Your Hear Works Harder When You're Hot
Ever notice that an easy run can suddenly feel difficult on a hot summer day?
You might be running the same pace, on the same route, with the same fitness level, yet your heart rate keeps climbing and the workout feels harder and harder.
That is not your imagination.
When you exercise in the heat, your cardiovascular system has to do two jobs at once: deliver blood to your working muscles and help your body get rid of heat.¹
The result is a phenomenon called cardiovascular drift, where your heart rate gradually rises while the amount of blood pumped with each heartbeat falls during prolonged exercise.²
So what exactly is happening?
Your Heart Has More Work to Do
During exercise, your muscles need blood to deliver oxygen and remove metabolic waste. But when your body gets hot, another system starts demanding blood flow too: your skin.
As your core and skin temperatures rise, blood vessels near the skin dilate. This increases blood flow to the skin, allowing heat from your body to be transferred toward the surface.¹
This is an important part of thermoregulation. Your cardiovascular system is essentially helping turn your skin into a giant radiator.
The problem is that your heart has a limited amount of blood to distribute.
Your muscles need blood to keep working. Your skin needs blood to release heat. Your heart has to keep up with both.
Your Heart Rate Starts to Climb
As exercise continues in the heat, heart rate gradually increases even if you maintain the exact same pace or power.
This is cardiovascular drift.
Under prolonged moderate-intensity exercise, cardiovascular drift is characterized by a progressive increase in heart rate and a decrease in stroke volume, meaning the amount of blood pumped with each heartbeat.²
Your heart compensates for the lower stroke volume by beating faster.
Think of it like this:
Cool conditions
More blood per heartbeat → lower heart rate
Hot conditions
Less blood per heartbeat → higher heart rate
The amount of blood your heart pumps per minute is called cardiac output. It is determined by heart rate multiplied by stroke volume.
Cardiac Output = Heart Rate × Stroke Volume
So when stroke volume begins to fall, increasing heart rate helps your body maintain the cardiac output needed to keep you moving and regulate your temperature.²
Why Does Stroke Volume Fall?
One major reason is that less blood is returning to your heart.
As your body heats up, more blood is directed toward the skin. At the same time, you are sweating and gradually losing fluid.
That loss of fluid reduces your plasma volume, which means there is less fluid circulating through your bloodstream.³
Less circulating blood volume can reduce the amount of blood returning to the heart. With less blood entering the heart, there is less available to pump out with each beat.
This is one reason dehydration can make cardiovascular strain in the heat even greater. Research has found that dehydration can account for a substantial portion of the reduction in stroke volume during prolonged exercise in the heat.⁴
Your Skin Becomes Part of the Equation
Your skin plays a surprisingly large role in this process.
When you get hot, blood vessels in your skin dilate, increasing skin blood flow. This brings warm blood from the body's core toward the surface where heat can be transferred to the environment.¹
But there is a catch.
During exercise, your muscles already require substantial blood flow. Now your skin is demanding more too.
This creates a cardiovascular balancing act between exercise and thermoregulation.¹
And as heat stress becomes more severe, the cardiovascular system can eventually struggle to satisfy all of these demands. Cardiac output, blood pressure, and blood flow can become compromised during prolonged, intense exercise in severe heat and dehydration.¹
This Is Why the Same Workout Feels Harder
Imagine running 5 miles at the same pace on two different days.
On a cool morning, your body can focus primarily on supplying your working muscles.
On a hot afternoon, your body has another problem to solve.
Your heart is beating faster. More blood is being directed toward your skin. You are losing fluid through sweat. Your plasma volume is decreasing. Your stroke volume can fall. Your core temperature is rising.
The result?
The same external workload can require a greater cardiovascular effort.
This is one reason heart rate and perceived exertion can be higher in the heat even when your pace or power stays exactly the same.²˒⁵
Your body has not suddenly become less fit.
The environment has simply increased the physiological cost of the workout.
The Good News: Your Body Can Adapt
This is also why heat acclimatization is so powerful.
With repeated exposure to exercising in the heat, your body makes cardiovascular adaptations that help reduce the strain.
Heat acclimatization can increase total body water and plasma volume, improve the ability to maintain stroke volume, lower heart rate during exercise, and improve skin blood flow and sweating responses.⁶
In other words, your cardiovascular system becomes better at handling the heat.
That does not mean heat becomes harmless. It means your body becomes more efficient at managing the additional challenge.
The Takeaway
When a workout feels harder on a hot day, there is a physiological reason.
Your heart is not just supporting your muscles. It is also helping your body fight the heat.
As your temperature rises, more blood is sent toward the skin. Sweat causes fluid loss and can reduce plasma volume. Stroke volume can fall, and your heart compensates by beating faster.
That gradual rise in heart rate is cardiovascular drift.
So the next time your normal pace feels unusually difficult on a hot day, remember:
You are not just running your normal workout. Your cardiovascular system is running a cooling system at the same time.
Sources
González-Alonso J, Crandall CG, Johnson JM. The cardiovascular challenge of exercising in the heat. Journal of Physiology. 2008.
Wingo JE, Ganio MS, Cureton KJ. Cardiovascular drift during heat stress: implications for exercise prescription. Exercise and Sport Sciences Reviews. 2012.
Harrison MH. Heat and exercise. Effects on blood volume. Sports Medicine. 1986.
Coyle EF, González-Alonso J. Cardiovascular drift during prolonged exercise and the effects of dehydration. International Journal of Sports Medicine. 1998.
Wingo JE. Exercise intensity prescription during heat stress: a brief review. Scandinavian Journal of Medicine & Science in Sports. 2015.
Périard JD, Eijsvogels TMH, Daanen HAM. Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies. Physiological Reviews. 2021.
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