Natriuretic Peptide

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The heart is responsible for producing natriuretic peptides.
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Natriuresis is the process of excretion of abnormally large amounts of salt in the urine. The word comes from the Latin term natrium, which means "sodium" and the Greek term ouresis, which means "making water." Natriuresis is similar to diuresis, the excretion of an unusually large quantity of urine, except that in natriuresis the urine is exceptionally salty.

There are three naturally occurring protein hormones in the body that cause natriuresis. They are called A-type, B-type, and C-type natriuretic peptides (small protein molecules). These peptides lower the concentration of sodium in the blood, which tends to lower blood volume because the sodium takes water with it into the urine. Many diuretic drugs take advantage of this mechanism to treat medical conditions like high blood pressure. In fact, the natriuretic peptides are sometimes called the body's "natural diuretics."

Natriuretic peptides are produced by the heart and the blood vessels. The body produces more of them when it's affected by diseases -- like heart failure, kidney failure, and liver disease -- that are characterized by an expanded fluid volume. .

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­­The natriuretic pe­ptides work in opposition to the renin-angiotensin-aldosterone system, a hormonal system that plays an important role in controlling the amount of sodium the body excretes. The interaction between these two systems is complex, and their interplay affects many physiological processes, including heart function, relaxation and contraction of blood vessels, blood pressure, the excretion of sodium and water, and aspects of the nervous system.

On the next page we'll learn about the different types of natriuretic peptides.

Types of Natriuretic Peptides

  • A-type natriuretic peptides are also called atrial natriuretic peptides. They're made, stored and released largely by the atria, muscle cells in the upper chambers of the heart. The body makes more A-type natriuretic peptide in response to high blood pressure and stretching of the atria. These conditions are often seen in congestive heart failure, where increased blood volume and higher levels of extracellular fluid (fluid overload) stretch the heart's walls. In patients with heart failure, A-type natriuretic peptides are also made by the ventricles, cells in the heart's lower chambers -- the pumping chambers.
    human heart cross section Images
    A- and B-type natriuretic peptides are produced in the heart's ventricles -- the
    lower chambers.

    A-type natriuretic peptide causes a reduction in blood volume and therefore a reduction in heart output and blood pressure. Excretion of sodium by the kidneys and the breakdown of fat cells are also increased.

  • B-type natriuretic peptides are also called brain natriuretic peptides. B-type natriuretic peptides are made, stored and released mainly by the ventricles. It's produced in response to stretching of the ventricles due to the increased blood volume and higher levels of extracellular fluid (fluid overload) that accompany congestive heart failure. It acts as a natural diuretic, eliminating fluid, relaxing blood vessels and funneling sodium into the urine.

    When your heart is damaged, your body secretes very high levels of B-type natriuretic peptide into your bloodstream in an effort to ease the strain on your heart. The levels may also rise if you have new or increasing chest pain (unstable angina) or after a heart attack.

  • C-type natriuretic peptides are produced by cells that line the blood vessels. They cause relaxation of blood vessels, helping to lower blood pressure. Unlike A-type and B-type natriuretic peptides, they don't have direct natriuretic activity.

The B-type Natriuretic Peptide Test

The B-type Natriuretic Peptide (BNP) Test is a blood test for heart failure. It measures the level of B-type natriuretic peptide in your blood. Your heart's ventricles produce extra B-type natriuretic peptide when they can't pump enough blood to meet your body's needs. So if your B-type natriuretic peptide level is high, you probably have heart failure. The higher the levels are at diagnosis, the worse your heart failure is likely to be. The finding of low levels tends to exclude heart failure.

A variation of B-type natriuretic peptide called NT-proBNP (N-terminal prohormone B-type natriuretic peptide) has also been approved as a test for diagnosing congestive heart failure. When a molecule of B-type natriuretic peptide is released into the bloodstream by heart muscle cells, a molecule of NT-proBNP is also released. Both BNP and NT-proBNP tests may be useful in evaluating the risk of heart attack and other problems in patients who already have heart disease.

Doctors also use B-type natriuretic peptide levels to monitor the effectiveness of treatment. Once a heart failure patient has been on medication for a while and his heart function has improved and his heart size returned to normal or near-normal, his B-type natriuretic peptide level may drop because the heart produces B-type natriuretic peptide in response to abnormal pressures and heart enlargement. Other treated heart patients will show higher-than-normal levels even with a return to normal heart size and pressures.

Why is it important to avoid fluid overload?

In heart failure, the heart weakens and becomes unable to keep blood moving adequately through the heart and blood vessels. As a consequence, the flow of blood to the body's tissues decreases. Because the tissues don't receive the level of oxygen and nutrients they require, their efficiency and endurance decline. Poor circulation causes the kidneys not to be able to remove enough water, salt and waste products from the blood. In addition, decreased blood flow causes the kidneys to retain even more salt and water in an effort to increase blood volume.

In its attempt to satisfy the body's hunger for oxygen-rich blood, the heart may enlarge and beat faster. An increased blood volume causes blood vessels to distend with fluid and places an additional burden on the already overworked heart.

The normal balance of pressure between fluids inside and outside the blood vessels shifts. As a result, fluid that normally stays in the bloodstream leaks into surrounding tissue, causing fluid overload in the lungs, abdomen and/or legs, which often accompanies heart failure.

To learn more about natriuretic peptides, take a look at the links on the next page.

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