Fish have a single kidney which is the same length as the coelom. It can be divided up into cranial and caudal parts; the cranial part has endocrine and haematopoietic functions and the caudal is where filtration occurs. It is not uncommon for some species to have no glomeruli however as a rule freshwater fish have larger glomeruli in greater numbers. Some species also have renal portal veins.
- Fish have no loop of henle and water movement is by osmosis
- Ammonia is removed via the urine and the gills
As the environment is hypotonic compared to the body of the fish ions are lost and water is gained across the gills therefore the kidney excretes water and has a very high glomerular filtration rate. The gills also undertake active uptake of NaCl and excrete ammonia and the diet is also very important for maintaining NaCl levels
The environment is hypotonic compared to the body of the fish therefore water is lost across their gills so they drink sea water to replace this which results in a large intake of salt (activates Angiotensin 2). They excrete both ammonia and NaCl across their gills and further NaCl across their skin. Their kidneys have small or absent glomeruli and their main function is the elimination of excess divalent ions e.g. Mg2+