Blood is the red fluid that flows through every large or small vein and artery in the body. The main function of blood is to transport oxygen and nutrients to cells. However, it plays many other important roles as well, for example in the immune system. Blood can't be produced artificially or replaced with any synthetic fluid. People who need a blood transplant can only get it from donors.

The human body consists of billions of cells and each one of them needs a constant stream of oxygen and nutrients in order to survive. Since blood transports all of these compounds, its role is crucial. No tissue can live without blood, not even the muscles that form the heart walls. After it brings oxygen and nutrients to cells, the blood loads waste products such as carbon dioxide. These are then transported to organs that are able to process and eliminate them, such as the kidneys, lungs and digestive system.

Obviously, a lack of blood would disrupt several essential body functions. Without it, there would be no defence against infection, waste would not be eliminated and there would be no way to regulate body temperature.

In medical terms, blood is also known as whole blood and consists of three main cell types: the red blood cells, white blood cells and platelets.

The ones that give blood its distinctive color are red blood cells, also known as erythrocytes and abbreviated as RBCs. These mainly consist of hemoglobin, which is a special type of protein with a high content of iron. The red cells actually turn red after they become loaded with oxygen from the lungs, which is then supplied to tissues and cells. Red blood cells look like discs with a flat appearance and a small indent. Erythrocytes are the most numerous type of cells in the human body. They have a pretty long life cycle, of about 4 months and every day new ones are produced to replace lost cells.

White blood cells are also named leukocytes or abbreviated as WBCs. They defend the body against pathogens, being one of the main elements of the immune system. Red blood cells are always a lot more numerous than white ones but the proportion of WBCs can increase in cases of infection. The blood stream allows white cells to travel to any location that is under attack but they can also move out of the blood in order to reach germs.

Several types of white blood cells exist and they usually have specialized roles. They are produced every day in the bone marrow and can live from a few days to several months. Granulocytes and lymphocytes are some of the white cells that move along the walls of blood vessels to patrol against various forms of infection. Besides killing bacteria, viruses and other pathogens, they also attack any cells of the body that is infected or it has become a tumour.

Some varieties of white blood cells are armed with antibodies, which are a particular type of proteins that allow them to identify and attack objects that are not part of the body. Lymphocytes are able to learn how to fight some types of pathogens. For example, if the body has defeated a particular viral infection in the past, they will remember how to produce antibodies against the same type of attack, which will be eliminated very fast. Since WBCs are needed to fight infections, people who suffer from them will have a much higher amount in the blood, compared to a healthy person.

Platelets or thrombocytes are the third type of blood cells. These are also produced in the bone marrow and play a vital role in repair. As soon as a blood vessel is damaged, these small cells with an oval shape rush to the area and seal the breach to prevent blood leaking. They have a short life cycle, of only 9 days, so new platelets have to be produced constantly. However, major blood vessel damage also requires some special proteins found in the blood, named clotting factors. Platelets can seal minor damage or stop bleeding in emergency situations, but permanent repairs need these proteins, which are able to build durable clots that seal the breach for good.

The repair mechanism of platelets and clotting factors is not only active on the exterior but also inside our bodies. Any scratch, wound, cut or internal bleeding is sealed by them. The system is like a complex mechanism, with various cells and proteins working together to create a clot, which only becomes effective after the last part of the puzzle reaches its location. The mechanism has its limits, however. Clotting can't seal massive damage, for example when one of the main blood vessels is completely severed. Outside help is needed to stop a potentially lethal bleeding, such as stitches or bandages.

Besides the three main types of cells, our blood is full of numerous other compounds. Some of these are the nutrients being transported to tissues, as well as the waste generated by them. Hormones are also transported in the blood stream. These are special chemicals produced in the endocrine glands that send signals to tissues or organs.

While blood can't be produced artificially, it is possible to replace it through a transfusion. This procedure means that a person is supplied with blood taken from somebody else, which can't fully replace its own blood but only some parts of it. It is also possible to only supply one of the blood components through a transfusion. Instead of the whole blood, the patient only gets one, the red blood cells or the platelets for example. Modern science is now able to separate blood from donors into these special components, in order to use them for targeted transfusions.