Blood is a vehicular system, with the vessels of the circulatory system arteries, veins, and capillaries functioning as roadways. As blood travels throughout the body it transports oxygen from lungs to all body cells. From the intestine, blood uploads and then distributes digestive products used for cellular metabolism. Blood also removes waste products, such as carbon dioxide and other by-products of cellular metabolism, and transports them to the lungs, kidney and skin for excretion. The average adult human has 4 to 6 litres of blood, which accounts for approximately 7% of the total body weight. Formed elements (cells) compose 40-45% of this total volume and plasma, the liquid medium, constitutes the balance.
Types of blood cells
The formed elements are red cells, white cells and platelets. Blood also contains membrane fragments, macromolecules and ions. Each type of blood cell has special functions and differs morphologically (in size and shape)", physically (elastic properties), bio-chemically (comprising molecules), and physico-chemically (surface properties) from the others. White cells are components of the immune system, which protects the body against foreign matter. Red cells transport oxygen and carbon dioxide. Platelets are essential in preventing blood loss because they readily coagulate as one of the initial steps in hemostasis (clotting).
Red blood cells
Red blood cells are called erythrocytes or red corpuscles. They are produced and released from hematopoietic bone marrow into the circulation as fully mature blood cells. Red cells transport oxygen from lungs to body tissues and CO2 from body tissues to the lungs. Haemoglobin molecules to which oxygen and carbon dioxide are bound, and cellular organelles occupy the enclosed space of a red cell. The Red cell is a flexible, biconcave disk with a diameter of ~7 µm and a thickness of ~2 µm. This shape imparts a large surface area for the transfer of respiratory gases into and out of red cells. The flexibility of red cells enables them to deform as they travel through capillaries which have diameters of 4-5 µm. Once a red cell is released into the circulation, it has a life span of approximately 120 days. The number density of red cells in the blood of an average adult ranges from 4.2(10)6 to 6.2(10)6 /µL. The average is 5.5(10)6 /µL for males and 4.5(10)6 /µL for females.
White blood cell
White blood cells (leukocytes) are very close to spherical in shape with diameters of 5 to 15 µm depending on their types. There are several types of white cells, each having special functions related to seeking out and destroying foreign micro-organisms (antigens). White cells communicate among themselves via a highly complex network of glycoprotein molecules called cytokines. Most white cells circulate within the vascular and lymphatic channels; however, some reside in body tissues. White cells have varying life spans that range from a few hours to years, possibly for one’s lifetime. Total number range of white cells in an adult is 4.5(10)3 to 11(10)3 /µL, that is, about three orders of magnitude fewer than red cells in the blood. The three major groups of white cells are; granulocytes, monocytes and lymphocytes, which includes T and B cells.
Platelets are called thrombocytes! They are small, discoid in shape and enucleated (lacking a nucleus). These formed elements play a vital role in preventing blood loss from damaged blood vessels, a function known as hemostasis. Platelets contain numerous organelles. Platelets range in size from 2 to 4 µm. When activated, they change from a flattened to a spheroid shape. They also extend pseudo-pods on their outer membrane. The normal platelet count ranges from 1.5(10)5 to 4.5(10)5 / µL.
Plasma is the aqueous portion of blood in which, various types of blood cells are suspended. The main function of plasma is to transport blood cells and solutes to and from tissue cells. Within plasma are dissolved solutes. Solutes provide tissue cells with nutrients and other vital substances. Solutes dissolved in plasma include proteins, carbohydrates, lipids, vitamins, electrolytes (salt) and hormones. Solutes make up about 10% of the plasma volume. Plasma is composed mostly of water. Approximately 4% of an individual’s total body weight is plasma. Normally plasma is straw colored. The term serum is sometimes used interchangeably with plasma, but the fluids are not the same. Serum is plasma without fibrinogen. Fibrinogen is one of the important proteins in plasma.
Blood transfusions are involved in a variety of surgical and medical treatments. Blood is given to replace losses during trauma or surgery, to treat anemia, hemophilia, and internal bleeding and to replace a specific component destroyed by disease or chemotherapy.