Circulatory system: blood

An adult has about 5 liters of blood, roughly the volume of five large water bottles. Blood is a tissue, a fluid connective tissue, and it has several parts that each do a different job. If you let blood stand in a tube after a special treatment that prevents clotting, it separates into layers: a pale yellow liquid on top and red cells below.

The liquid part is plasma, which makes up a little over half of your blood. Plasma is mostly water, and it carries nutrients, hormones, wastes, and proteins around the body. The rest of the blood is cells and cell pieces. Red blood cells are the most numerous. Each is shaped like a disc that is thinner in the middle, which gives it a large surface area. Red blood cells contain hemoglobin, a protein that carries oxygen, and mature red blood cells have no nucleus, which leaves more room for hemoglobin. They live about 120 days, and your red marrow makes millions of replacements every second. Iron is needed to make hemoglobin; too few red blood cells or too little hemoglobin is called anemia, and it can make a person feel tired.

White blood cells defend against disease. They are fewer than red cells, but they can leave the blood vessels and move into tissues to attack germs. You will study them in the immune system lessons. Platelets are small cell fragments that stop bleeding. When a vessel is cut, platelets stick to the damaged spot and form a plug, and a mesh of protein fibers called fibrin traps blood cells and forms a clot.

Blood has three major jobs. It transports oxygen, nutrients, hormones, and wastes. It protects the body through clotting and white blood cells. And it helps regulate body temperature by carrying heat from working organs to the skin.

Blood types come from marker molecules on the surface of red blood cells. The main markers are called A, B, and Rh. A person who is type A has A markers, type B has B markers, type AB has both, and type O has neither. A "positive" or "negative" tells whether the Rh marker is present. The immune system attacks markers it does not recognize, so a transfusion must be a safe match. Type O negative red cells have none of the markers, so they can be given to nearly anyone in an emergency.

Common mistake: thinking blood is just a red liquid. It is a tissue with a liquid part and three kinds of cells or pieces, and each part has its own job. Next lesson: the respiratory system.