Excretory system
Fish utilize kidneys to filter out the wastes from their blood and then use their skin and gills to excrete nitrogenous wastes, ammonia and excess water (they rely a lot on diffusion). They continuously keep a homeostasis of water and ions that differs between salt and fresh water fish. Freshwater fish maintain a hypertonic environment in their body: they gain water and lose ions (through diffusion) because their body has a lower concentration of water compared to their environment. To compensate that they produce a dilute urine and the gills actively take up irons. This means that they have a high filtration rate in their kidneys and therefore their kidneys have a high amount of glomeruli. On the contrary saltwater fish maintain a hypotonic environment: they lose water and gain ions like Na+, Cl- and K+ (through diffusion) because they have a higher water concentration compared to their environment. To compensate that they produce a very concentrated/salty urine and the ions get actively transported (away) through the gills. Fish do not have a loop of Henle in their nephron since they do not face the problem of water shortage in their aquatic habitat, but rather having a too high concentration of water in their body (especially freshwater fish).
In amphibians the nephrons in the kidneys filter out the nitrogenous wastes and excess water that get transported away along the ureter. Their waste products include ammonia and urea. The kidneys of amphibians function similarly to those of a freshwater fish. When they are in the water, they absorb specific salts through their skin, and the kidney then produces very dilute urine. When amphibians are on land they conserve water by reabsorbing water across the epithelium of the urinary bladder. Reptiles have very small kidneys and in diapsids the main nitrogenous waste product is uric acid. Many reptiles live in arid or hot environments and therefore need to be able to conserve water very well. They have adapted to this by their kidneys working very efficiently. They are specially adapted to be able to concentrate their wastes into uric acid, which has very low amounts of water in it. They additionally do not loose a large amount of water when excreting their waste because the uric acid does not absorb and requires very little water when expelled from the body. Reptiles do not have the ability to produce liquid urine that is more concentrated than their own body fluid because they lack a structure in their nephron called the loop of Henle. Therefore they sometimes use their colon to help in the reabsorption of water. The reason why they are able to produce such concentrated uric acid is because the glomerulus is situated in the medulla, which means that not so much water gets filtered out initially out of the blood. Therefore no loop of Henle is required to reabsorb any excess water. Birds require everything to be much lighter to assist them in flying more easily. Therefore they do not have a bladder, as it is too heavy. Instead the excretory system joins the digestive system by connecting the cloaca and the ureter, which results in a liquid excrement (urine and feces are mixed). Birds seem to be the first animals to possess a loop of Henle, which assist in more efficient usage of water by reabsorbing some of it back into the blood. Mammals have like birds specialized nephrons with a loop of Henle because their kidney has adapted to life on land due to facing a terrestrial problem: water shortage. They also possess kidneys, ureters, a bladder and a urethra. The kidneys of those mammals that live in the sea can maintain a constant salt level by excreting urine that contains high amounts of salt. This is one of the reasons why sea mammals can drink saltwater, because the salt levels in their urine are higher than those of the seawater. They have highly specialized nephrons with a glomerulus, Bowman’s capsule, loop of Henle, and a collecting duct, not to mention malpighian tubules that surround the loop of Henle. It can be considered the most complex excretory system of the animals. |