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Circulatory System

CIRCULATORY SYSTEM (A) FORMATION OF TISSUE FLUID AND EXCHANGE OF MATERIALS IN THE CAPILLARY NETWORK In a capillary network, two opposing forces mainly determine the movement of fluid between the blood and tissue fluid: (1) the hydrostatic pressure difference and (2) the osmotic potential difference between the blood and the tissue fluid. In the part of the capillary network near the arterial end, blood pressure is much higher than that of the tissue fluid so that the difference in hydrostatic pressure exceeds the osmotic difference between the two fluids. As a result, some plasma is filtered out of the capillaries under pressure into the tissue space to form tissue fluid. As blood moves along the narrow capillaries, the blood pressure drops continuously so that the difference in hydrostatic pressure between the blood and tissue fluid decreases steadily or may even be reversed. At the venous end of the capillary network, most the tissue fluid formed at the arterial end is reabsorbed back into the capillaries by osmosis This mechanism results in a continuous formation of tissue fluid from the plasma by filtration and the return of tissue fluid back to the blood by osmosis. This circulation of tissue fluid is essential for the regulation of blood pressure and blood volume.

For instance, when there is a drop in blood pressure such as due to hemorrhage, less tissue fluid would be formed at the arterial end and more tissue fluid will be absorbed back into the blood at the venous end of the capillary network. The net flow of tissue fluid into the capillary network results in a rise in blood volume and blood pressure to normal .The opposite occurs when the blood pressure or blood volume increases. When the osmotic potential of the blood is raised, e.g. due to a low plasma protein level, the osmotic difference between the blood and tissue fluid decreases. This results in a net formation of tissue fluid from the plasma as the volume of tissue fluid formed exceeds that returned to the plasma As the tissue fluid volume (about 10 litters) is three to four times larger than the volume of plasma (About 3 liters), the tissue fluid serves as a reservoir which can supply additional fluid to the Circulatory system or draw off excess.

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This mechanism of tissue fluid formation and withdrawal is Important in maintaining a constant plasma volume. This process, however, plays a relatively minor role in the exchange of nutrients and metabolic Wastes between the blood and the tissue. Although some nutrients and wastes are carried by mass Flow during this fluid movement, most nutrients and metabolic wastes are transported between blood and tissue cells by diffusion according to the concentration gradient of these substances. Thus Glucose diffuses from the blood to the tissue cells across the capillary wall along the whole length of the capillary network not just restricted to the arterial end. Similarly, waste substances produced by The body cells diffuse into the blood along the capillaries in the opposite direction this Process of diffusion is facilitated by the very thin capillary wall and the numerous branches of the Capillary network which present a very large surface area for diffusion to occur. The low rate of Blood flow along the capillaries also allows enough time for diffusion to take place.

Many local textbooks elaborate on the formation of tissue fluid but give little description on the Diffusion of substances between the blood and the tissue. Thus many students develop the mistaken Belief that the formation and absorption of tissue fluid in the capillary network is essential for the Transport of materials between blood and body cells, and fail to appreciate the importance of diffusion in the exchange of materials in the capillary network. (B) ADAPTIVE FEATURES OF THE CAPILLARY NETWORK It is a well known fact that blood flows very slowly along the capillaries and this low flow rate Facilitates the exchange of materials by diffusion between the blood and tissue fluid. In accounting for The low blood flow rate in the capillaries, many students wrongly think that it is due to the narrow Diameter of the capillaries, which presents a high resistance to blood, flow. In a closed circulation, it must be realized that the volumes of blood flowing through any cross section Of the circulatory system per unit time (V) is constant, and the flow rate of the blood at different Points of the system varies according the following relationship: V = A X R Where A is the total cross sectional area at any point and R is the flow rate of blood at this point. As V is constant at all points of the circulatory system; it follows that the flow rate is inversely Proportional to A.

The very low rate of blood flow in a capillary network is therefore a result of the Large total cross-sectional area rather than due to the narrow diameter of the capillaries. The very Narrow diameter of the capillaries results in a high resistance to blood flow, and this leads a Significant drop in the blood pressure along the capillaries. However, this rapid drop of blood Pressure in the capillaries does not affect the blood flow rate in this region, which is inversely Proportional to the total cross-sectional. The blood velocity is very high in the aorta, becomes progressively lower in the Arteries and arterioles, and then drops rapidly to a low level as it pass through the capillaries, Which have a total cross-sectional area 1000 times that of the aorta. The speed then increases steadily in the venues and veins as the cross-sectional area Decreases.

The significance of the very slow blood flow in the capillaries (0.07 cm per second) is that it allows adequate time for the exchange of nutrients and metabolic wastes between the blood and tissue cells by diffusion. Irrespective of the total cross-sectional area and the resistance of the Blood vessels; blood always flows along a concentration gradient, i.e. from a point of higher Pressure to that of lower pressure. The greater the resistance to blood flows between these two Points, the greater will be the drop in blood pressure as blood moves along. In a closed system such as the circulatory system, the rate of fluid flow at any point is not determined By the pressure at that point, but by its relative total cross-sectional area.

Thus blood flow rate is Lowest at the capillary network because of its large total cross-sectional area. On the other hand, the High resistance of the narrow and numerous capillaries does not affect the blood flow rate at the Capillaries, but will result in a rapid drop in blood pressure along the capillaries as more energy is Required moving the blood along this part. To help students to integrate the above ideas, the table below provides a summary of the adaptive Features of the capillaries and their significance: Adaptive features of capillaries and their significance, Adaptive features Effect Significance 1. Large total cross-sectional area slow blood flow rate, Adequate time for exchange of Substances 2. Thin capillary wall, Readily permeable to dissolved substances, Rapid diffusion of substances Plastic anemia is a disease of the bone marrow- the organ that produces the body’s blood cells.

Approximately .Two thousand people in the U.S. are diagnosed each year with plastic anemia. The symptoms of plastic anemia are fatigue, bruising, infections, and weakness. Although these symptoms are much like those associated with leukemia. Plastic anemia is not a form of cancer. In-patients, with plastic anemia the bone marrow stops producing, or produces too few red blood cells, white blood cells, and platelets.

Without sufficient red blood cells, oxygen cannot reach organs and tissues throughout the body. A decrease in the number of white blood cells causes the body’s ability to fight infection as well as it should. Platelets are needed to help blood clot. Although the exact cause of plastic anemia is not known, most evidence points to a combination of factors. The first factor is damaged stem cells.

These are the primitive cells in the bone marrow that produce blood cells. Another factor is damage to the bone marrow environment in which blood cells develop. Ot …


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