Question - dehydration

Why does a human drinking sea water lead to dehydration?

Is it because sea water has a very high salt conc. (so low water potential), so when this reaches the collecting duct, it is still very conc. so the water potential gradient is less steep, so less water can be reabsorbed form collecting duct into blood capillaries, so large volumes of watery urine are produced?

Reply 1

GermanScientist

9

Original post by ILoveUSA

Why does a human drinking sea water lead to dehydration?

Is it because sea water has a very high salt conc. (so low water potential), so when this reaches the collecting duct, it is still very conc. so the water potential gradient is less steep, so less water can be reabsorbed form collecting duct into blood capillaries, so large volumes of watery urine are produced?

Yes I think it's because the high sodium level means that less water is reabsorbed into the body and instead passes out in the urine.

A fun fact our lecturer told us was that if you are stuck at sea and thirsty, don't drink sea water - drink fish blood :tongue:

Reply 2

prettytoes:)

Have an enema if you prefer, i know I would :wink:

Reply 3

FiniteMr

13

Original post by ILoveUSA

Why does a human drinking sea water lead to dehydration?

Is it because sea water has a very high salt conc. (so low water potential), so when this reaches the collecting duct, it is still very conc. so the water potential gradient is less steep, so less water can be reabsorbed form collecting duct into blood capillaries, so large volumes of watery urine are produced?

It's not quite what you've put here. You can't consider the ingested water as passing through the nephron-it passes into the blood and then alters the way the blood is filtered. Here is my explanation:

Sea water is hypertonic - this means its ion concentration is higher than the average ion concentration of the body's fluids. (The ion concentration being mainly made up by sodium ions-salt of course) When you drink it, it therefore raises the osmolality of your body. (I don't know what level you are at - osmolality is a measure of the ion concentration in the body's extracellular fluid, mainly the blood). This is detected by the brain (the hypothalamus) and causes release of ADH, the main trigger for producing a concentrated urine. This increases the proportion of solute (salt here) in the urine but decreases the amount of urine produced, so that you can eventually decrease osmolality-but slowly There is a maximum concentration of solute possible in urine, and it can be demonstrated that for sea water, the minimum amount of fluid that needs to be excreted in the urine to flush the salt content of 1L of sea water is greater than 1L. This means that to get rid of the salt in the water you drank, you need to excrete a higher volume than you actually drank, resulting in a net loss.

In addition, the body doesn't react well to hypertonic sea water. It is hypertonic, which causes water to be drawn into the gastointestinal tract (water follows salt). That will cause vomiting and diarrhoea, leading to further fluid loss and dehydration

(Here is a bit more detail on the calculation - sea water is about 1000mOsm and urine is at most 1200mOsm, with 600mOsm made up of the urea used to create concentrated urine. So maximum 600mOsm can be expelled per litre of urine. Since 1L of sea water has 1000mOsm, that leaves 400mOsm still to be removed from the body after 1L of maximally concentrated urine is formed, such that an extra 2/3 of a L is needed to remove the rest. So, by taking in 1L of sea water, you lose 1 2/3 L of water in the urine, hence a net loss of 2/3 of a L of urine is lost.)

A TLDR; the salt taken into the body needs to be removed. The volume of water lost in the urine as you remove the salt from the body is greater than the volume of water ingested, causing a net loss and dehydration.

Source: I am obsessed with renal physiology.

Question - dehydration

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