Blood of different species
There are differences in blood between species.
Humans
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. Endocrine hormones also travel in the blood. (see List of human blood components)
Human blood is red, ranging from bright red when oxygenated to dark red when not. It owes its colour to hemoglobin, a respiratory protein containing iron in the form of heme, to which oxygen binds. It is a popular misconception that deoxygenated blood is blue because veins are blue and that blood only becomes red when it comes into contact with oxygen.
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.
Blood also transports metabolic waste products, drugs and other foreign chemicals to the liver to be degraded and to the kidney to be excreted in urine.
Insects
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.
Small invertebrates
In some small invertebrates, oxygen is simply dissolved in the plasma. All other animals use respiratory proteins to increase the oxygen carrying capacity. Hemoglobin is the most efficient respiratory protein found in nature. Hemocyanin (blue) contains copper and is used in crustaceans. Sea squirts, among others marine life, use a vanadium chromagen (bright green, blue, or orange) for its respiratory pigment.
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.
Anatomy of blood
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 corpuscles are:
- Red blood cells or erythrocytes (about 99%). These corpuscles lack a nucleus and organelles, so are not cells strictly speaking. They contain the blood's hemoglobin and distribute oxygen. The red blood cells also give rise to the system of blood types.
- Platelets or thrombocytes (0.6 - 1.0%) are responsible for blood clotting or coagulation.
- White blood cells or leukocytes (0.2%), are part of the immune system; they destroy infectious agents.
Blood plasma is essentially an 
