Blood is a bodily 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 the cells. It circulates constantly around the body and enables communication between tissues.

Blood transports:

  • Oxygen
  • Nutrients
  • Hormones
  • Protective substances
  • Clotting factors
  • Heat

Blood is composed of plasma in which different types of cells are suspended. Blood plasma is the pale yellow liquid component of blood that holds the blood cells in whole blood in suspension. It makes up about 55% of the body’s total blood volume.


Human blood cells at 1000x magnification

Blood constitutes about 7% of body weight. In a healthy man weighing 70 kg, about 5.6 litres of blood is present. In women this proportion is less. On the other hand, the proportion is considerably higher in children. It decreases gradually with age until adult level is attained.

Blood cells and plasma can be separated by centrifugation. Cells and plasma can also be separated by gravity i.e. sedimentation. Cells are heavier and sink to the bottom of the blood sample.

Blood in the body is constantly in motion. The heart constantly pumps blood and maintains a constant internal environment for the cells. Homoeostatic mechanisms maintain the volume and composition of blood within narrow limits. The blood transfers heat from metabolically active organs such as skeletal muscles and liver and distributes it around the body. It plays an important role in maintaining core body temperature. Core temperature is normally maintained within a narrow range so that important enzymatic reactions can occur.

Topics in this Section

  • Composition of blood
    • Plasma
    • Cellular content of blood
      • Erythrocytes
      • Leukocytes
      • Thrombocytes
    • Haemostasis
  • Erythrocyte disorders
    • Anemias
      • Iron deficiency anemia
      • Megaloblastic anemia
        • Vitamin B12 deficiency anemia
        • Folic acid deficiency anemia
      • Hypoplastic and aplastic anemia
      • Haemolytic anemia
        • Congenital hemolytic anemia
        • Acquired hemolytic anemia
      • Normocytic normochromic anemia
    • Polycythemia
      • Polycythemia rubra vera
  • Leukocyte disorders
    • Leukopenia
      • Granulocytopenia (neutropenia)
    • Leucocytosis
    • Leukemia
      • Types of leukemia
  • Hemorrhagic diseases
    • Thrombocytopenia
    • Vitamin K deficiency
    • Disseminated intravascular coagulation (DIC)
    • Congenital disorders