C. Modes of Respiration 1. A gill has a moist, thin, vascularized epidermis. External gills project from the body surface of a few amphibians and some insects. The internal gills of adult fishes are positioned where water can enter the mouth and then flow over them as it exits just behind the head. Lungs contain internal respiratory surfaces shaped as a cavity or sac.
Lungs provide a membrane for gaseous exchange with blood.
Gases diffuse across the inner respiratory surfaces of the lungs. Pulmonary circulation enhances the diffusion of dissolved gases into and out of lung capillaries. Human Respiratory System A. Airways to the Lungs 1. Blood vessels warm the air and mucus moistens it. Alveoli provide a tremendous surface area for gaseous exchange with the blood located in the dense capillary network surrounding each alveolar sac.