Blood Pressure


Homeostasis is the tendency for the body to maintain itself in a stable state. Many control mechanisms accomplish this. For instance, at the biochemical level, end products of a chemical reaction chain may feed back to the beginning of the chain to suppress an overproduction of the end product. Buffers prevent the body pH from changing too radically. Negative feedback loops in the neuronal circuitry prevent the impulses along a neural pathway from having too much of an effect. Rather than rote memorization of the reactions of the body to all kinds of stress (which will increase your own stress), you can save a lot of studying time by appreciating, logically, that in just about all areas of body function there are homeostatic mechanisms that will do what one would expect: namely, act efficiently to prevent reactions from going too far and to restore stability to body function. Such mechanisms are especially prominent in the interactions of the cardiovascular, renal, and pulmonary systems.

What Is Blood Pressure?

In the usual context, blood pressure refers to brachial artery pressure, which in the average adult is about 120/80. The "120" (in mm Hg) refers to systolic pressure, the brachial arterial pressure during cardiac ventricular contraction. The "80" is diastolic pressure, the brachial arterial pressure during cardiac ventricular relaxation. The average between the two, thus, is about 100. Such an average of 100 may be abnormal if the pulse difference (termed the arterial pulse pressure) is wide. For instance:

1) A pressure of 160/40 for which the average is also 100, may occur in arteriosclerosis, wherein hardening of the arteries permits little flexibility of the vessels. The pressure then may dramatically rise in systole, due to vascular rigidity, and dramatically fall back in diastole.

2) In aortic regurgitation, where blood flows backward through an incompetent aortic valve, the diastolic pressure may be low, due to the runoff of blood back to the heart, whereas the systolic pressure may be elevated due to the added volume of (the returned) blood that the heart has to pump with each beat.

3) Patent ductus arteriosus (Fig. 2-1). Before birth, blood largely bypasses the lungs and flows from the pulmonary artery directly into the aorta via the ductus arteriosus, because the collapsed lungs resist pulmonary blood flow; the fetus does not oxygenate its blood via the lungs. Normally the ductus closes after birth. If this does not occur, blood flows from the aorta redundantly into the pulmonary circulation. This back-flow causes an attempt to increase the cardiac output, with limited tolerance to exercise. In each diastole of patent ductus, there is relatively little resistance to blood flow, as blood, in addition to its normal flow down the aorta, also runs back into the pulmonary circulation, thereby reducing diastolic pressure. The extra volume that the heart then has to pump out during systole results in a higher systolic pressure.

Fig. 2-1. Patent ductus arteriosus.

Fig. 2-2. Mean blood pressure* varies in different parts of the circulation, being about 100 in the major arteries where it is usually tested, and close to 0 where the venae cava enter the heart. This progressive drop

Figure 2-1

in mean blood pressure is the result of resistance to blood flow, particularly at the arteriolar level, where the vessels narrow to become capillaries. By the time blood has reached the capillaries, mean blood pressure is down to about 30mm Hg. The difference in

*The term "mean arterial blood pressure" means something slightly different from the average between the sys


Aorta and Large Arteries

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