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Vascular System and Foetal Circulation Vascular system
1. The vascular tree Blood is sent mostly 'in parallel' from the heart. This means each organ receives blood fresh from the heart and not from other tissues. However, a few organs are connected 'in series', and obtain their blood from the venous outflow of another organ. This is called a portal system. The main advantage of this system is it allows a solute to be transported from one place to another without dilution in the general circulation. For example, the hepatic portal vein in vertebrae carries blood from the gastrointestinal tract to the liver, which allows filtration of newly absorbed compounds. The vascular tree is split into two parts - the pulmonary circulation and the systemic circulation. In the systemic circulation, arteries extend from the aorta and transport blood to the individual organs. Each artery branches several times, increasing the number, whilst the diameter of the individual arteries gradually decreases. The smallest arteries are called arterioles. The pressure in arteries and arterioles is relatively high, and they contain around 15% of the blood volume. Each of the arterioles branch into even smaller capillaries. Each capillary has a diameter similar to the size of a single erythrocyte. Exchange of nutrients and gases takes place across the capillary wall. The pressure in capillaries is intermediate, and they contain around 5% of the blood volume. The capillaries empty into venules, which merge into larger veins, which then finally empty into the heart. The pressure in veins and venules is relatively low, and they contain around 80% of the blood volume. Some organs receive an anastomosed blood supply. This is a collateral blood supply, which avoids the organ becoming necrotic if blood supply is blocked. Such systems are present in the brain (the circle of Willis) and the heart. All blood vessels, except capillaries, are under neural and hormonal regulation.
2. Structure of blood vessels The general structure of a blood vessel is as follows ...
a) Arteries and arterioles Arteries and arterioles contain relatively thicker walls, more smooth muscle and more elastin fibres than veins and venules. The high level of elastin and collagen in the aorta dampens the oscillating cardiac output when the blood is pushed from the heart. This is achieved as the elastic properties of the aorta allow it to stretch to accommodate the sudden inrush of blood, decreasing the pressure a little. Arterioles are have high amounts of smooth muscle in their walls. Contraction of the smooth muscle decreases the radius of the arterioles. This would result in a corresponding increase in blood pressure. In this way, contraction of the arterioles allows regulation of blood pressure. Resistance to blood flow is higher in arterioles than in arteries. b) Capillaries Capillaries do not contain any collagen or elastin fibres in their walls. Pressure in capillaries is relatively low. This is important as capillaries are very fragile, so high pressure would damage them. They are also very permeable, and high pressure would force a lot of fluid out through the capillary wall. Individual capillaries have a very small radius, but there are a large number of capillaries. This results in a large surface area and small blood flow that allows efficient exchange. The slower flow of blood through capillaries allows time for diffusion of molecules. There are three types of capillaries ...
Continuous - continuous lining of endothelial cells except for clefts between cells. This most common type present in the body.
Fenestrated - fenestrations are points where the cell membrane is compressed to permit greater fluid transmission. An example is the glomerular capillaries in the kidney.
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