11.9 Pressures in the body

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• Explain the concept of pressure the in human body.
• Explain systolic and diastolic blood pressures.
• Describe pressures in the eye, lungs, spinal column, bladder, and skeletal system.

Pressure in the body

Next to taking a person’s temperature and weight, measuring blood pressure is the most common of all medical examinations. Control of high blood pressure is largely responsible for the significant decreases in heart attack and stroke fatalities achieved in the last three decades. The pressures in various parts of the body can be measured and often provide valuable medical indicators. In this section, we consider a few examples together with some of the physics that accompanies them.

[link] lists some of the measured pressures in mm Hg, the units most commonly quoted.

Typical pressures in humans
Body system Gauge pressure in mm Hg
Blood pressures in large arteries (resting)
Maximum (systolic) 100–140
Minimum (diastolic) 60–90
Blood pressure in large veins 4–15
Eye 12–24
Brain and spinal fluid (lying down) 5–12
While filling 0–25
When full 100–150
Chest cavity between lungs and ribs −8 to −4
Inside lungs −2 to +3
Digestive tract
Esophagus −2
Stomach 0–20
Intestines 10–20
Middle ear <1

Blood pressure

Common arterial blood pressure measurements typically produce values of 120 mm Hg and 80 mm Hg, respectively, for systolic and diastolic pressures. Both pressures have health implications. When systolic pressure is chronically high, the risk of stroke and heart attack is increased. If, however, it is too low, fainting is a problem. Systolic pressure increases dramatically during exercise to increase blood flow and returns to normal afterward. This change produces no ill effects and, in fact, may be beneficial to the tone of the circulatory system. Diastolic pressure can be an indicator of fluid balance. When low, it may indicate that a person is hemorrhaging internally and needs a transfusion. Conversely, high diastolic pressure indicates a ballooning of the blood vessels, which may be due to the transfusion of too much fluid into the circulatory system. High diastolic pressure is also an indication that blood vessels are not dilating properly to pass blood through. This can seriously strain the heart in its attempt to pump blood.

Blood leaves the heart at about 120 mm Hg but its pressure continues to decrease (to almost 0) as it goes from the aorta to smaller arteries to small veins (see [link] ). The pressure differences in the circulation system are caused by blood flow through the system as well as the position of the person. For a person standing up, the pressure in the feet will be larger than at the heart due to the weight of the blood $\left(P=\mathrm{h\rho g}\right)$ . If we assume that the distance between the heart and the feet of a person in an upright position is 1.4 m, then the increase in pressure in the feet relative to that in the heart (for a static column of blood) is given by

$\Delta P=\Delta \mathrm{h\rho g}=\left(1.4 m\right)\left(\text{1050 kg}{\text{/m}}^{3}\right)\left(9.80 m{\text{/s}}^{2}\right)=1\text{.}4×{\text{10}}^{4}\phantom{\rule{0.25em}{0ex}}\text{Pa}=\text{108 mm Hg}.$

Increase in pressure in the feet of a person

$\Delta P=\Delta \mathrm{h\rho g}=\left(1.4 m\right)\left(\text{1050 kg}{\text{/m}}^{3}\right)\left(9.80 m{\text{/s}}^{2}\right)=1\text{.}4×{\text{10}}^{4}\phantom{\rule{0.25em}{0ex}}\text{Pa}=\text{108 mm Hg}.$

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