Composition of Blood and Its Functions

Blood is a remarkable and essential fluid in the human body. Its composition and functions are critical to sustaining life. The blood is red and circulates in the body like a river. The blood consists of a homogenous fluid called plasma constituting 55% and the remaining 45% by three types of blood corpuscles.

The Components of Blood

Red Blood Cells (RBCs)

Red blood cells, or RBCs, are the most abundant cells in the blood.

• The red blood cells are biconcave circular and enucleated.
• The cell organelles like the Golgi apparatus, mitochondria, RNA, and centriole are present.
• The average diameter is 7.5 µm. The average number of RBCs is about 5 million/mm³ of blood.
• The number of R.B.C. increases in acute exercises and high altitudes.
• The red blood cells consist of a respiratory pigment called Haemoglobin.
• It is a conjugated protein composed of a protein called globin, united to a prosthetic group called haeme. Haeme is a porphyrin derivative and consists of four iron atoms.
• The amount of hemoglobin will be 14-16 gm/100 ml of blood.
• The oxygen combines with hemoglobin to form oxyhemoglobin in the form of which it is transported to different tissues of the body.
• Oxyhaemoglobin is dissociated at the tissue level to release oxygen into tissues and retain hemoglobin in the blood.
• Carbon dioxide is also transported in the blood in the form of carbamino compounds when combined with hemoglobin.
• The life span of R.B.C. is 120 days.
• The spleen helps in trapping the old cells and broken fragments.
• The dead cells are disposed of by macrophages present in the liver, bone marrow, and spleen.
• The hemoglobin of dead R.B.C. is also broken down into haeme and globin.
• The iron is liberated from the haeme and once again used in the formation of R.B.C.
• The development of R.B.C. is called erythropoiesis. In the fetus, the liver, red bone marrow, and spleen are centres for the formation of R.B.C.
• After birth, they are developed exclusively from red bone marrow.
• The red bone marrow consists of precursors of R.B.C. like proerythroblasts or haemo erythroblasts, which transform into erythrocytes. The vitamins B₁₂ and folic acid are necessary for erythrocyte production.
• A decrease in the total count of RBC causes anemia and the increase in RBC causes polycythemia.

White Blood Cells (WBCs)

• The white blood cells are larger than R.B.C. but they are few.
• They are nucleated blood cells. The ratio of red cells to white blood cells is 600 to 1.
• The number of leucocytes varies from 5000 to 9000/mm³ of blood.
• The number increases at times of stress and pain.
• The increase in the number of leucocytes is called Leucocytosis.
• In certain diseases, the leucocytes decrease in number.
• The reduction in the leucocyte count below the normal level is called Leucopenia.
• An abnormal increase in the number of leucocytes with plenty of immature cells is called Leukemia.
• The W.B.C. are of two types based on the nature of cytoplasm and nucleus. They are Granula leucocytes and Agranular leucocytes.
  1. Granular Leucocytes:
     • They are provided with granular cytoplasm and lobed nucleus.
     • Hence they are called granular-polymorphonuclear leucocytes.
     • They constitute up to 64.7% of the total leucocytes. The granular leucocytes are divided into three types based on staining properties.
     • They are:
       1. Neutrophil:
          • The neutrophils constitute about 62% of the total leucocytes.
          • The nucleus is many-lobed. They are phagocytic in function.
          • They protect the body against infection, hence they are called Microscopic policemen.
       2. Eosinophils or Acidophils:
          • They constitute up to 2.3%. The nucleus is bilobed.
          • They help in the destruction of foreign bodies and toxins produced from them.
          • They are stained with acidic dye like eosin.
          • During infection, their number increases, and this condition is called eosinophilia.
          • A decrease in the total count of eosinophils is called eosinopaenia.
       3. Basophils:
          • These cells constitute up 0.4%. The cytoplasm consists of bluish-black granules of variable size.
          • They are stained with alkaline dye like methylene blue. The nucleus is bilobed or ribbon-like.
          • It is believed that they are transported to tissues where they become mast cells.
  2. Agranular Leucocytes: They are without granules in the cytoplasm. The nucleus is single. Hence they are called Agranular polymorphonuclear leucocytes. They constitute up to 35.3% of the total leucocytes. Agranular leucocytes are two types.
     1. Lymphocytes:
        • The lymphocytes are round, and nongranular with large nuclei.
        • They constitute about 27% of the total leucocytes.
        • The lymphocytes are of two types namely (a) B-lymphocytes and (b) T-lymphocytes.
        • They protect the body against infection by producing antibodies.
     2. Monocytes:
        • These are larger leucocytes with a horse-shoe-shaped nucleus that is eccentrically situated.
        • These cells constitute 5.3% of the leucocytes and are phagocytic in function.
        • They clean the blood from debris. Hence they are called Internal scavengers.
• The development of leucocytes is called Leucopoiesis.
• In the embryonic stage, the leucocytes are derived from the mesodermal cells.
• In the adult, the granulocytes are produced exclusively from the red bone marrow.
• The lymphocytes and monocytes are developed from the lymphoid tissues of the body and to some extent from the bone marrow.
• The primitive white blood cells present in the bone marrow are called Myelocytes.
• These are precursors of white blood cells. The life span of W.B.C. is one day.

Platelets

Platelets are tiny but crucial for blood clotting. They prevent excessive bleeding when you get a cut or injury.

• Platelets are small colorless bodies that are spherical, oval, or rod-shaped.
• They measure up to 2.5 µm in diameter. Their number varies from 2,50,000 to 5,00,000/mm³.
• The giant cells or megakaryocytes in red bone marrow are broken down into pieces called blood platelets.
• The life span of platelets is a few days. They are destroyed in the spleen.
• The blood platelets produce thromboplastin which helps in the coagulation of blood.
• They also help to close vascular wounds by forming platelet plugs.

Plasma: The Liquid Portion of Blood

• The plasma of the blood consists of 90% of water and the remaining 10% is constituted by organic and inorganic substances.
• The organic substances are plasma proteins, glucose, urea, creatinine, amino acids, lipids, antibodies, vitamins, enzymes, Antitoxins, cholesterol, etc.
• The plasma proteins are fibrinogen, albumins, and globulins.
• The inorganic substances constitute Na, K, Mg, Cl ~ HPO₄, HCO_3, etc.
• The plasma is slightly saltish and possesses a pH of about 7.2.

Function of Plasma

• The plasma maintains the acid-base equilibrium of the blood.
• The fibrinogen of plasma brings about the coagulation of blood.
• Albumins and globulins help in the retention of water in the plasma, by their osmotic effect and are also protective in function.
• A fall in the level of these proteins leads to the filtering out of excessive volumes of water from the blood into the tissues, leading to edema.