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Disorders of Blood Pressure Regulation 3 Disorders of Blood Chapter 23 Pressure Regulation CAROL M. PORTH THE ARTERIAL BLOOD PRESSURE ➤ Blood pressure is probably one of the most variable but best- Mechanisms of Blood Pressure Regulation regulated functions of the body. The purpose of the control of Short-Term Regulation blood pressure is to keep blood flow constant to vital organs such Long-Term Regulation as the heart, brain, and kidneys. Without constant blood flow to Blood Pressure Measurement these organs, death ensues within seconds, minutes, or days. HYPERTENSION Although a decrease in flow produces an immediate threat to Essential Hypertension life, the continuous elevation of blood pressure that occurs with Constitutional Risk Factors hypertension is a contributor to premature death and disability Lifestyle Risk Factors Target-Organ Damage because of its effects on the heart, blood vessels, and kidneys. Diagnosis The discussion in this chapter focuses on determinants of Treatment blood pressure and conditions of altered arterial pressure— Systolic Hypertension hypertension and orthostatic hypotension. Secondary Hypertension Renal Hypertension Disorders of Adrenocortical Hormones Pheochromocytoma THE ARTERIAL BLOOD PRESSURE Coarctation of the Aorta Oral Contraceptive Drugs Malignant Hypertension After completing this section of the chapter, you should High Blood Pressure in Pregnancy be able to meet the following objectives: Classification Diagnosis and Treatment ■ Define the terms systolic blood pressure, diastolic High Blood Pressure in Children and Adolescents blood pressure, pulse pressure, and mean arterial Diagnosis and Treatment blood pressure. High Blood Pressure in the Elderly ■ Explain how cardiac output and peripheral vascular Diagnosis and Treatment resistance interact in determining systolic and dia- ORTHOSTATIC HYPOTENSION stolic blood pressure. Pathophysiology and Causative Factors ■ Causes Describe the mechanisms for short-term and long- Diagnosis and Treatment term regulation of blood pressure. ■ Describe the requirements for accurate and reliable blood pressure measurement in terms of cuff size, deter- mining the maximum inflation pressure, and deflation rate. The arterial blood pressure reflects the rhythmic ejection of blood from the left ventricle into the aorta.1–3 It rises during sys- tole as the left ventricle contracts and falls as the heart relaxes during diastole. The contour of the arterial pressure tracing shown in Figure 23-1 is typical of the pressure changes that occur in the large arteries of the systemic circulation. There is a rapid rise in the pulse contour during left ventricular contrac- tion, followed by a slower rise to peak pressure. Approximately 70% of the blood that leaves the left ventricle is ejected during the first one third of systole, accounting for the rapid rise in the pressure contour. The end of systole is marked by a brief down- 1 2 Unit VI Disorders of Cardiovascular Function Systolic pressure (peak) are referred to as the resistance vessels because they can selec- 120 tively constrict or relax to control the resistance to outflow of Mean Dicrotic notch blood into the capillaries. The body maintains its blood pressure arterial Pulse by adjusting the cardiac output to compensate for changes in pressure pressure peripheral vascular resistance and changes the peripheral vascu- lar resistance to compensate for changes in cardiac output. 80 In hypertension and disease conditions that affect blood Diastolic pressure, changes in blood pressure usually are described in pressure (minimum) terms of the systolic and diastolic pressures, pulse pressure, and mean arterial pressure. These pressures are influenced by the stroke volume, the rapidity with which blood is ejected from the 40 heart, the elastic properties of the aorta and large arteries and their ability to accept various amounts of blood as it is ejected from the heart, and the properties of the resistance blood ves- sels that control the runoff of blood into the smaller vessels and capillaries that connect the arterial and venous circulations. 0 (mm sec) Mechanisms of Blood FIGURE 23-1 • Intra-arterial pressure tracing made from the bra- Pressure Regulation chial artery. Pulse pressure is the difference between systolic and Although different tissues in the body are able to regulate their diastolic pressures. The darker area represents the mean arterial pres- sure, which can be calculated by using the formula of mean arterial own blood flow, it is necessary for the arterial pressure to remain pressure = diastolic pressure + pulse pressure/3. relatively constant as blood shifts from one area of the body to another. The mechanisms used to regulate the arterial pressure depend on whether short-term or long-term adaptation is needed2 ward deflection and formation of the dicrotic notch, which (Fig. 23-2). occurs when ventricular pressure falls below that in the aorta. The sudden closure of the aortic valve is associated with a small Short-Term Regulation rise in pressure caused by continued contraction of the aorta and other large vessels against the closed valve. As the ventricles The mechanisms for short-term regulation of blood pressure, relax and blood flows into the peripheral vessels during dias- those acting over minutes or hours, are intended to correct tem- tole, the arterial pressure falls rapidly at first and then declines porary imbalances in blood pressure, such as occur during slowly as the driving force decreases. physical exercise and changes in body position. These mecha- In healthy adults, the pressure at the height of the pressure nisms also are responsible for maintenance of blood pressure pulse, called the systolic pressure, ideally is less than 120 mm Hg, at survival levels during life-threatening situations such as dur- and the lowest pressure, called the diastolic pressure, is less than ing an acute hemorrhagic incident. The short-term regulation 80 mm Hg (Fig. 23-2). The difference between the systolic and of blood pressure relies mainly on neural and humoral mecha- diastolic pressure (approximately 40 mm Hg) is called the pulse nisms, the most rapid of which are the neural mechanisms. pressure. The magnitude of the pulse pressure reflects the volume of blood ejected from the left ventricle during a single beat (stroke Neural Mechanisms. The neural control centers for the volume) and the total distensibility of the atrial tree. The mean regulation of blood pressure are located in the reticular forma- arterial pressure (approximately 90 to 100 mm Hg), represents tion of the medulla and lower third of the pons, where integra- the average pressure in the arterial system during ventricular con- tion and modulation of autonomic nervous system (ANS) traction and relaxation and is a good indicator of tissue perfusion. responses occur.2 This area of the brain contains the vasomo- Notice that the mean arterial pressure is not a simple mathemat- tor and cardiac control centers and is often collectively referred ical average of systolic and diastolic pressures. This is because a to as the cardiovascular center. The cardiovascular center greater fraction of each cardiac cycle is spent in diastole rather transmits parasympathetic impulses to the heart through the than in systole. vagus nerve and sympathetic impulses to the heart and blood The mean arterial blood pressure is determined mainly by vessels through the spinal cord and peripheral sympathetic the cardiac output (stroke volume × heart rate) and the peripheral nerves. Vagal stimulation of the heart produces a slowing vascular resistance, and can be expressed as the product of the of heart rate, whereas sympathetic stimulation produces an two (mean arterial blood pressure = cardiac output × peripheral increase in heart rate and cardiac contractility. Blood vessels vascular resistance). The peripheral vascular resistance reflects are selectively innervated by the sympathetic nervous system. changes in the radius of the arterioles as well as the viscosity or Increased sympathetic activity produces constriction of the thickness of the blood (see Chapter 21 for a discussion of cardiac small arteries and arterioles with a resultant increase in periph- output and peripheral vascular resistance). The arterioles often eral vascular resistance. Chapter 23 Disorders of Blood Pressure Regulation 3 Arterial blood pressure Cardiac output Peripheral vascular resistance Sympathetic activity Stroke volume Heart rate Vagal and Heart sympathetic activity Baroreceptors Venous return Angiotensin II Blood volume Adrenal gland Aldosterone FIGURE 23-2 • Mechanisms of blood pres- Salt and water sure regulation. The solid lines represent the retention mechanisms for renal and baroreceptorcon- trol of blood pressure through changes in cardiac output and peripheral vascular resis- Renin-angiotensin tance. The dashed lines represent the stimu- mechanism lus for regulation of blood pressure by the Kidney baroreceptors and the kidneys. The ANS control of blood pressure is mediated through vasoconstriction that causes an increase in peripheral vascular intrinsic circulatory reflexes, extrinsic reflexes, and higher neural resistance. control centers. The intrinsic reflexes, including the barorecep- The arterial chemoreceptors are chemosensitive cells that tor and chemoreceptor reflexes, are located in the circulatory monitor the oxygen, carbon dioxide, and hydrogen ion content system and are essential for rapid and short-term regulation of of the blood. They are located in the carotid bodies, which lie in blood pressure. The sensors
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