A large surface area allows for more gas exchange while a thin cuticle allows for faster diffusion of gases into and out of the blood. As blood flows through the gill lamellae, the blood takes up more and more oxygen and approaches the oxygen tension of the incoming water. Aquatic organisms regulate respiration based on oxygen concentration. The oxygen content can vary considerably in water in variable conditions such as temperature, environment and water depth. Aquatic animals cannot use carbon dioxide to regulate respiration due to carbon dioxide being highly soluble in the surrounding water, resulting in carbon dioxide never building to appreciable levels in the blood.
Mammalian respiration is regulated by carbon dioxide concentration.
Increased carbon dioxide concentrations result in increased carbon dioxide diffusion into the blood and cerebrospinal fluid resulting in increased acidity. Chemoreceptors in the brain are sensitive to this increases in acidity which results in increased respirations. During inhalation, most of the air flows directly to the caudal sacs. The cranial sacs expand during inhalation but only receive air from the lung and not the outside. On exhalation, air from the caudal sacs flows into the lung.