Transport
Blood transports oxygen from the lungs to the capillaries and nutrients from the intestine to the capillaries, where they enter tissue fluid. From the tissue fluid, blood also absorbs carbon dioxide and other wastes given off by the cells and carries them away. Carbon dioxide exits the blood at the lungs. The liver produces urea, a waste product that travels by way of the bloodstream to the kidneys, where it is excreted.
Exchanges between Blood and Tissue Fluid
At the arterial end of a capillary, blood pressure (40 mm Hg) is higher than the osmotic pressure of the blood (25 mm Hg). Osmotic pressure is caused by the presence of salts and particularly by the plasma proteins. Because blood pressure is higher than osmotic pressure, water exits a capillary at the arterial end. This is a filtration process because large substances, such as red blood cells and plasma proteins, remain, but small substances, such as water molecules, leave the capillaries. Tissue fluid, created by this process, consists of all the components of plasma except the plasma proteins.
Along the length of the capillary, molecular diffusion follows a concentration gradient. The area of greater concentration for oxygen and nutrients is always blood. Therefore, they diffuse out of a capillary. The cells use glucose (CGH120G) and oxygen (02) in the process of cellular respiration, and they use amino acids for protein synthesis. Following cellular respiration, the cells give off carbon dioxide (C02) and water (H20). Tissue fluid is always the aT' a of greater concentration for these waste materials; Before, they diffuse into blood at the capillary.
Because blood pressure is much reduced (10 mm Hg) at the venous end of the capillary, osmotic pressure (25 mm Hg) tends to pull water back into the capillary. Retrieving water by means of osmotic pressure is not completely effective; thus, a portion of the fluid is not picked up at the venous end. This excess tissue fluid enters the lymphatic capillaries and is called lymph. Lymph is returned to the systemic venous blood when the major lymphatic vessels enter the subclavian veins.
Water oxygen and nutrient molecules (for example. glucose and amino acids) exit a capillary near the arterial end. Water and waste molecules (for example. carbon dioxide) enter a capillary near the venous end.
Blood Clotting
When a blood vessel is cut, it immediately constricts to reduce blood flow to the area. Then, the platelets stick to collagenous fibers in the connective tissue layer beneath the endothelium. As more and more platelets congregate, they form a platelet plug that can fill a small break.
Another response to blood vessel injury is the formation of a blood clot. At least 12 clotting factors in the blood participate in blood clot formation. However, only the roles played by the platelets, prothrombin, and fibrinogen are discussed here. Prothrombin and fibrinogen are plasma proteins produced by the liver. Vitamin K is necessary for prothrombin production, and if it is missing from the diet, hemorrhagic disorders develop.
The clotting process consists of a cascade of enzymatic reactions in which each reaction leads to the next. The main events can be summarized as follows: The clotting process is initiated when platelets and damaged tissue release prothrombin activator, an enzyme that converts prothrombin to thrombin. This reaction requires the presence of calcium (Ca2+). Thrombin, in turn, acts as an enzyme that brings about a change in fibrinogen so that it forms long threads of fibrin. Once formed, fibrin threads wind around the platelet plug and provide a framework for the clot. Red blood cells trapped within the fibrin threads make a clot appear red.
Hemophilia
A is the most common of the severe dotting disorders that are inherited. The bleeding problems associated with hemophilia A are the result of a deficiency of factor VIII:C. If the level of factor VIII:C is less than 1 % of normal, the affected individual often bleeds in infancy and is afflicted throughout life by hemarthroses (hem-arth-ro's-es) (sing., hemarthrosis), or bleeding into joints. This leads to cartilage degeneration in the joints and resorption of underlying bone. Bleeding into muscles leads to necrosis of overlying tissue, as well as pressure-induced nerve damage and muscular atrophy. Gastrointestinal bleeding occurs, and blood also appears in the urine. The most common cause of death is intracranial bleeding accompanied by neurological damage.
If blood is placed in a test tube and allowed to clot, a yellowish fluid rises above the clotted material. This fluid, called serum, contains all of the components of plasma except fibrinogen and prothrombin. Since a number of different terms have been used to refer to portions of the blood. A blood vessel is a thrombus, but when it dislodges and is transported in the blood, it is called an embolus. If thromboembolism is not treated, a heart attack can occur. In time, an enzyme called plasmin destroys the fibrin netvvork of a clot and restores the fluidity of plasma.
Blood clotting requires platelets, plasma proteins, and a variety of other materials. A blood clot consists of red blood cells entangled vvithin fibrin threads.
