Under Pressure: Why Stress Raises Blood Pressure for Some, but Not Others

Mental stress doesn’t just affect the mind, it can heighten emotions, trigger anxiety, and raise blood pressure, leaving individuals feeling tense and overwhelmed. When stress becomes chronic, it can affect the cardiovascular system, raising the risk of hypertension, heart disease, and other heart-related problems over time.

But not everyone suffers the physical effects of mental stress to the same extent. Why is it that the tension in our minds ends up in our arteries for some of us, but not others?

Dr. Philip Millar’s lab in the Department of Human Health Sciences is taking a contemporary approach to understanding how mental stress impacts the sympathetic nervous system — the system that controls many of our automatic or “unconscious” bodily functions, including blood vessel dilation and restriction. Their research focuses on how the neural signals directed towards the kidneys and skeletal muscle respond to stress differently among individuals, which they believe may explain why stress increases blood pressure in some people more severely than others.

“We want to understand the mechanisms that explain why people respond so differently to stress,” Millar said. “Sometimes a person’s neural activity will change a lot, but their blood pressure response doesn’t.”

The complex connections between stress, organs and blood pressure may hold the key to this mystery. Blood pressure in organs like the kidneys and skeletal muscle are regulated by the sympathetic neural activity, by adjusting the size of blood vessels. The constriction of these blood vessels results in elevated blood pressure. However, an organ’s neural responses to stress can vary widely between individuals, which may explain why some individuals have different overall physical responses to stress.

In a recent study involving 33 healthy adults, Millar and his team tracked sympathetic nerve activity and blood flow in muscle and kidneys during mental stress — in this case, the stress of answering a series of rapid math questions. Using advanced ultrasound imaging and nerve recording tools, the researchers could observe the body’s internal responses in real time.

By comparing nerve impulses and blood flow patterns, the team determined that muscle and renal (kidney) systems — both important indicators of how the body regulates blood flow and pressure under stress — don’t always respond in a similar fashion during stress.

“We found no consistent relationship between the muscle response and the renal response,” says Millar. In other words, depending on the individual, stress may cause a reduction in blood flow in one organ, but not the other.

In fact, only about half of the participants (52 per cent) reported similar results in kidney and muscle responses to mental stress; in the other half of the group, the two organs reacted differently. 

“We did not expect to see such huge variability between individuals,” says Millar, noting that this was the first time such a study had been undertaken. “The results were quite surprising, particularly since our participants were all young and healthy. Normally, we’d expect to see more consistent responses in a group like this.” The results highlight just how diverse human stress reactions can be, even under identical conditions.

Looking ahead, the Millar lab aims to determine whether these different stress response patterns are stable or fluctuate over time. “We need to understand how reliable these responses are,” he says. “This will be key to knowing if we can use them as tools to help predict or manage stress.”

These discoveries could have important clinical applications. By identifying which parts of the body are most reactive to stress, researchers may eventually find ways to help physicians treat people with exaggerated blood pressure responses — a key risk factor for cardiovascular disease — more effectively.

Through this work, Millar and his team hope to understand how stress shapes the body’s internal communication systems and why, for some, the pressure hits harder than for others.

Read the full study in the Journal of Applied Physiology.

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