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 (erythrocytes) are present in the blood and help carry oxygen to the rest of the cells in the body
Red blood cells (erythrocytes) are present in the blood and help carry oxygen to the rest of the cells in the body

Blood is a circulating tissue composed of fluid plasma and cells (red blood cells, white blood cells, platelets). Medical terms related to blood often begin in hemo- or hemato- (BE: haemo- and haemato-) from the Greek word "haima" for "blood".

The main function of blood is to supply nutrients (oxygen, glucose) and constitutional elements to tissues and to remove waste products (such as carbon dioxide and lactic acid). Blood also enables cells (leukocytes, abnormal tumor cells) and different substances (amino acids, lipids, hormones) to be transported between tissues and organs. Problems with blood composition or circulation can lead to downstream tissue dysfunction.


Blood of different species

There are differences in blood between species.


Human blood is a liquid tissue; its major function is to transport oxygen necessary to life throughout the body. It also supplies the tissues with nutrients, removes waste products, and contains various components of the immune system defending the body against infection. Hormones also travel in the blood. There are about 4-6 liters of blood in an average human body, accounting for approximately 8% of body mass. Adult humans have about 60 milliliters of blood per kilogram of body weight.

Human blood is red, ranging from bright red when oxygenated to dark red when not. It owes its colour to hemoglobin, a metalloprotein compound containing iron in the form of heme, to which oxygen binds. There exists a popular misconception that deoxygenated blood is blue and that blood only becomes red when it comes into contact with oxygen. Blood is never blue, but veins appear blue because light is diffused by skin. Moreover, the blood inside is dark red and exhibits poor light reflection. From a physiological perspective, veins and arteries appear similar when skin is removed and are seen directly.

Blood moves in blood vessels and is circulated by the heart, a muscular pump. It passes to the lungs to be oxygenated, and then is circulated throughout the body by the arteries. It diffuses its oxygen by passing through tiny blood vessels called capillaries. It then returns to the heart through the veins. See circulatory system for a more detailed description of this circulation.

Blood also transports metabolic waste products, drugs and other foreign chemicals to the liver to be degraded and to the kidney to be excreted in urine.

A buffer solution of carbonic acid (H2CO3) and bicarbonate (HCO3-) is present in the human blood stream, in order to maintain a pH between 7.35 and 7.45.

Small invertebrates

In some small invertebrates like insects, oxygen is simply dissolved in the plasma. Larger animals use respiratory proteins to increase the oxygen carrying capacity. Hemoglobin is the most common respiratory protein found in nature. Hemocyanin (blue) contains copper and is used in crustaceans and mollusks. It is thought that tunicates (sea squirts) might use vanabins (proteins containing vanadium) for respiratory pigment (bright green, blue, or orange).

In many invertebrates, these oxygen-carrying proteins are freely soluble in the blood; in vertebrates they are contained in specialized red blood cells, allowing for a higher concentration of respiratory pigments without increasing viscosity or damaging blood filtering organs like the kidneys.


In insects, the blood (more properly called hemolymph) is not involved in the transport of oxygen. (Openings called tracheae allow oxygen from the air to diffuse directly to the tissues). Insect blood moves nutrients to the tissues and removes waste products.

Anatomy of blood

Blood is composed of several kinds of corpuscles; these formed elements of the blood constitute about 45% of whole blood. The other 55% is blood plasma, a yellowish fluid that is the blood's liquid medium. The normal pH of human arterial blood is approximately 7.40.

The corpuscles are:

  • Red blood cells or erythrocytes (96%). In mammals, these corpuscles lack a nucleus and organelles, so are not cells strictly speaking. They contain the blood's hemoglobin and distribute oxygen. The red blood cells (together with endothelial vessel cells and some other cells) are also marked by proteins that define different blood types.
  • White blood cells or leukocytes (3.0%), are part of the immune system; they destroy infectious agents.
  • Platelets or thrombocytes (1.0%) are responsible for blood clotting or coagulation and are involved in inflammation.

Blood plasma is essentially an aqueous solution containing 96% water, 4% blood plasma proteins, and trace amounts of following:

Together, plasma and corpuscles form a non-Newtonian fluid whose flow properties are uniquely adapted to the architecture of the blood vessels.

Physiology of blood

Production and degradation

Blood cells are produced in the bone marrow; the process is termed hematopoiesis. The proteinaceous component is produced overwhelmingly in the liver, while hormones are produced by the endocrine glands and the watery fraction maintained by the gut and the kidney.

Blood cells are degraded by the spleen and the Kuppfer cells in the liver. The liver also clears proteins and amino acids (the kidney secretes many small proteins into the urine).

Transport of oxygen

The amount of oxygen dissolved in blood is directly proportional to the PO2 of the blood.

The hemoglobin molecule is the primary transporter of oxygen. 98.5% of the oxygen is chemically combined with the Hb. Only 1.5% is physically dissolved.

Transport of carbon dioxide

When systemic arterial blood flows through capillaries, carbon dioxide diffuses from the tissues into the blood. Some carbon dioxide is dissolved in the blood. Some carbon dioxide reacts with hemoglobin to form carbamino hemoglobin . The remaining carbon dioxide is converted to bicarbonate and hydrogen ions. Most carbon dioxide is transported through the blood in the form of bicarbonate ions.

Transport of hydrogen ions

Some oxyhemoglobin loses oxygen and becomes deoxyhemoglobin. Deoxyhemoglobin has a much greater affinity for H+ than does oxyhemoglobin so it binds most of the hydrogen ions.

Health and disease

Ancient medicine

Hippocratic medicine considered blood one of the four humors (together with phlegm, yellow bile and black bile). As many diseases were thought to be due to an excess of blood, bloodletting and leeching were a common intervention until the 19th century (it is still used for some rare blood disorders).

In classical Greek medicine, blood was associated with air, springtime, and with a merry and gluttonous (sanguine) personality. It was also believed to be produced exclusively by the liver.


Blood pressure and blood tests are amongst the most commonly performed diagnostic investigations that directly concern the blood.


See also blood diseases

Problems with blood circulation and composition play a role in many diseases.

  • Wounds can cause major blood loss. The thrombocytes cause the blood to coagulate, blocking relatively minor wounds, but larger ones must be repaired at speed to prevent exsanguination. Damage to the internal organs can cause severe internal bleeding, or hemorrhage.
  • Circulation blockage can also create many medical conditions from ischemia in the short term to tissue necrosis and gangrene in the long term.
  • Hemophilia is a genetic illness that causes dysfunction in one of the blood's clotting mechanisms. This can allow otherwise inconsequential wounds to be life-threatening, but more commonly results in hemarthrosis , or bleeding into joint spaces, which can be crippling.
  • Leukaemia (more often called leukemia) is a group of cancers of the blood-forming tissues.
  • Major blood loss, whether traumatic or not (e.g. during surgery), as well as certain blood diseases like anemia and thalassemia, can require blood transfusion. Several countries have blood banks to fill the demand for transfusable blood. A person receiving a blood transfusion must have a blood type compatible with that of the donor.
  • Blood is an important vector of infection. AIDS, the virus of which, HIV, is transmitted through contact between blood, semen, or the bodily secretions of an infected person. Hepatitis B and C are transmitted primarily through blood contact. Owing to blood-borne infections, bloodstained objects are treated as a biohazard.
  • Infection of the blood is bacteremia or sepsis. Malaria and trypanosomiasis are blood-borne parasitic infections.


Blood transfusion is the most direct therapeutic use of blood. It is obtained from volunteers by blood donation. As there are different blood types, and transfusion of the incorrect blood may cause severe complications, crossmatching is done to ascertain the correct type is transfused.

Other blood products administered intravenously are platelets, blood plasma, cryoprecipitate and specific coagulation factor concentrates.

Many forms of medication (from antibiotics to chemotherapy) are administered intravenously, as they are not readily or adequately absorbed by the digestive tract.

As stated above, some diseases are still treated by removing blood from the circulation.

Mythology and religion

Due to its importance to life, blood is associated with a large number of beliefs. One of the most basic is the use of blood as a symbol for family relationships; to be "related by blood" is to be related by ancestry or descendance, rather than marriage. This bears closely to bloodlines, and sayings such as "blood is thicker than water" and "bad blood", as well as "Blood brother".

Blood and blessing

Among the Germanic tribes (such as the Anglo-Saxons and the Vikings), blood was used during the sacrifices, the Blóts. The blood was considered to have the power of its originator and after the butchering the blood was sprinkled on the walls, on the statues of the gods and on the participants themselves. This act of sprinkling blood was called bleodsian in Old English and the terminology was borrowed by the Catholic Church becoming to bless and blessing.


In Judaism, blood cannot be consumed even in the smallest quantity (Leviticus 3:17 and elsewhere); this is reflected in the dietary laws. Blood is purged from meat by salting and pickling.

Other rituals involving blood are the covering of the blood of fowl and game after slaughtering (Leviticus 17:13); the reason given by the Torah is: "Because the soul of every animal is [in] his blood" (ibid 17:14), although from its context in Leviticus 3:17 it would appear that blood cannot be consumed because it is to be used in the sacrificial service (known as the korbanot), in the Temple in Jerusalem.

Ironically, Judaism has historically been the religion to be most affected by blood libels.


Christians believe that the Eucharist wine either is or represents the blood of Jesus shed for their salvation.

Jehovahs Witnesses

Jehovah's Witnesses take very literally the Biblical injunction against deriving benefit from blood ("Because the soul of every animal is [in] his blood", Leviticus 17:14). The religious movement maintains that apart from consuming blood, it is also forbidden to receive blood transfusions.

Vampire legends

Vampires are fictional beings thought to cheat death by drinking the blood of the living. Porphyrias, a group of inherited or acquired disorders, may have been the source of vampire legends.

See also

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