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Learn the function of excretion

Learn the difference between excretion and elimination

Main idea

All day long, every cell in our body performs thousands of chemical reactions. Together, these are our metabolism.  Some of these reactions produce waste molecules which need to be removed. The process of removing these is called excretion.

Excretion is not elimination

Elimination = getting rid of unused material from your digestive system. Waste comes out of the anus as feces (fecal matter.)

Excretion = removing metabolic waste chemicals. Removed through lungs, skin and kidneys.

Excretory organs

Lungs excrete CO2 gas

Skin excretes urea

kidneys filter out urea, which is then stored in the bladder as urine. When appropriate, the urine is excreted through your urethral orifice (“peeing”.)

Major organs excretion


Homeostasis and kidneys

Organisms need to control things like body temperature, water level, salt level, and sugar level. None of their levels should get too high or too low, otherwise the organism will get sick or die. Keeping levels within a safe zone is called homeostasis – the maintenance of a constant internal environment.

(a) maintain blood pH (acid/base levels)

(b) regulate the water content of the blood.

Let’s say that it is hot and you haven’t been drinking much water. So you shouldn’t lose water. In this case, chemical signals are sent to your kidneys to reabsorb as much water as possible. Thus you get less urine creation, and your body loses less water.

On a normal day you make about 1.5 liters of fluid.

(c) regulates glucose levels

If blood glucose levels rise well above normal, then kidneys excrete glucose into the urine.

Failure of homeostasis = disease

Disruption of kidney function leads to health issues:

  • Kidney stones occur when minerals or uric acid salts crystallize, and obstruct a ureter.

  • Kidney damage may be caused by high blood pressure or diabetes.

  • When a patient’s kidneys can no longer maintain homeostasis, the patient is said to be in kidney failure.


Worksheet Excretion 2 page worksheet

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