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What Is Blood Pressure?
Author: Cleaves M. Bennett MD FACP
Monday, January 07, 2008

Stephen Hales was the first man ever to measure blood pressure.
Although he was a Church of England clergyman, he was fascinated by scientific experiment. In 1711 he measured the blood pressure in the carotid artery of a horse—using a brass needle and a hollow glass tube nearly thirteen feet long.

To the amazement of the crowd that had gathered around him, the horse's blood rose up the tube to a height of over eight feet.

It was the first measurement of blood pressure—and also the first demonstration of the effect that stress has on blood pressure. Initially, the horse was frightened and in pain. But as it became calmer its pressure settled down to about five feet, which is closer to what we now know, is normal for a horse.

What exactly is blood pressure?

Pressure is defined by the dictionary as "the force exerted by one body on another." The bodies, of course, may be fluid or gaseous, as well as solid. The pressure in your tires is the force that the air pumped into them exerts on the walls. Water pressure is the force that pushes water along the pipes in your house and out through the faucet. If your water pressure is low, all you get when you turn on your shower is a trickle.

Your blood pressure is the pressure inside the blood vessels, which builds up, and then slackens a little, and then builds up again, every time your heart pumps. This pressure moves the blood along the vessels (or pipes) in your body, from the heart to tissues and organs and back again.

The heart is an amazing organ. It starts pumping blood before you're born, when you're still only a few inches long. And it grows as you do, adapting in size to handle whatever work your body needs it to do.

Although your heart is only the size of your fist, it pumps some 40,000 gallons of blood through your body each day, along 60,000 miles of blood vessels. Unbelievable! And it manages all this by beating 100,000 times a day, for as much as eighty or ninety years at a stretch. The finest pumps that engineers have designed simply don't measure up to it. It's phenomenal.

When your heart beats, it pumps blood out through the aorta, an artery about the size of a garden hose. Smaller and smaller arteries branch out from the aorta and carry the blood to parts of the body. The smallest branch arteries, which are called arterioles, are about a thousandth of an inch wide. You'd need a microscope to see them. And there are millions of them in every organ and tissue of your body.

From the arterioles, the blood passes into even more microscopic vessels, the capillaries. Here an exchange takes place between blood and tissue, the blood passing oxygen and nutrients to the tissues of the body, and the tissues unloading carbon dioxide and wastes into the blood.

The blood then returns to the heart via the veins, and the cycle begins again.

All this movement of blood through your blood vessels is accomplished by pressure that is generated by your heartbeat. The pressure in the blood is highest at the aorta, and the entire system of aorta, branch arteries, and arterioles through which the blood is pumped all over the body is a high pressure system. The network of capillaries, and the veins which bring the blood back to the heart, are low pressure systems.

Now here is the really ingenious part—the control system for your circulation. The purpose is to provide adequate blood flow to every organ and tissue in the body, recognizing that their needs change fairly often. The heart, in order to respond to changing needs, can beat slower or faster and can pump more or less blood with each beat. In the tissues and organs, each of those millions and millions of tiny arterioles has a muscle in it, so it can open up or close down slightly. These tiny muscles are all under the control of your endocrine and nervous systems, making it possible to redirect the flow and send more blood wherever it is needed.

Your endocrine and nervous systems get the message that more blood is needed in the kidneys. They will signal the arterioles in the kidneys to open up, maybe close down the arterioles in the spleen a bit, and send some extra blood to the stomach. That sort of supply and demand is going on all the time—delicate and precise adjustments to tissue and organ blood flow, to keep everything working smoothly. It's a beautiful system! It's marvelous, in the sense of something to be marveled at and appreciated. Your wonderful body!

There is one other aspect of the system you need to know about. This is a reflex protective action in the arterioles themselves that opens them up as the overall blood pressure falls and closes them down (or constricts them) as the pressure goes up too high. This process is called the myogenic reflex, meaning that it originates in the muscles of the arterioles themselves. Nowhere is it more vigorously operative than in the brain and retinal circulation. As the blood pressure rises in hypertension, the microcirculation is closed down; that is, the arterioles constrict all over the body, especially in the brain and eyes. Your doctor can actually see this constriction when he looks inside your eyes using an ophthalmoscope (the instrument with a bright light and a magnifying glass). This constriction is a homeostatic reflex—that is, it tends to keep the organ and tissue blood flow constant, in spite of fluctuations or changes in the pressure.

The little arterioles control the runoff from the major arteries. Thus their tiny muscles play an important part in regulating your circulation and in controlling blood pressure.

But of course it is your heart that causes the pressure, because it's the pressure generated by your heart pumping that sends the blood to its destination. No heartbeat, no pressure. If you have a weak heartbeat, low blood pressure. So high blood pressure actually implies a strong heartbeat. Remember that. We'll talk more about it later.

Let's see how the heart and the arterioles work together to maintain the circulation and the blood pressure.

When the heart gets filled with blood, it beats. And each beat forces approximately two or three ounces of blood out of the heart and into the arteries. That extra amount of blood, added to what is already there, affects the arteries the way water coming into a hose affects the hose. There's a sudden increase in pressure inside the hose. Most people, particularly young people whose bodies are still in pretty good shape, have fairly elastic arteries. When that extra blood comes in, the larger arteries all expand a little, and the pressure doesn't rise quite as much as it would if the arteries were like the stiff pipes in your house. You can feel that expansion at your neck or wrist—it's your pulse.

Many older people who suffer from arteriosclerosis (the medical name for hardening of the arterial wall) have less flexible, less elastic, less expandable major arteries. And when the heart empties blood into them the pressure rises much more than it does in younger people— because there is less elasticity, less slack to take it up. But whatever the state of your arteries, the blood pressure always goes up when the heart beats.

Now during the short period of time in which the heart is beating, the blood is draining from the little arterioles into the capillaries and veins. As the heart relaxes and begins to fill again, the blood is still draining out of the arterioles, so the pressure in the arteries (blood pressure) falls rapidly, and keeps on falling until the next beat of the heart drives it back up. So the pressure in your arteries is always fluctuating, rising and then falling, rising and falling again like a wave.

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When we measure your blood pressure we obtain two figures. We measure the pressure at its highest and at its lowest—at the top of the wave and at the bottom. Read More