Describe as many functions of gills as possible?

Gills of vertebrates are developed in the walls of the pharynx along a series of gill slits opening to the exterior. In fish, the gills are located on both sides of the pharynx. Gills are madel of filaments which help increase surface area for oxygen exchange. In bony fish, the gills are covered by a bony cover called an 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 either by the use of spiracles or ram ventilation (ventilation by constantly swimming).

In most species, a countercurrent exchange system is employed to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other. Water taken into the mouth passes out of the slits, bathing the gills as it passes.

Some fish utilize the gills for the excretion of electrolytes. Gills' large surface area tends to create a problem for fish seeking to regulate the osmolarity of their internal fluids. Saltwater is less dilute than these internal fluids; as a consequence, saltwater fish lose large quantities of water osmotically through their gills. To regain the water, they drink large amounts of seawater and excrete the salt. Freshwater is more dilute than the internal fluids of fish, however, so freshwater fish gain water osmotically through their gills.

The gill slits of fish are believed to be the evolutionary ancestors of the tonsils, thymus gland, and Eustachian tubes, as well as many other structures derived from the embryonic branchial pouches. In some amphibians, the gills occupy the same position on the body but protrude as external tufts.