Blood

Blood (血液) is a bodily fluid within humans and other vertebrates, which is circulated throughout the body, delivering necessary substances to the cells and transporting metabolic waste products away from them.

As well as being crucial for many forms of life, there are various parts of magic and science which involve blood in some way.

General
Blood consists of blood cells (, and ) suspended in. The plasma is mostly water, with varying amounts of proteins, glucose, mineral ions and carbon dioxide, with the plasma being the main medium for transporting excretory waste products.

The red blood cells are the most abundant and use the iron-containing protein haemoglobin (responsible for the red) to facilitate the transportation of oxygen from the lungs to the cells, reversibly binding with the respiratory gas and increasing solubility. The white blood cells (also known as leukocytes) are cells of the body's immune system which protect the body against infectious diseases and foreign invaders). Platelets are involved in clotting when bloods vessels are damaged and bleeding occurs.

The blood is transported at pressure through the body's circulatory system, consisting of a network of throughout the body and the heart, which pumps blood around the system. Of these blood vessels, typically carry oxygenated blood, carried away from the heart and delivered to the rest of the body via  and, with  and  carrying deoxygenated blood back to the heart. The flow of blood through the body is also referred to as the bloodstream.

Blood performs many important functions in the body. One of the most important is supplying oxygen and nutrients (e.g. glucose, amino acids, fatty acids etc, dissolved in blood or bound to proteins such as blood lipids) to the cells, and removing metabolic waste products (carbon dioxide, urea, lactic acid etc). Others include immunological functions through the circulation of white blood cells and antibodies, the regulation of core body temperature, coagulation, messenger functions through the transportation of hormones, and hydraulic functions.

Bleeding (also known as haemorrhaging or blood loss) is when blood escapes from the circulatory system, due to damage to the blood vessels, which can occur as a result of injury, infection or other factors. Bleeding can occur both internally and externally. A considerable loss of blood can cause the body's blood pressure to drop and with a large loss of blood (in addition to the loss from the functions it would have been serving), this can cause a person to go into shock, potentially fatally. Under the right circumstances, a can be used to make up for a lack of blood, with donated blood transferred to a recipient intravenously.

There are multiple human blood types, and knowledge of which blood falls under which type is necessary if a blood transfusion is being performed - if the wrong blood type is used in a transfusion, the recipient's body will reject it and they will experience a potentially life-threatening reaction. It is possible for a person's blood type to change through a bone marrow transplant.

There are various forms of artificial blood, fake blood and blood substitutes, which have been created on both the science and magic sides for various purposes, including transfusion, films,  training, deception, and the pre-cleaning obfuscation of residual information from bloodstains, among others.

Blood, its circulation and other aspects, have been used for analogies in describing certain subjects, such as the activity of a city and the transportation of resources along its routes. Its color has led to certain shades of red being described in terms relating to blood, such as blood-red or pigeon blood (the brightest and most valuable shade of red in rubies). The term is often used with regards to, and the term  has been used to refer to.

Magic
Given the typical color of blood in humans and most other creatures, blood is symbolically associated with the color red (which also represents the element of fire), and also symbolizes the activity of the heart.

Around the end of the 19th century and the beginning of the 20th century, 'true' magicians started to set down rules for themselves in research and experimentation, among which was prohibiting the usage of live creatures as ingredients, which included the use of human or animal blood in magic circles. Although technically banned, this did not mean that such actions were not performed, with some magicians deeming it necessary to break these rules.

One such magician was Aleister Crowley, who is experienced in the use of blood sacrifices in magic, a sacrifice which he has also referred to as his Blood Sign (血の供儀). Such a blood sacrifice was used to empower the curse which he used to destroy the Golden Dawn, and the death of Crowley's Hazards were used as a planet-wide blood sacrifice to optimize the prime Aleister's magical power. While in a female body, Aleister made use of a blood sacrifice and 'scarlet power' derived from. Perhaps most notably, he used the blood of three doves/pigeons in the magic circle used to summon Coronzon in Africa in 1909.

In Christianity, the blood of the Son of God (said to have healed the eyes of the blind Longinus) and Saints are commonly ascribed potent healing qualities. One story involving it is utilized in the anti-air defense system protecting Canterbury Cathedral, magic based on the miracle of Thomas Becket's blood. In, bread and wine symbolically represent flesh and blood respectively, and this symbolism has been utilized in magic involving the body of the Son of God or the regular human body.

In Norse mythology, the oceans were said to be created from the blood of the primordial giant. This story formed the basis of the spell Ymir's Ocean, which allowed its user to control water and salt in the ocean by changing her blood's salt concentration (concentrating the salty blood in her hand and preventing it from flowing to prevent it from damaging her organs, the restriction of blood flow meaning she couldn't use it for long durations). As well as runes potentially being drawn in blood, the Teiwaz rune, a representative example of runes carved on weapons, can have its effects increased by a slain foe's blood running through the rune's grooves and dying it red. The sword Hrunting is said to become tougher and sharper from the blood of slain enemies (with the Knight Leader's version using Telesma in its place).

Some forms of fortune telling are based on peoples' blood types. The corpse wax used in the Necessarius Underground Labyrinth makes use of a target's blood to form a connection to them.

The legendary immortal creatures known as Vampires are said to consume blood. Certain flesh-and-blood demons such as the Bologna Succubus are capable of providing a blood transfusion to human, but cannot receive one from a human. A drop of Anna Sprengel's blood can apparently produce unknown miracles.

Science
One method used in medical research involving artificial blood involves synthetic preservative derived from the structure of egg white, which has also been applied in other ways in several other fields of medical research, such as collagen for treating burns and a catalyst for cold sleep.

There are a number of esper abilities which involve blood, including Deep Blood, possessed by Gemstone Himegami Aisa, Amahebi Saeka's Scorpion Tail and Yugure Tsumebakei's Macro Dying.

As blood samples are a potential way of acquiring DNA, there have been attempts to illicitly obtain the genetic information of espers in Academy City through blood,  as well as measures taken to prevent such attempts.