Blood Composition and Function

Human Physiology

Lesson 29 : Blood Composition , Function

Blood Composition and Function

Blood is fluid connective tissue comprising of formed elements (blood cells) and fluid medium (blood plasma). Physically blood is red coloured fluid (due to red blood cells), slightly viscous than water, specific gravity of 1.052 and pH of 7.35-7.45. Physically blood is also defined as suspension of cells in plasma. Plasma constitutes about 55-60% and cellular elements are 40-45% of blood.

Interesting features are:

  • The human heart beats more than 3,000 million times in an average lifetime.
  • At rest, each heartbeat pumps out around 80 milliliters of blood.
  • A tiny pinhead-sized drop (one micro liter) of blood contains 5,000,000 red cells, 5,000 white cells, as well as 300,000 platelets.
  • The average person’s body contains 4-5 liters of blood.
  • At any moment, three-quarters (75%) of the blood in the body is in the veins, one-fifth (20%) is in the arteries, and one-twentieth (5%) is in capillaries.
Important Functions of Blood are:
  1. Transport of oxygen to tissue and removal of carbon dioxide from tissues.
  2. Transport of nutrients and waste products through different organs.
  3. Provides protection against disease producing microbes through immune system and phogocytosis.
  4. Thermoregulation by distribution of heat from center of body to periphery and vice-versa.
  • Cellular components

For fractionation, whole blood mixed with suitable anticoagulant (a chemical substance that prevents clothing of blood) in a test tube is subjected to centrifugation. After centrifugation the blood gets separated into following fractions due to difference in their specific gravity.

  1. Plasma: topmost clear, translucent layer comprising about 55-60% total blood volume.
  2. Buffy Coat: middle creamish coloured layer comprising of white blood cells(WBCs) or leucocytes & platelets (thrombocytes) and constitute 0.5-1.0 % blood volume. It is visible only after high speed centrifugation
  3. Packed Cell Layer: bottom red colour layer comprising of red blood cells (RBCs) or erythrocytes and occupies about 40-45% of blood volume.
  4. Blood cells (formed elements) are continuously produced by a process called haematopoiesis which occurs in different organs as listed below:





Organ of production

Bone marrow

Bone marrow except lymphocytes (lymphoid organs)

Bone marrow


Round, biconcave, Non-nucleated pink coloured

Irregular motile, nucleated colourless

Irregular shaped, non-nucleated colourless

Size (diameter in micrometer)




Number (per micro liter of blood)

4.5- 5.0 x 106

7000- 10000

1,50,000 to 3,00,000

Life span

120 days

Few hours to several hours

Days to weeks

Erythrocytes are most numerous cells of blood. Erythrocytes are regularly produced in the bone marrow of flat bones (e.g. scapula, pelvic girdle, ribs, and vertebrae) in adult individual and long bones in young ones. Approximately 2x1011 RBCs are produced daily and equal number is removed from blood after completing their life span. The colour of RBCs is red due to presence of hemoglobin, a respiratory pigment which is involved in transport of oxygen and carbon dioxide.

Hemoglobin + Oxygen ? Oxyhemoglobin (oxygenated blood)
Hemoglobin + Carbon dioxide ? Carbamino hemoglobin (deoxygenated blood)

Hemoglobin concentration of healthy men and women is 16g/100 ml and 14 g/100 ml, respectively. Men have higher higher haemoglobin content and RBC number due to action of male sex hormone. Erythropoietin hormone produced from kidneys stimulates production of RBCs. Bile pigment, bilirubin is catabolic and product of hemoglobin breakdown and it is excreted in feces. Iron (ferrous form), cobalt or vitamin B12, folic acid and protein are the nutrients required for hemoglobin synthesis. Deficiency of any of these nutrients can lead to anemia.

White blood cells or leucocytes are mainly involved in body defense activities. Based on their appearance and functions WBCs are of following categories.

  1. Granulocytes:
    1. Neutrophils: are most numerous of all WBCs comprising about 60% of all leucocytes. They are involved in phagocytes of micro organisms.
    2. Eosinophils: are involved in allergic reaction of body and constitute about 3-5% ofall leucocytes.
    3. Basophils: are rarest of leucocytes with less than 2% of WBCs and are involved in development of allergies.
  2. Agranulocytes:
    1. Lymphocytes: are antibody secreting leucocytes and constitute about 30% of all leucocytes.
    2. Monocytes: are large leucocytes participating in phagocytosis of dead cells of body and constitute 5-8% of all leucocytes.

All leucocytes except lymphocytes are produced in the bone marrow and their life span is few hour to few days. Lymphocytes are produced from lymph nodes, thymus and spleen and life span is up to years.

Platelets are smallest cells of blood produced in bone marrow. They are involved in stoppage of blood from injuries and coagulation of blood (clot-formation). Deficiency of platelets occurs in some diseases like dengue, in which clotting of blood is delayed.

  • Plasma:
  • Plasma constitutes fluid part of blood. It is 92% water and rest is solids. Plasma is straw coloured, slightly alkaline fluid. Major portion of solids is plasma proteins (6-8%) of three major types viz. albumin, globulins and fibrinogen. Other solids include glucose, urea, amino acids, fats hormones, enzymes, electrolytes etc. The concentration of all these constituents is maintained within a narrow limit is healthy individuals.

    Last modified: Tuesday, 10 April 2012, 11:16 AM