High Blood Pressure (Hypertension)

How the Body Controls Blood Pressure

The body controls blood pressure in several ways.

• What factors affect blood pressure?
• Keeping the blood pressure normal
• Pulse pressure

What Factors Affect Blood Pressure?

Blood pumped through blood vessels is always under pressure, much like water that is pumped through a garden hose. This pressure is highest in the arteries closest to the heart and gradually decreases as the blood travels around the body.

Blood keeps moving around the body because there are differences in pressure in the blood vessels. Blood flows from higher-pressure areas to lower-pressure areas until it eventually returns to the heart.

Blood pressure is controlled by three things:

• How fast the heart beats (heart rate). The pace at which the heart beats, or heart rate, is counted in heartbeats per minute. Generally, when heart rate increases, blood pressure rises. When heart rate decreases, blood pressure drops.

  A number of things affect heart rate, including the body’snervous system; chemical messengers called hormones, body temperature, medications, and diseases.

• How much blood the heart pumps with each beat (stroke volume). The amount of blood pumped out of a ventricle with each heartbeat is called stroke volume. When you’re resting, stroke volume is about the same as the amount of blood that veins carry back to the heart. But under stressful conditions, the nervous system can increase stroke volume by making the heart pump harder.

  Stroke volume can also be affected by certain hormones, drugs, and diseases, as well as increases or decreases in the amount of blood in the body, called blood volume.

  Nice To Know: You might also hear the term “cardiac output” used to describe the amount of blood that’s pumped through the body. Cardiac output is simply the amount of blood pumped out of a ventricle in one minute: Cardiac output = Heart rate x Stroke volume (amount of blood pumped with each beat) As cardiac output increases, so does blood pressure. This is why heart rate and stroke volume are important ways for the body to control blood pressure.

• How difficult it is for blood to travel around the body (peripheral resistance). The third major component that affects the blood pressure is the caliber or width of the arteries. Blood traveling in narrower vessels encounters more resistance than blood traveling through a wider vessel (its harder for water to pass through a narrow pipe than a wide pipe).

  Depending on what a person is doing, the amount of blood the heart pumps varies enormously. Yet the blood pressure normally remains pretty stable. That’s mainly because the body adjusts the resistance of the arteries, either widening or narrowing them as appropriate, to prevent the blood pressure from swinging wildly.

  This ability to regulate the width of the blood vessels is called the peripheral resistance. Most of the resistance to blood flow in the circulation occurs in the small-diameter arteries called arterioles.

  These arterioles are especially important in the immediate regulation of blood pressure. That’s because they contain specialized smooth muscle in their walls that can relax or contract, allowing the blood vessel to get wider or narrower.

  These changes are caused by:

  • Nervous system stimulation (for example, stress, caffeine, or tobacco)
  • Hormones
  • Proteins
  • Substances derived from the inner lining, or endothelium of blood vessels
  • Substances released during the body’sinflammatory response, called inflammatory chemicals
  • Certain medications
  • Various diseases

  Nice To Know: A group of hormones called the renin-angiotensin-aldosterone system (RAAS) is another critical player in blood pressure control. They regulate the amount of fluid in the blood, the width of the blood vessel, and the sodium and water balance by their action on the kidneys and blood vessels. The kidneys play a vital role in long-term changes in blood pressure. The hormones act on the kidneys to control the amount of sodium and water they excrete. If too much sodium or water stays, the amount of fluid in the blood, called the blood volume, goes up. This increase in blood volume means that the heart has to pump harder to circulate more fluid, and blood pressure goes up.

Keeping The Blood Pressure Normal

Generally, a change in any factor that may cause the blood pressure to rise is balanced by a change in another factor. This is how the body keeps blood pressure in a normal range.

For example, when you begin to exercise, your heart rate increases, as does the amount of blood pumped out of the heart with each beat (the stroke volume). This would normally increase the blood pressure.

But the blood pressure remains normal because the blood vessels widen in order to increase the capacity for the extra blood being pumped while exercising. This helps offset the increase in blood pressure associated with the increase in heart rate and stroke volume associated with exercise.

On the other hand, if blood pressure suddenly drops, a series of changes restores normal blood pressure. These include short-term increases in heart rate, the strength of the heart’s contractions, and peripheral resistance. Over a longer time period, blood volume also increases due to the actions of hormones on the kidneys.

Pulse Pressure

There is another dynamic component of blood pressure called pulse pressure. Pulse pressure is the difference in pressure between when the ventricles of the heart contract and when they relax. It can be felt as a throbbing beat in an artery, called a pulse.

When the ventricles contract, blood is pumped out of the left ventricle into the main artery leading away from the heart to the body, called the aorta. This creates the highest pressure that occurs in the aorta, called the systolic blood pressure.

The increased pressure and increased blood volume cause the aorta to stretch. Because the blood pressure in the aorta is higher than the pressure in more distant vessels, blood moves forward toward the body’s tissues.

When the ventricles relax, blood stops flowing into the aorta and the pressure drops to its lowest level. This is called the diastolic blood pressure.

But blood continues to move forward in the circulation even when the ventricles are relaxed. Because the walls of the aorta and other elastic arteries bounce back, they maintain pressure on the blood moving through them.