What is the blood? It isn't just a red liquid. It is five litres of a careful mixture of plasma and blood cells. These cells come in three varieties: red, white and platelets.
The plasma makes up most of the blood. It is mainly water but carries lots of other essential ingredients.
The following substances are carried in the plasma:
- Dissolved carbon dioxide: This is the waste gas produced by respiration in cells
- Dissolved glucose and amino acids: Food molecules for respiration, building and repairing cells
- Urea: Waste product of digestion, this is lost from the kidney
- Antibodies and antitoxins: Chemicals that protect us from disease and poisons
- Hormones: Chemicals that control some of our body functions
Plasma has a yellowish appearance. It sometimes oozes out of blisters. Nice!
The best known of the cells are the red blood cells, correctly called erythrocytes.
Erythrocytes contain the oxygen carrying molecule haemoglobin; this is a special pigment that gives blood its red colour. Iron is needed in the production of haemoglobin; if your diet lacks this mineral you can develop the condition anaemia.
Red blood cells are unlike other cells in that they do not contain a nucleus. They are really just a bag containing the haemoglobin. The cells have a doughnut-shape with a flattened centre instead of a hole.
In the diagram you can see a whole cell on the left and another cut in half on the right.
When blood picks up oxygen we say that it has been oxygenated. This happens because haemoglobin molecules form weak bonds with oxygen to make a new complex molecule called oxyhaemoglobin.
Although much less common than red blood cells, the white cells come in many different varieties.
There are thousands of different types each with a specific job in defending the body against disease. They form part of the immune system.
When a disease is detected the corresponding white blood cell is copied until there are millions produced which attack the invading foreign cell.
Some white blood cells 'eat' invading cells. Others produce chemicals to attack them.
These are not really cells in their own right. They are fragments of larger cells.
Their job is to form part of a clot so that they plug a wound and stop too much blood being lost. Without that clotting mechanism you would cut your finger and bleed to death!
So hooray for the platelets!