FISH RESPIRATORY SYSTEM
Given that fish live in the water, their respiratory system is no doubt different from land animals. The air we breathe is approximately 21% oxygen, while water is approximately 1% oxygen. Fish have no lungs or air sacs to hold air for gas exchange. The water that flows in through their mouth gets filtered through their gills, and gas exchange occurs in the gill filaments. The operculum is the hard external covering of the gills that protects them and roughly marks the difision between the head and the body
Osmoregulation
Freshwater Fish:
The internal surfaces of the fish have higher salt concentrations than the surrounding environment and water will diffuse into the body. Freshwater fish drink very little, to no water, and excrete large amounts of dilute urine due to the increased osmosis of water through the skin and gills.
Saltwater Fish:
The internal surfaces of the fish have lower salt concentrations than the surrounding enfironment and water will diffuse out of the body. Marine fish drink larger amounts of water to prevent dehydration, and excrete small amounts of concentrated urine, which also helps to conserve water.
How Do Gills Work?
Fish breathe by pulling
water in, or forcing
water into their mouth
while swimming, and it
gets filtered through
their gills and gill
filaments (primary and
secondary lamellae)
before it flows back out
into the environment.
The gill filaments are full of capillaries allowing gas exchange, and oxygen can diffuse from the water into the capillaries. It is then carried in the blood stream through the body, and back to the heart to be pumped through the gill filaments again. Much like the villi and microvilli of our small intestine, the primary and secondary lamellae provide a larger surface area for gas exchange. In each filament/lamella, oxygen rich water flows in the opposite direction of the oxygen poor blood. Carbon dioxide also diffuses out of the bloodstream, and out of the fish through the gills.
Evolution of Swim Bladders
Some fish are called "lungfish" because they will surface to gulp air into their airsac (lung). Their airsac may have evolved into what is now known as a swim bladder. Bony-skeleton fish have swim bladders to control their buyoancy through gas exchange with blood vessels. However, the swim bladders have no respiratory function. Sharks do not have swim bladders, which is why some must continuously swim to avoid sinking.
