For all vertebrates and many invertebrates, the blood plays a major role in gas transport. In vertebrate blood, oxygen is carried by hemoglobin, which is located within the red blood cells. Carbon dioxide, in contrast, is carried mainly as the bicarbonate ion, dissolved in the blood plasma. It is commonly agreed that the supply of oxygen is more critical than the elimination of carbon dioxide and that whenever the oxygen supply is adequate, there is no difficulty in eliminating carbon dioxide at the rate at which it is formed.

The main parameters that are of interest in connection with oxygen transport and scaling are (1) the concentration of hemoglobin, which determines how much oxygen can be carried by one unit volume of blood, (2) the total volume of blood in the body and thus the total amount of hemoglobin in the blood, (3) the size of the red blood cell, and (4) the affinity of hemoglobin for oxygen, which is of interest both for the uptake of oxygen in the lung and for its delivery in the tissues.

Hemoglobin concentration

The hemoglobin concentration and oxygen capacity of blood have been surveyed by Larimer (1959) and Burke (1966) for a wide range of mammals. As could be expected, the oxygen-carrying capacity of the blood is strictly proportional to its hemoglobin concentration. The average hemoglobin concentration in 18 mammals, ranging in size from a small bat to the horse, was 128.7 g hemoglobin per liter blood.