Blood is a specialised connective tissue consisting of a liquid component called plasma and formed cellular elements suspended within it. In humans the formed elements include red blood cells, white blood cells and platelets. Blood circulates through the cardiovascular system and performs transport, regulatory, protective and homeostatic functions. Plasma contains water, electrolytes, proteins, nutrients, hormones, gases, waste products and many other dissolved substances.
Red blood cells transport most of the oxygen carried in blood through haemoglobin and also contribute to carbon dioxide transport. White blood cells participate in immune defence, while platelets are central to haemostasis and clot formation. Blood composition is not fixed. Concentrations of cells and dissolved substances change according to hydration, nutrition, exercise, disease, hormonal state, bleeding and many other factors. The ability of blood to move rapidly between organs makes it a central medium for communication and exchange throughout the body.
Plasma forms the liquid matrix of blood and consists predominantly of water together with dissolved proteins and small molecules. Major plasma proteins include albumin, globulins and fibrinogen, each contributing to particular physiological functions. Albumin contributes substantially to colloid osmotic pressure and helps transport various substances. Globulins include proteins involved in transport and immune functions. Fibrinogen is a precursor of fibrin and participates in blood clot formation.
Plasma also contains sodium, potassium, calcium, chloride, bicarbonate and other electrolytes that influence osmotic balance, membrane function and acid base regulation. Nutrients such as glucose, amino acids and lipid related substances are transported through plasma, as are hormones and metabolic waste products. The composition of plasma is continuously modified as blood passes through organs. The kidneys, lungs, liver, digestive tract and endocrine organs all contribute to maintaining or changing the chemical composition of circulating plasma.
Red blood cells are specialised for gas transport. Their mature human form lacks a nucleus and most other internal organelles, leaving more space for haemoglobin and reducing metabolic demands that would otherwise consume transported oxygen. Their flexible biconcave shape provides a large surface area relative to volume and allows them to pass through narrow capillaries. Haemoglobin contains iron containing groups that bind oxygen reversibly, allowing oxygen loading in the lungs and unloading in tissues according to local conditions.
Carbon dioxide is transported partly in association with haemoglobin and proteins but is carried predominantly in plasma as bicarbonate generated through reactions involving red blood cells. Red blood cells are produced in bone marrow from precursor cells and have a limited lifespan before being removed and processed primarily by organs involved in the reticuloendothelial system. Iron and other components recovered from ageing cells can be conserved and reused.
White blood cells comprise several distinct cell types with different roles in innate and adaptive immunity. Some rapidly respond to invading microorganisms or damaged tissue, while others participate in antibody production, immune memory, antigen presentation or regulation of inflammatory responses. White blood cells can leave the bloodstream by crossing the vessel wall and entering tissues where immune activity is required. Their distribution changes during infection, inflammation, stress and other physiological conditions.
Some cells specialise in engulfing microorganisms and cellular debris, while others release signalling molecules or directly target infected or abnormal cells. The immune functions of blood therefore depend on coordinated interactions between circulating cells, tissues and soluble proteins. Blood is not simply a transport medium for immune cells. It also provides the route through which immune signals and molecular components move between different parts of the body, allowing local responses to become coordinated at the organism level.
Platelets are small membrane enclosed fragments derived from large precursor cells in the bone marrow and play a central role in haemostasis. When a blood vessel is damaged, exposed structures beneath the endothelial lining promote platelet adhesion and activation. Activated platelets change shape, release signalling substances and recruit additional platelets to form an initial plug. The coagulation system then generates a protein network that stabilises the developing clot.
Multiple regulatory mechanisms prevent uncontrolled clotting throughout the circulation and help limit the response to the damaged region. After repair, clot breakdown mechanisms remove unnecessary coagulation material. Abnormalities in platelet number or function can produce excessive bleeding, while excessive platelet activation or coagulation can contribute to thrombosis. Haemostasis therefore represents a balance between preventing blood loss and maintaining unobstructed circulation.
Blood groups are determined by inherited differences in molecules present on the surfaces of red blood cells and by corresponding antibodies or other immune recognition mechanisms in plasma. The ABO and Rh systems are particularly important in transfusion medicine. If incompatible blood is transfused, immune reactions can damage red blood cells and produce potentially serious consequences. Blood typing and compatibility testing therefore precede transfusion.
Blood can also be separated into components such as red blood cells, plasma and platelets so that specific components can be provided according to clinical requirements. Donated blood has a limited storage life and must be collected, tested, processed and stored under controlled conditions. Blood transfusion is therefore both a biological and logistical process requiring accurate identification, screening and compatibility management.
Blood tests provide information about many aspects of physiology because circulating blood reflects processes occurring throughout the body. Measurements can include cell counts, haemoglobin concentration, electrolyte levels, glucose, lipid concentrations, enzymes, hormones, proteins and markers associated with inflammation or organ function. Interpretation requires reference ranges and clinical context because a value outside a typical range does not by itself establish a particular diagnosis. Blood chemistry can change with meals, hydration, exercise, medication, disease and time of day.
Samples may be collected from veins, arteries or capillary sites depending on the measurement required. Laboratory methods include microscopy, automated cell counting, chemical assays, immunological techniques and molecular analysis. Blood therefore provides a practical window into systemic physiology, although its composition reflects many interacting processes and must be interpreted carefully.
Disorders affecting blood include anaemias, abnormalities of white blood cells, platelet disorders, clotting disorders and diseases involving abnormal blood cell production. Anaemia generally involves reduced oxygen carrying capacity and can result from inadequate red blood cell production, excessive loss or destruction of red blood cells, or insufficient haemoglobin related resources. Clotting disorders can increase bleeding or thrombosis depending on the underlying defect. Abnormal white blood cell production can impair immune function or produce malignant cell populations.
Blood disorders may arise from inherited genetic changes, nutritional deficiencies, infections, immune abnormalities, environmental factors or other diseases. Treatment varies according to cause and can involve replacement of deficient substances, medicines, transfusion, transplantation or targeted therapies. The complexity of blood means that disease can arise from interactions among cells, proteins, chemical composition and the organs responsible for producing or regulating its components.
