Blood is a liquid connective tissue that consists of both liquid and cells. The liquid part of blood is called plasma.     Plasma is a watery, golden-yellow fluid that contains many dissolved substances. Substances dissolved in plasma include glucose, proteins, and gases. There are three types of cells in blood: red blood cells, white blood cells, and platelets.     Red blood cells are shaped like flattened disks. There are trillions of red blood cells in your blood. Each red blood cell has millions of molecules of hemoglobin. Hemoglobin is a protein that contains iron. The iron in... Show more

Blood is a liquid connective tissue that consists of both liquid and cells. The liquid part of blood is called plasma.    
Plasma is a watery, golden-yellow fluid that contains many dissolved substances. Substances dissolved in plasma include glucose, proteins, and gases. There are three types of cells in blood: red blood cells, white blood cells, and platelets.    
Red blood cells are shaped like flattened disks. There are trillions of red blood cells in your blood. Each red blood cell has millions of molecules of hemoglobin. Hemoglobin is a protein that contains iron. The iron in hemoglobin gives red blood cells their red color. It also explains how hemoglobin carries oxygen. The iron in hemoglobin binds with oxygen molecules so they can be carried by red blood cells.    
White blood cells are larger than red blood cells, but there are far fewer of them. Their role is to defend the body in various ways. For example, white blood cells called phagocytes engulf and destroy microorganisms and debris in the blood.    
Platelets are small, sticky cell fragments that help blood clot. A blood clot is a solid mass of cell fragments and other substances that plugs a leak in a damaged blood vessel. Platelets stick to tears in blood vessels and to each other, helping to form a clot at the site of injury. Platelets also release chemicals that are needed for clotting to occur.

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