Introduction

In animals all waste products must be excreted from the body in some manner. The urinary system is involved in the excretion of nitrogenous waste products such as urea, uric acid or ammonia. The excretion of nitrogenous waste from animals is important because if it is allowed to build up, it can prove toxic. We obtain nitrogen from protein in the diet, when protein is metabolised it produces amino groups which are able to form the highly toxic ammonia. Different classes and species of animals deal with toxic ammonia in different ways. A summary:

Method of Excretion by Species:

Fish

Fish excrete nitrogenous waste as ammonia; this is unusual because ammonia is highly toxic therefore storage in the body can pose a risk. However, fish are able to cope due to their environment – the large volumes of water they reside in allow them to continuously excrete ammonia (without the need for storage) directly into the water, diluting the ammonia to non-toxic levels.

Fish deaminate (remove the amine group) amino acids (obtained in the diet by protein consumption) in the gills. The pure ammonia diffuses into the respiratory water which is leaving via the gills; it is therefore excreted at the site of production with minimal time in the body to reduce toxicity.

There are two extremes of environment which fish have to adapt to however – marine and freshwater. The difference in concentration of salt between these two environments has led to slightly different methods of dealing with nitrogenous waste. Osmoregulation of fish living in fresh water requires them to excrete large volumes of dilute urine. This is due to osmosis, which allows large volume of water to enter body fluid from the surrounding hypotonic freshwater.

The converse is true for marine fish, the hypertonic water means water leaves bodily fluid by passive osmosis, in an attempt to conserve water small amounts of concentrated urine are released.

Elasmobranches, such as sharks, primarily live in seawater and are able to produce urea as well as ammonia. This is important for osmoregulation, as said earlier the hypertonic seawater means water is able to leave bodily fluids by osmosis. By producing urea (which is less toxic than ammonia and therefore storable), elasmobranches are able to increase their osmolarity higher than the seawater and therefore take up water from the sea like freshwater species. This is due to the urea being retained in the blood and therefore increasing the osmolarity of the blood.

Fish still have kidneys however, despite nitrogen excretion being handled by the gills. Instead the kidneys are involved in the excretion of excess water and divalent ions, e.g. Mg²⁺ and SO₄^2-.

Lungfish, which are able to live out of water, have another adaption. They excrete ammonia in the usual way when in the water, but during times of drought when the lungfish buries itself in the mud the production of toxic ammonia could prove fatal. So instead, when buried in the mud, the lungfish produces urea which can be accumulated and excreted into the environment at less toxic levels.

Amphibians

As with fish the type of nitrogenous excretion depends entirely on environment. Typically tadpoles use the same method as fish – ammonia excretion via the gills. However adult amphibians produce urea as they do not remain constantly in the water. The urea is stored, diluted in the bladder, but as amphibians may be prone to desiccation they have adapted the ability to reabsorb water from the urine when needed. This requires them to have relatively large bladders for storage.

Amphibians do not drink to obtain water; instead they are able to absorb water through a region of the skin called the ventral pelvic area.

Birds

All birds excrete a paste like substance called uric acid (which contains the waste nitrogen) independent of habitat. It initially begins as watery urine which travels down the ureters, from the kidney to the cloaca. The cloaca is a unified exit for the gastrointestinal, urinary and reproductive systems.

In the cloaca, the urine mixes with faecal matter from the digestive tract and as much water is removed as possible to produce a dry, crystalline paste containing uric acid. A small amount of watery urine may be excreted as well.

Reptiles

The method of nitrogen excretion depends on the habitat; aquatic reptiles (e.g. turtles) will mainly excrete ammonia (possibly urea) as with fish. Whilst reptiles living in drier conditions (e.g. lizards, snakes & tortoises) excrete uric acid – the low toxicity, crystalline paste. This helps to conserve water and during development in the egg, a build-up is not fatal. Uric acid, as with birds, is excreted from the cloaca.

The kidneys of reptiles do not have loops of Henle and so they are unable to produce concentrated urine also, whilst a large bladder is present in chelonians (turtles) snakes and some lizards do not have a bladder at all.

Mammals

All mammals produce primarily urea (sometime ammonia) which is excreted in urine. Mammals are able to osmoregulate and maximise water conservation by varying the concentration of urine depending on the hydration of the body. Mammals living in environments with plentiful access to fresh water will excrete large amounts of dilute urine. Mammals living in dry or marine environments will excrete small amounts of concentrated urine.

Osmoregulation in mammals is mainly controlled by the hormone ADH, its release (when a mammal is dehydrated) will result in the reduction of water loss and increased reabsorption of water in the kidneys by the loop of Henle.

Osmoregulation

Osmoregulation has been mentioned above in some cases, it can be associated with the excretion of nitrogenous waste because without correct osmoregulation, levels of ammonia can build within the body. A well osmoregulated animal will be taking on water and balancing ions well enough to ensure that nitrogen can be excreted and not build up.

The primary problem facing osmoregulation is the build-up of salt ions, this can occur with animals living in marine habitats.

• Fish can actively excrete NaCl via gills
• Some cartilaginous fish such as sharks have specialised salt-excreting glands in their rectum
• Birds and reptiles have specialised salt-excreting glands on their heads
• Marine mammals are able to excrete salt in urine efficiently