Control of Blood Pressure

Blood pressure (BP) is determined by cardiac output (CO) and peripheral resistance (PR) according to this relationship: BP = CO x PR. Maintenance of normal blood pressure therefore requires regulation of these two factors.

Cardiac output depends on the volume of blood discharged from the left ventricle with each contraction

Measurement of Arterial Blood Pressure

Systemic arterial blood pressure usually is measured using an instrument called a sphygmomanometer (sfig"mo-mah-nom'e-ter) (fig. 15F). This device consists of an inflatable cuff connected by tubing to a compressible bulb and a pressure gauge. The bulb is used to pump air into the cuff, and a rise in pressure is indicated on the pressure gauge. The pressure in the cuff is expressed in millimeters of mercury (mm Hg) based on earlier equipment that used a glass tube containing a column of mercury in place of a pressure gauge.

To measure arterial blood pressure, the cuff of the sphygmomanometer is usually wrapped around the arm so that it surrounds the brachial artery. Air is pumped into the cuff until the cuff pressure exceeds the pressure in that artery. As a result, the vessel is squeezed closed and its blood flow stopped. At this moment, if the diaphragm of a stethoscope is placed over the brachial artery at the distal border of the cuff, no sounds can be heard from the vessel because the blood flow is interrupted. As air is slowly released from the cuff, the air pressure inside it decreases. When the cuff pressure is approximately equal to the systolic blood pressure within the brachial artery, the artery opens enough for a small amount of blood to spurt through. This movement produces a sharp sound (Korotkoff's sound) that can be heard through the stethoscope. The height of the mercury column when this first tapping sound is heard represents the arterial systolic pressure (SP).

As the cuff pressure continues to drop, a series of increasingly louder sounds can be heard. Then, when the cuff pressure is approximately equal to that within the fully opened artery, the sounds become abruptly muffled and disappear. The height of the mercury column when this happens represents the arterial diastolic pressure (DP). The sound results from turbulence that occurs when the artery narrows.

The results of a blood pressure measurement are reported as a fraction, such as 120/80. In this notation, the upper number indicates the systolic pressure in mm Hg (SP), and the lower number indicates the diastolic pressure in mm Hg (DP). Figure 15G shows how these pressures decrease as distance from the left ventricle increases.

The difference between the systolic and diastolic pressures (SP-DP), which is called the pulse pressure (PP), is generally about 40 mm Hg.

The average pressure in the arterial system is also of interest because it represents the force that is effective throughout the cardiac cycle for driving blood to the tissues. This force, called the mean arterial pressure, is approximated by adding the diastolic pressure and one-third of the pulse pressure (DP + 1/3PP). ■

(stroke volume) and heart rate. Mechanical, neural, and chemical factors affect these actions.

The blood volume entering the ventricle, for example, affects the stroke volume. As blood enters, myocar-dial fibers in the ventricular wall are mechanically stretched. Within limits, the greater the length of these fibers, the greater the force with which they contract. This relationship between fiber length (due to stretching of the cardiac muscle cell just before contraction) and force of contraction is called Starling's law of the heart. Because of it, the heart can respond to the immediate demands placed on it by the varying volumes of blood that return from the venous system. In other words, the more blood that enters the heart from the veins, the greater the ventricular distension, the stronger the ventricular contraction, the greater the stroke volume, and the greater the cardiac output (fig. 15.37). The less blood that returns from the veins, the less the ventricular distension, stroke volume, and cardiac output, and the weaker the ventricular contraction.

Starling's law of the heart ensures that the volume of blood discharged from the heart is equal to the volume entering its chambers. This further ensures that the volume of blood pumped by both ventricles is, on average, equal.

Recall that baroreceptors in the walls of the aortic arch and carotid sinuses sense changes in blood pressure. If arterial pressure increases, nerve impulses travel from the receptors to the cardiac center of the medulla oblongata. This center relays parasympathetic impulses to the S-A node in the heart, and heart rate decreases in response. As a result of this cardioinhibitor reflex, cardiac output falls, and blood pressure decreases toward the normal level. Figure 15.38 summarizes this mechanism.

Figure

A sphygmomanometer is used to measure arterial blood pressure. The use of the column of mercury is the most accurate measurement, but due to environmental concerns is being replaced by alternative gauges and digital readouts.

Figure

Blood pressure decreases as the distance from the left ventricle increases.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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