To the venules occurs gradually. Immediately behind the capillaries are pericytic venules, the diameter of which varies from 0.1 to 0.5 mm, and the length - from 0.5 to 70 mm; pericytes are present in their wall.
Intimacy of these vessels consists of an endothelium and a very thin subendothelial layer. The endothelial cell compounds in these venules are the most permeable in the entire vascular system.
The middle membrane of these venules can contain only contractile pericytes. According to some signs, post-capillary venules are close to capillaries, for example, they participate in inflammatory processes and the exchange of cells and molecules between blood and tissues.
Muscular veins: structure, histology
Most venul are muscular and contain at least individual smooth muscle cells in their walls. These vessels usually accompany arterioles, from which they are easily distinguishable on sections of tissue due to their thinner wall and collapsed lumen of irregular shape. Such venules can also affect blood flow in arterioles through the production and secretion of vasoactive substances that diffuse into the tissue.
From venules blood gathers in veins of increasing size, which are arbitrarily divided into small, medium and large. Most veins are small or medium in size; their diameter is 1–9 mm. Intima usually contains a thin subendothelial layer, which may sometimes be absent. The middle membrane consists of small bundles of smooth muscle cells mixed with reticular fibers and a thin network of elastic fibers. Adventitia containing collagen fibers is well developed.
Venous trunks large, located close to the heart, are large veins. The latter contain a well-developed intimacy, but their middle shell is much thinner, with a small number of layers of smooth muscle cells and a sharp predominant connective tissue. Adventitia is the most developed membrane in the veins; it often contains longitudinally located bundles of smooth muscle cells.
Such veins, especially the largest ones, are equipped with valves inside. The valves are formed by 2 folds of intimacy of a lunar shape that protrude into the lumen. They consist of connective tissue rich in elastic fibers and are lined with endothelium on both sides.
Valves, which are especially numerous in the veins of the limbs, direct venous blood to the heart. The force that promotes the movement of blood in the veins, due to the activity of the heart, increases due to the contraction of the skeletal muscles that surround these veins.
Lecture No. 16 Anatomy and physiology of the heart. Blood vessels of the pulmonary circulation.
THE CARDIOVASCULAR SYSTEM
It consists of the heart and blood vessels - blood and lymph.
Blood vessels:
capillaries
Arteries branch to:
Smaller ones are arterioles, which also branch into precapillaries, which branch into capillaries. In the capillaries, gas exchange takes place - oxygen and nutrients are given away, waste products are taken. Blood flows from the capillaries to the post-capillaries, then to the venules, and then to the veins.
Blood carries nutrients, hormones, metabolic products - transport function. Also a function - gas exchange (due to the fact that the walls of the capillaries are very thin).
The walls of the aorta and veins are thick, so they play the role of tubes through which blood flows.
Microvasculature includes all vessels from arterioles to venules. In them, blood performs its main functions - gas exchange and metabolism occur.
Arteries
Artery - This is a blood vessel through which blood flows from the heart.
Artery wall structure
The inner layer - intima (endothelium - provides a smooth surface), the middle layer - smooth muscle, the outer - adventitia (loose high-torque).
Smooth intima prevents blood clots from forming inside the blood vessels. If the smoothness of the vessel is disturbed, platelets, clinging to irregularities, are destroyed and blood clots form.
The wall of the artery is firm and elastic.
Veins
Vein - A blood vessel through which blood flows to the heart.
The walls of the veins are thinner and weaker than those of the arteries.
Vein classification
Depending on the structure of the walls of the veins, veins are distinguished:
muscle - there are such majority in the body, their wall consists of 3 layers: intima, smooth muscle) the layer is much thinner than arteries), adventitia.
armless - have 2 layers - intima and adventitia. Such veins are located inside the parenchymal organs and in the periosteum. They are tightly fused with the parenchyma and periosteum and therefore do not subside.
Depending on the structure of intima, veins are secreted:
with valves - on the inner wall there are valves formed from the endothelium. When there is a pulse wave under the pressure of the blood, the blood pushes the valves inward, preventing the reverse flow of blood. Valves are needed when blood rises from the bottom up - in the abdominal cavity and in the lower extremities.
without valves - veins of the head, neck, upper limbs.
Veins are also:
superficial - which we can see through the skin. They form networks between which there is an anastomosis - bridges from blood vessels. The drawing of superficial veins is strictly individual.
deep - located deep, usually parallel to the bones, like arteries. Names correspond to bones when it comes to limbs. Such veins accompany the large arteries. On the limbs, each major artery is accompanied by two companion veins.
According to the degree of development of muscle elements in the walls of veins, they can be divided into two groups: veins of the muscular (fibrous) type and veins of the muscle type. Muscle type veins, in turn, are divided into veins with weak, medium and strong development of muscle elements.
Fibrous veins (non-muscular) - are located in organs and their areas having dense walls with which they are firmly fused with their outer shell. The veins of this type include the non-muscular veins of the meninges, retinal veins, bone veins, spleen and placenta. The veins of the meninges and retina are malleable when the blood pressure changes, they can stretch very much, but the blood accumulated in them is relatively easy to flow into larger venous trunks under the influence of its own gravity. The veins of the bones, spleen and placenta are also passive in the movement of blood through them. This is because all of them are tightly fused with the dense elements of the corresponding organs and do not fall off, so the outflow of blood through them is easy.
The wall of non-muscular veins is represented by the endothelium, surrounded by a layer of loose fibrous connective tissue, fused with the surrounding tissues. Smooth muscle cells are absent.
Muscle type veins characterized by the presence in their membranes of smooth muscle cells, the number and location of which in the vein wall are due to hemodynamic factors.
There are veins with weak, medium and strong development of muscle elements.
Veins with poor muscle development - These are small and medium veins of the upper body through which blood moves passively, under the influence of gravity.
Small and medium-sized veins with poor muscle development elements have a poorly expressed subendothelial layer, and a small amount of muscle cells are contained in the middle membrane. In some small veins, for example, in the veins of the digestive tract, smooth muscle cells in the middle membrane form separate "belts" located far from each other. Due to this structure, veins can expand greatly and perform a depot function. In the outer membrane of small veins there are single longitudinally directed smooth muscle cells.
Among the veins of large caliber, in which muscle elements are poorly developed, the most typical superior vena cava, in the middle membrane of the wall of which there is a small number of smooth muscle cells. This is partly due to the upright posture of a person, due to which blood flows through this vein to the heart due to its own gravity, as well as respiratory movements of the chest. At the beginning of diastole, even a slight negative blood pressure appears in the atria, which, as it were, draws in blood from the vena cava.
Veins with moderate muscle development characterized by the presence of single longitudinally oriented smooth muscle cells in the intima and adventitia and bundles of circularly located smooth myocytes, separated by layers of connective tissue - in the middle membrane. Inner and outer elastic membranes are absent. Collagen and elastic fibers of the outer sheath are directed mainly longitudinally. In addition, in the outer shell there are individual smooth muscle cells and small bundles of them, which are also located longitudinally.
To veins with a strong development of muscle elements include large veins of the lower half of the trunk and legs. They are characterized by the development of bundles of smooth muscle cells in all three of their membranes, and in the inner and outer shells they have a longitudinal direction, and in the middle - circular. Numerous valves are available. This structure is due to the flow of blood in the veins against gravity.
Veins - carry out the outflow of blood from organs, participate in the exchange and depositing functions. Distinguish superficial and deep veins. Veins widely anastomose, forming plexuses in the organs.
Hemodynamics: In veins, in comparison with arteries, other hemodynamic conditions: both pressure and its drops are much lower in magnitude, and changes in pressure are not pulsating - they are associated not with contractions of the heart, but with a change in the position of parts of the body or with breathing.
Blood Composition: In addition, veins have a different blood composition: in particular, less oxygen and more carbon dioxide.
Structure Features
The named functional features lead to the following structural features.
Elastic Elements - Veins contain fewer elastic elements (due to lower pressure drops).
Muscle Elements — Typically, veins contain less muscle elements. Moreover, the relative content of the latter depends not so much on the caliber of the vessel as on the location of the vein. So, in the veins of the upper half of the body there are few (or not at all) muscle elements. On the contrary, in the veins of the lower extremities and the lower half of the body there are significantly more muscle elements (to overcome the gravity of the blood) and their number increases as the veins enlarge.
Valves - Approximately 50% of the veins have valves - to prevent retrograde blood flow when the pressure gradient changes.
Vasa vasorum - feeding arteries are found in all shells of the vessel wall (and not just in t. Externa, as in arteries) and capillaries open directly into the lumen of the veins.
Shells Tunica intima1. In t. intima subendothelial layer is relatively weak.
2. Usually there is no internal elastic membrane (except for the inferior vena cava and heart veins).
3. Valves, if any, are derivatives of the inner shell at the base of the valve are muscle elements, the thickness of the leaf itself is loose fibrous connective tissue, and from the surface the valve is covered with endothelium.
Tunica mediaThis layer is much thinner than arteries of a similar caliber.
Tunica externa This shell, on the contrary, is more pronounced and often amounts to the main part of the vessel wall
(being several times thicker than the two previous shells combined).
II. Muscle bundle orientation
at t. media - as in almost all vessels (paragraph 18.1.5.1), circular, but in t. intima and t. externa (if they have muscle elements) - longitudinal.
Exceptions are veins of the heart: in t. media - the longitudinal direction of the muscle bundles; portal vein: in t. media are bundles of both circular and longitudinal orientation.
1. According to the structure of the walls of the veins are divided into 2 types
(muscleless and muscle type); and taking into account the further division of the muscle type veins, 4 subtypes are obtained.
2. Thus, veins differ in the content of muscle elements:
III. contain in two shells (t. media and t. externa) - veins with average development of muscle elements,
Non-muscular (fibrous) veins: no myocytes. These include the veins of the meninges, bones, spleen, and placenta. These veins are tightly fused with the stroma of the corresponding organs. Therefore, in a number of organs (for example, in bones, the dura mater), the veins, despite the absence of muscle elements, do not collapse.
The wall of such veins includes: t. intima - endothelium on the basement membrane, t. externa - a thin layer of loose connective tissue. The middle shell is absent.
Veins with poor muscle development:
myocytes - only in t. media
Almost all veins of the upper half of the body belong to this group of veins - from small to the largest of them, the superior vena cava.
Here already beyond t. intima (endothelium and poorly developed subendothelial layer) follows
t. media - with a small amount of muscle elements. The main thickness is t. externa. It is represented almost exclusively by loose connective tissue. In those veins of this type, which are located in the lower half of the body and in the lower extremities, valves may be contained.
Veins with an average development of muscle elements: myocytes - in t. media and t. externa
This group of veins in humans includes the brachial vein and the middle veins of the lower extremities, and in animals (in particular cats) the femoral vein (in which hemodynamics is similar to that in the brachial vein). Thus, these are veins through which blood moves upward (against gravity), but it does not cover a very large distance.
Shell. T. intima (including the endothelium and the subendothelial layer) forms valves in most of these veins. In t. media - 2-3 layers of myocytes. T. externa contains not only connective tissue, but also muscle elements.
IV. Veins with a strong development of muscle elements: myocytes - in all three membranes
In the last group - large veins of the legs and lower half of the trunk: femoral veins (in humans), deep veins of the male penis, iliac veins, inferior vena cava.
Shell. They contain muscle elements in all three shells, including at t. intima - in the subendothelial layer. But, despite the strong development of muscle elements, a significant effect on the blood flow in these veins has a reduction in the muscles of the legs and pelvis. There are valves in the femoral vein; however, they are not present in the inferior vena cava (as in the superior vena cava).
End of work -
This topic belongs to the section:
Histology
Histology from the buckwheat histos tissue logos teaching the science of the structure of development and vital activity of tissues of living organisms ... The formation of histology is closely related to the development of microscopic technology and ... In the history of the study of tissues and the microscopic structure of organs, two periods of pre-microscopic and ...
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poor development of the circular muscle layer; more frequent longitudinal arrangement of smooth myocytes;
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the presence of valves.
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According to the degree of development of muscle elements in the walls of veins, they can be divided into two groups: veins of the muscular (fibrous) type and veins of the muscle type. Muscle type veins, in turn, are divided into veins with weak, medium and strong development of muscle elements.
Fiber type veins (non-muscular) - are located in organs and their areas that have dense walls, with which they are firmly fused with their outer shell. The veins of this type include the non-muscular veins of the meninges, retinal veins, bone veins, spleen and placenta. The veins of the meninges and retina are malleable when the blood pressure changes, they can stretch very much, but the blood accumulated in them is relatively easy to flow into larger venous trunks under the influence of its own gravity. The veins of the bones, spleen and placenta are also passive in the movement of blood through them. This is because all of them are tightly fused with the dense elements of the corresponding organs and do not fall off, so the outflow of blood through them is easy.
The wall of non-muscular veins is represented by the endothelium, surrounded by a layer of loose fibrous connective tissue, fused with the surrounding tissues. Smooth muscle cells are absent.
Muscle type veins characterized by the presence in their membranes of smooth muscle cells, the number and location of which in the vein wall are due to hemodynamic factors.
There are veins with weak, medium and strong development of muscle elements.
Veins with poor development of muscle elements are small and medium veins of the upper body, through which blood moves passively, under the influence of gravity.
Veins of small and medium caliber with poor development of muscle elements have a poorly expressed subendothelial layer, and a small number of muscle cells are contained in the middle membrane. In some small veins, for example, in the veins of the digestive tract, smooth muscle cells in the middle membrane form separate "belts" located far from each other. Due to this structure, veins can expand greatly and perform a depot function. In the outer membrane of small veins there are single longitudinally directed smooth muscle cells.
Among large-sized veins, in which muscle elements are poorly developed, the most typical is the superior vena cava, in the middle membrane of the wall of which there is a small number of smooth muscle cells. This is partly due to the upright posture of a person, due to which blood flows through this vein to the heart due to its own gravity, as well as respiratory movements of the chest. At the beginning of diastole, even a slight negative blood pressure appears in the atria, which, as it were, draws in blood from the vena cava.
Veins with an average development of muscle elements are characterized by the presence of single longitudinally oriented smooth muscle cells in the intima and adventitia and bundles of circularly located smooth myocytes, separated by layers of connective tissue - in the middle membrane. Inner and outer elastic membranes are absent. Collagen and elastic fibers of the outer sheath are directed mainly longitudinally. In addition, in the outer shell there are individual smooth muscle cells and small bundles of them, which are also located longitudinally.
Veins with strong development of muscle elements include large veins of the lower half of the trunk and legs. They are characterized by the development of bundles of smooth muscle cells in all three of their membranes, and in the inner and outer shells they have a longitudinal direction, and in the middle - circular. Numerous valves are available. This structure is due to the flow of blood in the veins against gravity.
54. The vessels of the microvasculature. Histohematological barrier. Organospecificity of capillaries.