5.1 Blood consists of plasma, which constitutes 55 percent of its volume, and red blood cells which make up roughly 45 percent of the remaining volume, with a small percentage of the volume contributed by white cells and platelets (Applegate, 2000, 227). The plasma is about 90 percent water, and the other 10 percent is composed of more than 100 different organic and inorganic solutes. The constituents of plasma are constantly changing because it is a transport medium. The most abundant solutes are plasma proteins which normally remain in the blood or interstitial fluid. The three major classes of plasma proteins are albumins (60 percent), globulins (36 percent), and fibrinogen (4 percent). Other solutes of plasma include nutrients, hormones, oxygen, carbon dioxide, antibodies, and electrolytes.
Interstitial fluid is composed of the fluid and solutes which have escaped from the capillaries (Applegate, 2000, 263). Its composition varies depending on the blood flow through the capillaries, the amounts and types of nutrients flowing from the capillaries to the tissue cells, and the amounts and types of waste products flowing from the tissue cells into the capillaries. All these flow through the interstitial fluid, so its compositions is dynamic. Large proteins cannot pass through the capillary walls, so there tend to be less of them in the interstitial fluid than in the blood. Lymph is similar in composition to blood plasma, and is derived from plasma as it passes through the capillary walls at the arteriole end (Applegate, 2000, 287). As the interstitial fluid begins to accumulate, it is picked up by the lymphatic vessels and returned to the circulation.
5.2 Lymph nodes are small, bean-shaped structures distributed throughout the body along the lymphatic pathways (Applegate, 22000, 289-290). Lymph nodes are not present in the central nervous system. They cluster in three superficial areas on each side of the body: the ...