Blood is a liquid tissue; it transports substances around the body and defends it against disease. This page is about the composition and functions of blood.

Composition of Blood:

  • Red Blood Cells
  • White Blood Cells
  • Platelets
  • Plasma which contains:
  1. Glucose
  2. Amino-acids
  3. Vitamins
  4. Hormones
  5. Mineral salts
  6. Antibodies
  7. Antitoxins
  8. Urea
  9. Phospholipids
  10. Lipoproteins
  11. Hydrogencarbonate ions
  12. Fibrinogen

Blood is a liquid tissue. This means that it contains cells suspended in a liquid. Red blood cells carry oxygen and help to carry carbon dioxide. White blood cells are involved in the body’s defense mechanisms. Platelets are fragments of cells; they help blood to clot.. The liquid is called plasma. It contains many important substances which must be carried around the body.

There are about five million red blood cells in one cubic millimetre of blood, perhaps a bit more in boys and less in girls. If you are accessing this page from high altitude and you have lived up in the mountains for years, there will be many more red blood cells in your blood, possibly seven million.

Functions of Blood:

  • Transport of:
  1. The products of digestion
  2. Respiratory gases
  3. Hormones
  4. Vitamins
  5. Urea
  6. Heat
  • Defense:
  1. Production of antibodies and antitoxins
  2. Phagocytosis of “germs”
  3. Clotting
  4. Antibodies kill bacteria.

You may have a natural immunity to some diseases, but other diseases are likely to kill you if you do not receive medical attention. The problem is that the germs multiply faster than you white blood cells kill them. If you cannot make the right antibodies you will get very ill and could die. You can teach your body to recognise diseases like smallpox by “inoculating” yourself (get your doctor to do it) with a very similar disease like cowpox. Edward Jenner discovered this. Cowpox does not make you very ill and will certainly not kill you. When your white blood cells have learnt how to make the antibodies needed to destroy cowpox they can also destroy smallpox. Get yourself inoculated now especially if you are going to a part of the world where smallpox is endemic.

When you cut yourself, the damaged blood vessels trigger the clotting process. Fibrinogen, which is a soluble protein in your blood, turns into fibrin which is an insoluble fibrous protein. Platelets get caught in the network of fibrin and form a clot which dries up into a scab. This is quite a good thing because it prevents any more blood from getting out and it prevents germs from getting in.