• Post last modified:December 30, 2022
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Blood is a mobile fluid connective tissue composed of a fluid, the plasma and the cells, the blood corpuscles. Blood is a body fluid in humans and other animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells.

Vertebrate blood is bright red when its haemoglobin is oxygenated and dark red when it is deoxygenated. Blood is circulated around the body through blood vessels by the pumping action of the heart. Blood accounts for 7% of the human body weight.

Composition of Blood

  • The average adult has a blood volume of roughly 5 litres, which is composed of plasma and several kinds of cells.
  • Plasma is a slightly alkaline non-living intercellular substance that constitutes about 60%part of the blood.
  • It is a pale yellow but transparent and clear fluid. Water alone forms about 90 to 92% of the plasma.
  • Plasma proteins constitute about 7 to 8% part of the plasma.
  • Plasma proteins mainly include albumin, globulin, prothrombin, and fibrinogen. Plasma without clotting factors is called serum.
  • Erythrocytes, leukocytes, and platelets are collectively called formed elements.

Erythrocytes or red blood cells (RBCs) have red-coloured, an iron-containing complex protein called haemoglobin.

Leucocytes, also known as white blood cells (WBCs), are colourless, that lack haemoglobin. WBCs are involved in protecting the body against both infectious disease and foreign invaders. They are of two types: –

I. Granulocytes: It contains granules in their cytoplasm. They are also called polymorphonuclear leukocytes or polymorphonuclear neutrophils because of the varying shapes of the nucleus. Granulocytes are of three types:

(a) Neutrophils (b) Eosinophils (c) Basophils  

II. Agranulocytes: Agranulocytes or non-granulocytes, also mononuclear leukocytes, are one of the two types of white blood cells.

Agranular cells are noted by the absence of granules in their cytoplasm, which distinguishes them from granulocytes. Agranulocytes are of two types:

(a) Lymphocytes (b) Monocytes

  • Neutrophils are the most abundant cells (60-65%) of the total WBCs.
  • Neutrophils and monocytes (6-8%) are phagocytic cells that destroy foreign organisms entering the body.
  • Basophils (0-1%) secrete histamine, serotonin, heparin, etc., and are involved in inflammatory reactions. 
  • Eosinophils (2-3%) resist infections and are also associated with allergic reactions.
  • Lymphocytes (20-25%) are of two major types – ‘B’ and ‘T’ forms responsible for immune responses of the body.
  • Platelets also called thrombocytes, can release a variety of substances most of which are involved in the clotting of blood.

Karl Landsteiner reported ABO blood groups first time in human beings, which are determined by the gene “I”(isoagglutinin). There are three alleles, IA, IB and IO of this gene. Proteins produced by IA and IB alleles are called an antigen and B antigen respectively.

Rh Factor or Rh Blood Grouping

A protein named rhesus antigen is present on the surface of red blood corpuscles in many persons. When the Rh-ve mother is pregnant with Rh+ve fetus, Rh antigens of the fetus do not get exposed to the Rh-ve blood of the mother in the first pregnancy as the blood of the mother and blood of the fetus are well separated by the placenta.

  • However, during the delivery of the first child, there is a possibility of exposure of the maternal blood to small amounts of the Rh+ve blood from the fetus.
  • In such cases, the mother starts preparing antibodies against Rh antigens in her blood.
  • In the case of her subsequent pregnancies, the Rh antibodies from the mother (Rh-ve) can leak into the blood of the fetus (Rh+ve) and destroy the fetal RBCs.
  • This could be fatal to the fetus or could severe anaemia and jaundice to the baby. This condition is called erythroblastosis foetalis.

Blood Clotting

Blood clotting, or coagulation, is an important process that prevents excessive bleeding when a blood vessel is injured.

  • At the site of an injury, the platelets disintegrate and release a phospholipid, called platelet factor-3 (platelet thromboplastin). 
  • Injured tissues also release a lipoprotein factor called thromboplastin. These two factors combine with calcium ions (Ca++) and certain proteins of the blood plasma to form an enzyme called prothrombinase.
  • Prothrombinase catalyzes the breakdown of prothrombin (inactive plasma protein) into an active protein called thrombin and some small peptide fragments.
  • Thrombin brings about the depolymerization of fibrinogen into its monomers. 
  • Later thrombin stimulates the re-polymerization of these monomers into long insoluble fibre-like polymers called fibrin.
  • The thin, long and solid fibres of fibrin form a clot that seals the wound and stops bleeding.

Blood Vessels

The blood vessels are the part of the circulatory system, and microcirculation, that transports blood throughout the human body. Three types of blood vessels are – arteries, veins, and capillaries.

Arteries are elastic vessels that transport blood (oxygenated) away from the heart towards the body tissues. The pulmonary artery is the only artery that carries deoxygenated blood. Veins transport blood (deoxygenated) to the heart.

Pulmonary vein is exceptional, conveys oxygenated blood from the lungs to the left atrium of the heart. Veins contain valves that prevent the backflow of blood. Arteries and veins are joined together by capillaries in the tissues.

Walls of the arteries and veins consist of three layers:

  1. Tunica interna – The innermost layer consisting of endothelium and an elastic membrane.
  2. Tunica media – A middle layer of smooth involuntary muscle and elastic fibres.
  3. Tunica Externa – An external layer consisting of loose connective tissue.