Even though it's such a basic and important part of us, many of us do not actually know what our blood is made of. The main components of blood include three types of cells and the plasma, which is the liquid in which the cells float. Some secondary components of blood are nutrients, clotting agents, various hormones, and waste products that have not yet been removed from the body. The three types of blood cells that are included in blood are red blood cells, white blood cells, and platelets.

The red blood cells, or erythrocytes, are probably what most people think of when they think about the composition of blood. These cells use hemoglobin to transport oxygen throughout the body. It's this hemoglobin, actually the iron in the hemoglobin, that gives blood its red color. Red blood cells are relatively round with indentations in the center. When the blood goes from the heart to the lungs, the hemoglobin molecules bind with the oxygen in the lungs. After the blood passes through the lungs, it briefly returns to the heart to be pumped to the rest of the body, gradually delivering the oxygen to the rest of the body's cells.

White blood cells, or leukocytes, is one of the primary components of blood as well. These cells fight infection. Granulocytes, lymphocytes, and monocytes are the three types of white blood cells. Lymphocytes, in turn, have two types — T cells and B lymphocytes. T cells are responsible for directing what the immune system does, while the B lymphocytes manufacture antibodies. The third type of white blood cells, monocytes, pass through the cell walls and become macrophages, which then eat both harmful bacteria and the damaged and dead cells that are native to the body.

Platelets, or thrombocytes, are also cells. In fact, they are the smallest of the three types of cells. The primary function of platelets is to make the blood clot. When platelets are exposed to the air, which happens when a wound bleeds, they break down to release a substance into the blood. This substance indirectly causes fibrinogen, a protein that is another of the components of blood, to become fibrin. The fibrin turns into long strings, which mat together to form a clot that keeps red blood cells from escaping the body through the wound. This makes them one of the most important of the components of blood, as they prevent excessive blood loss.

Blood plasma, makes up more than half of the total components of blood, and itself is about 90% water. Plasma's main purpose is to transport the other components of the blood throughout the body. While doing this, it also transports various proteins, nutrients, electrolytes, hormones, cholesterol, vitamins, and chemicals like iron.