During the blood donation process the National Blood Service (NBS) aim to take just under a pint (about 470mls) of blood, which works out at no more than 13 per cent of your blood volume. After donation, your body has an amazing capacity to replace all the cells and fluids that have been lost
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
Your body makes about two million new red cells every second, so it doesn't take long 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
Why Wait?
So why wait 16 weeks between whole blood donations? Well, unlike white cells and platelets, it takes several weeks for all the red cells to be replaced. There's an important link between your red cells and your health because it's these cells, or rather the red-coloured haemoglobin they contain, that take oxygen around your body
Haemoglobin contains iron and some is lost with each blood donation. To compensate, iron is mobilised from the body's iron stores, and the body also increases the amount of iron it absorbs from food and drink. Any iron deficiency can result in reduced haemoglobin levels, and eventually, if not treated, in iron deficiency anaemia
This deficiency can make you feel tired, which is why, as well as asking you to wait 16 weeks to donate whole blood, the NBS staff will also test your haemoglobin levels every time you give. They will make sure that your haemoglobin level is above 125g/l for women and135g/l for men
Iron levels
The body stores iron in the form of two proteins -ferritin (in men it accounts for about 70 per cent of stored iron, in women 80 percent) and haemosiderin. The proteins are found in the liver, bone marrow, spleen and muscles. If too much iron is taken out of storage and not replaced through dietary sources, iron stores may become depleted and haemoglobin levels fall
After a donation, most people's haemoglobin levels are back to normal after six to twelve weeks. We ask you to wait 16 weeks to ensure that if you are a dedicated loyal donor who never misses a donation, we don't risk lowering your haemoglobin levels over the long term
You can help your iron levels by eating a variety of iron-rich foods. On average men need to replace about 1mg of iron per day, women 2mg. With a balanced diet, getting enough iron shouldn't be a problem
Foods such as lean red meat, poultry, fish, leafy green vegetables, brown rice, lentils and beans can all boost your haemoglobin. Vitamin C helps with iron absorption, so to get the most from the food you eat, drink a glass of vitamin C-rich fruit juice with your meal. CLICK HERE for further details
Drink up
Blood volume makes up approximately eight per cent of your body weight. About 55 percent of blood is comprised of plasma, of which 90 per cent is water. So although you donate less than a pint of blood at a time, almost half of this is water
That's why it is important for you to drink plenty of water (we would like you to drink at least 500ml), just before you donate and immediately after you've donated. It's important to replace fluids after you've donated, to help bring your blood volume levels back to normal. The kidneys also play their part in controlling blood volume by regulating the amount of sodium and water lost in urine
Feeling faint
After donation some people can feel faint. When the body loses blood, special nerve cells in the walls of the arteries of the neck, called baroreceptors, sense that your blood pressure has dropped. The blood vessels constrict to compensate for this loss and to keep the blood pressure normal
Standing up too quickly, for example, can cause an abrupt drop in your blood pressure and make you feel light headed. Lying on the couch restores blood flow to the brain as your head will be at the same level as your heart. Sitting on the edge of the donation bed with your feet hanging down for at least two minutes will also help, as it allows your blood pressure to stabilise itself before you stand up
If you are feeling faint, our staff will ask you to stay at the session until you feel well again
Quick Facts about Human Blood
On an average there are about 5 - 6 litres of blood in an individual. Here are some quick facts about blood
• Blood contributes about 8% of the human body weight
• Every second, two million blood cells die
• If our blood vessels are measured, they would be 100,000 miles in length
• Each day 350 gallons of blood is filtered by the kidneys
• Four lives can be saved by donating one unit of blood, which is about 470 ml
• Native Americans use blood as a paint
• The average life span of a red blood cell (RBC) is 120 days
• Half of our blood cells are replaced every week
• Donated platelets can be used only for 5 days and constant donors are needed
• Sickle cell patients can require frequent blood transfusions throughout their live
