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The Human Body:
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Blood vessels
There are a number of different types of blood vessels in the body. Each type serves a specific purpose. There are Arteries that bring blood to tissues (high speed, low resistance), Arterioles that carry out similar functions(low-medium resistance), gaseous exchange occurs in capillaries (higher resistance), Venules and Veins back to the heart.
The arteries’ main functions are to buffer against changes in the blood supply and blood pressure, maintain the blood pressure and help to create continuous flow. Thus, arteries have a much larger lumen compared to the other blood vessels. An aorta’s circular lumen is about 10mm. In the aorta, there is a small amount of smooth muscle to prevent it’s rupture and is thick and muscular. It is impermeable to fluid and red blood cells. The oxygenated blood flows at a rapid rate in the arteries and has low blood volume but at immense pressure. The pulse there is strong and pulsatile.
The smooth muscle of the arterioles is sensitive to local chemical changes and hormonal signals and brings about either vasodilation or vasoconstriction as follows. Smooth Muscle relaxation causes Vasodilatation when there is low oxygen, and high carbon dioxide and metabolites, low sympathetic stimulation and presence of heat. While Smooth Muscle contraction causes Vasoconstriction when there is
high oxygen levels, low carbon dioxide and metabolites, high sympathetic stimulation, when one is Coldand if Endothelin is present.
The capillaries are sites of material exchange between blood and body fluids and cells, so high surface area is important. Thus its circular lumen is much smaller, 7-9um, so as to allow the exchange to be sufficiently carried out. Furthermore, it is only 1 cell thick, so exchange is enhanced. It is thus, semi-permeable. Blood flow is slow and blood volume is high, with falling blood pressure. It is non-pulsatile and moves away and towards the heart. Contraction of the pre-capillary sphincter regulates entry of blood into the capillary. Red blood cells here move in a single file.
Diagram 1.3 Red blood cells in single file in capillary
There is net exchange occurring in the capillaries. It is broguth about by various forces. One of which, the Oncotic pressure gradient. Oncotic pressure is usually 25 mmHg in capillaries, 3 mmHg in interstitium, as some plasma proteins leak out of pores so opg = 25 mmHg . So, net water flow into capillary (down water [] gradient or osmosis). Another is the Hydrostatic pressure gradient , hydrostatic pressure is usually 35 mmHg at arterial end of capillary and 15 mmHg at venous end (due to resistance to flow of blood through capillary), causing fluid to flow out of capillary. Oncotic pressure opposes hydrostatic pressure , thus moving water back into capillary.
Veins, on the other hand is very much different. It acts as a main distensible reservoir for blood. So, the vena cava’s diameter is about 12.5mm. It is in a distorted oval shape, relatively thin and slightly muscular. It is impermeable to fluid and red blood cells. Blood flow towards the heart and is mainly deoxygenated. Furthermore, blood flow is slow and of low pressure. The volume is high but is non pulsatile. Dilation and constriction is limited.
The Venous capacity is the amount of blood veins can hold. It increases when there is more blood spends longer time in veins, thus, lower circulating volume. It decreases when more blood returns to heart and subsequently pumped out. It ultimately depends on the distensibility of the vessel walls and influence of the externally applied pressures squeezing veins. Venous return is the volume of blood returning to atria.
Venous system pressure is 17 mmHg while atrial pressure 0 mmHg (goes down pressure gradient). If atrial pressure increases (eg with faulty AV valve), there will be lower venous return. Blood accumulation in venous system, can cause congestive heart failure .
Thus regulation of venous return is important. A number of systems are present to control it. Firsly, Sympathetic innervation. Veins have low tone, but high sympathetic innervation. So, vasoconstriction occurs and there is raised venous pressure. It is to be noted that vasoconstriction in veins means high flow (from decrease capacity, squeezing out more blood already present); while arteriole vasoconstriction lower flow (from increase resistance). Secondly it’s Skeletal muscle activity. Many large veins lie between skeletal muscles in arms or legs. When muscle contracts, veins compressed (skeletal muscle pump), so lower venous capacity, higher venous pressure (extra way that blood returns to heart during exercise). Thirdly it’s Gravity. It is largely dependent on the position a person is in. If lying down, then pressure is equal. But if standing up, the vessels below heart are subjected to gravity (pressure caused by weight of blood from heart to vessel). Thus the distensible veins yield under hydrostatic pressure causing elevated capacity (arteries same pressure, but not distensible). Whereas in leg, post-capillary blood pools in extended veins, decreases venous return and, lowers cardiac output. Fourthly, it’s the pressure within chest cavity that is 5 mmHg less than atmospheric pressure. Blood goes down pressure gradient. Thus, squeezes blood from lower veins to chest veins, increasing venous return. Lastly, it’s Cardiac suction. During ventricular contraction, AV valves drawn downward, enlarging atrial cavity so atrial pressure transiently drops lower than 0mmHg – sucking more blood into atria.
In veins, there are semi-lunar valves. These valves stop blood from going backwards. They are one-way valves that are spaced 2-4 cm away which also helps to counteract gravity (minimize back flow).
In Varicose veins, the venous valves are incompetent as it can no longer support the column of blood above it, and collapses. Such conditions are aggravated by frequent, prolonged standing, very distended veins especially in pregnant women. It can be so distended that blood pools in them and edges of valves cannot form seal.
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