Blood is a specialised connective tissue that circulates through the cardiovascular system and consists of a fluid matrix called plasma together with cellular and cell derived components. Its major functions include transport, regulation and protection. Blood carries oxygen, carbon dioxide, nutrients, hormones, metabolic waste products, heat, immune cells and numerous proteins throughout the body. It also participates in maintaining pH, temperature, fluid balance and chemical composition.
The cellular components include red blood cells, white blood cells and platelets, while plasma contains water, proteins, electrolytes, nutrients, gases, hormones and waste products. Blood is therefore much more than a simple fluid. Its composition is dynamically regulated, and changes in blood composition can provide important information about physiological state and disease.
Plasma makes up slightly more than half of the volume of whole blood and is composed mainly of water. It also contains proteins such as albumin, globulins and fibrinogen, as well as electrolytes, nutrients, hormones, dissolved gases and metabolic products. Albumin contributes substantially to plasma oncotic pressure and helps maintain fluid distribution between the bloodstream and surrounding tissues. Globulins include antibodies and other proteins involved in transport and defence.
Fibrinogen is important for blood clotting and is converted into fibrin during coagulation. Plasma composition changes according to food intake, hydration, metabolism, endocrine activity and disease. The fluid component of blood therefore provides a transport medium and an important part of the extracellular environment surrounding cells.
Red blood cells, or erythrocytes, are the most numerous cellular components of blood. Their primary function is transporting oxygen from the lungs to tissues and carrying a substantial proportion of carbon dioxide back towards the lungs. They contain large quantities of haemoglobin, an iron containing protein capable of binding oxygen reversibly. Mature human red blood cells lack nuclei and most organelles, providing more internal space for haemoglobin and allowing the cells to adopt a flexible biconcave shape.
Their unusual structure increases surface area relative to volume and allows them to pass through narrow capillaries. Red blood cells are produced mainly in the bone marrow and normally circulate for approximately 120 days before being removed primarily by macrophages in the spleen and liver.
White blood cells, or leukocytes, are involved primarily in immune defence and include several distinct cell types with different functions. Neutrophils can rapidly respond to many bacterial infections and are important phagocytes. Lymphocytes include B cells, T cells and natural killer cells and perform specialised roles in adaptive and innate immunity. Monocytes circulate in blood and can enter tissues where they differentiate into macrophages or related cells. Eosinophils participate in responses involving parasites and allergic inflammation, while basophils contain and release mediators involved in inflammatory and allergic responses. The number and relative proportions of these cells can change in response to infection, inflammation, stress, medication and disease.
Platelets are small cell fragments derived from large bone marrow cells called megakaryocytes. They are essential for haemostasis, the set of processes that limit blood loss after vascular injury. When a blood vessel is damaged, platelets adhere to exposed structures, become activated and release signalling substances that recruit and activate additional platelets. Coagulation proteins then participate in a cascade that generates thrombin and ultimately converts fibrinogen into fibrin. Fibrin forms a network that strengthens the developing clot. Anticoagulant and fibrinolytic mechanisms subsequently limit and remove clots when appropriate. Haemostasis therefore involves interactions among vessel walls, platelets, plasma proteins and regulatory systems rather than being produced by one component alone.
Blood participates extensively in homeostasis. It distributes heat generated by metabolism and transports hormones that regulate distant tissues. It contributes to acid base balance through dissolved substances, plasma buffers and the transport of carbon dioxide between tissues and lungs. The cardiovascular system carries blood through the kidneys, where plasma is filtered and its composition is adjusted. Blood volume and electrolyte concentrations influence cardiovascular function and tissue hydration. Plasma proteins contribute to osmotic balance, while red and white cells provide specialised functions. Because blood connects organs through circulation, changes produced in one tissue can influence distant organs through transported substances. Blood is therefore a major communication and regulatory medium within the organism.
Blood must remain fluid while circulating but must be capable of forming clots rapidly after injury. This apparent contradiction is resolved through tightly regulated haemostatic systems. Endothelial cells normally produce signals that discourage inappropriate clotting, while vascular injury exposes structures and signals that activate platelets and coagulation. Natural anticoagulant pathways and fibrinolysis limit clot formation and help prevent excessive obstruction of vessels. Disorders of these mechanisms can produce either excessive bleeding or inappropriate thrombosis.
Blood composition also changes with physiological conditions. Anaemia can reduce oxygen carrying capacity, while abnormal white cell numbers can reflect immune or haematological disorders. Changes in clotting factors or platelets can alter bleeding risk. Blood tests therefore provide a major window into systemic physiology and disease.
Blood has been studied for centuries, but modern understanding developed through discoveries concerning circulation, microscopy, cellular components, haemoglobin, coagulation and immunology. The recognition that blood circulates through a closed cardiovascular system transformed physiology, while microscopic examination revealed cells and later enabled identification of specialised blood cell types. Modern haematology studies blood cells, plasma proteins, coagulation, blood formation and disorders affecting these components.
Blood transfusion depends on compatibility between donor and recipient blood, including major blood group systems such as ABO and Rh. Blood also provides material for biochemical, genetic, immunological and infectious disease testing. Its combination of transport, defence, regulation and repair makes it one of the most important connective tissues in the human body.
