Gas Exchange in Fish Gills

The life of a fish Fish live in freshwater or saltwater A few fish like the mudskipper can survive for a short amount of time out of water. Fish carry out gas exchange under water and must therefore extract oxygen from the water. Water contains a lot less oxygen compared to air. Fish have gills to carry out gas exchange

Water versus Air The water surrounding a fish contains a small percentage of dissolved oxygen. In the surface waters there can be about 5 ml. of oxygen per litre of water. This is much less than the 210 ml. of oxygen per litre of air that we breath, so the fish must use a special system for concentrating the oxygen in the water to meet their physiological needs.

Water & Oxygen Oxygen enters the water by absorption directly from the atmosphere or by aquatic plant and algae photosynthesis. Oxygen is removed from the water by respiration and decomposition of organic matter. Temperature, pressure, and salinity affect the dissolved oxygen capacity of water. After dissolving at the surface, oxygen is distributed by currents and turbulence.

Oxygen & Temperature Water temperature is a key factor in the regulation of water's oxygen levels. Warm water contains a lot less oxygen concentration than cold water.

Different fish, different metabolisms, different environments, different lifestyles… Carp and trout are examples of the two extremes of fish oxygen requirements.   Trout have a small gill area and a high metabolism, so they only live in the ocean and in fast moving streams where the oxygen levels are high.   Carp, in contrast, have a small gill area and a slower metabolism, so they can withstand low levels of oxygen and live in small lakes and ponds.  

The structure of Gills The gills are made up of filaments. Each filament is divided into lamellae. The lamellae help increase the surface area. The lamellae are thin, ensure that the diffusion distance between the blood, in the lamellae, and the water is small. The gills are bright red due to the large amount of blood vessels

Bony fish & cartilaginous fish Operculum

When a fish breathes, it opens its mouth at regular times and draws in a mouthful of water. It then draws the sides of its throat together, forcing the water through the gill openings. The water passes over the gills on the outside. Valves inside the mouth keep the water from escaping through the mouth again. The operculum can be very important in adjusting the pressure of water inside of the pharynx to allow proper ventilation of the gills. Lampreys and sharks lack an operculum, they have multiple gill openings. Also, they must use different methods to force water over the gills. In sharks and rays, this ventilation of the gills is achieved by ram ventilation (ventilation by constantly swimming).

Counter-current exchange The blood flows thorough the gill filaments in the opposite direction from the water passing the gills. This is very important for getting all of the available oxygen out of the water and into the blood.  If the blood flowed in the same direction as the water passing it, then the blood would only be able to get half of the available oxygen from the water. The blood and water would reach an equilibrium in oxygen content and diffusion would no longer take place. 

By having the blood flow in the opposite direction, the gradient is always such that the water has more available oxygen than the blood, and oxygen diffusion continues to take place after the blood has acquired more than 50% of the water's oxygen content. The system gives fish an 80-90% efficiency in acquiring oxygen. 

The benefits of being a fish Counter current Biozone pp 193 - 194