Why does gaseous exchange happen?

Answer 1

Gills and lungs have capillaries exposed to water or air to allow gas exchanges, due principally to gasses' partial pressure. Gas exchange is more difficult for fish than for mammals because the concentration of dissolved oxygen in water is less than 1%, compared to 20% in air. (By the way, all animals need molecular oxygen for respiration and cannot break down water molecules to obtain oxygen.) Fish have developed specialised gas-exchange organs called gills, which are composed of thousands of filaments. The filaments in turn are covered in feathery lamellae which are only a few cells thick and contain blood capillaries. This structure gives a large surface area and a short distance for gas exchange. Water flows over the filaments and lamellae, and oxygen can diffuse down its concentration gradient the short distance between water and blood. Carbon dioxide diffuses the opposite way down its concentration gradient. The gills are covered by muscular flaps called opercula on the side of a fish's head. The gills are so thin that they cannot support themselves without water, so if a fish is taken out of water after a while the gills will collapse and the fish suffocates. Fish ventilate their gills to maintain the gas concentration gradient. They continuously pump their jaws and opercula to draw water in through the mouth and then force it over the gills and out through the opercular valve behind the gills. This one-way ventilation is necessary because water is denser and more viscous than air, so it cannot be contained in delicate sac-like lungs found in air-breathing animals. In the gill lamellae the blood flows towards the front of the fish while the water flows towards the back. This countercurrent system increases the concentration gradient and increases the efficiency of gas exchange. About 80% of the dissolved oxygen is extracted from the water.