What Sends Your Blood Pressure Soaring?

Excerpted from
What Your Doctor May Not Tell You About Hypertension : The Revolutionary Nutrition and Lifestyle Program to Help Fight High Blood Pressure
By Mark Houston, M.D., Barry Fox, Ph.D., Nadine Taylor, M.S., R.D.

Sixty, seventy, perhaps eighty times a minute, your heart contracts and pumps freshly oxygenated blood through the aorta, the giant artery rising up and away from your heart. Some of that blood flows out into a network of progressively smaller arteries that snake their way through your chest, arms, neck, and head. Another portion of the blood finds its way to your liver, kidneys, and other mid-body areas, while the rest travels through the aorta until the artery forks, with either branch continuing down through your lower body and into each corresponding leg.

No matter what its destination, the blood follows a similar path: from the aorta, through larger then progressively smaller arteries, then finally into the tiny capillaries where oxygen and nutrients are exchanged for waste products at the cellular level. Having performed these duties, the blood begins its journey back to the heart through a series of ever larger veins. The entire process is dependent upon the ceaseless beating of the heart, plus a fairly constant amount of pressure in the system.

A Look at the Plumbing

When we talk about elevated blood pressure, the first thing we think of is the heart. After all, there's a lot going on in the heart. It beats sixty to ninety times per minute, or about 100,000 times a day. During an average lifetime, it will beat more than 2.5 billion times, pumping about 1 million barrels worth of blood throughout your body. But there's a lot going on in the blood vessels, too. They automatically deliver greater or lesser amounts of blood to the tissues exactly when it's needed.

Together, the heart and the blood vessels make up the cardiovascular system, a complex but highly efficient system that delivers oxygen and nutrients to every cell in the body, exchanging them for waste products that are swept away for disposal or recycling.

The heart is essentially four empty chambers surrounded by muscle. Used blood from all over the body enters into the top right chamber (right atrium), rests there for just a moment, then drops down into the bottom right chamber (right ventricle). When the heart beats (when the muscle surrounding the heart squeezes), the used blood in the bottom right chamber shoots out of the heart and through an artery into the lungs. There, the blood releases the load of carbon dioxide it's been carrying, picks up a fresh load of oxygen, and returns to the heart.

Now entering the heart for the second time, the oxygen-rich blood goes into the top left chamber (left atrium). It rests there for just a moment before dropping into the bottom left chamber (left ventricle). The bottom left chamber is the real workhorse of the heart, as it has to propel fresh blood completely out of the heart and through an incredibly long series of arteries that tunnel throughout the body into every limb and organ.

We normally think of the arteries and veins as being like the plumbing pipes in our houses, just lying there passively while things pass through them. But the vascular system is more than just a bunch of pipes. It functions as an organ, just like the heart, with a series of tasks it must perform in order to keep the blood flowing smoothly. And like all living tissue, it can be damaged, which causes certain important body functions to go awry.

Although the vascular system is made up of both arteries and veins, it's the arteries that are the real problem in hypertension. Look at the diagram of the artery on the previous page. It's not just one substance like a hose-a certain thickness of "artery stuff." Instead, it's made up of several layers of different substances, each with different capabilities, different tasks, and different vulnerabilities. In hypertension, we're primarily concerned with the innermost wall of the artery, the lining-called the endothelium - and with the muscular media.

The endothelium is, in fact, an endocrine organ, which means that it manufactures and secretes hormones locally and directly into the bloodstream. These hormones deliver messages, instructing arteries, blood, body tissues, and other organs to do, or not to do, various things. And the endothelium is a pretty big organ, in fact, it's the largest in the body, weighing in at nearly five pounds. If you were to peel it out of the arteries and lay it Hat, it would take up about 14,000 square feet in surface area-that's the size of six and one half tennis courts!

Besides releasing numerous hormones and other substances, the cells of the endothelium serve as a physical barrier between the flowing blood and the muscle portion of the artery wall. These cells are tightly interlocked, like tiles on a newly laid countertop. Anything that travels from the blood to the tissues has to pass either through these endothelial cells or through tiny gaps between them.

Just behind the endothelium, inside the artery wall, lies the media, a layer of smooth muscle cells that contracts and relaxes on demand. The media squeezes the artery passageway to make it narrower, and relaxes the pressure to let it widen. It does this at the command of the endothelium, which constantly monitors the environment, checking out the blood pressure and other factors. When something isn't quite right, the endothelium sends out certain hormones and mediators to set things straight. For example, if the blood pressure is too high, the endothelium may release nitric oxide (NO for short). The NO tells the media to relax. When the media stops squeezing, the artery gets a little bit wider and the blood pressure falls to a safer level. On the other hand, if the endothelium notices that the blood pressure has dropped too low, it can release various contracting substances that order the media to constrict, thus raising the blood pressure.

In addition to the contraction and dilation of blood vessels, the endothelium helps to control the function of the platelets, clotting elements in the blood. It also dictates the thickness and thinness of the blood, how white blood cells stick to the artery walls, the inflammatory process, the growth, thickness, and stiffness of the vascular muscle, and other important factors.

Another important job of the endothelium is to help control oxidative stress. Oxidative stress is akin to biochemical thievery. Electrically unbalanced molecules in the body snatch electrons from other substances in order to balance themselves. This may make the thieving molecules "feel better," but the molecules that were "mugged" may not be able to function properly anymore, leading to weakness or destruction of body cells and tissues. The body manufactures antioxidants to prevent (or at least control) this kind of damage, and we also absorb various antioxidants from our food. Often, however, the thieving molecules are too much for the body to handle, and we suffer from oxidative stress that can damage the endothelium and encourage hypertension.

In short, the thin layer of cells lining the inside of the arteries acts like a monitoring and correcting station, constantly working to ensure that the blood flows and behaves properly.