What is blood?

Somebody asked the above question in the discussions section. This is my answer, which may be a bit oddly structured.

Blood is a water-based body fluid that many multicellular life forms have. It carries dissolved molecules, some of them quite large, and even whole cells.

Blood serves many purposes. It brings to cells the substances they need, and transports off waste substances that hinder their life functions and would poison the cells if they were accumulated too much. Very important among these substances are those central to the energy metabolism. Cells need sugar and (di-)oxygen and must get rid of CO2. Blood also transports information expressed by specific molecules that regulate the function of cells/organs (hormones?), so it helps to coordinate a multi-organ body. Blood transports molecules and cells belonging the immune system; those serve to inactivate or kill pathogens, i.e. life forms and viruses that are harmful to the body. If an organism that works with has/uses blood loses the fluid through a wound, that is typically a big problem. Therefore blood has a mechanism to form a solid barrier where it exits the body, so that minor wounds are self-sealing.

Blood circulates through the body by canals which branch and split until they are very fine, and then reunite to bigger canals. These canals are called venesveins or blood vessels. Their fine branching serves to guide the bloodflow past as many cells as feasiblepossible. In the examples I know, the circular flow of the blood is driven by a dedicated hollow organ, called the heart, that acts as a pulsating pump through movement of its muscles. In tactAlong with the strokes of the heart, the venesveins also help a bit to pump the blood forward with muscles of their own.

An interesting detail about blood is that the dioxygen gas is transported by means of a carrier molecule. This is due to the fact that dioxygen cannot be dissolved in water as well as CO2 can. Oxygen transport is the bottleneck of gas transport in the blood and the body has come up with the carrier molecule to enable more of it. This enables the whole body to use chemical energy faster in all the cells, allowing for overall higher performance with all life functions.