Good news if you like taking a hot bath as new research, published in the European Journal of Applied Physiology I, investigated how our blood vessels respond to heat compared with exercise.

Researchers recruited 25 adults aged between 45 and 70, with an average age of 59 who were all classified as overweight or obese and were relatively physically inactive.

Each volunteer took part in two different experiments.

In the first experiment, participants sat in a hot 40°C bath for 30 minutes.  

In the second experiment, the same people spent 30 minutes cycling on a stationary exercise bike at a moderate intensity, working at around 60–70 percent of their maximum heart rate.

The scientists measured their heart rates, core body temperatures and blood pressure before and during the experiments and had the volunteers swallow tiny electronic temperature sensors that transmitted readings from inside their bodies.

The researchers used optical coherence tomography which allowed them to examine microscopic blood vessels in the skin of participants' forearms, measuring their width, the speed at which blood travelled through them and how much blood was flowing.

They also used laser Doppler flowmetry, to independently measure changes in skin blood flow. 

After 30 minutes in the hot bath, participants' tiny skin blood vessels had widened by an average of 5.2 microns, allowing more blood to flow through them compared to a decrease of 1.1 microns for the cyclists. 

They also measured 14 percent more blood-perfused vessel density after the bath, compared with a decrease of 3 percent after cycling. 

After the hot bath, participants experienced an average reduction of approximately 6 mmHg in systolic blood pressure (the top number in a blood pressure reading) and 10 mmHg in diastolic pressure (the bottom number). 

After cycling, the average systolic reading increased by approximately 1 mmHg, while diastolic pressure decreased by around 2 mmHg. 

When you exercise, your muscles produce heat as they work and your body responds by moving blood towards the skin, where that heat can be released. 

When you sit in hot water, you absorb heat from your surroundings, your body still needs to regulate its temperature, so it responds by widening blood vessels near the skin and increasing circulation. 

Wider blood vessels generally create less resistance to blood flow, which can help reduce blood pressure. 

While heart rate increased more during cycling, showing that although the bath produced a stronger response in the skin's small blood vessels, the two activities were putting different demands on the cardiovascular system. 

The findings suggest that heat exposure alone can create a significant circulatory response, even without exercising your muscles. 

Of course, physical exercise provides many benefits that passive heating cannot replicate, including improvements in muscular strength, endurance and physical fitness. 

However, this is particularly interesting for people who find conventional exercise difficult, including those with mobility limitations or conditions that restrict physical activity. 

So while a hot bath probably isn't going to replace your daily walk, it's reassuring to know that relaxing in warm water isn't necessarily doing nothing. 

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