The generation of ATP by cellular respiration, to replace the energy used by vertebrates in obtaining food or escaping predators, consumes oxygen. The respiratory and circulatory systems of vertebrates show adaptations which support the mitochondria of muscles and other active tissues. The heart pumps the blood through the system. The blood becomes oxygenated as it passes through the capillaries of the gills or lungs. The more active the life style, the larger the amounts of organic molecules necessary to produce energy.
Vertebrates have several adaptations for feeding, digestion and nutrient absorption.
For example, muscles in the walls of the digestive tract move food from organ to organ along the tract. The first three of these classes are fishes, while the remaining four are the tetrapod vertebrates. An animal possessing two pairs of limbs that support it on land. In addition to being tetrapods, the reptiles, birds and mammals have other adaptations for a terrestrial lifestyle which are not found in the amphibians. Most mammals do not lay eggs but retain other features of the amniotic condition, consequently, they are also considered amniotes along with the birds and reptiles.