Consider this scenario:

You are competing in the Comrades Marathon, the largest and oldest ultramarathon in the world that is held in South Africa. It involves running 90km from Pietermaritzburg to Durban, running up and down five hills along the way. You start running at 5:30AM and you have 12 hours to finish the course.

Even though the weather is cool with a top of 16°C (60.8°F), you still feel sweaty as you run the course. What would you need to consider when you are running in the Comrades Marathon?

Strenuous activity such as running produces sweat that cools the body down. However, when sweating stops functioning, the person suffers from heat stress where the body continues to heat up. This leads to symptoms such as dizziness, obscured vision and fainting as the body struggles to function normally. Over two separate episodes, heat stress was resolved in two different ways. In episode 11, the cells are saved by an injection of intravenous fluid into the body. However, in a special episode aired exclusively in Japan, the cells are saved by Pocari Sweat, a Japanese sports drink.

How does taking in such fluids treat heat stress? In this blog post, we will look at how the body normally scavenges fluids and salts from urine to keep sweating and how taking in fluids helps or hinders this process.

Summarising the role of sweat

As the body heats up above the temperature set point, the hypothalamus initiates various mechanisms to cool the body down. One of these mechanisms involves sweat glands which are activated to release sweat. Sweat consists of water and salts that are released on the surface of the skin. Heat in the skin delivered by dilated blood vessels converts the sweat to a gas which takes away the heat, cooling the body.

However, without taking in fluids, the body will eventually run out of sweat. When it does, the body cannot effectively cool itself down. As a result, the body continues to warm up, causing heat stress.

The body’s normal response to retaining fluid

Sweat is replenished by taking out water from plasma in the blood. This reduces blood volume, resulting in a decrease in blood pressure (as less fluid pushes onto the blood vessel) and an increase in blood osmolarity (salt concentration). The body responds to these stimuli in various ways.

The kidney, sensing a reduction in blood pressure, secretes renin. Renin is an enzyme that cleaves angiotensinogen in the blood to angiotensin I. In turn, angiotensin I is converted by angiotensin-converting enzyme (ACE) to angiotensin II. Angiotensin II acts on the adrenal glands (located on top of the kidneys) to secrete aldosterone. Aldosterone works in the kidneys to increase sodium and chloride reabsorption from urine. This draws in water from the urine to compensate for the increased salt concentration in the kidney.

In addition, osmoreceptors in the hypothalamus sense an increase in blood osmolarity. This results in the release of ADH (antidiuretic hormone, also known as vasopressin) which acts on the kidneys to increase water reabsorption from urine. As a result of the actions of both aldosterone and ADH, less water and salt are lost from urine, producing a more concentrated, yellow urine. In the process, water and salts are taken from urine to the body which restores blood volume, allowing the body to keep sweating.

Did you know? Drinking alcohol or caffeine inhibits the release of ADH. As a result, more water is lost through urine which is why you may feel dehydrated during a hangover.

How does external fluids influence the fluid retention process?

Thirst is the state of needing to drink fluids to restore blood volume and osmolarity back to normal. It is driven by the thirst centre of the hypothalamus which is activated by various stimuli including osmoreceptors in the hypothalamus, a dry mouth and angiotensin II. Thirst directs the person to drink more fluids such as water to restore fluid balance.

The type of fluid consumed to satisfy thirst will impact on how somebody performs during strenuous activity and exercise. During exercise when both water and salts are lost by sweating, drinking plain water only does not fully restore fluids. This is because plain water increases blood volume but reduces blood osmolarity too much which would reverse responses to retain fluids and salts in the body. While the thirst disappears, water and salts stop being reabsorbed in the kidney, resulting in them being lost in urine. As salts are not recovered from urine, they are instead drawn out from tissues such as muscles, resulting in impaired function and muscle cramps which are a sign of heat cramps.

External fluids contain salts and substances that are suspended in water. These fluids act to mimic plasma, providing both water and salts to the body. Even though blood volume still goes back to normal, blood osmolarity does not change, allowing responses to retain fluid to be maintained. Hence, these fluids can compensate for the water and salts lost by urine and sweat. External fluids can be administered in two ways:

• Intravenous fluids: as shown in episode 11, the cells are saved when a needle is injected into the body and fluids emerge from it, nourishing the cells. The fluid injected is usually saline which consists of water and salts (usually sodium and chloride) to replace those lost during sweating. Dextrose (a form of glucose) may also be added to provide an energy source while the person recovers from heat stress.
• Sports drinks: in the side episode, the cells are saved when bottles of Pocari Sweat arrive to dispense the sports drink. Although there are various sports drinks available on the market (including Gatorade and Powerade), all sports drinks consist of water, salts and sugars. Water and salts in the drink act to replenish water and salts lost from sweat and urine without compromising blood osmolarity, allowing sweating and fluid retention to continue. Sugars are also present to sustain the active muscles and brain as they produce energy to keep the body moving during exercise.

How should you prepare for the Comrades Marathon?

Given that you are running an ultramarathon which will take almost all day to complete, it is important to keep your fluids up while running. At the same time, the muscles need a continual supply of sugar so that they can keep functioning for the whole 90km. Hence, you would be drinking sports drinks quite often which would continually replenish water and salts lost from sweat, allowing you to keep running. Sugars can also be delivered from the sports drink to the brain and muscles so that they can keep producing energy, allowing you to keep running.

In addition, you may also consume food from rest stations in the ultramarathon such as bananas, potato chips and energy bars. This would provide you with an additional source of energy to keep running while slowing down digestion. By slowing down digestion, fluids, salts and sugars can be released more slowly to sustain running over longer distances during an ultramarathon.

Conclusion

Over the last two blog posts, we looked at how the body responds to high body temperatures. In the previous blog post, we looked at responses such as dilating blood vessels and sweating that allowed the body to cool down and what happens when these mechanisms break down. In this blog post, we investigated how fluids are retained from the body to reduce fluid loss and how drinking different fluids helps or hinders this process. Collectively, these responses allow normal fluid balance to be maintained while allowing the body to keep sweating to cool down, preventing the body from suffering heat stress. This is important not only for surviving hot days but also while doing strenuous activity or exercise as is seen in the scenarios.

In the next blog post, we will look at how the body responds to massive blood loss during traumatic injury. See you there!