Toads, like all adult amphibians, possess lungs and a respiratory system. Amphibians are vertebrates characterized by a “double life,” transitioning from an aquatic larval stage to a semi-terrestrial adult form. This dual existence necessitates a highly adaptable system for gas exchange, allowing them to acquire oxygen and expel carbon dioxide from both air and water. The unique respiratory adaptations of toads allow them to survive in diverse environments by utilizing multiple surfaces for breathing.
The Function and Structure of Toad Lungs
The lungs in adult toads are structurally simpler than those of mammals, resembling elastic, sac-like organs. They contain a lower surface area for gas exchange because they lack the highly divided alveolar structure seen in more terrestrial vertebrates. Toads must actively force air into these lungs because they lack a diaphragm, which creates negative pressure for inhalation in mammals. This positive-pressure method is known as buccal pumping.
The toad first lowers the floor of its mouth, drawing air in through the nostrils while the glottis remains closed. It then closes the nostrils and raises the mouth floor, actively pushing the trapped air into the lungs. Expiration relies on the elastic recoil of the lungs and contraction of the body wall muscles to expel the spent air. Pulmonary respiration is often insufficient to meet the toad’s oxygen demand, particularly during periods of high activity.
The Necessity of Breathing Through the Skin
Because the toad’s lungs are relatively simple, a significant portion of gas exchange occurs through the skin, a process called cutaneous respiration. This method is mandatory for survival, supplementing the oxygen absorbed by the lungs and serving as the primary route for carbon dioxide elimination. The skin must remain consistently moist and thin to facilitate the diffusion of gases directly across the surface.
This specialized respiratory skin is highly vascularized, meaning it is richly supplied with a dense network of capillaries lying close to the epidermis. For cutaneous respiration to function, the toad must prevent the skin from drying out, which ties the animal to moist environments or water sources. If the skin desiccates, gas exchange ceases, leading to respiratory failure. Toads eliminate carbon dioxide through the skin much more efficiently than they absorb oxygen, sometimes accounting for over 90% of total carbon dioxide excretion.
Respiratory Changes from Tadpole to Adult
The respiratory system undergoes a dramatic transformation as the toad transitions from its aquatic larval stage to its terrestrial adult form. Newly hatched tadpoles breathe primarily using external or internal gills, extracting dissolved oxygen from the water. Their permeable skin also contributes a small degree of gas exchange during this aquatic phase.
As the tadpole begins metamorphosis, the gills are gradually absorbed and become non-functional. Simultaneously, the paired lungs begin to develop, and the creature starts making periodic trips to the water surface to gulp air. By the time metamorphosis is complete, the terrestrial adult relies on the lungs for primary oxygen uptake and the highly developed skin for supplementary respiration and waste gas expulsion.