Why is sweat composed of water and salt?

[fog37]

Hello Everyone,
First of all, my well wishes for everyone in the path of hurricane Florence. My reflection today is about the natural body perspiration mechanism. Please let me know if my understanding is mistaken or correct on this topic:

Our bodies shed thermal energy using four mechanisms: radiation, conduction, convection and perspiration.

Perspiration (evaporative cooling) is always working while the other three mechanisms do not if the ambient temperature is higher than the body's temperature.

During perspiration, I think the water in the sweat reaches the skin surface already warm/hot having absorbed heat internally from the body. Or does the water in sweat absorb the body heat once it makes it to the skin surface? The water in sweat then evaporates from liquid to gas. Heat is removed from the body and we feel cooler. I know water has a high heat capacity. Is it really the best fluid for perspiration? Alcohol would be better but our body does not contain alcohol.

Perspiration works when the ambient temperature is lower, equal or higher than the body's temperature. But it stop working when the ambient relative humidity RH is 100%. I guess it is possible for RH to be 100% even when it is cool outside. The worst situation for the body is when the ambient temperature is super high (higher than the body's temperature) and the RH=100%.

Is that correct?

Sweat is water+salt. Apparently, salt, which is a waste product, does not help perspiration since it increases the evaporation point slowing evaporation down.

Thanks!

 

[BillTre]

Alcohol would be better but our body does not contain alcohol.

Sometimes my body contains alcohol. When it does I would expect it to show up in sweat, but I don't know of any actual evidence for this.

During perspiration, I think the water in the sweat reaches the skin surface already warm/hot having absorbed heat internally from the body. Or does the water in sweat absorb the body heat once it makes it to the skin surface?

The sweat coming out of sweat glands will be at the temperature of the body where the sweat glands are located (body temps can vary based on location, core temperatures usually being highest).
As the sweat evaporates, heat is used for the waters transition from liquid to gas (heat of vaporization), which is large for water compared to lots of other chemicals. This will cause the temperature of the sweat to drop and heat in the skin will conduct more rapidly to those cooler areas due to the increased temperature gradient, removing more heat.
Sweat the drips off of you will not have this second effect.

I do not think that sweat is always happening. There are physiological controls for temperature regulation and this should be turned off when it would not be beneficial.

Additionally, heat can be moved around the body by the circulation of blood so that heat from higher temperature areas like the bodies core can be transported to lower temp areas like the skin or to areas where the body can dump it if needed. Or similar strategies can be used to conserve heat in colder conditions. For example, this can is done by control of blood vessels to peripheralize the blood flow when appropriate. I would think of this as somewhat like convection (moving heat by moving the heated matter, however it is not driven by density differences as is usually the case in non-biological systems, it is driven by biological controls).
Thus, when you are hot you might notice that the blood vessels on your arms are more prominent (due to more blood going through them). This is also why icing areas of the body can sometimes reduce swelling, and it is easier to poke blood vessels int he arm when it is warm.

The worst situation for the body is when the ambient temperature is super high (higher than the body's temperature) and the RH=100%.

Yes. Try living in the US SE in the summer without AC.
But worse would be a hot water bath.

Apparently, salt, which is a waste product, does not help perspiration since it increases the evaporation point slowing evaporation down.

Yes, you would need something like a kidney to remove the salt before perspiring it out.

 

[russ_watters]

Perspiration (evaporative cooling) is always working while the other three mechanisms do not if the ambient temperature is higher than the body's temperature.

Since radiation works at a distance, it could still come into play even with a high ambient air temperature. But that would be a minor factor.

During perspiration, I think the water in the sweat reaches the skin surface already warm/hot having absorbed heat internally from the body. Or does the water in sweat absorb the body heat once it makes it to the skin surface? The water in sweat then evaporates from liquid to gas.

It is already at body temperature when in the body -- when it evaporates, it absorbs more heat, right from our skin.

I know water has a high heat capacity. Is it really the best fluid for perspiration? Alcohol would be better...

Water's heat of vaporization is higher than that of alcohol.

Perspiration works when the ambient temperature is lower, equal or higher than the body's temperature. But it stop working when the ambient relative humidity RH is 100%. I guess it is possible for RH to be 100% even when it is cool outside. The worst situation for the body is when the ambient temperature is super high (higher than the body's temperature) and the RH=100%.

Is that correct?

Mostly, yes. The dew point is what tells you if sweating is possible. Any dew point below body temperature allows for sweat to evaporate. This includes when the RH is 100% and the air is cool. When sweat evaporates in that case, it is enabled by your body heating up the air to allow it to carry more moisture.

 

[jim mcnamara]

As a note to @russ_watters point, when temperatures are above your skin temperature, you may still radiate. But. You are the coldest object in the ambient environment. Since every object around you is warmer than you, they radiate heat and you are a good "target".

A general overview of the content of sweat content: https://www.ncbi.nlm.nih.gov/pubmed/18156662

 

[russ_watters]

As a note to @russ_watters point, when temperatures are above your skin temperature, you may still radiate. But. You are the coldest object in the ambient environment. Since every object around you is warmer than you, they radiate heat and you are a good "target".

I'm not certain we are saying the same thing, so I'll clarify:

When discussing body temperature, "the ambient temperature" is usually referring to the temperature of the air around you. But since air is largely transparent to infrared light - unless the humidity is high - your ability to lose (or gain) heat via radiation is not always connected to your ability to lose or gain heat via convection -- an "object around you" could be 140 million km away or even be nonexistent when it comes to radiation. Here's three common scenarios, one where it is the same and two where it isn't:

1. If you are indoors and there isn't a large infrared source like a fireplace or space heater, the "the ambient temperature" is the same for convection and radiation.
2. If you are outside on a sunny day, in the sun, your radiation heat transfer is dominated by the sun radiating toward you.
3. If you are outside at night on a clear day, you radiate significantly to space, where "the ambient temperature" is 5C. This is so significant that objects that produce little or no heat of their own end up colder than the air around them, cooling faster than the air does at night (they actually are what causes the air to cool).

What I'm not certain of is in case 2 if the transparency of the atmosphere allows you to radiate heat away in directions not facing the sun or if the blue sky is bright enough for a significant radiation of heat toward you.

 

[jim mcnamara]

2. True, I believe. Maybe for an additional reason, too. Pick a desert. Ancient paper on the surface temperature of the Earth surface in summer near Tucson AZ: 71°C This is higher than air temperature recorded anywhere. AFAIK. The ground radiates immense amounts of heat.
https://www.jstor.org/stable/2255549?seq=1#page_scan_tab_contents
[opinion]
The sunny side of large surface rocks do the same. I live in the desert and I've been roasted in rock-lined arroyos. I do not recommend it.
[/opinion]