Blood is the fluid that is pumped by heart through the circulatory system. Its major function is to transports oxygen into human and animal bodies. Blood has two major parts including the plasma and blood cells. Plasma makes up 55% of the blood, and the blood cells make up 45% of the blood. Also, there are three parts of blood cells which are the red blood cells, white blood cells, and platelets.

Plasma is straw-color fluid which is about 92% of water. It also contains lots of substances essential to life, such as proteins. The types of proteins include albumin, globulins, and fibrinogen. The red blood cells make up most of the cellular portion of the blood. They have the shape of oval discs. When they carry lots of oxygen, the hemoglobin give them brighter red color. After hemoglobin delivered its oxygen to tissues, the red blood cells get darker. The white blood cells are the complete cells that can move independently unlike the red blood cells. They protect body from infections, and they digest bacteria, cellular debris, old cells, and other foreign bodies. Platelets are the smallest blood cells that have round or oval shape. They are involved with blood clotting. They adhere to the rough surface and release substances that help forming the blood clot when a blood vessel is cut.

Blood is involved with many important body systems. In cell membrane, it has proteins and carbon hydrates that determine the blood type. There are many blood diseases which are divided into five types. They are anemias, polycythemias, leukemias, leukosarcomas, and hemorrhagic, or bleeding, disorders.