Pre Knowledge


Blood is the main transport medium of the body, it even carries heat around the body (blood has a temperature of 38 degrees and a pH of 7.35) An adult human has approximately 5 litres of blood. Blood also carries nutrients, wastes, gases, hormones, antibodies and clotting factors.

Composition of Blood

Red Blood cells
There are approximately 4-6 million per mL of blood and their main function is to transport blood. Red blood cells are round, but biconcave rather than spherical. Have a lifespan of 4 months. Contain haemoglobin, which aids in the transportation of oxygen. Red blood cells can also carry small amounts of carbon dioxide which helps maintain the pH balance of the blood.

Is an iron-containing protein molecule which gives blood its red colour. Haemoglobin has an affinity for oxygen and combines with it to for oxyhaemoglobin, it also releases oxygen easily in areas of low oxygen concentration.

White blood cells (leucocytes)
They function as part of the immune system. Approximately 4000- 11000 per mL of human blood (higher levels often indicate infection). White blood cells are around 50% bigger than red blood cells and they all have a nucleus , no definite shape.

Platelets (thrombocytes)
They are disc shaped, half the size of red blood cells and there are around 400000 per mL of blood. Platelets function in the clotting of blood, they stick to each other and to blood fibres at the site of a wound. This contact breaks them open, causing them to release and enzyme, thromboplastin, which beings the process of sealing the blood vessels and causing the blood to clot – reducing blood loss.

Plasma is the yellow watery fluid part of blood and consists of 90% water and 10% proteins. Plasma carries many substances in either dissolved or suspended form.
- Plasma proteins : clotting factors, immunoglobins (antibodies), albumen and enzymes
- Nutrients : the end products of digestion – amino acids (from proteins), glucose (carbohydrates) glycerol and fatty acids (lipids) and cholesterol
- Gases: oxygen and carbon dioxide
- Excretory waste products: nitrogenous wastes such as urea, uric acid and ammonia
- Ions: sodium chloride, calcium and magnesium phosphate
- Hormones : chemical messengers involved in the co-ordination of body systems