Waste products contain traces of ammonia that are harmful to organisms. Ammonia is known to be very toxic because it can impair the organs and disrupt homeostasis. This is the reason why animals immediately eliminate waste from their body.

Aquatic animals release ammonia by diffusing it out of their bodies into the water. However, aquatic animals living in the freshwater diffuse ammonia through their skin. On the other hand, aquatic animals in the saltwater diffuse ammonia through their gill membranes or body surfaces.

a. Annelids and mollusks
The nephridia found in annelids and mollusks is responsible for removing the metabolic wastes from the body. They excrete wastes from the body in the form of urine.

b. Insects and arachnids
Excretion takes place in a saclike organ called Malpighian tubules, which extract waste from the blood. These terrestrial invertebrates expel uric acid and other digestive wastes in the form of a thick paste.

c. Mammals and land amphibians
The kidneys are responsible for removing the metabolic wastes from the body. They excrete wastes from the body in the form of urine.

d. Reptiles and birds
They are unable to excrete ammonia directly. Instead, they convert ammonia into uric acid. First, the urine is reduced to crystals of uric acid while the excess water in the urine is absorbed by the cloaca. Then, the crystals form a thick, white paste.

Aquatic animals in the freshwater environment must excrete wastes while facing the osmotic challenge. Since the concentration of the water in the external environment is higher than the concentration of water in their bodies, the water leaves their bodies through osmosis; hence, they excrete dilute urine.

On the other hand, fishes in the saltwater environment tend to lose more water via osmosis since their bodies are less salty than the water in the external environment. To prevent them from losing too much water, they excrete salt across their gills and produce urine in small quantities.

The diagram below shows how the kidneys interact with the circulatory system to maintain homeostasis and to eliminate nitrogenous wastes.

The diagram below shows how the kidneys interact with the circulatory system to maintain homeostasis and to eliminate nitrogenous wastes. (Click to see the diagram)

Taking a lot of salt in the body can strain the kidneys and disrupt the homeostasis. For most animals, one effect is dehydration. This happens when too much water exits the body due to a high concentration of salt in the blood.

Unfortunately, some animals do not have the ability to eliminate concentrated salt in their body. Instead, they make their kidneys retain more fluid in the body to prevent dehydration. This can be very fatal because once the kidneys are too strained, it would stop performing its function in eliminating waste products. This will lead to the body being poisoned by the toxins.

The nitrogenous wastes produced by the large herds of cattle are converted by the bacteria into nitrogen compounds that can be utilized by other organisms. In this case, the nitrogen in the soil is absorbed by the plants in order for them to grow and develop.

However, excess nitrogen can be harmful to the surrounding water ecosystems. Once the nitrogen is carried by the runoff or seeped into the groundwater, it goes into the streams, rivers, lakes, and oceans. High concentrations of nitrogen would give rise to the algae bloom.