About half of your blood consists of plasma. The other half consists of blood cells.
Blood cells are made in the bone marrow (the inside of your bones). Once the cells are fully formed, they swarm out into the bloodstream where plasma transports them through the body. Once in the bloodstream, most blood cells have a limited lifespan. Red blood cells live for 120 days. White blood cells live for 2 days on average, and platelets only ten days. Enormous numbers of cells are broken down and replaced by new ones all the time in your body. There are three kinds of blood cells: red blood cells, white blood cells and platelets.
Red blood cells
Red blood cells (erythrocytes) are the largest proportion of blood cells. They transport oxygen through the body using haemoglobin, a protein that uses iron to bind oxygen to itself, making it the perfect transport medium for oxygen. A lack of haemoglobin and iron is called anaemia.
White blood cells
White blood cells (leukocytes) are primarily designed to combat everything that does not belong to the body. When a person is given a blood transfusion, their white blood cells may create antibodies against the white blood cells in the donor blood. All being well, patients do not notice this. But often the antibodies trigger fevers or other, more serious, side effects. That is why we filter white blood cells out of donated blood as much as possible. The filtering is done in all blood donations and is called general leukocyte depletion (GLD).
Platelets (thrombocytes) allow blood to clot. If damage occurs to a blood vessel, the platelets stick to the blood vessel wall and to each other. They form a scab that closes the leak. Platelet deficiency can lead to significant bleeding.
Plasma consists of water in which proteins, minerals, fats and hormones are dissolved. It transports blood cells through the body and contains more than one hundred proteins, each of which has a different function. For example, the protein albumin has a water attracting function: it ensures water remains in the blood vessels and does not leak into the tissues. Plasma also contains clotting factors, proteins that work together with platelets in the clotting process.
Another important type of proteins in the plasma are the antibodies. These proteins protect the body against infectious diseases by binding to intruding viruses and bacteria. In this way they help the immune system to recognise intruders as foreign bodies, after which they are destroyed.