Sweating. It’s often seen as an unpleasant side effect of pushing yourself in the gym, or getting nervous on a first date, but this often undervalued bodily function is essential for our species to survive and perform in hot environments.
But how does it actually happen?
Well, depending on how big you are, you’ll have anywhere between 1.5 and 5 million sweat glands on your body, so it’s no wonder your shirt is soaked after a heavy gym session! However, not all sweat glands are the same; there are actually two main types: eccrine and apocrine. Your eccrine glands are the ‘classic’ sweat glands that produce most of the sweat you can see when you train. They are tiny, numerous and are found almost everywhere on the body, although they are most concentrated on the palms, soles and face.
Apocrine glands, on the other hand, secrete a slightly different type of sweat into a hair follicle which then makes its way up to the skin’s surface. Because of this, apocrine glands are bigger and are only found in your armpits and a few other places where hair grows. They secrete a thicker solution that is largely to blame for the distinctive odor associated with sweating. Sweat itself doesn’t smell at all, however it begins to when bacteria on the skin and hair begin to break it down (yeah, gross...). Males tend to have significantly more apocrine sweat glands than females, which perhaps lends some credibility to the stereotype that men smell more than women!
Eccrine glands are the ones involved in actually cooling us down, so these are the ones we’ll focus on for now. You can have as many as 700 of these per square centimetre of skin, although in some places there may only be around 20 per square centimetre.
Although eccrine glands always respond to stimulation in the same way, the way in which they are stimulated or ‘turned on’ actually varies quite a lot.
What gets these glands going?
As you might expect, the main cause of sweating is an increase in body temperature (mainly in the inner core, but also on the skin). Neurons sensitive to heat in the hypothalamus section of the brain detect temperature changes and stimulate the release of a neurotransmitter called acetylcholine from neurons local to the sweat glands. This causes a complex ‘cascade reaction’ that results in the secretion of sweat, known as ‘hypothalamic’ sweating.
Another cause of sweating is emotional stress, something which most of us have experienced at some point in our lives, think before a job interview or when speaking in public. When we get nervous, our hands and feet in particular can become slick with sweat. This is known as ‘cortical’ sweating and may be an evolutionary mechanism that helped our ancestors maintain their grip while running and climbing to get out of harm's way. Although no doubt helpful back then, nowadays this can be very uncomfortable and embarrassing when greeting your potential new boss with a clammy handshake.
The third and final type of sweating is known as ‘medullary’ sweating and will be familiar to anyone who, like myself, starts sweating at the mere thought of a hot curry. When we eat spicy food the compound capsaicin that is found in chilli peppers binds to sensors in the mouth and causes us to start sweating from the face.
Humans are actually the exceptions in the animal kingdom with regards to how we sweat. Most other animals have more apocrine glands than eccrine glands as more of their bodies are covered in hair. In horses and marsupials these apocrine glands function in a similar way to human eccrine glands, i.e. as a cooling mechanism. Other animals such as dogs only have a few eccrine glands on their paws and therefore must rely on other forms of cooling, such as panting.
It’s thought that our ability to regulate our body temperature through efficient sweating has contributed a great deal to our success as a species. The ability to easily regulate our core temperature gave us an advantage when hunting in the wild as our prey would tend to overheat far quicker than we would, and therefore be relatively easy for us to catch. That’s something to bear in mind next time the sweat is dripping into your eyes as you chase down the person in front of you in a race…
Russell Davies is an undergraduate in Sport & Exercise Science at the University of Bath and is currently on placement with our friends at Porsche Human Performance, Silverstone.