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.

Principles
Blood consists of blood cells (red blood cells, white blood cells and platelets) suspended in blood plasma. The plasma is mostly water, with varying amounts of proteins (the main one being Albumin which regulates the blood's colloidal osmotic pressure), 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 blood vessels throughout the body and the heart, which pumps blood around the system. Of these blood vessels, arteries typically carry oxygenated blood, carried away from the heart and delivered to the rest of the body via arterioles and capillaries, with venules and veins carrying deoxygenated blood back to the heart.

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.

Science
There are a number of esper abilities which involve blood, including Deep Blood, possessed by Gemstone Himegami Aisa.