Blood

Blood is a fluid that circulates through the body via blood vessels. It carries oxygen, nutrients, hormones, and heat, while removing waste products such as carbon dioxide. The heart pumps the blood, while the blood vessels (arteries, veins, and capillaries) serve as its pathways.

Blood consists of two major components: a liquid part called plasma, and cells suspended within it. Plasma is mostly water and contains proteins, sugars, and other substances being transported.

Blood cells

Red blood cells (erythrocytes) contain hemoglobin, an iron-containing protein that binds oxygen. This gives blood its red colour. White blood cells (leukocytes) are part of the body's immune system and fight infection. Blood platelets participate in clotting when injury occurs, stopping blood loss.

Blood type

Human blood is classified into systems, the most common being the ABO system (A, B, AB, O) and the Rh system (positive or negative). The classification is based on antigens present on the surface of red blood cells. When transfusing blood, the donor type must match the recipient's; otherwise the body may attack the new cells.

Production and circulation

Blood cells are primarily produced in bone marrow. The liver and kidneys participate in purifying and regulating substances in the blood. The lungs are where oxygen is taken up and carbon dioxide is released.

Health and testing

Blood testing is a standard health tool, as the number of cells and levels of various substances can indicate different conditions, such as anaemia or infection. Blood donation supports patients, difficult births, and treatment of illness.