9. Illustrates the process of transportation of oxygenated and deoxygenated blood.
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The cardiovascular or circulatory system is designed to ensure the survival of all cells of the body at every moment and it does this by maintaining the immediate chemical environment of each cell in the body (i.e., the interstitial fluid) at a composition appropriate for that cell's normal function. The term “homeostasis” is used to denote the approximate constancy of the internal environment (Claude Bernard, 1866).
First consider the simple hypothetical case of a single spherical cell suspended in a large (>times the cell volume), well-stirred volume of aqueous medium in equilibrium with room air and containing other nutrients. Oxygen availability is often a limiting factor for cell survival, and it is generally supplied to a cell by passive diffusion. As oxygen molecules diffuse into the cell, they are consumed, so that there is a progressive fall in oxygen concentration from the surface of the cell to the lowest concentration which occurs at the center of the cell. For a spherical cell with a typicaldiffusion coefficient for oxygen (≈10−5 cm2/s) and an oxygen consumption of resting skeletal muscle (≈10−2 ml O2 cm−3 min−1), the critical size (radius) which is just adequately supplied with oxygen from the surrounding medium is about 1 mm. Thus, we find that diffusion puts an upper limit on the size of cells in regard to their need for oxygen.
Although diffusion is an efficient transport process over short distances (<100 μm) as seen by the average time required for a molecule to diffuse a distance x (t ≈ x2/2D), how can a much larger multicellular organism, such as the human body containing about 100 × 1012 cells, be adequately supplied with oxygen? For mammals, the bathing medium for cells is water and total body water is about 60% of body weight. For a 70-kg person, total body water is distributed among three compartments with the following approximate volumes: intracellular ≈23 l (33% of body weight); interstitial ≈16 l (22.5% of body weight); and circulating plasma ≈3 l (4.5% of body weight). Cells are bathed in interstitial fluid (ISF), but interstitial fluid volume is only a little more than half the intracellular fluid volume. Thus, ISF cannot be considered a large reservoir of fluid, and its composition is directly influenced by cellular metabolism.
An organism is faced with the following problem: How can the composition of ISF be maintained near its desired value? The solution of this problem is to introduce a circulatory system which continuously refreshes the ISF by putting it in intimate contact with “fresh, reconditioned” fluid (i.e., arterial blood). The circulating blood must be brought close to the cells (<10 μm) since nutrient and metabolic waste exchange takes place by passive diffusion, a transport mechanism which is most efficient over short distances. Thus, the cardiovascular system uses bulk flow (convection) to reduce the effective distance between the pumping action of the heart and the various parts of an organism.
In order for this system to be practical and do its job efficiently, two important conditions must be satisfied: (1) there must be adequate blood flow through the smallest blood vessels, capillaries, which are in contact with the cells comprising a tissue; and (2) the chemical composition of the incoming blood must be controlled to be that which is desired in the ISF. The design and operation of the cardiovascular system fulfill these conditions. Two important functions of the cardiovascular system are to move material (the carrier is blood) and to move heat (tissue metabolism generates heat that must be brought from the body's core to the cutaneous vascular bed at its surface, where it is radiated away from the body).
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