Composition of Blood

Blood is composed of:

1. Plasma
2. Red blood cells
3. White blood cells
4. Platelets

Plasma

• Plasma is the liquid part of the blood.
• It is approximately:
  • 90% water
  • 7% proteins
  • 3% dissolved substances.
• Proteins include antibodies and clotting proteins.
• The function of plasma is transport.
  • It transports dissolved materials like glucose, vitamins, minerals, and hormones.
  • It transports wastes including urea, salts and CO₂.
  • Plasma also transports heat. It is involved in homeostasis.

Red Blood Cells (Erythrocytes)

• Red blood cells transport oxygen.
  • They contain haemoglobin.
  • Haemoglobin is a globular protein pigment made from iron, which carries oxygen.
  • Red blood cells are biconcave in shape. This increases their surface area for oxygen transport.
  • They are made in the bone marrow of long bones.

• When first created, red blood cells have a nucleus. When they mature, the nucleus dies.
• As a result, red blood cells are unable to repair themselves and live for only four months (approx.).
• They contain no mitochondria and do not respire.
• Red blood cells are broken down in the liver and spleen.
• When broken down:
  • Haemoglobin is converted into the pigments bilirubin and biliverdin, which are sent to the gall bladder and secreted into bile.

White Blood Cells (Leucocytes)

• White blood cells fight infection.
• They are large, flattened discs.
• They have a nucleus and mitochondria.
• They are made in the bone marrow and some mature in the lymph system.
• There are two main types of white blood cells:
  1. Lymphocytes
  2. Monocytes

Lymphocytes:

• Produce antibodies to fight disease.
• Made in the bone marrow.
• Stored in the spleen and lymphatic system.
• Have a large, rounded nucleus that almost fills the entire cell.

Monocytes:

• Engulf foreign bodies (phagocytosis).
• Live for less time than lymphocytes.
• Have a kidney-shaped nucleus.

Platelets (Thrombocytes)

• Platelets allow blood to clot.
• Made in the bone marrow.
• Do not have a nucleus.
• Blood clotting prevents the entry of foreign cells and pathogens into the body.
• Clotting forms scabs on wounds, which reduces the risk of infection.

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