Blood is technically defined as a specialized bodily fluid which delivers oxygen to the cells of a living being. In vertebrate animals and humans, it is made of cells that are suspended in a liquid matrix called plasma. Many refer to blood as a tissue, and in fact, it is the only liquid tissue in the body.
Human blood is made of 55 percent fluid, which is largely water. The other 45 percent is comprised of red and white blood cells, platelets, proteins and other dissolved vitamins, minerals and gases. Human blood, and the blood of all other vertebrate animals, is bright red when it is exposed to oxygen. This is because a factor contained in the red blood cells, called hemoglobin, binds oxygen and takes on a red color when it does so. Blood that is deoxygenated has a darker, purple-red color.
Erythrocytes, commonly known as red blood cells, are responsible for carrying oxygen to the body's tissues. They are the most numerous of all of the blood cells. Leukocytes, or white blood cells, are responsible for fighting infection. They are the major portion of the body's immune system. There are many different types, all of which have different functions. There are far fewer white blood cells than there are red blood cells.
Platelets, which are formally known as thrombocytes, are the third type of blood cell. These are responsible for clotting the blood. They create a fibrin mesh that traps other blood cells and prevents blood from leaving the body.
Within the body, human blood is maintained at a pH between 7.35 and 7.45. Changes in blood pH are very harmful and can easily lead to death. Other major health concerns related to the blood are sepsis, which is an infection of the blood, and leukemia, which is a cancer of the white blood cells.