Blood is a type of connective tissue composed of different types of cells embedded in a fluid medium called plasma. The average human adult carries about 5 liters of blood. Blood contributes about 8 percent to the total body weight. Cells in blood make up about 45 percent of the total volume of blood. The liquid portion of blood or plasma accounts for 55 percent of the total blood volume. Blood carries out important functions such as transport of substances around the body, maintenance of a constant environment inside the body, blood clotting and defense against diseases.
Blood plasma is a pale, straw-colored liquid that carries about 90 percent water and 10 percent substances dissolved or suspended in it. The most important components in plasma include water, plasma proteins and mineral ions as they are involved in homeostasis. These components are maintained at constant concentrations in the plasma as they affect blood pressure, blood pH, body temperature etc. Digestive products (glucose, amino acids, fatty acids etc), vitamins, hormones and excretory products (e.g. urea, uric acid) occur in varying concentrations in the plasma.
Water in the plasma is the medium for transport of dissolved substances in plasma, around the body. It serves as a source of water for body cells. The concentration of solutes such as mineral ions and plasma proteins defines the solute potential of blood. The direction of water movement is affected by solute potential. Blood plasma carries a number of mineral ions such as sodium, potassium, calcium and magnesium. Sodium is the most abundant solute in the plasma. Mineral ions collectively participate in the regulation of solute potential. Water and solutes in the plasma is regulated to achieve the required blood volume and pressure.
Plasma proteins account for seven to nine percent of blood plasma and they help maintain blood pH within a narrow range (between 7.35 and 7.45). Among plasma proteins, albumin is the most abundant. Produced in the liver, albumins contribute to solute potential of blood. Globulin is another type of plasma protein. There are three main subtypes of globulins present in plasma. Alpha globulins and beta globulins are produced in the liver. These bind to certain substances and transport them in the blood. Gamma globulins are antibodies produced by a type of blood cells called lymphocytes. There are several other plasma proteins, including prothrombin, fibrinogen and enzymes.
Red blood cells, white blood cells and platelets are the three main types of blood cells. Red blood cells or erythrocytes are the most abundant of blood cells and are highly involved in oxygen transportation and gaseous exchange. Approximately, there are five million red blood cells in one cubic millimeter of blood. Two types of white blood cells (leucocytes) are present in blood. Among the two types of leucocytes, granulocytes account for 72 percent of the total white cell count. The remaining 28 percent is agranulocytes. White blood cells play a vital role in the body in defending against diseases. Compared to red blood cells, white blood cells are present in much smaller numbers (about 7000 per millimeter cube of blood).
Erythrocytes are produced in the bone marrow and appear as small, biconcave discs lacking nuclei. There are several other features to these cells that make them efficient for gaseous exchange. Red blood cells are highly packed with hemoglobin which is an iron-containing protein. Their chemical and physical features and large number in the blood, provides a large surface area to volume ratio for gaseous exchange. Hemoglobin carries oxygen and carbon dioxide (in small quantities) in the blood.
Granulocytes are made in the bone marrow and specialize into neutrophils, eosinophils and basophils. Seventy percent of granulocytes are neutrophils. There are about 4900 neutrophils in one millimeter cube of blood. Neutrophils protect against diseases by engulfing microbes such as bacteria. Eosinophils contribute 1.5 percent to the total number of granulocytes. Per cubic millimeter of blood there are about 105 eosinophils. Basophils contribute the smallest percent (0.5 percent) to the total granulocyte count. Eosinophils and basophils are inflammatory cells involved in allergic reactions.
Agranulocytes are subdivided into monocytes and lymphocytes. Compared to lymphocytes (which contribute 24 percent), monocytes only contribute about four percent to the total of agranulocytes in blood. Both these types originate in the bone marrow. Monocytes become macrophages when they enter into tissues. Macrophages engulf pathogens destroying them (phagocytosis). Lymphocytes are produced in the thymus gland and lymphoid tissue. They produce antibodies that fight against pathogenic micro-organisms.
Platelets are irregularly shaped cell fragments that lack normal cellular features. They are produced in the bone marrow and play an important role in the process of blood clotting. There are about 250,000 platelets in a millimeter cube of blood. Rupture of platelets, as they occur in wounds, release substances called thromboplastin (a lipoprotein) which initiates clotting mechanism. Blood clotting prevents excessive blood loss from the body and also prevents entry of pathogens into the body.