Our bodies take in water from food and drinks. We even get some water when we respire by burning glucose to release energy. We lose water in sweat, faeces, urine and when we breathe out, on a cold day you can see this water as it condenses into vapour.
For the cells of our body to work properly, it is important that their water content is maintained at the correct level. This means our body must maintain a balance between the water we take in and the water we lose. This is done by the kidneys. Blood is brought to the kidneys to be filtered, and then returned, to be circulated around the body. As the blood passes through the kidneys, all the small molecules are filtered out of the blood.
This includes molecules of water, salt, glucose and urea. The kidneys then reabsorb all of the glucose and as much water and salt as the body needs, putting them back into the blood. This leaves some water and salt, and all of the urea, which is now called urine. The urine passes from the kidneys to the bladder, where it is stored prior to being excreted from the body.
The kidneys do more than just control the body water balance. They also control the level salts in the blood and the excretion of urea and other metabolic waste. The kidneys maintain our water balance by producing urine of different concentrations. When the water level of our blood plasma is low, more water is reabsorbed back into the blood and the urine becomes more concentrated. When the water level of our blood plasma is high, less water is reabsorbed back into the blood and our urine is more dilute.
The level of water in the blood plasma can vary depending on:
* External temperature - when it is hot, we sweat more and lose water, thereby making the blood plasma more concentrated.
* Amount of exercise - if we exercise, we get hot and increase our sweating, so we lose more water and the blood plasma becomes more concentrated.
* Fluid intake - the more we drink, the more we dilute the blood plasma. The kidneys respond by producing more dilute urine to get rid of the excess water.
* Salt intake - salt makes the plasma more concentrated. This makes us thirsty, and we drink more water until the excess salt has been excreted by the kidneys.
Drugs that affect water balance are Alcohol which causes the kidneys to produce a greater volume of more dilute urine that can lead to dehydration and Ecstasy causes the kidneys to produce a smaller volume of less dilute urine. This can result in the body having too much water.
The concentration of our urine is controlled by a hormone called ADH. ADH is produced by the pituitary gland that is situated just below the brain. The pituitary gland monitors the concentration of the blood plasma. It releases ADH into the bloodstream, which travels in the blood to the kidneys. The more concentrated the plasma, the more ADH is released into the blood. When the ADH reaches the kidneys, it causes them to reabsorb more water. This keeps more water in the body and produces more concentrated urine.