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Elimination versus Excretion
( Adapted from  http://www.biology4kids.com/files/systems_excretory.html )

Your digestive system is for getting food into your body, digesting it, absorbing nutrients you need, and elimination of the materials you don’t need (feces).

Once you put food in your mouth, you begin mechanical digestion: grinding food down into a pulp. Your body releases enzymes that start chemical digestion – the breakdown of biological molecules. Some of this happens in the stomach.

The food moves into the small intestine, where the larger organic molecules are broken down into smaller ones. These small compounds and nutrients are then absorbed into the blood stream. Whatever you don’t absorb continues into the large intestine. Here, water is removed (re-absorbed) from the material. Whatever is left is feces, and can be eliminated (defecation) at your convenience.

Excretion Is Not Elimination:
Excretion is removing toxic chemicals, thru your lungs, skin and kidneys.

Skin releases salts, urea.

“Peeing” is excretion of urine through your urethral orifice: What is urine & where does it come from?

Your body processes food protein into human proteins, but there is a leftover waste molecules, called urea. Urea floats out of your cells, into your blood. Your kidneys filter urea out of the blood, and send it to your bladder.

Also, throughout the day, you need to balance the amount of water and salts in your body. Your kidneys are the core organs involved in the excretory system. Related body parts include the ureters, bladder, and urethra.

{ http://www.mixbag777.com/2014/05/human-urinary-system.html }

Kidneys act as a filter. Eventually all of the blood in your body passes through the kidneys. They filter harmful molecules out of your blood and leave the ones that are good for you.


Kidneys are key players in hydration (water) levels. Let’s say you are in the desert and you haven’t been drinking much water. Chemical signals are sent to your kidneys to reabsorb as much water as possible. The result is less urine creation, so your body loses less water. A normal day has you creating about one and a half liters of fluid.

The Kidneys and Homeostasis
The kidneys remove urea, maintain blood pH (acid/base levels), and regulate the water content of the blood.

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

Disruption of kidney function leads to health issues such as kidney stones, kidney damage, and kidney failure.

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


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