Pressure in Physics

Pressure

• Pressure is defined as force per unit area.
• Units for pressure include Pascals (Pa), atmospheres (atm), mmHg, and cmH₂O.
• One Pascal is equal to one Newton per square meter (N/m²).
• One atmosphere (atm) is equal to 1.013 x 10⁵ Pa, 760 mmHg, or 1033 cmH₂O.
• Pressure in a liquid column (P) can be calculated using the formula P = pgh, where p is the density of the liquid, g is the acceleration due to gravity, and h is the height of the column.
• Mercury has a density of approximately 13.6 g/cm³ and water has a density of 1 g/cm³. A water column would need to be 13.6 times higher than a mercury column to exert the same pressure.
• Blood has a density of approximately 1.06 g/cm³ at 37°C.

Types of Pressures

• Blood pressure.
• Pressure inside the skull (intracranial pressure).
• Eye pressure (intraocular pressure).
• Pressure in the skeleton.
• Pressure in the urinary bladder.
• Ear pressure.
• Pressure in the lungs.
• Hyperbaric oxygen therapy pressure.
• Pressure in the digestive system.

Negative Pressure

• Negative pressure is any pressure lower than atmospheric pressure.
• During inhalation, the pressure in the lungs must be lower than atmospheric pressure.
• The pressure in the lungs during inhalation is a few centimeters of water pressure.

Calculating Atmospheric Pressure

• Example calculation: Calculate atmospheric pressure when the height of the mercury column is 76 cm.
• P = pgh = 13.6 g/cm³ * 980 cm/s² * 76 cm = 1.013 x 10⁶ dyn/cm² = 1.013 x 10⁵ N/m².

Blood Pressure

• Sphygmomanometer is the most common clinical instrument for measuring blood pressure.
• Two types of pressure gauges used in sphygmomanometers are mercury manometers and aneroid gauges.
• Recently, a digital sphygmomanometer is used.
• Blood pressure is a routine clinical measurement of health.
• It is measured using a sphygmomanometer, which consists of a cuff, a bulb, and a meter to measure the pressure in the cuff.

Blood Pressure Ranges

• Blood pressure is measured in systolic/diastolic form.
• Normal blood pressure is less than 120/80 mmHg.
• Prehypertension: systolic < 120 and diastolic < 80
• Stage 1 High blood pressure : 140-159
• Stage 2 High blood pressure: 160 or higher

Measuring Blood Pressure

• Blood pressure is measured using a sphygmomanometer.
• Measurement takes place at the brachial artery of the upper arm.
• Two pressures are recorded: systolic and diastolic.
• Systolic pressure occurs during ventricular contraction.
• Diastolic pressure occurs between heartbeats, during ventricular relaxation.
• Normal blood pressure for young adults is typically 120/75 mm Hg.

Abnormalities of Blood Pressure

• Hypertension (high blood pressure): Chronic resting blood pressure greater than 140/90. Can weaken small blood vessels and cause aneurysms.
• Hypotension (low blood pressure): Chronic low resting blood pressure, may be caused by blood loss, dehydration, or anemia

Pressure Inside the Skull

• The brain is filled with approximately 150 cm³ of cerebrospinal fluid (CSF).
• Hydrocephalus is a disorder resulting from blocked CSF flow or trapped CSF in the skull, causing raised intracranial pressure.

Measuring Hydrocephalus

• Crude method: Measuring the circumference of the skull above the ear. Normal values for infants are 32-37 cm. Larger head size may indicate hydrocephalus.
• Qualitative method (transillumination): Using light scattering properties, method used to evaluate brain fluid levels.

Eye Pressure

• The eye contains aqueous and vitreous humors (fluid) that transmit light.
• Eye pressure (intraocular pressure) maintains the eye's shape and size.
• Normal eye pressure ranges from 12-23 mmHg.
• Glaucoma is a condition where fluid drainage is blocked leading to increased pressure and vision impairment. Glaucoma can be mild (tunnel vision) or severe (blindness).

Measuring Eye Pressure

• A physician can estimate eye pressure by feeling it with their fingertips.
• Tonometers are instruments that precisely measure intraocular pressure.