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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- (British English: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.

The blood is circulated around the lungs and body by the pumping action of the heart.

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 (normal range is 7.35-7.45). Blood that has a pH below 7.35 is acidic, while blood pH above 7.45 is alkaline. Blood pH along with paCO2 and HCO3 readings are helpful in determining the acid-base balance of the body. Blood is about 7% of the human body weight [1], so the average adult has a blood volume of about 5 liters, of which 2.7-3 liters is plasma. The combined surface area of all the erythrocytes in the human anatomy would be roughly 2,000 times as great as the body's exterior surface.

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 (coagulation)

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

Blood plasma is essentially an aqueous solution containing 96% water, 4% blood plasma proteins, and trace amounts of other materials. Some components are:

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 Kupffer cells in the liver. The liver also clears proteins and amino acids (the kidney secretes many small proteins into the urine). Erythrocytes usually live up to 120 days before they are systematically replaced by new erythrocytes created by the process of hematopoiesis.

Transport of oxygen[]

Blood oxygenation is measured with the partial pressure of oxygen. 98.5% of the oxygen is chemically combined with the Hb. Only 1.5% is physically dissolved. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species.

With the exception of pulmonary and umbilical arteries and their corresponding veins, arteries carry oxygenated blood away from the heart and deliver it to the body via arterioles and capillaries, where the oxygen is consumed; afterwards, venules and veins carry deoxygenated blood back to the heart.

Under normal conditions in humans, hemoglobin in blood leaving the lungs is about 96-97% saturated with oxygen; 'deoxygenated' blood returning to the lungs is still approximately 75% saturated.[2][3] A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures (about 20% of the level found in an adult's lungs) and so fetuses produce another form of hemoglobin with a much higher affinity for oxygen (hemoglobin F) in order to extract as much oxygen as possible from this sparse supply.[4]

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.

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.


In humans and other hemoglobin-using creatures, oxygenated blood is a bright red in color. Deoxygenated blood is a darker shade of red, which can be seen during blood donation and when venous blood samples are taken. However, due to an optical effect caused by the way in which light penetrates through the skin, veins typically appear blue in color.[5] This has led to a common misconception that before venous blood is exposed to air it is blue.

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 (see bleeding). 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.
  • 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. HIV, the virus which causes AIDS, 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 human donors 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.

It is the fluid part of the blood that saves lives where severe blood loss occurs, other preparations can be given such as ringers atopical plasma volume expander as a non-blood alternative, and these alternatives where used are rivalling blood use where used.

Cultural significance of blood[]

Blood is given widespread significance in culture, religion and mythologies

Main article: Cultural significance of blood

See also[]

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