Kidneys control how much water leaves in urine

1. Osmoregulation keeps the water and dissolved ion content of the blood within narrow limits.
2. Water enters the body in drinks, food and metabolic reactions such as aerobic respiration.
3. Water leaves in urine, sweat, faeces and exhaled air.
4. The kidneys provide adjustable water loss because they can change both the volume and concentration of urine.
5. Sweating and water vapour in exhaled air also remove water, but these losses cannot be adjusted precisely to maintain water balance.

Blood is filtered and useful substances are recovered

1. Blood enters each kidney through the renal artery and leaves through the renal vein.
2. Small molecules are filtered from the blood into kidney tubules, including water, glucose, ions and urea.
3. Blood cells and large plasma proteins remain in the blood because they are too large to pass through the filtration barrier.
4. Useful substances are reabsorbed from the tubules into the blood.
5. All glucose is normally reabsorbed, together with the ions and water needed by the body.
6. Urea, excess ions and excess water remain in the tubule fluid and form urine.

Water shortage produces concentrated urine

1. When the body is short of water, the kidneys reabsorb a larger proportion of the water from the tubule fluid.
2. Less water remains to carry the dissolved urea and ions out of the body.
3. The resulting urine has a small volume and a high concentration.
4. This response reduces water loss but cannot prevent some water from being excreted because waste substances must still be removed.
5. Dark urine can indicate concentration, although urine colour alone is not an exact measurement of hydration.

Excess water produces dilute urine

1. After a large water intake, the kidneys reabsorb a smaller proportion of the water from the tubule fluid.
2. More water remains in the tubules and is excreted.
3. The resulting urine has a large volume and a low concentration.
4. Removing the excess water returns the water content of the blood towards its normal range.
5. The concentration of urea in the urine changes with water content even if the amount of urea produced has not changed.

Urine data reveal kidney responses

1. A water-conserving response is shown by lower urine volume and higher urine concentration.
2. A water-removing response is shown by higher urine volume and lower urine concentration.
3. When comparing results, use the same time interval because a volume collected over two hours cannot be compared directly with a volume collected over one hour.
4. If urine production rate is required, calculate $urine\; production\; rate=\frac{urine\; volume}{time}$.