How does the Body Replace Blood?
During a whole blood donation we 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.
Male donors need to wait a minimum of 12 weeks between whole blood donations and female donors 16 weeks. So why wait? 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. Men normally have more iron stores than women. 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 male donors to wait 12 weeks and female donors to wait 16 weeks to donate whole blood, we also test your haemoglobin levels every time you give. We make sure that your haemoglobin level is above 125g/l for women and135g/l for men.
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.
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.
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.