22 9-11 Flashcards
Pulmonary ventilation is?
A mechanical process causing gas flow into and out of the lungs according to volume changes in the thoracic cavity.
*Boyle’s law
At a constant temperature, the pressure of a gas varies inversly with its volume. Gases always fill their container. In a large container the molecules of gas will be farther apart and pressure will be low. If the volume of the container is reduced, the molecules will be forced together and pressure will rise.
*Which two forces act to pull the lungs (visceral pleura) away from the thorax wall (parietal pleura) and cause the lung to collapse?
- The lungs natural tendency to recoil
(because of elasticity, lungs always assume the smallest size possible) - Surface tension of the alveolar fluid
(molecules of the fluid lining the alveoli attract each other - surface tension constantly draw alveoli to their smallest dimension)
*What force acts to pull the thorax outward and enlarge the lungs?
The elasticity of the chest wall.
Why is it important for Pip to remain negative? How is it kept negative?
An equalization of pressure would cause lung collapse. Kept negative by draining of the pleural fluid into the lymph system and by the tight coupling of the lungs to the thorax wall (surface tension of pleural fluid).
Intrapulmonary pressure
Pressure in the alveoli.
Rises and falls during respiration, but equalizes with atmospheric pressure.
Pressure inside the lung decreases as lung volume increases suring inspiration; pressure increases during expiration.
Intrapleural pressure
Pressure in the plural cavity.
Rises and falls with respiration, but is always about 4mmhg less thabn intrapulmonary pressure.
Pleural cavity pressure becomes more negative as chest wall expands during inspiraton; returns to initial value as chest wall recoils.
Result of dynamic interplay between lungs tendency to recoil + surface tension (collapse) vs. elasticity of chest wall (expand).
Transpulmonary pressure
Ppul - Pip The size of the transpulmonary pressure determines the size of the lungs at any moment - the greater the transpulmonary pressure, the larger the lungs.
Basic rule of pulmonary ventilation?
Volume changes lead to pressure changes and pressure changes lead to the flow of gases to equalize the pressure.
*Inspiration steps
And boyle’s law relation to inspiration
- Inspiratory muscles contract - diagphram descends, rib cage rises.
- Thoracic cavity volume increases.
- Lungs are stretched; intrapulmonary volume increases.
- Intrapulmonary/Ppul pressure drops (-1mmhg)
- Air flows into lungs down its pressure gradient until Pip is 0 (equal to atmospheric pressure)
Boyle’s law: when the thoracic cavity volume increases, the pressure drops creating a pressure gradient - higher Patm - which causes air to flow in.
*Expiration steps
And boyle’s law relation to expiration
- Inspiratory muscles relax - diagphram rises, rib cage descends from recoil of costal cartilage.
- Thoracic cavity volume decreases.
- Elastic lungs recoil passively; intrapulmonary/Ppul volume decreases
- Intrapulmonary pressure rises (+1mmhg)
- Air flows out of lungs down its pressure gradient until intrapulmonary pressure is 0.
Boyle’s law: when the thoracic cavity volume decreases, the pressure increases creating a pressure gradient - higher Ppul than Patm - which causes air to flow out.
What happens when Ppul
Air rushes into lungs. (Intrapulmonary pressure is less than the atmospheric pressure)
When does inspiration end?
When Ppul = Patm. At the same time, Pip declines to about -6mmhg relative to Patm
What are the determinants of Ppul?
- Quantity of air in the alveoli 2. Volume of alveoli (determined by the respiratory muscles)
*Quiet inspiration vs. forced inspiration
QUIET: Diagphram and external intercostals contract, ↑ thoracic volume, ↓ Ppul (below Patm), air flows in.
FORCED: Scalenes, sternocleidomastoid and pectoralis minor/thorax also contract, ↑ thoracic volume beyond the ↑ in volume during quiet inspiration
