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An Introductory Physics Course with Peter Eyland
Lecture 11 (Pressures in the body))

In this lecture the following are introduced:
• Pressure and the eyes
• Pressure and the lungs
• Pressure and digestion
• Pressure and the joints
• Blood Pressure
• Pressure gauges

Pressure and the eyes

the eye

The clear fluids in the eye are called the aqueous humour and the vitreous humour. The pressures they exert keeps the eyeball in a fixed size and shape. The pressure in the aqueous humour (anterior chamber) is normally 12->23 mm Hg (1.6 -> 3.0 kPa). This is measured relative to atmospheric pressure.

Aqueous humour is produced by the eye continuously. When it does not drain properly then there is greater pressure inside the eye which is symptomatic of glaucoma.

This increased pressure affects the retina at the back of the eye.

normal eye eye with glaucoma

The blood supply to the nerves of the retina is reduced and the nerves start to die. As the optic nerve degrades, blind spots develop and vision changes. Peripheral vision (sight at the edges) is affected first and then the central vision.

peropheral vision affected central vision affected

Without treatment, glaucoma can cause blindness.

The Air Puff Test

To test for glaucoma your chin rests on a padded stand to secure the position of your eyes. You look into the examining instrument without blinking. The optometrist then shines a bright light into your eye to align the instrument. A brief puff of air is blown at your eye. The instrument calculates pressure from the change in the light reflected off your corneas as the air puff is blown.

The Lungs and Intrapleural Cavity

You breathe about 25,000 times a day and inhale about 10,000 litres of air. Inside the air spaces of the lungs there is normally air at atmospheric pressure, P0.

The outside of the lungs and the inside of the chest cavity is lined with a membrane called the pleura. The space inside the pleura (the intrapleural cavity) is almost empty and has only a few millilitres of fluid to lubricate the pleura during breathing. In this intrapleural cavity, the pressure is 5->10 mm Hg below atmospheric (0.7 -> 1.3 kPa). When you lower your diaphragm, or expand your rib cage, this momentarily expands the pleural cavity and the pressure in the air spaces of the lungs forces the lungs to increase in size and draw more air in.

People with asthma have sensitive airways that overreact to pollens, dust mites, cold air etc. When their airways are exposed to these, the linings of the airways become inflamed and swell thus restricting the air flow. Also, the muscles surrounding the airways tighten and narrow the airways further.


Emphysemia is a problem where the normal elasticity of the air sacs and the walls of the airways are destroyed. Such people have difficulty in getting air out of their lungs. Gasping for breath usually doesn't help much because the lungs are already inflated. It is caused by smoking, working in coal mines, etc.

Pneumothorax is the lung collapse that happens when the pleural cavity is opened to atmospheric pressure by a puncture: This diagram is from the Mayo Clinic

Digestive system

The digestive tract is coupled to the lungs at the esophagus where pressure is less than atmospheric. The rest of the digestive tract is mostly above atmospheric, food being propelled by peristalsis. Peristalsis is a wave of smooth muscle contractions where there is a contraction above and relaxation below a segment of the tract. A (disposable) camera the size of a large vitamin pill has been developed which can be swallowed and pictures recorded along the entire length of the digestive tract.

Pressure in the stomach increases when stretched by eating. Trapped air in the stomach causes belching. Gas is also generated in the gut by bacterial action on fibre. Fibre is important because it keeps things moving. Here is an article on flatus called Tail wind


The highest pressures are in the weight bearing bone joints. Walking creates knee pressures of more than 10 Atmospheres (1 MPa).

Urinary bladder

At 30 cm H2O (3 kPa) the micturation ("gotta go") reflex occurs. Contraction can produce pressure up to 150 cm H2O (15 kPa). Normal voiding pressure is 20->40 cm H2O (2 -> 4 kPa). In male prostate obstruction the pressure can rise to 100 cm H2O (10 kPa).

Blood pressure

Rev. Stephen Hales 1733 (G.B.) made the first known experimental measurement of blood pressure. He connected a 9' (2.7m) vertical glass tube to an artery of a horse. He used a sharpened goose quill to puncture the artery and the goose's trachea for the flexible connection to the glass tube. He found blood rose to an average height of 8' (2.4m).

If you did that to a standing human, you would find:
pressure in the head gives a height of 0.8m (60 mm Hg)
pressure in the upper arm gives a height of 1.3m (100 mm Hg)
pressure in the ankle gives a height of 2.6m (200mm Hg)

The heart

The heart is a living pump


De-oxygenated (blue) blood comes back from the body to the right side of the heart. This is the low pressure side. Blood then gets pumped out to the lungs at 25/10 mm Hg (3.3/1.3 kPa). It takes about a minute for the total blood volume (5 litres) to flow through the lungs. Oxygenated (red) blood comes back from the lungs into the left (high pressure) side. Blood is pumped out from the left hand side to the body at systolic pressure (normal is less than 130mm Hg or 17.3 kPa). Blood is taken into the heart at diastolic pressure when it relaxes (normal is less than 85 mm Hg or 11.3 kPa).

Blood eventually goes out to the capillaries at about 0.5 mm Hg (0.07 kPa). For an animated picture, see the heartsite

Various gauges to measure pressure
The manometer

U tube

The manometer is a "U tube" filled with Mercury. It comes from a Greek word meaning "thiness" and measures pressure differences. A tube connects the short side to the pressurised vessel (or pipe) and the other end is open to the air. The difference in the heights of mercury, gives the pressure above atmospheric pressure.

The Bourdon gauge

bourdon mechanism

In 1849 Eugene Bourdon invented the Bourdon tube, which is an oval tube bent into a circular arc. Pressure in the tube will tend to straighten it out. This movement is usually then translated by gears and linkages into a 300 degree rotation.

Nowadays a helically wound elastic element is used that is attached directly to the pointer shaft (as shown in the green picture). This eliminates the need for any movement-amplifying gears or linkages, which are the parts that fail most frequently.

Usually a series of pressure sensing devices are used to protect against too much pressure and sudden returns to the zero. In contrast to the Mercury manometer, a Bourdon gauge can measure "absolute" pressure, i.e. pressure above vacuum.

The Sphygmomanometer


The "sphymomanometer" (sphygmos = "pulse") is a specialised Mercury manometer which measures blood pressure.

Using a sphygmomanometer to measure blood pressure.

measuring blood pressure

• Position pressure cuff on upper arm, a stethoscope is placed over a lower arm artery to listen for sounds.


• Pump up the pressure towards 180 mm Hg.
• Release the pressure slowly till tapping sounds are heard. This gives the systolic ("together") pressure to about 2mm accuracy. The sounds are due to turbulent flow of blood and are called Korotkoff sounds.
• Note the pressure when the sounds fade and the artery is completely open. This is the diastolic ("through") pressure to about 5mm accuracy.

You can get electronic devices which automatically give you the pressures and pulse rate. Less than 130/85 is normal (except for people over 65 less than 140/90 is considered normal). Hypertension or high blood pressure is greater than 140/90.


Normal eye pressure 12->23 mm Hg (1.6->3.0 kPa).
Interpleural pressure 5->10 mm Hg below atmospheric (0.7->1.3 kPa).
Pressure in the digestive tract is above atmospheric.
Pressure in the knee joint can be greater than 10 Atmospheres (> 1 MPa).
At 30 cm H2O (3 kPa) the micturation reflex occurs.
On the high pressure side of the heart, the normal range is less than 130/85 mm Hg.
On the low pressure side of the heart, the normal range is around 25/10 mm Hg.
A Mercury manometer is a "U tube" with Mercury; the difference in height gives the pressure above atmospheric.
The Bourdon gauge is an oval tube bent into a circular arc or helix. Pressure in the tube will tend to straighten it and it can give an "absolute" pressure.
The "sphymomanometer" is a specialised Mercury manometer which measures systolic and diastolic blood pressure.

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