SWEAT

Perspiration or sweat is your body's way of cooling itself, whether that extra heat comes from hardworking muscles or from over stimulated nerves.

The average person has 2.6 million sweat glands Sweat glands are distributed over your entire body, except for your lips, nipples and external genital organs. Your sweat gland is in the layer of skin called the dermis.

The sweat gland is a long, coiled, hollow tube of cells. The coiled part in the dermis is where sweat is produced, and the long portion is a duct that connects the gland to the opening or pore on your skin’s outer surface. Nerve cells from the sympathetic nervous system connect to the sweat glands.

There are two types of sweat glands:

• Eccrine - the most numerous type that are found all over the body, particularly on the palms of the hands, soles of your feet and forehead
• Apocrine - mostly confined to your armpits and your anal-genital area. They typically end in hair follicles rather than pores.

These two glands differ in size, the age that they become active and the composition of the sweat that they make. Compared to apocrine glands, eccrine glands are smaller, are active from birth (Apocrine glands become active only at puberty) and produce a sweat that is free of proteins and fatty acids

We are constantly sweating, even though we may not notice it. Sweating is your body's major way of getting rid of excess body heat, which is produced by metabolism or working muscles. The amount of sweat produced depends upon your state of emotion and physical activity. Sweat can be made in response to nerve stimulation, hot air temperature, and/or exercise.

When your sweat gland is stimulated, the cells secrete a fluid, that is similar to plasma, that is, it is mostly water and it has high concentrations of sodium and chloride and a low concentration of potassium -- but without the proteins and fatty acids that are normally found in plasma. The source of this fluid is the spaces between your cells (which get the fluid from the blood vessels (capillaries) in the dermis. This fluid travels from the coiled portion and up through the straight duct.   What happens in the straight duct depends upon the rate of sweat production or flow:

Sweat is produced in apocrine sweat glands in the same way. However, the sweat from apocrine glands also contains proteins and fatty acids, which make it thicker and give it a milkier or yellowish color. This is why underarm stains in clothing appear yellowish. Sweat itself has no odour, but when bacteria on the skin and hair metabolize the proteins and fatty acids, they produce an unpleasant odour. This is why deodorants and anti-perspirants are applied to the underarms instead of the whole body.

The maximum volume of sweat that a person who is not adapted to a hot climate can produce is about one litre per hour.  If you move to a hot climate, your ability to produce sweat will increase to about two to three litres per hour within about six weeks! This appears to be the maximum amount that you can produce.

When sweat evaporates from the surface of your skin, it removes excess heat and cools you. This is actually due to a neat principle in physics, which goes like this. To convert water from a liquid to a vapour, it takes a certain amount of heat called the heat of vaporization. This heat energy increases the speed of the water molecules so that they can escape into the air. Typically, all of the sweat does not evaporate, but rather runs off your skin. In addition, not all heat energy produced by the body is lost through sweat. Some is directly radiated from the skin to the air and some is lost through respiratory surfaces of your lungs.

A major factor that influences the rate of evaporation is the relative humidity of the air around you. If the air is humid, then it already has water vapour in it, probably near saturation, and cannot take any more. Therefore, sweat does not evaporate and cool your body as efficiently as when the air is dry.

When the water in the sweat evaporates, it leaves the salts (sodium, chloride and potassium) behind on your skin, which is why your skin tastes salty. The loss of excessive amounts of salt and water from your body can quickly dehydrate you, which can lead to circulatory problems, kidney failure and heat stroke. So, it is important to drink plenty of fluids when you exercise or are outside in high temperatures. Sports drinks contain some salts to replace those lost in the sweat.

Also sweating responds to your emotional state. So when you are nervous, anxious or afraid, there is an increase in sympathetic nerve activity in your body as well as an increase in epinephrine secretion from your adrenal gland. These substances act on your sweat glands, particularly those on your palms of your hand and your armpits, to make sweat. Thus, you feel a "cold" sweat.

The increased sympathetic nerve activity in your skin also changes its electrical resistance, which is the basis of the galvanic skin response used in lie detector tests.

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