3 Applications of pressure

• 1.
• Blood pressure. Blood is a viscous fluid pumped by the heart through arteries and veins.
• Blood flow is normally steady.
• If it becomes turbulent it produces noise, detected by a stethoscope on the artery while measuring blood pressure.
• Two values are recorded:

• Systolic pressure: the maximum pressure in the artery when the heart muscle contracts and pushes blood from the left ventricle into the aorta. Normal: about 120 torr.
• Diastolic pressure: the minimum pressure when the heart muscle relaxes. Normal: about 80 torr. (The cuff is placed on the upper arm at heart level so that the height of the blood column does not add or subtract ρgh to the reading. Long sitting increases the risk of leg blood clots because blood flow in the legs is slow; moving and exercising the legs helps.)

• 2.
• Car-tyre pressure. A tyre filled with air at high pressure has a small contact area with the road.
• A tyre with too low a pressure has a large contact area, so more friction and more heat, and wear.
• Pressure is measured by a tyre-pressure gauge (a piston pushed against a spring with a graduated scale).

Example 1 (tyre). A tyre gauge shows 2.5 × 10⁵ N/m² above the atmosphere. Absolute pressure = 2.5 × 10⁵ + 1.013 × 10⁵ = 3.51 × 10⁵ N/m² (3.5 atm).

Example 2. Blood pressure of 120 torr = 120 mm Hg = 0.12 × 13 600 × 9.8 = 1.6 × 10⁴ N/m².