Consider the following scenario:

A 19-year-old man works as a traffic controller, holding a stop/slow sign to direct traffic while his team repairs a road lane. The weather is hot and humid with a temperature high of 38°C. In the morning, he feels hot and sweaty but is well enough to direct traffic. However, later in the day he has a headache and feels disoriented. His fellow workers berate him for letting traffic through which disrupts their work. A few minutes later, he collapses. Everyone stops what they were doing and comes to see if he is OK. What happened and what should everyone do next? 

The human body works at a strict temperature range of around 37°C (98.6°F) to ensure that the body can function properly. When the body gets too hot or cold, the body activates various mechanisms to return body temperature to normal. In the anime episode, when these mechanisms fail, the body temperature can continue to increase, making it a struggle for the cells and human body to function properly. How does your body keep itself cool while working under hot, humid conditions? This blog post will explain how the body regulates its temperature and what happens when such mechanisms break down.

An introduction to homeostasis

For cells inside the human body to optimally function, the body maintains a consistent internal environment where variables such as temperature are regulated to a set value (also known as the set point). This describes the concept of homeostasis: the maintenance of a stable internal environment within a narrow optimum range despite changes in the internal and external environments.

In most cases, homeostasis is maintained by negative feedback loops. These loops involve the body responding to a stimulus (a change in the internal or external environment) to bring a variable such as temperature back to its set point. For example, if the body is too hot, it will try to cool itself down to reduce the body temperature back to normal. There are three components to negative feedback loops:

• Sensors which sense internal or external stimuli and relay information to the control centre;
• Control centres which compare the stimulus to the set point of a variable and send responses to effectors (if required); and
• Effectors which receive messages from the control centre to appropriately respond to the stimulus.

The body’s normal response to high temperatures

The human body maintains a consistent temperature of 37°C (98.6°F) by controlling both heat production from metabolism (chemical reactions that produce heat and energy) and heat loss coming out of the body. Information on body temperature is collected from thermoreceptors (temperature sensors) located around the human body, particularly the skin. This information is sent to the hypothalamus inside the brain which acts as the control centre of temperature regulation. The hypothalamus compares information collected from thermoreceptors to the temperature set point and sends messages to adjust heat production and heat loss to adjust body temperature if necessary.

If the human body becomes too hot such as working outside during a hot day, the body will attempt to cool itself down by reducing heat production and increasing heat loss. To reduce heat production, the hypothalamus will diminish the release of hormones that promote the secretion of the hormone thyroxine from the thyroid gland (located on the neck). By diminishing the secretion of thyroxine, metabolism around the body decreases.  This lowers heat production in the body, but it also reduces the amount of energy produced around the body. This is why you become tired and work less efficiently during a hot day.

The hypothalamus will also send messages around the body to increase heat loss, resulting in the following responses:

• Smooth muscle cells in the arterioles will relax, dilating the blood vessels. This increases blood flow to the skin. As blood carries heat around the body, increased blood flow to the skin will send heat to be dissipated from the skin, increasing heat loss.
• Sweat glands, which store sweat consisting of water and salts, are also activated. Consequently, sweat glands secrete sweat on the surface of the skin. Heat from the skin evaporates the sweat to a gas which takes the heat away, cooling the body.

In addition, the person may also change behaviour to cool their body down. This may include wearing cool, thin clothing; sitting or working in the shade and eating cold foods and drinks.

Did you know? During humid days, sweating does not work well in cooling the human body. That is because under humid conditions, the air is already saturated with water. This makes it more difficult for the sweat to evaporate, disabling the body’s cooling system.

Heat stress

During a hot day, the body will activate various mechanisms to cool the body down. However, when these mechanisms break down (particularly when sweat runs out as shown in the episode), the body continues to heat up, resulting in heat stress. Heat stress is when the body cannot cool itself down, resulting in the body continuing to heat up. This can affect normal body function which can produce various symptoms. There are three levels of heat stress:

• Heat cramps: low salt levels in the muscles due to sweating produces muscle pains and spasms, usually in the abdomen, arms and legs. This can emerge while the person is doing strenuous activity or exercise during a hot day without replenishing lost fluids and salts.
• Heat exhaustion: excessive sweating during a hot day depletes the body of fluids and salts, resulting in dehydration. This not only results in muscle cramps (as seen in heat cramps) but the person may also exhibit pale, sweaty skin; a rapid, weak pulse and fast, shallow breathing. Dilation of the blood vessels also has the side-effect of reducing blood flow to the brain. As a result, the person may also be tired and dizzy and they may also faint.  
• Heat stroke: a life-threatening condition, this happens when the body temperature rises above 40.5°C. As a result, organs such as the brain are damaged and start to shut down. This not only results in the person having red hot skin with no sweat but it also leads to overt symptoms such as headache, dizziness, nausea, confusion and unconsciousness.

How should people respond to heat stress?

At the beginning of the blog post, I described a typical scenario of a person working during a hot day. Here, we see that the person can cool themselves off by activating mechanisms to increase heat loss. However, as time progresses, these mechanisms can break down if fluids are not being replenished. As a result, the person suffers from heat exhaustion, where the person has a headache and feels disoriented and confused. This also leads to fainting which prompts his fellow workers to do something.

In this case, the workers should lie the traffic controller in a recovery position in a cool or shady area. They should remove excessive clothing from the person and loosen any remaining clothing. Following this, they should cool the person down by spraying cold water to moisten the skin and applying cool packs on the skin. As soon as the traffic controller regains consciousness, the person should replenish their fluids by drinking water and rest until they are okay to stand up and walk around.

Following the ordeal, his fellow workers check up on him to see if he is okay, apologising for not noticing what actually happened earlier when he was struggling to direct traffic. Someone else offers to take over his work. The person is thankful for him as he can go home early to recover for the next day.

Conclusion

In this blog post, we looked at homeostasis and how it can be applied to explain how the body cools itself during a hot day. The body has various mechanisms that it uses to cool the body by reducing heat production and increasing heat loss to decrease body temperature back to normal. However, when these mechanisms break down, heat stress can result which affects body function and produces various symptoms. If not treated, this can lead to heat stroke which is a life-threatening condition. Thankfully, if treated right away, the person can fully recover from heat stress.

We looked at the various ways that the body cools itself down during a hot day. What we did not look at; though, is how important fluid and salt balance is and what happens when lost fluids and salts are not replenished. In the next blog post, we will look at fluid loss as a result of sweating and see how they can be replenished. See you there!