Respiration

Internal respiration is the exchange of oxygen and carbon dioxide between blood and cells in different tissues of an animal's body. Internal respiration occurs in animals with a circulation system (categories 2, 4, and 5 above). Animals with gills or lungs take up oxygen and transport oxygen-rich blood throughout the body; they transport carbon dioxide-rich blood from the body back into the respiratory organs where it is expelled. The oxygen-rich blood and carbon dioxide-rich blood do not mix, making for an efficient internal respiration system. Mammals and birds have a double circulation system for blood, in which separate pumps in the left and right chambers of the heart move the oxygen-rich blood in the arteries and carbon dioxide-rich blood in the veins.

The blood of vertebrates and some invertebrates contains a protein (such as hemoglobin, hemocyanin, or chlorocruorin), which binds oxygen and transports it from the respiratory organs throughout the body. These oxygen-binding proteins greatly improve the oxygen carrying ability of blood. For example, human hemoglobin contains about 98% of the oxygen in a human's blood.

Hemoglobin is a red protein which binds oxygen and occurs in the red blood cells of vertebrates. Each molecule of hemoglobin contains an iron atom and can bind up to four molecules of oxygen. In muscles, hemoglobin passes its oxygen to myoglobin. Myoglobin is an oxygen-binding protein that makes muscles red and transports oxygen to the cells of the muscle. In turn, muscle cells use the oxygen from myoglobin to power muscle movement by cellular respiration.

Some segmented worms (annelids) have a green blood protein, called chlorocruorin, which binds iron and serves as an oxygen carrier. Some invertebrates have a blue blood protein, called hemocyanin, which binds copper and serves as an oxygen carrier.