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.
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.
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.
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:
Together, plasma and corpuscles form a non-Newtonian fluid whose flow properties are uniquely adapted to the architecture of the blood vessels.
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.
The amount of oxygen dissolved in blood is directly proportional to the PO2 of the blood.
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.
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.
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.
See also blood diseases
Problems with blood circulation and composition play a role in many diseases.
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.
As stated above, some diseases are still treated by removing blood from the circulation.
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".
Among the Germanic tribes (such as the Anglo-Saxons and the Vikings), blood was used during the sacrifices, the Blts. 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.
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.
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.