The orientation of the gill filaments and lamellae ensures that the water flowing over them moves in the opposite direction to the flow of blood through the capillaries, maintaining a diffusion gradient.
Plants can control the opening of their stomata to limit this, and xerophytes may have additional adaptations such as: hairs, waxy cuticle, small leaves, sunken stomata, rolled leaves.
The alveolar epithelium is adapted for gas exchange by having a large surface area, good blood supply, thin walls and elastic fibres which help recoil.
Inspiration: external intercostal muscles contract, rib cage moves up & out, diaphragm contracts, volume of the thorax is increased, pressure in the thorax decreases so the atmospheric pressure is greater than the pulmonary pressure and air is forced into the lungs.
Expiration: internal intercostal muscles contract, ribs move down and inwards, diaphragm relaxes, volume of the thorax is decreased, pulmonary pressure is greater than atmospheric pressure, air is forced out of the lungs.
Water evaporates from the leaves creating tension (transpiration), and the cohesive nature of water moves the whole column of water up the xylem (cohesion-tension theory).