Physics: Pressure and Fluid Dynamics

10 Questions

What is the typical maximum volume in the bladder before voiding?

500 ml

What is the method of measuring bladder pressure by inserting a needle through the wall of the abdomen into the bladder?

Direct cystometry

What is the relationship between the increase in radius of the bladder and the increase in volume?

Volume increases as R3, radius increases

What is the highest pressure in the body found in?

Weight-bearing bone joints

Why does the pressure in the knee joint increase when walking?

Because the force is concentrated on a smaller area

How do bones adapt to reduce pressure?

By changing the shape of the bone

What is the relationship between the increase in radius of the bladder and the increase in pressure?

Pressure increases as R2, radius increases

What is the pressure in the esophagus usually compared to?

Intrathoracic pressure

What is the normal voiding pressure in the bladder?

20 to 40 cm H2O

What is the method of measuring bladder pressure by passing a catheter with a pressure sensor into the bladder?

Indirect cystometry

Study Notes

Pressure

Pressure (P) is a force per unit area: P = F/A
Units of pressure: pascal (Pa), pounds per square inch (psi), bar (bar), atmospheric pressure (atm), millimeters of mercury (mmHg), inches of mercury (inHg), torr

Blood Pressure

Measured using a sphygmomanometer
Systolic pressure: maximum pressure driving blood at the peak of the pulse
Diastolic pressure: lowest blood pressure between pulses
Normal blood pressure: 120 mmHg systolic, 80 mmHg diastolic

Pressure in the Body

Pressure in the circulatory system: affected by pumping action of the heart and losses associated with arterial expansion and contraction, and viscous friction
Hydrocephalus: increased pressure in the skull due to trapped cerebrospinal fluid
Glaucoma: increased pressure in the eye due to partial blockage of the drain system
Pressure in the digestive system:
- Increases during eating due to air swallowed and bacterial action
- Coupled to intrathoracic pressure through the diaphragm
- External factors such as belts, girdles, flying, and swimming affect gut pressure
Pressure in the urinary bladder:
- Increases with volume due to accumulation of urine
- Measured using indirect cystometry (passing a catheter with a pressure sensor) or direct cystometry (inserting a needle through the abdomen)
Pressure in the skeleton:
- Highest pressures found in weight-bearing bone joints (up to 10 atm)
- Bone adaptation reduces pressure by increasing surface area (e.g., flat finger bones)

Practice Problems

Example 1: Calculate the pressure exerted by a person on the ground
Example 2: Calculate the force exerted by the atmosphere over an area
Example 3: Calculate the pressure exerted by the air on the bottom surface of a rectangular box
Example 4: Calculate the mass of a vertical column of air that exerts a pressure of 1 atm over a square surface
Example 5: Calculate the gauge pressure and absolute pressure at the oil-water interface and at the bottom of a container
Example 6: Calculate the difference in blood pressure between the head and feet of a person