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Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Blood is a specialized biological fluid consisting of red blood cells (also called RBCs or erythrocytes), white blood cells (also called leukocytes) and platelets (also called thrombocytes) suspended in a complex fluid medium known as blood plasma.
By far the most abundant cells in blood are red blood cells. These contain hemoglobin which gives blood its red color. The iron-containing heme portion of Hemoglobin facilitates hemoglobin-bound transportation of oxygen and carbon dioxide by selectively binding to these respiratory gasses and greatly increasing their solubility in blood. White blood cells help to resist infections, and platelets are important in the clotting of blood.
Blood is circulated around the body through blood vessels by the pumping action of the heart. Blood is pumped from the strong left ventricle of the heart through arteries to peripheral tissues and returns to the right atrium of the heart through veins, blood then enters the right ventricle and is pumped through the pulmonary artery to the lungs and returns to the left atrium through the pulmonary veins, blood then enters the left ventricle to be circulated again. Arterial blood carries oxygen from inhaled air in the lungs to all of the cells of the body, and venous blood carries carbon dioxide, produced as a waste product of metabolism by cells, to the lungs to be exhaled.
Medical terms related to blood often begin with hemo- or hemato- (BE: haemo- and haemato-) from the Greek word "haima" for "blood." Anatomically, blood is considered a connective tissue from both its origin in the bones and its function.
- Supply of oxygen to tissues (bound to hemoglobin which is carried in red cells)
- Supply of nutrients such as glucose, amino acids and fatty acids (dissolved in the blood or bound to plasma proteins)
- Removal of waste such as carbon dioxide, urea and lactic acid
- Immunological functions, including circulation of white cells, and detection of foreign material by antibodies
- Coagulation, which is one part of the body's self-repair mechanism
- Messenger functions, including the transport of hormones and the signalling of tissue damage
- Regulation of body pH (the normal pH of blood is in the range of 7.35 - 7.45)
- Regulation of core body temperature
- Hydraulic functions
Problems with blood composition or circulation can lead to downstream tissue dysfunction. The term ischaemia refers to tissue which is inadequately perfused with blood.
The blood is circulated around the lungs and body by the pumping action of the heart. Additional return pressure may be generated by gravity and the actions of skeletal muscles. In mammals, blood is in equilibrium with lymph, which is continuously formed from blood (by capillary ultrafiltration) and returned to the blood (via the thoracic duct). The lymphatic circulation may be thought of as the "second circulation".
Constituents of human blood
Blood accounts for 7% of the human body weight, with an average density of approximately 1060 kg/m³, very close to pure water's density of 1000 kg/m3 The average adult has a blood volume of roughly 5 litres, composed of plasma (see below) and several kinds of cells (occasionally called corpuscles); these formed elements of the blood are erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). The red blood cells constitute about 45% of whole blood by volume.
Each litre of blood contains:
- 5 × 1012 erythrocytes (45.0% of blood volume) : In mammals, mature red blood cells lack a nucleus and organelles. 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 glycoproteins that define the different blood types. The proportion of blood occupied by red blood cells is referred to as the hematocrit. The combined surface area of all the red cells in the human body would be roughly 2,000 times as great as the body's exterior surface.
- 9 × 109 leukocytes (1.0% of blood volume) : White blood cells are part of the immune system; they destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents (pathogens) and foreign substances.
- 3 × 1011 thrombocytes (<1.0% of blood volume) : Platelets are responsible for blood clotting (coagulation). They change fibrinogen into fibrin. This fibrin creates a mesh onto which red blood cells collect and clot. This clot stops more blood from leaving the body and also helps to prevent bacteria from entering the body.
The other 55% (making up a total of 2.7-3.0 litres in an average human) is blood plasma, a fluid that is the blood's liquid medium, appearing golden-yellow in color. Blood plasma is essentially an aqueous solution containing 92% water, 8% blood plasma proteins, and trace amounts of other materials. Some components are:
- Serum albumin
- Blood clotting factors (to facilitate coagulation)
- Immunoglobulins (antibodies)
- Carbon dioxide
- Various other proteins
- Various electrolytes (mainly sodium and chloride)
Together, plasma and cells form a non-Newtonian fluid whose flow properties are uniquely adapted to the architecture of the blood vessels. The term serum refers to plasma from which the clotting proteins have been removed. Most of the protein remaining is albumin and immunoglobulins.
The normal pH of human arterial blood is approximately 7.40 (normal range is 7.35-7.45), a weak alkaline solution. Blood that has a pH below 7.35 is considered overly acidic, while blood pH above 7.45 is too alkaline. Blood pH along with arterial carbon dioxide tension (PaCO2) and HCO3 readings are helpful in determining the acid-base balance of the body. The respiratory system and urinary system normally control the acid-base balance of blood as part of homeostasis.
Production and degradation
Blood cells are produced in the bone marrow; this process is termed hematopoiesis. The proteinaceous component (including clotting proteins) is produced overwhelmingly in the liver, while hormones are produced by the endocrine glands and the watery fraction is regulated by the hypothalamus and maintained by the kidney and indirectly by the gut.
Blood cells are degraded by the spleen and the Kupffer cells in the liver. The liver also clears some proteins, lipids and amino acids. The kidney actively secretes waste products into the urine. Healthy erythrocytes have a plasma half-life of 120 days before they are systematically replaced by new erythrocytes created by the process of hematopoiesis.
Transport of oxygen
Blood oxygenation is measured in several ways, but the most important measure is the hemoglobin (Hb) saturation percentage. This is a non-linear (sigmoidal) function of the partial pressure of oxygen. About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at normal pressure is chemically combined with the Hb. Only 1.5% is physically dissolved in the other blood liquids and not connected to Hb. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species (for exceptions, see below).
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.
Differences in infrared absorption between oxygenated and deoxygenated blood form the basis for realtime oxygen saturation measurement in hospitals and ambulances.
Under normal conditions in humans at rest, hemoglobin in blood leaving the lungs is about 98-99% saturated with oxygen. In a healthy adult at rest, deoxygenated blood returning to the lungs is still approximately 75% saturated. Increased oxygen consumption during sustained exercise reduces the oxygen saturation of venous blood, which can reach less than 15% in a trained athlete; although breathing rate and blood flow increase to compensate, oxygen saturation in arterial blood can drop to 95% or less under these conditions. Oxygen saturation this low is considered dangerous in an individual at rest (for instance, during surgery under anesthesia): "As a general rule, any condition which leads to a sustained mixed venous saturation of less than 50% will be poorly tolerated and a mixed venous saturation of less than 30% should be viewed as a medical emergency."
A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures (about 21% 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 function under these conditions.
Substances other than oxygen can bind to the hemoglobin; in some cases this can cause irreversible damage to the body. Carbon monoxide for example is extremely dangerous when absorbed into the blood. When combined with the hemoglobin, it irreversibly makes carboxyhemoglobin which reduces the volume of oxygen that can be carried in the blood. This can very quickly cause suffocation, as oxygen is vital to many organisms (including humans). This damage can occur when smoking a cigarette (or similar item) or in event of a fire. Thus carbon monoxide is considered far more dangerous than the actual fire itself because it reduces the oxygen carrying content of the blood.
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 in an open system.
Other invertebrates 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 found 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.
Giant tube worms have extraordinary hemoglobins that allow them to live in extraordinary environments. These hemoglobins also carry sulfides normally fatal in other animals.
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 and other proteins to form carbamino compounds. The remaining carbon dioxide is converted to bicarbonate and hydrogen ions through the action of RBC carbonic anhydrase. 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 hydrogen ion (H+) than does oxyhemoglobin so it binds most of the hydrogen ions.
Blood circulation transports heat through the body, and adjustments to this flow are an important part of thermoregulation. Increasing blood flow to the surface (e.g. during warm weather or strenuous exercise) causes warmer skin, resulting in faster heat loss, while decreasing surface blood flow conserves heat.
The restriction of blood flow can also be used in specialized tissues to cause engorgement resulting in an erection of that tissue. Examples of this would occur in a mammalian penis, clitoris or nipple.
Another example of a hydraulic function is the jumping spider, in which blood forced into the legs under pressure causes them to straighten for a powerful jump.
In humans and other hemoglobin-using creatures, oxygenated blood is bright red. This is due to oxygenated iron-containing hemoglobin found in the red blood cells. Deoxygenated blood is a darker shade of red, which can be seen during blood donation and when venous blood samples are taken.
The blood of most molluscs, and some arthropods such as horseshoe crabs, is blue. This is a result of its high content of copper-based hemocyanin instead of the iron-based hemoglobin found, for example, in mammals. While mammalian blood is never blue, there is a rare condition (sulfhemoglobinemia) that results in green blood. Skinks in the genus Prasinohaema have green blood due to a buildup of the waste product biliverdin.
Health and disease
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.
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.
- Overproduction of red blood cells is called polycythemia.
- 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.
- Bacterial infection of the blood is bacteremia or sepsis. Viral Infection is viremia. 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, eg. haemochromatosis.
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 when used.
- Blood substitutes often called Artificial blood
- List of human blood components
- blood phobia
- Medical technologist
- ↑ 1.0 1.1 Alberts, Bruce (2005). "Leukocyte functions and percentage breakdown". Molecular Biology of the Cell. NCBI Bookshelf.
- ↑ "Density of Blood". The Physics Factbook. 2004. External link in
- ↑ Martini, Frederic, et al (2006). Human Anatomy. 5th ed. Page 529. San Francisco, California: Pearson Education, Inc. ISBN 0-8053-7211-3
- ↑ Ventilation and Endurance Performance
- ↑ Transplant Support- Lung, Heart/Lung, Heart MSN groups
- ↑ J Physiol. 2005 July 1
- ↑ The 'St George' Guide To Pulmonary Artery Catheterisation
- ↑ Oxygen Carriage in Blood - High Altitude
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