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Q: What happens inside the body when a person faints?

A: Fainting occurs when something interrupts blood flow to the brain. Although usually harmless, fainting can cause injuries and sometimes signals a problem with the heart or circulatory system.

If you faint, it’s worth talking with your doctor, especially if there doesn’t seem to be an explanation.

Some basic physiology helps explain what happens when a person faints. It’s harder for the heart to pump blood up to the brain than down to the toes. Blood pressure helps overcome the downward tug of gravity and push blood to the head.

We tend to think of blood pressure as a relatively stable entity. It isn’t. It changes every time you stand up, sit down, bend over, eat, sneeze, get stressed or relax. The human body is designed to counter these shifts and quickly bring blood pressure back to its usual point.

This requires an intricate feedback system that involves sensors in the aorta and in the carotid arteries leading to the brain. These sensors stimulate a host of nerve and hormone signals that change how fast the heart beats, how much blood it pumps with each beat, and the contraction and relaxation of blood vessels. 

When blood pressure falls for any reason, heart rate should speed up and blood vessels should contract to maintain blood flow to the brain.

The brain requires a constant supply of sugar and oxygen to function. If the heart and blood vessels don’t respond to low blood pressure as they should, the brain quickly goes into an energy-conserving shutdown. The brain’s signals to nerves and muscles stop, and the person slumps to the ground.

Once the body is horizontal, it’s easier for the heart to pump blood to the head. Usually the brain “wakes up” again within seconds. That may not be true if loss of consciousness was due to a heart problem.

Role of the vagus nerve

Most fainting episodes result from excessive stimulation of the vagus nerve. Known as vasovagal syncope, it happens when blood vessels relax and blood pressure starts to fall. 

The vagus nerve sends signals to the heart to slow down, lowering blood pressure even more. Blood flow to the brain halts, and the person blacks out and collapses.

Many situations can trigger an episode of vasovagal syncope. Standing for a long time in a hot, crowded environment is a common one. 

Sometimes the trigger is a strong emotion, such as the response to bad news. Some people faint when they see blood, have a coughing spell, laugh hard or strain to urinate or move their bowels.

Vasovagal syncope is more common in people under 35, in part because the nervous system is more sensitive in younger people.

Q: I am getting more forgetful. I will sometimes walk into a room and not remember what I wanted to do. When are memory changes like these worrisome?

A: Memory lapses like what you describe are very common as we get older. They can be unsettling, but they don’t necessarily herald impending dementia. The key is in how often these slips occur.

Keep track of the pattern. Is it happening several times a week, or is it happening once or twice a month? Is it a change compared to a few years ago? Is it getting gradually worse?

Forgetfulness can be a normal part of growing older. Memory lapses can also stem from several other conditions, including lack of sleep, stress, medications, alcohol or depression.

Any of these conditions can be treated. For example, you can adjust your sleep schedule, try deep breathing or other techniques to reduce stress, change the dose or type of medications you take, cut down on your drinking, or get treated for depression.

Don’t be alarmed by occasional forgetfulness. The time to call your doctor is if it’s more persistent or if you develop worsening memory loss that’s interfering with your daily activities and routine.

What you can do now 

Clinical studies continue to show that exercising daily, eating a healthy diet, such as the Mediterranean diet, and getting enough sleep are the best ways to maintain brain health.

Regular exercise can slow the progression of and may even prevent cognitive decline. Exercise is thought to shield the brain from damage in a number of ways: by improving blood flow, protecting the blood vessels that feed the brain, and reducing stress hormone levels.

There have been some large studies showing a benefit of the Mediterranean-style diet in slowing and perhaps preventing cognitive decline as we age. The Mediterranean diet includes fresh fruits and vegetables, whole grains, olive oil, nuts, legumes, fish and small amounts of red wine.

Growing evidence suggests that enough good quality sleep leads to greater clearance of amyloid from the brain, which can help prevent Alzheimer’s disease. Aim for seven to nine hours per night.

Other ways to help maintain cognitive fitness include staying socially connected and learning new things, such as taking a course at a local college or picking up a new instrument.

Howard LeWine, M.D., is an internist at Brigham and Women’s Hospital in Boston and assistant professor at Harvard Medical School. For additional information, visit health.harvard.edu.

© 2020 President and Fellows of Harvard College. Distributed by Tribune Content Agency, LLC.