Q: What is the role of blood pressure, how does it work, and what are the factors that affect it?

A: A simple way to understand it might be imagining this scenario: You’re in your back yard, holding a hose. When you turn on the water, it shoots through the hose and out the nozzle. The water is moving because it’s under pressure. Similarly, when your heart beats it creates pressure that enables your blood to “water” your body with the nutrients that are in your blood.

Three things affect the amount of pressure that pushes blood through your body: how forcefully the heart pumps, how much blood there is, and how narrow the smallest blood vessels are. That last one may need some explanation. When you’re watering plants, and you want the stream of water to go farther, what do you do? You make the nozzle smaller. Narrowing the opening through which the water flows increases the pressure, and the water then is able to reach the distant flowerbed.

What affects how hard the heart pumps? Several body hormones, particularly epinephrine and norepinephrine, “whip” the heart to beat faster and harder. These hormones are made by your adrenal glands (located just on top of your kidneys). Exercising and fear cause increases in these two hormones, which is good: your heart needs to work harder when you exercise, or when you may need to run away from something frightening. However, chronic anxiety and stress also increase these hormones, and that’s not good. During a stressful day, no good purpose is served by your heart beating faster and harder, and by your blood pressure going up. Some uncommon diseases - not just chronic anxiety and stress - also can raise levels of these hormones.

What affects how much blood you have? Blood is part water. Therefore, the amount of fluid you drink and the amount you lose through sweating and urination affect how much fluid there is in your blood. Blood also is part salt. Salty foods, and salt added to your foods, can increase the fluid in your blood. That’s why people with high blood pressure are urged to go light on salt, and often given diuretic drugs to help their kidneys eliminate excess fluid and salt.

What affects how narrow your small blood vessels (arterioles) are? These blood vessels have circles of muscle in their walls that can clamp down and narrow the vessels. The same hormones — epinephrine and norepinephrine — that whip your heart to beat faster and harder also cause the blood vessels to clamp down, raising blood pressure.

In other words, blood pressure is influenced by many factors. Maintaining normal blood pressure — one that can rise and fall as your body needs it — is essential for good health.

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