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The Human Body:
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Blood
Blood is 8% of the total body weight of a person and is made up of plasma-a yellowish watery fluid of 90% water that contains digested food, minerals, salts and hormones. About 45% of the blood is made up of blood cells. There are 3 types of blood cells, red blood cells(1% of blood volume), white blood cells and platelets.
Blood has many important roles to play in the body. It’s a delivery service that transports oxygen and nutrients to every part of the body, from the head to the tips of the toes. As it goes around, blood also collects carbon dioxide and waste from cells. It carries carbon dioxide to the lungs to breathe out and waste to the kidneys and liver to be regulated and subsequently, discharged from the body. Blood also transports hormones which control growth. Vitamins, salts and minerals are taken to the parts of the body wherever needed. The water in the plasma bathes cells and keeps everything lubricated. Blood too contains special chemicals-plasma proteins which seal off cuts in the skin, forming clots to stop blood from excessively escaping from the body. These plasma proteins also serve to exert osmotic pressure. The plasma proteins Not dissolved; colloidal dispersion can also serve as reserve supply of amino acids for cell nutrition. The specific functions are as follows. Albumin binds and transports many substances for example bilirubin, bile salts, penicillin. Globulins exists in the Alpha and beta form which binds and transports substances eg, thyroid hormones, cholesterol, iron and are blood clotting factors. The Gamma form acts as anti-bodies (immunoglobulins). Fibrinogen is important in blood clotting process whereby it is the inactive precursor for fibrin meshwork.White blood cells fight infection from harmful bacteria and viruses that are disease causing. Moreover, it controls the body temperature by absorbing heat from busy organs like the liver and the heart, taking it to cooler, resting parts, so that heat is spread more evenly and a constant working temperature can be maintained.
Red blood cells have an unusual shape and are basically a plasma-membrane filled sac of haemoglobin. They look like flat discs that are thinner at the centre for they do not have any nucleus. This allows them to be flexible so that they can squeeze through very narrow capillaries. They are constantly renewed, have a short life-span of 120 days because they are easily wear out quickly as they travel around the body. The red blood cells have a quaternary protein structure that makes the cells look red, haemogloblin because of iron. It picks up oxygen easily. Each of the 4-iron atoms can combine reversibly with one oxygen molecule. Each hg molecule can pick up 4 oxygen molecules. Oxygen not very soluble in plasma, so most (98%) is carried in haemoglobin. It also carries carbon dioxide back to lung. Carbon monoxide (occupies O2 binding site on heme: CO poisoning). The H+ of carbonic acid buffers this acid so not to change PH of blood. The nitric acid in Heme binds to sulphur. Released at tissues to relax arterioles- stabilize blood pressure. The shape also provides a large surface area so that oxygen can be absorbed quickly. Each red blood cell can carry 1 billion oxygen molecules. They also contains some enzymes like Glycolytic enzymes and Carbonic anhydrase.
There are 2 populations of White blood cells, the T and B cells. T cells reside in the thymus, called thymocytes, synthesize and release lymphokines which help to attach and eliminate foreign particles. Circulating T-lymphocytes leave the thymus and travel to the lymph nodes and the spleen. T cells may regularly leave lymphoid tissues to circulate in the blood and tissue fluid. Any encounter with antigens will stimulate the T cell to divide to form a clone of cells, all of which can attach to the antigen molecules and destroy them. On the other hand, B cells can differentiate into the plasma cell series and memory cells. The mature plasma cell actively synthesizes and secretes antibodies of the same kind. Memory cells carry the memory of the first contact with the antigen and somehow enable the individual to respond more promptly and vigorously in a second encounter. The antigen bounds to the antibody into a antigen-antibody complex via non-covalent bonds for example H-bonds, van der Waals forces, hydrophobic interactions etc. Antigen-combining sites on the antibody molecule correspond to sites on the antigen molecels called antigen determinants. The reaction may be manifested as precipitation.
Blood in the body is constantly reconditioned. Its composition remains constant despite ongoing drain of supplies. Reconditioning organs receive more blood than needed to perform homeostatic adjustments to blood, for example, 20 % of cardiac output go to digestive tract to pick up nutrient supplies and another 20% to kidneys to eliminate waste, adjust water and electrolyte composition.
The lymphatic system is another set of vessels whose primary function is to return excess fluid which leak out of capillaries into interstitium, back to blood. It also return of filtered plasma proteins to blood such that proteins can leak out, but not back into capillaries and can go back via lymphatics.
The presence of the overlapping endothelial cells allow one-way openings in wall. The fluid pressure outside vessel pushes innermost edge of pair of overlapping edges, creating gap, permitting interstitial fluid to enter while the fluid pressure inside vessel pushes gap closed, forming “lymph”.
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