Blood

Blood is a special kind of connective tissue and body fluid. It moves through the circulatory system of humans and other vertebrates, which are animals with backbones. Blood carries important things like nutrients and oxygen to the body's cells. It also takes waste away from those cells.

Blood is made of blood cells floating in a liquid called plasma. Plasma makes up 55% of blood. It is mostly water, about 92% by volume. Plasma also contains proteins, glucose, mineral ions, and hormones. The most common blood cells are red blood cells. They carry oxygen using a substance called hemoglobin. White blood cells help the body fight infections and parasites. In mammals, platelets help blood clot, which means they help it thicken and stop bleeding.

The heart pumps blood through blood vessels all around the body. In animals with lungs, blood carries oxygen from the air we breathe to the body's tissues. It also carries carbon dioxide, a waste gas made by cells, back to the lungs so it can be breathed out. Blood looks bright red when it has oxygen and dark red when it does not.

Many medical words about blood begin with "hemo-" or "hemato-." These come from the Greek word "haima," which means "blood." Scientists who study the body's structures think of blood as a special form of connective tissue. This is because it is made of cells and cell pieces floating in plasma.

Functions

Blood does many important jobs in the body. It carries oxygen to the tissues. Inside the red cells, the oxygen is held by hemoglobin. Blood also brings food materials such as glucose, amino acids, and fatty acids. Some of these dissolve in the blood. Others attach to plasma proteins, such as blood lipids. At the same time, blood carries away waste like carbon dioxide, urea, and lactic acid.

Blood also helps protect the body. It moves white blood cells around, and antibodies find foreign material. When a blood vessel breaks, coagulation turns the blood from a liquid into a semisolid gel. This helps stop the bleeding. Blood carries hormones as messengers. It also signals when tissue is damaged. Blood helps control body temperature and has hydraulic functions.

Constituents

See also: Reference ranges for common blood tests

Blood makes up about 7% of the human body's weight. An average adult has roughly 5 litres of blood volume, made of plasma and cells. The cells include red blood cells, which carry oxygen using hemoglobin; white blood cells, which are part of the immune system and fight pathogens; and platelets, which help the blood clot. Red blood cells make up about 45% of blood, plasma about 54.3%, and white blood cells about 0.7%.

Further information: Complete blood count

About 55% of blood is blood plasma, a pale yellow liquid that is mostly water. Plasma carries nutrients such as glucose, amino acids, and fatty acids. It also removes waste products such as urea and lactic acid. Plasma contains antibodies and electrolytes, mainly sodium and chloride. Blood pH is kept in a narrow range, and hormones travel through the plasma to reach different parts of the body. In other vertebrates, red blood cells are flattened and keep their nuclei, and thrombocytes help with clotting instead of platelets.

See also: Acid–base homeostasis

Physiology

Main article: Circulatory system

Blood is pushed around the body through blood vessels by the heart. In humans, blood leaves the heart's strong left ventricle through arteries to the body's tissues. It then returns through veins and travels to the lungs and back to the heart. Blood carrying oxygen from the air is sent to all the cells of the body. Blood also carries carbon dioxide, a waste product made by cells, to the lungs so it can be breathed out. Moving skeletal muscles can also help push blood back toward the heart. William Harvey described how blood moves around the body in 1628.

Further information: Oxygen saturation (medicine)

Most blood cells are made in the bone marrow. In young children, almost every bone makes red blood cells. In adults, this happens mainly in the larger bones. Healthy red blood cells live for about 120 days. After that, they are broken down by the spleen and the liver. The kidney removes waste products into the urine. Hemoglobin, the main molecule that carries oxygen, is found in most mammals.

Main article: Lymphatic system

Color

Main article: Hemoglobin

Hemoglobin is the main thing that gives blood its color. Each molecule has four heme groups. How they interact with other molecules changes the exact shade. Blood that is full of oxygen, found in arteries and capillaries, is bright red. Blood with less oxygen is a darker red. This darker blood is found in veins. You can see it when blood is donated or when a doctor takes a blood sample from a vein. This is because hemoglobin absorbs different light when it has oxygen and when it does not.

Blood in carbon monoxide poisoning is bright red, because carbon monoxide forms carboxyhemoglobin. In cyanide poisoning, the body cannot use oxygen, so the blood in veins stays full of oxygen and looks redder. Some conditions that affect the heme groups can make the skin look blue. This is called cyanosis. If the heme is changed by oxidation, methemoglobin forms. It is more brownish and cannot carry oxygen. In the rare condition sulfhemoglobinemia, blood in arteries is partly full of oxygen and looks dark red with a bluish tint. Veins near the skin look blue for several reasons. These reasons are linked to how the skin scatters light and how the visual cortex makes sense of what the eyes see. They are not caused by the actual color of the blood in veins. Skinks in the genus Prasinohaema have green blood. This happens because of a buildup of the waste product biliverdin.

Disorders

Many things can go wrong with blood. Injuries can cause bleeding, and a person can lose a lot of blood before shock sets in. Thrombocytes help coagulation form clots that stop bleeding. Injuries inside the body can cause internal bleeding. Dehydration means not having enough water. It can lower blood volume and cause orthostatic hypotension and fainting. Shock happens when body tissues do not get enough perfusion. This can be caused by blood loss, infection, or poor cardiac output. Atherosclerosis happens when a buildup called atheroma narrows the arteries. It becomes more common with age. Smoking, hypertension, hyperlipidemia, and diabetes can make it worse. A clot can form a thrombosis that blocks a blood vessel. When tissue does not get enough blood, it can die. This is called necrosis.

Anemia means there are not enough red blood cells. It can be caused by bleeding, thalassemia, nutritional deficiencies, or a genetic disorder. A person with anemia may need one or more blood transfusions. A blood test can confirm it. Many countries have blood banks that supply blood for transfusions. A patient must have a compatible blood type. Sickle-cell anemia is another condition. Leukemia is a group of cancers of the blood-forming tissues and cells. Having too many red cells (polycythemia vera) or too many platelets (essential thrombocytosis) may be premalignant. Myelodysplastic syndromes involve poor production of blood cells. Hemophilia is a genetic illness that affects clotting. It often causes bleeding into joints (hemarthrosis). Other clotting problems include coagulopathy and thrombophilia.

Blood can also carry infections. HIV spreads through contact with blood and other body fluids. Hepatitis B and C spread mainly through blood. Because of blood-borne infections, blood-stained objects are treated as a biohazard. Bacterial infections of the blood are called bacteremia or sepsis. Malaria and trypanosomiasis are parasitic infections that spread through blood. Carbon monoxide can also harm blood. It binds to hemoglobin, which leaves less of it free to carry oxygen. This can cause suffocation.

See also: Hematology

Main article: Carbon monoxide poisoning

Treatments

Blood for transfusion comes from people who give it, through blood donation. It is stored in a blood bank. The most important blood types are the ABO blood group system and the Rhesus blood group system. Giving the wrong blood type can cause serious problems, so crossmatching is done to make sure the blood is a good match. Other blood products given intravenously include platelets, blood plasma, cryoprecipitate, and special clotting factor concentrates. Many medicines, from antibiotics to chemotherapy, are also given through a vein, because the digestive tract does not absorb them well enough.

After severe blood loss, liquids called plasma expanders can be given through a vein. In these emergencies, a plasma expander can sometimes help more quickly than a blood transfusion, because transfused red blood cells do not start working right away. In modern evidence-based medicine, bloodletting is used for a few rare diseases, including hemochromatosis and polycythemia. In the past, bloodletting and leeching were common treatments until the 19th century, even though they were not proven to work. Many diseases were wrongly thought to be caused by having too much blood, according to Hippocratic medicine.

Etymology

The English word blood (Old English blod) comes from Germanic. Other Germanic languages have similar words, such as German Blut, Swedish blod, and Gothic blōþ.

Experts do not agree on where the word comes from before the Germanic languages. There is no accepted origin in Indo-European.

History

Robin Fåhræus, a Swedish doctor who created the erythrocyte sedimentation rate, suggested that the Ancient Greek system of humorism was based on watching blood clot in a clear container. When blood is left still in a glass for about an hour, four layers can be seen: a dark clot at the bottom, a layer of red blood cells, a whitish layer of white blood cells, and clear yellow liquid on top. Greek thinkers generally believed that blood was made from food. Plato and Aristotle are two important sources for this idea, but it goes back to Homer's Iliad. Plato thought that a fire in our bellies turned food into blood, and that the air we breathed carried the fire. Aristotle believed that food was turned into blood in the heart.

The ABO blood group system was discovered in 1900 by Karl Landsteiner. Jan Janský is credited with first sorting blood into four types (A, B, AB, and O) in 1907. This system is still used today. In 1907, the first blood transfusion was carried out using the ABO system to predict whether blood types would work together. The first non-direct transfusion took place on 27 March 1914. The Rhesus factor was discovered in 1937.

Culture and religion

See also: Blood libel

Main article: Jehovah's Witnesses and blood transfusions

Main article: Vampire

Blood is very important to life, so it is linked to many beliefs. One basic idea is that blood shows family ties through birth. To be "related by blood" means being related by family line, not by marriage. This connects to bloodlines, "blood is thicker than water," "bad blood," and "Blood brother." Blood is given special importance in the Islamic, Jewish, and Christian religions. The book of Leviticus 17:11 says "the life of a creature is in the blood." Some Christian churches, including the Eastern Orthodox Church, teach that blessed Eucharistic wine actually becomes the blood of Jesus. This belief is based on the Last Supper, when Jesus spoke to his disciples. Most Methodist and Presbyterian churches see the wine only as a symbol. In Judaism, animal blood may not be eaten. This is part of Kashrut. In Islam, the Qur'an forbids eating blood. Many Jehovah's Witnesses do not accept blood transfusions.

Blood also appears in traditions around the world. In many Aboriginal Australian traditions, ochre and blood are put on dancers for ceremonies. These peoples believe this links the dancers to the Dreamtime. In old Germanic tribes, blood was used in sacrifices. The Ancient Greeks believed the blood of the gods, ichor, was poisonous to humans. Mythical vampires drink blood. The Nosferatu legend comes from Eastern European folk stories. Real animals such as ticks, leeches, female mosquitoes, and vampire bats also drink blood. Vampire bats are New World creatures, found long after the European myths began.

Invertebrates

Main article: Hemolymph

Many invertebrates have a body fluid like blood, called hemolymph. The main difference is that hemolymph does not flow through a closed system of blood vessels. Hemolymph may carry oxygen, but it does not always use hemoglobin. Crustaceans and mollusks use hemocyanin instead of hemoglobin to carry oxygen.

In most insects, the hemolymph does not carry oxygen. Their bodies are small enough that their tracheal system can bring oxygen directly to the body.

Other uses

Blood residue can help forensic investigators find objects and understand what happened at a crime scene. Through bloodstain pattern analysis, investigators can also learn things from how bloodstains are spread out. Blood residue analysis is also a technique used in archeology. Blood has also been used in art. Artists such as Viennese Actionist Hermann Nitsch, Istvan Kantor, Franko B, Lennie Lee, Ron Athey, Yang Zhichao, Lucas Abela and Kira O'Reilly, and the photographer Andres Serrano, have made blood a striking visual part of their work. Marc Quinn has made sculptures from frozen blood, including a cast of his own head made with his own blood.

In family history, the word bloodline is used in genealogy to talk about a person's ancestry, origins, and ethnic background. Other phrases that use blood in this way are blue-blood, royal blood, mixed-blood and blood relative.