Blood

Blood is the red, viscous connective tissue fluid that circulates through the cardiovascular system of vertebrates, serving as the primary medium for transporting oxygen, nutrients, hormones, and immune cells throughout the body while carrying away carbon dioxide and metabolic waste.

Composition

Human blood is composed of a liquid portion called plasma and several types of formed cellular elements suspended within it. Whole blood accounts for roughly 7–8 percent of an adult human's total body weight, amounting to approximately 4.5 to 5.5 litres in an average adult, though individual figures vary with body size, sex, and health status.

Plasma

Plasma constitutes about 55 percent of blood volume. It is a pale yellow, water-based solution containing dissolved proteins (primarily albumin, fibrinogen, and globulins), electrolytes, glucose, clotting factors, hormones, and waste products. Plasma without clotting factors is called serum.

Red blood cells

Red blood cells (erythrocytes) make up the majority of the cellular component of blood. In humans they are biconcave, anucleate discs approximately 6–8 micrometres in diameter. Their principal function is oxygen transport, achieved through the iron-containing protein haemoglobin, which reversibly binds oxygen in the lungs and releases it in peripheral tissues. Erythrocytes are produced in the bone marrow through a process called erythropoiesis and have a lifespan of approximately 120 days, after which they are removed by the spleen and liver.

White blood cells

White blood cells (leukocytes) are the cellular agents of the immune system. They are far less numerous than erythrocytes and are classified into two broad groups: granulocytes (neutrophils, eosinophils, and basophils) and agranulocytes (lymphocytes and monocytes). Leukocytes defend the body against infection, foreign substances, and abnormal cells.

Platelets

Platelets (thrombocytes) are small, anucleate cell fragments derived from megakaryocytes in the bone marrow. They are essential for haemostasis, the process by which bleeding is stopped: upon vessel injury, platelets aggregate at the wound site and initiate the clotting cascade, ultimately forming a fibrin clot.

Functions

Blood performs several overlapping physiological functions that are critical for survival.

Transport

The most prominent function is transport. Haemoglobin-bound oxygen is carried from the lungs to every metabolically active tissue, while carbon dioxide produced by cellular respiration is returned to the lungs for exhalation. Blood also transports glucose and other nutrients absorbed from the gastrointestinal tract, distributes hormones secreted by endocrine glands, and conveys metabolic waste such as urea to the kidneys for excretion.

Immune defence

Leukocytes, antibodies, and complement proteins circulating in blood constitute the primary humoral and cellular arms of immune defence. Monocytes differentiate into macrophages that engulf pathogens and cellular debris through phagocytosis, while lymphocytes mediate adaptive immunity, including the production of antigen-specific antibodies.

Thermoregulation

Blood redistributes heat generated by metabolic activity in core organs and working muscles to the skin surface, where it can be dissipated. Vasodilation and vasoconstriction of peripheral blood vessels regulate how much heat is lost or conserved, maintaining core body temperature within a narrow physiological range.

Homeostasis and pH buffering

Blood maintains the body's acid–base balance (normal arterial pH approximately 7.35–7.45) through bicarbonate buffering, respiratory regulation of carbon dioxide, and renal excretion of hydrogen ions. Plasma proteins also contribute to oncotic pressure, regulating fluid exchange between blood and interstitial compartments.

Blood types and groups

Human blood is classified into groups based on the presence or absence of specific antigens on the surface of erythrocytes. The two most clinically important systems are the ABO blood group system and the Rhesus (Rh) system.

ABO system

First described by Karl Landsteiner in 1901, the ABO system defines four main blood groups—A, B, AB, and O—based on the presence of A and/or B antigens on red cell surfaces and corresponding anti-A and anti-B antibodies in plasma. Transfusing incompatible blood causes a potentially fatal haemolytic transfusion reaction.

Rhesus system

The Rh system, also identified largely through the work of Landsteiner and Alexander Wiener in the early 1940s, is defined by the presence (Rh-positive) or absence (Rh-negative) of the RhD antigen. Rh incompatibility between a pregnant person and fetus can cause haemolytic disease of the newborn.

Other blood group systems

The International Society of Blood Transfusion recognises over 40 blood group systems, including the Kell, Duffy, and Kidd systems, all of which carry clinical significance in transfusion medicine and organ transplantation.

Blood disorders

A wide range of conditions affect blood and its components.

Anaemia

Anaemia is characterised by a reduction in haemoglobin concentration or erythrocyte count, impairing oxygen delivery. Causes include iron deficiency, vitamin B12 or folate deficiency, haemolysis, bone marrow failure, and chronic disease.

Clotting disorders

Deficiencies in clotting factors lead to bleeding disorders such as haemophilia A (factor VIII deficiency) and haemophilia B (factor IX deficiency). Conversely, excessive clotting activity can cause thrombosis, resulting in conditions such as deep vein thrombosis and pulmonary embolism.

Blood cancers

Malignancies originating in blood-forming tissues include leukaemia (uncontrolled proliferation of abnormal leukocytes), lymphoma (cancer of lymphocytes typically presenting in lymph nodes), and multiple myeloma (cancer of plasma cells). These are collectively classified under the broader category of haematological malignancies.

Infections

The bloodstream can be invaded by bacteria (bacteraemia), viruses, fungi, or parasites. Systemic spread of infection through blood, known as septicaemia or blood poisoning, is a life-threatening emergency requiring prompt antimicrobial treatment.

Blood in medicine and transfusion

Blood transfusion — the intravenous administration of donated blood or its components — is one of the most common medical procedures worldwide. Modern transfusion practice uses component therapy, separately transfusing packed red cells, platelets, fresh frozen plasma, or cryoprecipitate according to the patient's specific needs. Blood banks and national collection services such as the American Red Cross and NHS Blood and Transplant in the United Kingdom manage donation, testing, and distribution. All donated blood is routinely screened for infectious agents including HIV, hepatitis B, hepatitis C, and syphilis.

Cultural and symbolic significance

Blood has carried profound symbolic meaning across cultures and throughout history. In many religious traditions it represents life force, sacrifice, and covenant — notably in the Abrahamic faiths, where blood sacrifice and ritual purity laws feature prominently in scripture. In secular contexts, expressions such as "blood ties" and "blood oath" reflect its association with kinship and binding commitment. Blood imagery recurs extensively in literature, mythology, and art as a symbol of mortality, courage, and passion.

Frequently asked questions

Is blood actually a connective tissue?

Yes. Biologically, blood is classified as a specialized connective tissue because it consists of cells and cell fragments (the formed elements) suspended in an extracellular matrix — in this case, plasma — fulfilling the defining structural criteria of connective tissue, even though it is fluid.

Is human blood always red?

Oxygenated blood is bright red due to oxyhaemoglobin, while deoxygenated blood is a darker crimson. Blood is never truly blue inside the body; the bluish appearance sometimes attributed to veins through the skin is an optical effect caused by the differential absorption of light wavelengths by tissue.

Is donating blood safe for the donor?

For healthy adults, donating a standard unit of whole blood (approximately 450–500 ml) is considered safe. The body replenishes plasma within days and red blood cells within four to six weeks. Most donors experience no significant side effects beyond mild, transient lightheadedness.

Is blood type inherited?

Yes. ABO and Rh blood group antigens are genetically determined and follow Mendelian inheritance patterns, meaning a person's blood type is inherited from their biological parents and remains fixed throughout life.

Is artificial blood a viable alternative to donated blood?

Research into blood substitutes — including haemoglobin-based oxygen carriers and perfluorocarbon emulsions — has been ongoing for several decades, but as of the mid-2020s no product has achieved widespread clinical approval as a full replacement for donated human blood. Several products remain under investigation for specific emergency or military applications.