What is Composition?

Blood consists of red blood cells, white blood cells, and platelets, suspended in a straw-colored liquid called plasma. If whole blood is centrifuged, about 40% of the total volume will be made up of packed cells, and the remainder will be plasma. The percentage of packed blood cells is referred to as the hematocrit. People who live at high altitudes accommodate to the low oxygen levels by forming more red blood cells, and consequently they have a higher hematocrit count.

Red Blood Cells

The red blood cells involved in oxygen transport are called erythrocytes. There are normally about 5 million red blood cells per microliter of blood. They are packed with hemoglobin, an iron-containing molecule that readily combines with oxygen and carbon dioxide. The gases are thus transported between lungs and body tissues by the circulatory system.

Red blood cells are formed by undifferentiated stem cells that occur in bone marrow. When the hemoglobin content of a stem cell reaches about 30%, the cellular structure disassembles itself, and forms a new structure - a bi-concave flexible disk with no nucleus, mitochondria, endoplasmic reticulum, or other structures commonly found in the cytoplasm. The lack of these organelles leaves room for more hemoglobin. A red blood cell simply functions as a container for the large molecules of hemoglobin and gases.

The unique shape of the red blood cell allows it to move easily through the tiniest capillaries. However, because it lacks the usual organelles, its life is limited to about 120 days in circulation. Disintegrated cell parts are removed by the liver and spleen, and the left-over iron is carried to the bone marrow, where it is recycled.

White Blood Cells

White blood cells, or leukocytes, are part of the immune system that protects the body against infection. There are usually 5 to 10 thousand white blood cells per microliter of blood, but this number can increase by several times during a severe systemic infection. They are formed in the bone marrow, lymph nodes, and spleen. Leukocytes are able to squeeze through capillary walls, and once free, are active in policing the tissues outside the vascular system. A leukocyte's function is to clean and remove dead cells and fragments as well as invading pathogens. They move by ameboid motion in response to chemicals released by damaged cells.

There are different types of white blood cells, each performing different functions. Most white blood cells are large cells with multilobed nuclei, called neutrophils, which ingest and destroy bacteria. Eosinophils collect in great numbers in response to allergic and parasitic infections. Lymphocytes produce antibodies that destroy foreign cells. Basophils produce the anticoagulant heparin, and also release histamines, substances responsible for inflammation. Monocytes give rise to macrophages, large cells that ingest nonbacterial foreign matter.

Platelets

Blood platelets, known as thrombocytes, are fragments of cells that bud off from large cells in the bone marrow. There are about 250,000-400,000 platelets found in each microliter of blood.

When a blood vessel is damaged, collagen fibers are released. In response to the presence of collagen, platelets swell, become sticky, and release substances called clotting factors. Clotting factors combine with a circulating enzyme, prothrombin, to form an active enzyme, thrombin. Thrombin causes circulating molecules called fibrinogen to polymerize and form fibrin threads. Fibrin threads form blood clots that collect red blood cells and serve as the framework for scar formation.

Plasma

Plasma has many functions, and as such, is a complex substance. Although it is about 90% water, it contains many other organic and inorganic materials. Some of the substances that plasma contains include:

1. proteins such as albumin, which are important in maintaining the osmotic pressure of the blood hormones
2. dissolved nutrient molecules such as glucose, amino acids, lipids and cholesterol enzymes
3. inorganic ions such as sodium, potassium, chloride, carbonate and bicarbonate, calcium and trace elements gases
4. carrier molecules such as transferrin, which carries iron to where it is needed in the tissues
5. waste products such as urea and creatinine; agglutinins which are responsible for clotting
6. proteins called antibodies that provide protection against diseases.