscle, collagen and elastic fibres. Structures of the veins all throughout are not same and differ from one place to another. Some veins do not possess smooth muscle. These are the cerebral veins, meningeal veins, retinal veins, etc.
Valves of the Veins:
Valves are present in most of the veins particularly of those of the lower limb. These valves prevent backflow from the heart. These are semilunar pocket like flaps (Fig. 7.20) formed by the local folding of the intima.
5. Blood Vessels of Blood Vessels (Vasa Vasorum):
Large arteries and veins of diameter above 0.1 mm are generally supplied with nutrient blood vessels which arise from the adjacent small arteries. In large arteries the vasa vasorum penetrates or may penetrate all the layers of the blood vessels and ends ultimately with the lumen of the artery.
In true sense, the vasa vasorum actually supplies the tunica adventitia and the external part of the tunica media.
