Describe how humans maintain homeostasis with respect to water and ions when...

The kidneys regulate the amount of water in the body. A special gland in the brain called the hypothalamus monitors water levels. If there is too little water in the body, the hypothalamus sends a single to the pituitary gland to communicate the need to conserve water to the kidneys. This is achieved by the secretion of a chemical called antidiuretic hormone. Once the kidneys receive this message, they reabsorb more water and concentrate the urine. At the same time, we are prompted to drink by the sensation of thirst, which is signalled by the brain. Alternatively, if there is too much water in the body, the hypothalamus instructs the kidneys - via the pituitary gland - to dilute the urine and get rid of the excess. Coffee acts as a diuretic, which means it expels fluids from the body. Diuretic drugs work to release water in a variety of ways. Some inhibit the kidney’s ability to reabsorb sodium so that the body must rid itself of the sodium and the water goes with it in the urine.

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Specialized nerve receptors, called osmoreceptors, located in the hypothalamus detect changes in osmotic pressure.When you decrease water intake or increase water loss—by sweating, for example—blood solutes become more concentrated. This increases the blood’s osmotic pressure. Consequently, water moves into the bloodstream, causing the cells of the hypothalamus to shrink (Figure Below). When this happens, a nerve message is sent to the pituitary, signalling the release of ADH, which is carried by the bloodstream to the kidneys. By reabsorbing more water, the kidneys produce a more concentrated urine, preventing the osmotic pressure of the body fluids from increasing any further.

The shrinking of the cells of the hypothalamus also initiates a behavioural response: the sensation of thirst. If more water is taken in, it is absorbed by the blood and the concentration of solutes in the blood decreases. The greater the volume of water consumed, the lower the osmotic pressure of the blood. As the blood becomes more dilute, fluids move from the blood into the hypothalamus. The cells of the hypothalamus swell, and nerve messages to the pituitary stop. Less ADH is released, and less water is reabsorbed from the nephrons.

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Coffee contains the xanthine, caffeine. Caffeine inhibits reabsorption of Na+ and increases the glomerular filtration rate. Thus, it has an effect on the tubules of the nephron leading to urine formation.

Caffeine inhibits the reabsorption of sodium, forcing it to be excreted. This directly causes the water to follow the sodium through the kidneys and to be excreted.

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