(14) Explain the difference in gas exchange in fish compared to mammals. Why is there a difference?

A.Fish does gas exchange in water using gills. Mammal do gas exchange in the atmosphere to obtain oxygen using lungs. Fish gills are a countercurrent exchange system. The water flow goes in one direction and blood goes in the opposite direction. This maximizes the amount of oxygen the fish gain from the water.
There is a difference because the oxygen levels in water are lower than in the air and water is a liquid and air is gas. Therefore, fish needs a different method of getting oxygen than mammals.

B.Fishes use gills to take oxygen out of water. Gills have the blood flow directly by them where the blood is oxidized and loses its CO2. Mammals use lungs to exchange oxygen in the atmosphere with the CO2 in their lungs. Fish live in liquid environments where Oxygen is not as abundant. Mammals either come to the surface to breath or live in area where there is a lot of air.

C.http://www.mrothery.co.uk/exchange/exchange.htm#GAS%20EXCHANGE
http://www.scribd.com/doc/8303894/26-Gas-Exchange

as 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.

D.Fish obtain oxygen from water by their gills. Gill filaments have flattened plates called lamellae. Blood flowing through the capillaries within the lamella pick up oxygen from the water through a countercurrent exchange. Water flows across the lamellae in a direction opposite to the blood flow. Blood entering the gills is low in oxygen so when it flows through the lamellae, it picks up more and more oxygen from the water. Majority of mammals just use their lungs. There is a difference because fish are aquatic animals with gills. Mammals do not have gills as their respiratory organ.