Blood is a body fluid that contains numerous cells and plasma (the liquid portion of blood). It circulates through the entire body through a vast network of arteries, veins, and capillaries. Blood constitutes 7% of a human body’s weight and an average adult has around 5 liters of blood. Blood is slightly alkaline. There are many cells in blood, which constitute 45% of blood tissue by volume and the rest of the 55% constitutes plasma. 92% of the plasma in blood contains water and the rest of it contains proteins, glucose, carbon dioxide, fats, vitamins, and mineral salts. Blood includes erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (thrombocytes). White blood cells include monocytes, eosinophils, basophils, neutrophils, and lymphocytes. The red blood cells account for 40% of blood and carry oxygen to each and every muscle, tissue, and cell. It contains hemoglobin, which contains iron and thus gives it the red color. The lifespan of each red blood cell is 120 days. Apart from supplying oxygen, they pick up carbon dioxide from the cells and release it into the lungs. The white blood cells constitute for 1% of the blood volume. Their function is to provide immunity to the body. It protects the body from foreign bodies and infections. Neutrophils are a type of white blood cell. Its lifespan lasts between six hours to a few days. It protects the body from bacteria. Eosinophils account to 2-3% of blood and their function is to kill pathogens that enter the body. Their lifespan is around eight to 12 days. Basophils account for 0.8% of blood and their function is to secrete anticoagulants (See: Anticoagulants). Their other function is to dilate blood vessels and acts as a histamine (See: Histamine). They secrete heparin which is an anticoagulant and prevents blood clotting. The lifespan of a basophil is around a few hours to a few days. Lymphocytes are cells that account for 30% of the blood. Their function is to provide cell-mediated immunity (See: Cell-mediated immunity) in the form of T cells. B cells provide humoral immunity (See: Humoral immunity). Lymphocytes provide protection from cancer cells. Monocytes are the largest type of white blood cells. Their job is to destroy damaged and old cells. They turn into macrophages (cells that engulf debris of other cells). Platelets are used by the body to clot the blood. They secrete vasoconstrictors (See: Vasoconstrictor). They dissolve bacteria and destroy them and perform the job of maintaining the blood vessels. Due to the presence or absence of certain antigens, blood comes in eight groups or types (See: Blood Groups). There are many disorders and diseases of blood including anemia, clotting, cancers, and other diseases. People with diabetes suffer from higher/lower levels of blood glucose which causes many other disease conditions.