Blood and blood vessels are the transport system within the bodies of all mammals. Blood is made up of a number of different types of cells suspended in plasma. Plasma is mostly made up of water containing dissolved substances such as glucose, lipids and amino acids. The types of cells found within blood are red corpuscles, or red blood cells, white blood cells and platelets.
White blood cells are also called leukocytes. There are two main groups of leukocytes — phagocytes and lymphocytes — which are both involved in fighting disease and infection. The platelets aren’t actually cells, but fragments of cells that are involved in blood clotting.
Red corpuscles are also referred to as red blood cells, RBCs and erythrocytes. Their main function is to carry oxygen throughout the body. RBCs travel through the respiratory system to collect oxygen and then they move through the blood vessels to the other tissues in the body. The structure of the red blood corpuscles is modified for this function.
Erythrocytes have a distinct shape to allow a large number of them to occur within the blood. They do not contain a nucleus and the center of the cell is sunk, which gives them the appearance of biconcave discs. If a blood smear is looked at through a microscope, there are many red corpuscles, which are easy to pick out, since they look similar to donuts.
The interior of the cells is filled with hemoglobin. This is a complex protein molecule that has four heme groups, which contain iron. Hemoglobin is red, which is what gives the red corpuscles their color. Also, hemoglobin is the protein that is responsible for carrying oxygen within the blood. The fact that there is no nucleus allows there to be that many more hemoglobin molecules in the cell.
When the red corpuscles pass through the lung, there is a high concentration of oxygen molecules compared to within the cells. The oxygen molecules diffuse through the membrane of the corpuscles and combine with hemoglobin to form oxyhemoglobin. Each of the four heme groups can combine with an oxygen molecule, so each hemoglobin can carry four oxygen molecules. When the red corpuscles pass by the tissues, the lower concentration of oxygen causes the weak bond between the oxygen and hemoglobin to break. The oxygen molecules readily detach from the heme groups and diffuse out of the red blood cells. The molecules then diffuse into the cells of other tissues that need it.
Within the red corpuscles, there can be up to 250 million molecules of hemoglobin. This means that up to 100 million oxygen molecules can be carried by a single cell. The biconcave shape of the red corpuscles gives it a larger surface area, meaning that the absorption rate of oxygen is much higher. All of these factors make this a highly efficient method of transport of oxygen throughout the body.