The Human Kidney
Your kidneys are one of the main organs which help to maintain Homeostasis, keeping the conditions inside your body as constant as possible.
For example, you produce Urea in your liver when you break down excess Amino Acids. These come from protein in your food and from the breakdown of worn out body tissues. Urea is poisonous, but your kidneys filter it out of your blood and get rid of it in Urine.
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The Kidneys are also vital in the water balance of your body. If the concentration of your bodily fluids changes, water will move in or out of your cells by Osmosis. This can damage or destroy cells.
You gain water when you eat and drink. You lose water constantly when you breathe, from your lungs. Whenever you get hot and exercise, you lose water through sweat.
The kidneys balance all these changes by removing excess water and it leaves the body as urine. If you are in short supply of water, they conserve it.
If you're producing very little urine, then it means your kidneys are conserving it for your body, as you are not drinking enough. If you are producing a lot, you are drinking to much and a lot is needed to be removed.
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The ion concentration of your body is very important. You take in mineral ions with your food, the amount you take in varies depending on how much you eat.
If you eat processed foods, which are high in salt, you take in a big load of minerals. Some are lost through your skin in sweat. Again your kidneys are most important in keeping a mineral ion balance. They remove excess mineral ions (particularly salt) which are lost in the urine
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Your kidneys filter your blood and then re absorb everything your body needs. So sugar (glucose), amino acids, mineral salts and urea all move out of your blood into the Kidney Tubules by diffusion along a concentration gradient. The blood cells are left behind - they are too big to pass through the membrane of the tubule
Then, all of the sugar is reabsorbed back into the blood by Active Transport. But the amount of water and the dissolved mineral ions which are reabsorbed varies, depending on what is needed by the body. This is known as Selective Reabsortion
Both sugar and dissolved mineral ions move back into the blood by active transport and diffusion. This makes sure that no sugar is left behind in the urine and the right quantity of dissolved mineral ions is reabsorbed
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The amount of water reabsorbed depends on whats needed by your body. Its controlled by a very sensitive feedback mechanism. Urea is lost in your urine. However, some of it leaves the Kidney Tubules and moves back into the blood by diffusion along a concentration gradient.
Your kidneys have a very large blood supply, and they produce Urine all the time. It trickles into your bladder, where its stored until the bladder is full and you choose to empty it
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Your urine contains the waste urea along with excess mineral ions and water not needed by the body. The exact quantities depend on what you have taken in and given out.
For example on a hot day where you have done lots of exercise do not drink a lot, you will produce a small amount of concentrated, dark yellow urine. But on a cool day where you drink lots and do very little, you will produce a lot of colourless, dilute urine.