Blood pressure is the force exerted by the blood against the walls of the vessels. Blood pressure is determined by the heart’s output and resistance to blood flow in the vessels. If either of these factors changes and there are no compensating changes, blood pressure will change (Fig. 11-13).
The output of the heart, or cardiac output (CO) is the volume of blood pumped out of each ventricle in one minute. Cardiac output is the product of two factors:
* Heart rate, the number of times the heart beats each minute. The basic heart rate is set internally by the SA node, but can be influenced by the autonomic nervous system, hormones, and other substances circulating in the blood, such as ions.
* Stroke volume, the volume of blood ejected from the ventricle with each beat. The sympathetic nervous system can stimulate more forceful heart contractions to increase ejection of blood. Also, if more blood returns to the heart in the venous system, stretching of the heart muscle will promote more forceful contractions.
Figure 11-13 Factors that influence blood pressure.
Resistance to Blood Flow Resistance is opposition to blood flow owing to friction generated as blood slides along the vessel walls. Because the effects of resistance are seen mostly in small arteries and arterioles that are at a distance from the heart and large vessels, this factor is often described as peripheral resistance. Resistance in the vessels is affected by the following factors:
* Vasomotor changes. A narrow vessel offers more resistance to blood flow than a wider vessel, just as it is harder to draw fluid through a narrow straw than through a wide straw. Thus, vasoconstriction increases resistance to flow and vasodilation lowers resistance.
* Elasticity of blood vessels. Arteries normally expand to receive blood and then return to their original size. If vessels lose elasticity, as by atherosclerosis, they offer more resistance to blood flow. You’ve probably experienced this phenomenon if you’ve tried to blow up a firm, new balloon. More pressure is generated as you blow, and the balloon is a lot harder to inflate than a soft balloon, which expands easily under pressure. Blood vessels lose elasticity with aging, thus increasing resistance and blood pressure.
* Viscosity, or thickness of the blood. Just as a milkshake is harder to suck through a straw than milk is, increased blood viscosity will increase blood pressure. Increased numbers of red blood cells, as in polycythemia, or a loss of plasma volume, as by dehydration,
will increase blood viscosity. The hematocrit test is one measure of blood viscosity; it measures the relative percentage of packed cells in whole blood.
* Total blood volume, the total amount of blood that is in the vascular system at a given time. A loss of blood volume, as by hemorrhage, will lower blood pressure. An increase in blood volume will generate more pressure within the vessels. It will also increase cardiac output by increasing venous return of blood to the heart.
To summarize, all of these relationships are expressed together by the following equation: Blood pressure = cardiac output X peripheral resistance.
Measurement of Blood Pressure
The measurement and careful interpretation of blood pressure may prove a valuable guide in the care and evaluation of a person’s health. Because blood pressure decreases as the blood flows from arteries into capillaries and finally into veins, healthcare providers ordinarily measure arterial pressure only, most commonly in the brachial artery of the arm. They use an instrument called a sphygmomanometer (Fig. 11-14), or more simply, a blood pressure cuff or blood pressure apparatus. They measure two variables:
* Systolic pressure, which occurs during heart muscle contraction, averages about 120 and is expressed in millimeters of mercury (mmHg).
* Diastolic pressure, which occurs during relaxation of the heart muscle, averages about 80 mmHg.
The sphygmomanometer is an inflatable cuff attached to a device for reading pressure. The examiner wraps the cuff around the patient’s upper arm and inflates it with air until the brachial artery is compressed and the blood flow is cut off. Then, listening with a stethoscope, he or she slowly lets air out of the cuff until the first pulsations are heard. At this point, the pressure in the cuff is equal to the systolic pressure, and this pressure is read. Then, more air is let out until a characteristic muffled sound indicates the point where the vessel is open and the diastolic pressure is read. Sphygmomanometers originally displayed pressure readings on a graduated column of mercury, but alternate types display the readings on a dial, or measure blood pressure electronically and give a digital reading. Blood pressure is reported as systolic pressure first, then diastolic pressure, separated by a slash, such as 120/80 mmHg.
Considerable experience is required to ensure an accurate blood pressure reading. Often it is necessary to repeat measurements. Note also that blood pressure varies throughout the day and under different conditions, so a single reading does not give a complete picture. Some people typically have a higher reading in a doctor’s office because of stress, or “white coat hypertension.” They may need to take their blood pressure at home while relaxed to get a true reading.
Figure 11-14 Measurement of blood pressure. (A) A sphygmomanometer, or blood pressure cuff. (B) Once the cuff is inflated, the examiner releases the pressure and listens for sounds in the vessels with a stethoscope.
Abnormal Blood Pressure
Lowerthan- normal blood pressure is called hypotension. Because of individual variations in normal pressure levels, however, what would be a low pressure for one person might be normal for someone else. For this reason, hypotension is best evaluated in terms of how well the body tissues are being supplied with blood. A person whose systolic blood pressure drops to below his or her normal range may experience fainting episodes because of inadequate blood flow to the brain. The sudden lowering of blood pressure to below a person’s normal level is one symptom of shock; it may also occur in certain chronic diseases and in heart block. Hypertension, or high blood pressure, has received a great deal of attention in medicine. Hypertension normally occurs temporarily as a result of excitement or exertion. However, it may persist in a number of conditions, including the following:
* Kidney disease and uremia (excess nitrogenous waste in the blood) or other toxic conditions;
* Endocrine disorders, such as hyperthyroidism and acromegaly;
* Arterial disease, including hardening of the arteries (atherosclerosis), which reduces elasticity of the vessels;
* Tumors of the adrenal gland’s central portion (medulla) with the release of excess epinephrine.
Hypertension that has no apparent medical cause is called essential hypertension. Excess of an enzyme called renin, produced in the kidney, appears to play a role in the severity of this kind of hypertension. Renin raises blood pressure by causing blood vessels to constrict and by promoting the kidney’s retention of salt and water. It is important to treat even mild hypertension because this condition can eventually:
* Weaken vessels and lead to saclike bulges (aneurysms) in vessel walls that are likely to rupture. In the brain, vessel rupture is one cause of stroke. Rupture of a vessel in the eye may lead to blindness.
* Stress the heart by causing it to work harder to pump blood into the arterial system. In response to this greater effort, the heart enlarges, but eventually it weakens and becomes less efficient.
* Stress the kidneys and damage vessels in the kidneys.
* Damage the lining of vessels, predisposing to atherosclerosis.
Although medical caregivers often place emphasis on the systolic blood pressure, in many cases, the diastolic pressure is even more important. The total fluid volume in the vascular system and the condition of small arteries may have a greater effect on diastolic pressure.
Treatment of Hypertension
Even though there is much individual variation in blood pressure, guidelines have been established for the diagnosis and treatment of hypertension. The first stage of hypertension begins at 140/90 mmHg. Treatment at this point should be based on diet, exercise, and weight loss, if necessary. Drug therapy should be added to this regimen for people with pressure readings above 159/99 mmHg. Drugs used to treat hypertension include the following:
* Diuretics, which promote water loss;
* Drugs that limit production of renin;
* Drugs that relax blood vessels.