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Excretion is the removal of toxic substances and metabolic waste products[1]. Every organism, from the smallest protist to the largest mammal, must cleanse itself of the potentially toxic by-products of its own vital activities. This process in living things is named elimination, which may be considered to include all of the different mechanisms and processes by which life forms dispose of or gets rid of waste products, harmful substances, and perished portions of the organism[2].In vertebrate animals, this process is mainly carried out by the lungs, skin and the kidneys[3]., with the urinary system being the main excretory system. Excretion through the skin occurs through sweating, which results in the removal of water and small amounts of urea and salts. The kidneys filter about 180 litres of blood every day and removes urea, toxins, medications, and excess ions through the formation of urine[4].

An example of such is cell excretion process. The cell clears its waste products by bringing the waste products close to the cell membrane and then closing the cell membrane around the waste products, isolating it from the rest of the cell. Then the cell could open the cell membrane on the outside of the cell, letting the products out without losing any cytoplasm or letting anything else in[5].

Many unicellular organisms such as Amoeba excrete their waste products by diffusion from their body surface as they are small in size. Multicellular organisms such as Hydra carry out water excretion by creating a break in its body wall which is a result of strong contraction when its gut is distended with fluid[6][7].


Excretory System in Man:

Our excretory system consists of kidneys, blood vessels that join them, ureters, urinary bladder and urethra. They help produce and excrete urine.

There are two bean-shaped kidneys that lie in the abdominal cavity, one on either side of the vertebral column. The kidneys are reddish brown. Each of them is about 10 cm long and weighs about 150 g. Although they weigh less, they receive a lot of blood for filtration.

A volume of blood nearly equivalent to that in the whole body passes through the kidneys every four or five minutes. The kidneys produce urine to filter out the waste products, like urea and uric acid, from the blood.

(A) Excretory Organs of Man (B) Internal Structure of a Kidney

Urine leaves each kidney through a tube called ureters. The ureters from both the kidneys are corrected to the urinary bladder that collects and stores urine. Ureters carry urine from the kidneys into the urinary bladder. The urethra is a canal that carries urine from the bladder and expels it outside the body[8].

Homeostasis in relation with Excretion

Homeostasis is the maintenance of a steady state within the body. Excretion is the removal of waste products and this system must function properly in order to maintain homeostasis. Every organism carries out some form of regulation, although it may somehow find a place to live where it can tolerate variation in the external environment.

Humans regulate their body temperature with both physiological and behavioural responses. This is a form of homeostasis that does not involve the hypothalamus sending signals for the body to internally keep the set point at a constant. Humans prevent heat loss for example by putting on more clothes or by taking the clothes off to keep the body cool.

Behavioral responses

Behavioural responses include the responses undertaken by individuals, as a form of external homeostasis to keep their body at a constant temperature. These include turning on appliances like the aircon or fan when it is hot to keep the external temperature cool. Or in fact turning on the heater during the cold weather to keep the external temperature warm, therefore reducing the amount of work the internal body is required to do in order to remain at the ser point. Behavioural responses are therefore everyday response, some of these responses are carried out without notice as it has become sub-conscious.

Physiological responses

There are two major physiological mechanisms in the human body, this includes the nervous system which is an integration of sensory and motor systems in an organism and hormones which are endocrine systems that affect sensitive tissues at various parts of the body. Hormones are used to trigger actions taken by the body, allowing the body responds to a particular change in the environmental situation in a certain way. The body uses specific responses to defend itself from diseases so that if the disease is to attach the body in future, there would be mechanisms in place to fight against the attaching disease. Pathogens are unwanted bacteria as the attach the body, the human body has a few different responses to different pathogens be it specific or nonspecific responses. The nonspecific responses are used to kill any form of bacteria that invade the human body, on the other hand, specific defences include defences that are specifically set up to destroy certain pathogens that invade the human body.

Nonspecific responses as mentioned before are set up to kill all intruding pathogens, these responses include sweat, saliva and many more. In the human body, there are two types of specific responses, these include the humoral response and the cell-mediated responses. The humoral response is located in the human blood which is carried out by B cells which create antibodies that are able to mark antigens destructions of pathogens that attach the body. Antigens are toxins that are able to induce responses in the immune system of the human body. They are used as stimulants to the body’s immune system which further allow the body to fight against intruding pathogens. The antibodies are the proteins in the human body that are able to recognize the specific different antigens, this marks the antigen that is to be destroyed. It is with the help of these antigens and antibodies that the body is unable to contract diseases like chicken pox twice in the lifetime of the individual.

The cell-mediated response are used to target infected cells in the body and fight against the viruses in these cells to keep the body free from invading pathogens. The body uses the Cytotoxic T cell which is a type of white blood cell to kill of all the cells that have been infected by a virus in the body[9].


  1. Britannicacom. 1. Encyclopedia Britannica. [Online]. Available from: https://www.britannica.com/science/excretion [Accessed 5 December 2016].
  2. excretion. biology. Encyclopedia Britannica. 2017 [cited 1 December 2017]. Available from: https://www.britannica.com/science/excretion
  3. Wikipediaorg. 2. Wikipediaorg. [Online]. Available from: https://en.wikipedia.org/wiki/Excretion [Accessed 5 December 2016]
  4. Excretory System - Excretion In Humans. Science.jrank.org. 2017 [cited 4 December 2017]. Available from: http://science.jrank.org/pages/2626/Excretory-System-Excretion-in-humans.html
  5. Carr K, Carr K. How do cells get rid of waste? Excretion and cell biology –Quatr.us. 2017 [cited 4 December 2017]. Available from: https://quatr.us/biology/cells-get-rid-waste-excretion-cell-biology.htm
  6. http://www.biologydiscussion.com/essay/excretion-in-animals-humans-and-plants-with-diagram/1570
  7. https://www.ncbi.nlm.nih.gov/pubmed/4682252
  8. http://www.biologydiscussion.com/essay/excretion-in-animals-humans-and-plants-with-diagram/1570
  9. https://homeostasisinhumans.weebly.com/excretion-during-homeostasis.html
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