Kidneys and Blood Pressure

We have two kidneys, each one about the size of an average adult fist, located in the abdomen just under the back muscles. Although we can function well on one kidney, our built-in excess capacity of two suggests kidneys are critical organs; nature supplies two to ensure survival.

The kidneys process and clear about 50 gallons of fluid daily. Within each kidney there are millions of specialized cells called nephrons. Each nephron is a marvelous filtering unit.

Millions of nephrons working together make a filtering system with incredible capacity. The kidneys’ ability to filter sodium and chloride from the blood and regulate fluid levels is one of the three systems of blood pressure regulation in the body.

Kidneys influence blood pressure by regulating fluid volume, including blood, and controlling the amount of sodium, potassium, calcium, and magnesium in our system.

Because these minerals profoundly influence the state of tension or relaxation of the muscles in the arterioles, the kidneys affect peripheral resistance.

Blood passes through the kidneys, wastes are removed, and the remaining materials, including water, are returned to the bloodstream. Waste products and the liquid that contains them are the urine we void.

Most of the sodium and water that are removed get reabsorbed from the nephrons and returned to the blood. If there’s an excess of sodium and fluid volume, the kidney cannot eliminate it.

Blood pressure is elevated to overcome this situation and literally force the sodium out. This process is analogous to reverse osmosis used to purify water. If sodium didn’t get reabsorbed, the problem of high blood pressure wouldn’t exist.

Sodium Reabsorption Diet

Sodium gets reabsorbed as sodium chloride, or common table salt. Sodium reabsorption illustrates the body’s excellent ability to conserve nutrients. This ability probably evolved as a mechanism to conserve sodium and chloride.

Salt was so scarce just 2,000 years ago that it was a medium of exchange. In the Roman Empire, soldiers were paid with a salt ration. (The word salary comes from the Latin word for salt: sal.)

In a few isolated areas, salt cakes are still a medium of barter. In our modern world, salt is no longer rare and our excess consumption of those two once-rare elements works against us.

Only in the last 1,000 years has salt become readily available. And only in the last 400 years has it become cheap. In the evolutionary process, 100,000 years is a “blink of the eye,” let alone 2,000. In short, humanity’s the same, and our kidneys are the same as they were 10,000 years ago, but the availability of salt has changed.

Reabsorption of sodium as sodium chloride can work against us by precipitating diet-related high blood pressure. Most processed foods contain large quantities of salt, and people often liberally add salt to food.

Sodium and chloride as they occur naturally in unprocessed foods are probably not a serious problem. Unprocessed foods contain sodium in a myriad of forms, including very small amounts of sodium chloride.

For example, sodium is found as citrate in citrus fruits and glutamate in grains. As a result, fruits, vegetables, and whole grains contain naturally balanced forms of sodium and don’t have an excess of either sodium or chloride.

In fact, the amount of chloride naturally present in foods, along with the body’s ability to reabsorb 99 percent of sodium, suggests that very little dietary sodium is required. Normal active adults get along well on only about 300 milligrams of sodium daily, and some experts claim even less is sufficient.

Processed foods contain salt either as a preservative or to increase taste intensity. Return to natural foods, fruits, grains, vegetables, meat, fish, poultry—anything that grows from the ground, on the ground, on trees, walks, swims, or flies.

Do not prepare or eat anything with elaborate sauces or coatings. Boil, broil, barbecue, bake, or poach without adding salt. It works! Unsalted food may seem bland at first, but in a short time you’ll start savoring flavors that you didn’t know were present. A new world of taste will open up to you.

Hormonal Influences

A number of hormonal systems influence the kidneys. All these systems when not functioning properly or in synchrony can cause sodium reabsorption, but they can be influenced by diet or drugs. Excess insulin causes sodium reabsorption by the kidneys, indirectly elevating blood pressure.

This makes people who produce excess insulin candidates for high blood pressure. Consequently, many overweight people, people who habitually consume excess sugar, and some diabetics who do not control insulin correctly develop high blood pressure.

Another more elaborate hormone system that influences blood pressure is the angiotensin-renin-aldosterone system. This system includes the adrenal glands that produce aldosterone, the primary hormone that induces the kidneys to retain sodium and chloride and excrete potassium.

Aldosterone is produced by the two adrenal glands situated on top of each kidney. Aldosterone causes the kidneys and the sweat glands (which act somewhat like kidneys) to retain sodium. Although aldosterone is produced by the adrenal glands, it is, in part, regulated by the kidneys.

This regulation involves the hormone angiotensin and the enzyme renin. Kidneys release renin, an enzyme that causes the release of an other hormone, angiotensin. Angiotensin causes constriction of the arterioles and this signals the adrenals to release more aldosterone.

Constriction of the arterioles and the release of aldosterone elevates blood pressure by two mechanisms: Arteriole constriction causes increased peripheral resistance, and aldosterone causes salt retention. Stimulation of the sympathetic nervous system produces renin.

This is the nervous system that takes charge when we’re under stress. If someone attacks you, your kidneys release renin, and the entire angiotensin-aldosterone process is started. This takes our blood pressure discussion into the realm of stress.

Some physicians who specialize in hypertension talk of high-renin producers. High-renin producers are people, often with Type A personalities, who normally produce excess renin. Excess renin leads to elevated blood pressure.

In some cases, the only recourse is to control renin levels with drugs that block its production. Some evidence suggests that inadequate magnesium can cause excessive renin production. Some serious conditions can also cause excessive aldosterone.

These serious illnesses must be dealt with by modern medical intervention. In these cases, the high blood pressure is secondary to the illness (secondary hypertension) and cannot be dealt with by diet. There are other factors not as well understood that influence the rate of sodium excretion by the kidneys.

These materials, called natriuretic factors (meaning sodium-excretion factors), are produced in other parts of the body and influence how the kidneys handle sodium. Natriuretic factors are produced in response to increased blood sodium levels; therefore, diets high or low in sodium will influence their levels proportionately.