Alterations in Blood Pressure Benjamin J. Miller
K E Y Q U E S T I O N S • How do changes in cardiac output and systemic vascular
resistance affect blood pressure? • How is blood pressure regulated on a short- and long-term basis? • What are the risk factors for the development of primary
hypertension? • How is secondary hypertension defined, and what are the
common etiologies?
• How is hypertension detected, classified, and managed? • What are the end-organ consequences of inadequately controlled
hypertension? • What are the differences between hypertensive emergency and
hypertensive urgency, and how are they managed? • What are the risk factors for orthostatic hypotension, and how is
the condition managed?
C H A P T E R O U T L I N E Arterial Blood Pressure, 337
Determinants of Systemic Blood Pressure, 337
Measurement of Blood Pressure, 338
Components of Blood Pressure Measurement, 338 Direct Measurement of Blood Pressure, 339 Indirect Measurement of Blood Pressure, 339
Mechanisms of Blood Pressure Regulation, 341 Short-Term Regulation of Systemic Blood Pressure, 341
Long-Term Regulation of Systemic Blood Pressure, 341
Normal Fluctuations in Systemic Blood Pressure, 343
Hypertension, 343 Definition and Classification, 343
Primary Hypertension, 344
Subtypes, 344 Risk Factors, 344 Outcomes, 345 Treatment Interventions, 346
Secondary Hypertension, 348
Hypertensive Emergencies and Urgency, 349
Low Blood Pressure, 350
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Meeting the needs of the body’s tissues for oxygen and nutrients requires both adequate blood flow at the tissue level and sufficient perfusion pressure systemically to force that blood forward. The systemic arterial blood pressure provides the momentum, and the tissues depend on its preservation to ensure their metabolic needs are met. This maintenance requires a complex regulatory system. The body’s organs can be damaged if the perfusion pressure is insufficient or if it is excessive.
ARTERIAL BLOOD PRESSURE As described in Chapter 15, oxygenated blood is propelled from the left side of the heart into the arterial circulatory system, and following a pressure gradient, travels to the capillary beds of the body’s tissues (Fig. 16.1). There, oxygen and nutrients are exchanged for metabolic wastes, and the blood then returns to the right side of the heart via the venous circulatory system, where it passes through the lungs to repeat the process. It is the pressure difference between the left and right sides of the heart that produces the gradient allowing this systemic movement of blood. The arterial blood pressure is produced by the force of the
left ventricular contraction overcoming the resistance of the aorta to open the aortic valve, and is the pressure maintained in the arterial system throughout the cardiac cycle.
Determinants of Systemic Blood Pressure The systemic arterial blood pressure is the physiologic result of the cardiac output and the resistance to the ejection of blood from the heart. Cardiac output (CO) is the product of two variables: stroke volume (SV) and heart rate (HR) (CO = SV × HR). SV is the specific volume of blood leaving the heart with each contraction, which itself is deter- mined by the volume of blood in the heart before systole (end-diastolic volume) and the contractility of the myocardium. The end-diastolic volume is determined by the amount of blood returned to the heart between contractions, and is typically called the heart’s preload. Stroke volume multiplied by the number of contractions of the heart per minute (heart rate) determines the amount of blood leaving the heart—the cardiac output, measured in liters per minute. The resistance to ejection into the arterial circulation is known as the systemic vascular resistance (SVR) and is determined by the radius of arteries and the
• Review Questions and Answers • Glossary (with audio pronunciations for selected terms) • Animations
• Case Studies • Key Points Review
338 UNIT IV Oxygen Transport, Blood Coagulation, Blood Flow, and Blood Pressure
termed the pulse pressure. Therefore the pulse pressure for a systolic pressure of 110 mm Hg and a diastolic pressure of 70 mm Hg would be 40 mm Hg.
Systolic and diastolic values are normed by age. Standards for the identification of normal blood pressure and levels of abnormal elevation have been established. The most precise standards for children are those based on height, age, and gender (Table 16.1). Standards for blood pressure have likewise been determined for the adult (Table 16.2).
Mean arterial pressure (MAP) is the calculated average pressure within the circulatory system throughout the cardiac cycle. Because more time is spent in diastole than in systole, MAP is not the arithmetic average of diastolic and systolic pressure, but rather reflects the relative time spent in each portion of the cardiac cycle. The calculation may be performed by computer during direct arterial blood pressure measure- ment, as described later, but is most conveniently determined by a
degree of vessel compliance. SVR is synonymous with cardiac afterload, and can be altered by constricting or relaxing (dilating) arterial smooth muscle. It can be calculated by using a derivation of Poiseuille’s law (see Chapter 15). This physical law states that in a tube with laminar flow, resistance is primarily determined by three factors: the radius of the tube, the length of the tube, and the viscosity of the fluid. Applied to SVR, because the viscosity of the blood and the total length of the arterial system are normally relatively constant, the radius of the arterioles becomes the major determinant of resistance. Therefore the formula for blood pressure is BP = CO × SVR. Alteration in any one of these variables will result in a change in blood pressure. This basic concept is important to normal physiologic function, disorders of blood pressure, and the therapeutic interventions undertaken to treat them. The pul- monary vascular bed contributes minimally to total systemic resistance and is seen as a separate resistance system, called pulmonary vascular resistance. It has its own pathology discussed in Chapter 21.
Measurement of Blood Pressure Components of Blood Pressure Measurement Arterial blood pressure is measured from its highest point during cardiac systole to its lowest during diastole. These are referred to as systolic pressure and diastolic pressure, respectively, and are measured in mil- limeters of mercury (mm Hg). During ventricular contraction, the pressure in the aorta rises to an average peak value of approximately 110 mm Hg in the adult (see Fig. 16.1). Whatever this peak pressure may be, it is referred to as the systolic blood pressure. The smooth muscle of the aorta passively recoils from this point, ejecting blood forward into the peripheral arteries at that given pressure. Stroke volume is the primary factor affecting systolic pressure; an increase or decrease in SV produces a corresponding change in systolic blood pressure. During ventricular diastole, the pressure in the arterial system falls to an average minimum value of 70 mm Hg in the adult. The value of this minimum pressure is called the diastolic blood pressure. SVR is the major determinant of diastolic blood pressure; an increase or decrease in diastolic pressure is the result of a corresponding increase or decrease in arterial resistance (SVR). The difference between systolic and diastolic blood pressure is
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