How your body replaces blood?
The average adult has around 10 pints of blood (roughly 8% of your body weight). Making a blood donation uses about 1 pint, after which your body has an amazing capacity to replace all the cells and fluids that have been lost.
Red blood cells
Take red cells. Millions of them are being made and dying every second. When you give blood you lose red cells and the body needs to make more to replace them. Special cells in the kidneys, called peritubular cells, sense that the level of oxygen in the blood has decreased (due to the loss of red cells) and start secreting a protein called erythropoietin. This passes through the bloodstream until it reaches the bone marrow (the soft fatty tissue inside the bone cavities).
The bone marrow produces stem cells, the building blocks that the body uses to make the different blood cells – red cells, white cells and platelets. The erythropoietin sends a message to the stem cells telling more of them to develop into red blood cells, rather than white cells or platelets.
How fast does your body make blood?
Your body makes about 2 million new red cells every second, so it only takes a number of weeks to build up stores of them again.
What about your white cells and platelets? A number of other messenger proteins also stimulate the production of these cells in the bone marrow, and over the next few days levels return to normal.
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