Excretion and osmoregulation

Excretion is the removal of waste products of metabolism from the body. Most of the waste products are toxic when allowed to accumulate in the body.

Importance of excretion
To remove toxic waste products whose accumulation in the body poisons/harms the organisms
To remove excess materials in the body which when left to accumulate affects the body metabolism.

Excretory products are divided into two groups:

  1. Nitrogenous excretory products.
    These are excretory products, which contain the element nitrogen. They include ammonia, urea and uric acid.
    This is a highly toxic nitrogenous waste and it requires a lot of water for its elimination. It is very soluble in water and due to this it requires less energy to be excreted. Ammonia is excreted by organisms which live in fresh water and therefore have a lot of water in their bodies. Such organisms include bony fish, protozoans, and amphibians when in water,
    This is a less toxic nitrogenous waste. It requires less water for its excretion. It however requires a lot of energy for its excretion because of its low solubility in water compared to ammonia. Urea is excreted by terrestrial organisms, which have easy access to water, and marine organisms. Such organisms include terrestrial mammals, amphibians when on land, cartilaginous fish, etc.
    Uric acid:
    This is less toxic than urea and requires no water for its elimination from the body. It is insoluble in water. The demerit of excreting uric acid is that it requires a lot of energy for its excretion. Uric acid is excreted by birds, reptiles and insects and also common in desert animals.

2.Non nitrogenous excretory products.
These are excretory substances that do not contain the element nitrogen. Such products include Carbon dioxide, excess salts and excess water.

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Animals producing nitrogenous compounds in form of urea are those living on land but have easy access to water. This is because though urea is less toxic than ammonia, it needs a relatively high amount of water to reduce its toxicity to the body during excretion. Urea is excreted in form of urine, which is a mixture of urea, salts and water. Urea is excreted by mammals, amphibians when on land and marine vertebrates.
Animals producing nitrogenous wastes in form of uric acid are those living on land with little access to water. Uric acid is the least toxic and needs the least amount of water for its excretion. Such animals conserve their water because it is not lost during excretion. These animals include, bird, reptiles and insects.

Excretory organs
These are organs that release excretory products. They include the following.

Table showing excretory organs and their corresponding excretory products

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In man the excretory organs are the kidneys, skin and lungs. Their excretory waste products are as shown in the table below.

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