In the most simple terms, excretion is the process by which the body eliminates waste. We know based on the previous post on respiration, that carbon dioxide is released from the lungs on expiration. However carbon dioxide is not the only waste product that needs to be eliminated from the body. The unwanted remnants of food post digestion: nitrogen from protein, sodium from salty foods that we eat etc., and remnants of metabolic activities of the body are excreted via the kidneys and other organs, which together make up the urinary system. The urinary system consists of the kidneys, ureters, bladder and urethra.
Very closely related to excretion is the kidney’s role in water regulation of the body. The average person, in their lifetime drinks, what is equivalent to a tanker truck of water in their lifetime. Unless our bodies lost the equivalent of what we drank many functions of the body that depend on a delicate balance between water and electrolytes would be upset; metabolic reactions of the body would also be affected. For example your blood pressure is determined by the difference between the amount of fluid you take in and what you excrete–your body constantly varies the amount of water excreted as urine to aid in keeping the blood pressure constant. This concept is related to the essential process of homeostasis; keeping the systems of the bodies in balance so that all the processes can keep functioning as they should.
It is important to note that faeces is a waste product that is NOT excreted from the body; it is expelled. For a waste product to be considered excreted from the body it has to be a bi-product of metabolism. The food we eat and all the remnants of chemical reactions of the body are bi-products of metabolism.
If you have any questions or concerns comment below! The next topic in the Characteristics of Organisms series will be Growth and Irritability.