Blood is a specialised connective tissue and circulating body fluid that moves continuously through the cardiovascular system. It consists of a fluid component called plasma and cellular or cell-derived components known as formed elements, principally red blood cells, white blood cells and platelets. Plasma contains water together with proteins, electrolytes, nutrients, hormones, gases, waste products and many other dissolved substances. Blood therefore serves simultaneously as a transport medium, a regulatory fluid and an important component of immune defence and haemostasis. Its composition is tightly regulated because substantial deviations in volume, acidity, electrolyte concentration or cellular components can interfere with normal physiology.
Plasma makes up the liquid portion of blood and is composed predominantly of water. Important plasma proteins include albumin, which contributes substantially to colloid osmotic pressure and transports various substances; globulins, which include antibodies and transport proteins; and fibrinogen, which participates in blood clotting. Plasma also carries glucose, amino acids, lipids, hormones, electrolytes and metabolic waste. The composition of plasma changes continuously as substances enter from the digestive tract or tissues and are removed or transformed by organs such as the liver and kidneys. Serum is closely related to plasma but is plasma after clotting has occurred and therefore lacks most of the fibrinogen that has been consumed during coagulation.
Red blood cells are the most abundant formed elements and contain haemoglobin, an iron-containing protein that binds oxygen. They transport oxygen from the lungs to tissues and carry a substantial proportion of carbon dioxide back towards the lungs. White blood cells form part of the immune system and include several specialised cell types involved in recognising, attacking or coordinating responses to pathogens, abnormal cells and tissue injury. Platelets are cell fragments derived from megakaryocytes and participate in haemostasis by adhering to damaged vessel surfaces, becoming activated and helping form platelet plugs and blood clots.
Blood performs several major transport functions. It delivers oxygen and nutrients to cells, transports carbon dioxide towards the lungs, carries hormones and signalling molecules between organs, distributes heat and transports metabolic waste products towards organs responsible for processing or elimination. It also transports drugs, toxins and other substances. The circulatory system makes these transport functions possible because blood can move rapidly between distant tissues. The same transport network means that local changes in one organ can have systemic effects, because substances released into blood can reach many other parts of the body.
Blood contributes strongly to homeostasis. Its buffering systems help resist potentially dangerous changes in acidity, while circulating proteins and electrolytes contribute to fluid balance. Blood distributes heat and can alter its flow to the skin to assist thermoregulation. Plasma electrolytes contribute to electrical activity in nerves and muscles, while circulating hormones coordinate functions across distant organs. The kidneys, lungs, liver, endocrine organs, bone marrow and cardiovascular system all interact to maintain appropriate blood composition. Consequently, blood is not merely a carrier of oxygen but an active component of physiological regulation.
Blood also provides defence and repair. White blood cells and antibodies help identify and combat infectious organisms, while complement proteins and other plasma components participate in innate and adaptive immune mechanisms. When a blood vessel is damaged, platelets and coagulation proteins cooperate to limit blood loss. The process involves vasoconstriction, platelet adhesion and activation, formation of a platelet plug, and activation of coagulation pathways that generate fibrin to stabilise the developing clot. Later, mechanisms involved in fibrinolysis help remove clots when they are no longer required. These processes must remain carefully balanced because excessive bleeding and excessive clotting are both dangerous.
Blood is produced and maintained through specialised physiological systems. Most blood cells are generated in the bone marrow through haematopoiesis from haematopoietic stem and progenitor cells. Red-cell production is stimulated principally by erythropoietin, a hormone produced mainly by the kidneys in response to reduced oxygen availability. Platelet production depends on megakaryocytes in the bone marrow and is regulated in part by thrombopoietin. White-cell production is regulated through multiple growth factors and signalling pathways. Mature blood cells have different lifespans and are removed or recycled by organs and tissues including the spleen, liver and bone marrow.
Blood can be examined through laboratory testing because its composition provides information about many physiological and pathological processes. A full blood count measures red blood cells, white blood cells and platelets and can provide evidence relevant to anaemia, infection, bleeding disorders and other conditions. Plasma or serum tests can assess electrolytes, glucose, kidney and liver markers, hormones, lipids, inflammatory markers and many other substances. Blood-group systems, including ABO and Rh, are clinically important for transfusion and pregnancy care. Blood transfusion can replace particular components or whole blood when clinically indicated, but compatibility and careful testing are essential.
