BLOOD Everybody is familiar with the sight of blood - the red fluid that oozes out of your body when you've sustained a cut or a deep injury, which is slightly denser and approximately 3-4 times more viscous than water. • Blood volume Blood volume is variable, but tends to be about 8% of body weight. Factors such as body size, amount of adipose tissue, and electrolyte concentrations all affect volume. The average adult has about 5 liters of blood.
Blood composition • Blood consists of cells which are suspended in a liquid. The components of blood can be separated by filtration, however, the most common method of separating blood is to centrifuge (spin) it. • Three layers are visible in centrifuged blood. The straw-colored liquid portion, called plasma, forms at the top (~55%). A thin cream-colored layer, called the buffy coat, forms below the plasma. The buffy coat consists of white blood cells and platelets. The red blood cells form the heavy bottom portion of the separated mixture (~45%).
1-The plasma • The Plasma is a straw colored liquid, most of which is water. It makes up 55% of the blood and serves as a transport medium for blood cells and platelets. • By far the greatest constituent of plasma is water, which accounts for >90% of the total volume. Other constituents fall into the following main categories: • Ions (Na+, Cl-, etc) , • Plasma proteins (albumins, globulins, fibrinogen, etc) , • Dissolved gases (O2, CO2) , • Circulating Nutrients (glucose, amino acids, etc) , • Circulating Tissue products (urea, creatinine, lactate, etc) • Circulating Hormones (insulin, adrenaline, etc)
The water in the plasma is an important solvent for ions such as Na+, Cl-, K+, etc. It also dissolves nutrients like simple sugar, lipids, amino acids and vitamins. Sometimes, it would help transport enzymes and hormones, but that depends on the activities of the body.Serum: Plasma differs slightly from what is termed serum, which is generally taken to mean the fluid part of the blood which remains after the blood has coagulated (clotted).
2-The red blood cells • The red blood cells form a major part of the blood. These are the familiar discotic shaped cells which make up 99% of the cells in the blood. They are the principal carriers of the red colored hemoglobin molecules. Hemoglobin is an iron containing protein and binds about 97% of all oxygen in the body; hence, the most principled function of red blood cells is to carry oxygen gas around the body. Although they do carry a little carbon dioxide sometimes, most of this unwanted gas is transported by the plasma
3- The white blood cells • The white blood (leukocytes) cells are a vital source of defense against external organisms. White blood cells also serve as 'sanitary engineers' cleaning up dead cells and tissue debris that would otherwise accumulate to and lead to problems. There are five classes of leukocytes: neutrophil, eosinophil, basophil, monocyte and lymphocytes.
4-The platelets • These are not true cells but fragments of cytoplasm from certain bone marrow cells. They play a part in the clotting of blood.
Plasma and platelets in clotting: • If damage occurs to a blood vessel, circulating platelets immediately get trapped at the injury site. On accumulating the platelets 'plug' the leak in the vessel providing a first step in damage control. This mechanism is supplemented by 'blood coagulation', or clotting, which is the most important means of defense against bleeding. As mentioned plasma contains several dissolved proteins. Fibrinogen is a rod shaped soluble protein which in the presence of a catalyst thrombin gets converted to an insoluble protein fibrin.
The combined action of the platelets and 'fibrin web' is sufficient to prevent a dangerous loss of blood. In cases where the formation of fibrin and hence formation of a clot is impaired due to some reason (e.g. a genetic disorder as in hemophilia) a person is at great risk of bleeding to death.