Volume/Pressure & Inspiration/Expiration

1. Boyle's Law on Volume/Pressure Relationships
1. Volume is INVERSELY proportional to Pressure
• INCREASE in Volume -> DECREASE in Pressure
• DECREASE in Volume -> INCREASE in Pressure
• VOLUME change --> PRESSURE change----> gas flows to equalize the pressure
2. 2. Simple Example of Boyle's Law
• - plastic bag with plastic tube in the top
• - as bag expands by pulling, gas moves IN
• - as bag shrinks by squashing, gas moves OUT
2. Inspiration
1. diaphragm muscle contracts, increasing thoracic cavity size in the superior-inferior dimension
2. external intercostal muscles contract, expanding lateral & anterior-posterior dimension
3. INCREASED volume (about O.5 liter) -----> DECREASED pulmonary pressure (-1 mm Hg) -----> air rushes into lungs to fill alveoli
4. deep/forced inspirations - as during exercise and pulmonary disease
• scalenes, sternocleidomastoid, pectorals are used for more volume expansion of thorax
3. Expiration
1. quiet expiration (exhalation) - simple elasticity of the lungs DECREASES volume -----> INCREASED pulmonary pressure -> movement of air out of the lungs
2. forced expiration - contraction of abdominal wall muscles (ie. obliques & transversus abdominus) further DECREASES volume beyond relaxed point ----> further INCREASE in pulmonary pressure -----> more air moves out

Category: Physiology Notes