Usually, red blood cells will circulate for approximately 120 days in the body until wearing out. It is then that the cells will undergo destruction by the spleen. However, the body constantly regenerates new red blood cells to counteract this in the bone marrow, the most common site being in the femur; the largest bone in the human body. Although all red blood cells are fundamentally similar, the structures on their surfaces, often referred to as markers, do vary from one person to another. These differences serve as the basis for blood classification.
Leukocytes, or white blood cells, are the larger than both red blood cells and platelets. They are responsible for defending the body against infecting organisms and foreign agents, in both the tissues and the bloodstream. Converse to red blood cells, nuclei are present in leukocytes and they can be classified into two groups: granulocytes and agranulocytes. White blood cells are always present in our blood, however, if there is an infection, the amount of these cells will increase and particularly so when the invading infection retaliates and persists. Because leukocytes are so vital for human existence, defects or problems with the white blood cells are often very serious. For example with Leukaemia, a cancer of the blood-forming tissues, the blood contains an abnormally abundant amount of white blood cells because of an uncontrolled proliferation that produces these atypical and immature leukocytes. The cells produced do not function properly and the overproduction suppresses the production of normal blood cells.
Granulocytes account for approximately 70% of all white blood cells and are formed in the bone marrow. Neutrophils, eosinophils and basophils are all types of granulocytes and in their cytoplasm; all contain granules of varying consistencies. The main constituent of granulocytes is neutrophils, which are the first line of defence and can be distinguished by the fine granules in the cytoplasm and their lobed nucleus. This particular type of granulocyte is capable of ingesting and killing bacteria. Eosinophils and basophils both have coarse granules found in their cytoplasm and differ to neutrophils. Eosinophils act against infestations of parasitic larvae and are present in large numbers in covering or lining surfaces within the body. They are also involved in allergic responses. Basophils can release chemicals such as histamine and heparin and are also capable of ingesting foreign particles, as well as being involved to the inflammatory response to infection.
The other category of white blood cell is agranulocytes, including the monocytes and the lymphocytes. Monocytes have a kidney-shaped nucleus and are derived from the phagocytic cells that line many vascular and lymph channels. They also migrate into connective tissue where infection is present, becoming macrophages and in turn, engulf cell debris (often left behind by the granulocytes and lymphocytes), invading bacteria, and other foreign particles. In addition to this, monocytes remove dead red blood cells. They usually constitute for between 4% and 8% of all white blood cells, whereas lymphocytes ordinarily number 20 to 35%.
The amount of lymphocytes rapidly proliferates when faced with infection and there are two fundamental types of this kind of agranulocyte: the B-lymphocytes and the T-lymphocytes. Both sorts recognise surface markers on cells and target them for destruction if foreign to the body, however, B-lymphocytes have a tendency to transfer into the connective tissue, in which they develop into plasma cells to produce highly specific antibodies to fight against foreign antigens. Some B-lymphocytes operate as memory cells, preparing for subsequent infection by the equivalent organism. T-lymphocytes on the other hand, can destroy invading cells directly, while others interact immune system cells to regulate the immune response.
Blood cells are suspended in plasma - the fluid constituent of blood. Essentially, it consists of a solution of various dissolved substances, such as proteins, hormones, vitamins, waste materials and salts (predominantly: sodium, potassium chlorides and bicarbonates) in addition to food materials (glucose, amino acids and fats). The proteins in plasma also include those necessary for blood clotting and the immunological response to infection, for example, antibodies.
An important constituent of plasma is platelets, or thrombocytes as they are also known. These tiny plate-shaped cytoplasmic bags of blood-clotting chemicals are produced by megakaryocytes and serious problems can arise if their production is hindered, as is the case in chemotherapy and AIDS since the risk of bleeding will increase. The primary function of plasma is to transport the blood cells and other substances, as well as to remove excretory wastes, however, platelets found in the plasma are crucial since these cells are the mechanism by which blood is converted to a solid from a liquid state.
When a wound begins to bleed, platelets congregate at the site to restrict the blood flow and begin coagulating the blood. The minerals that are transported in plasma and a protein called fibrinogen, help the sticky and irregularly shaped cells form a clot and when they are exposed to air, they begin to break apart. Fibrin is then produced because the platelets react with the fibrinogen and this leads to the fibrin threads forming a web-like mesh in which blood cells are trapped. Once this mesh of blood cells has hardened and dry, a scab is produced.
Blood coagulation is extremely important, as without it we would bleed to death. Nevertheless, if blood clots in the wrong place, thrombosis may occur within a blood vessel of a diseased state. In an artery for example, a thrombus impedes the blood flow to the tissue it supplies and if the artery obstructed is one to the brain, this can cause a stroke since the brain cannot function normally without a steady supply of oxygen.
Blood is not only the fluid of life by providing means for gaseous exchange but also the fluid of growth and protection because it transports nutrients from digestion and hormones throughout the body in addition to carrying bacteria fighting substances and wastes to the kidneys. Without blood, the human body would die and thus this rather incredible fluid is of uppermost importance to humans.
