Mechanics of breathing Flashcards
What does changes in lung volume induce?
Changes in lung volume induce changes in alveolar pressure, which generate pressure gradients between alveoli & atmosphere, causing air to flow
What happens in inspiration?
Diaphragm contracts & thoracic cavity expands. Alveolar pressure decreases
What happens in expiration?
Diaphragm relaxes (and lung recoils). Thoracic cavity volume decreases, alveolar pressure increases.
Comment on the pressure at the end of expiration
At the end of expiration, Palveoli = Patmosphere, therefore there is no movement of air
What happens during inspiration?(In terms of volume and pressure)
The outer surfaces of the lung are pulled outwards (expansion).
↑volume = ↓alveolar pressure.
Palveoli < Patmosphere
Air flows from high (atmosphere) to low (alveoli) pressure.
What happens during expiration?(In terms of volume and pressure)
Air within the lung is compressed.
↓volume = ↑alveolar pressure.
Palveoli > Patmosphere
Air flows from high (alveoli) to low (atmosphere) pressure.
How are the lungs and chest attached?
The lungs and chest wall are indirectly attached via the pleural cavity
What is the pleural cavity?
fluid filled space between the membranes (pleura) that line the chest wall and each lung.
What generates negative pressure within the pleural cavity?
The opposing elastic recoil of the chest wall (outward) and lungs (inward) generates negative pressure within the pleural cavity (sealed cavity + increased volume = decreased pressure).
What does negative pressure act to do?
Negative pressure acts to pull the two membranes together (more negative pressure = more force pulling pleurae together).
What does pleurae provide to aid movement of the lungs?
Pleurae also provide a frictionless surface to aid movement of the lungs.
What is the system in at the end of expiration?
System is in equilibrium
When will lung volume remain constant?
If the force pulling the visceral pleura inwards (lung recoil) = the force pulling the visceral pleura outwards (negative intrapleural pressure)
What happens to parietal pleura during inspiration and what does it decrease?
During inspiration, muscular contraction pulls the parietal pleura outwards, stretching the cavity, decreasing PIP
What does a decreased intrapleural pressure create?
The decreased (more negative) intrapleural pressure creates a greater level of force acting to pull the pleurae together (effectively acting to pull the visceral pleura outwards).
What happens when the force generated by decreased intrapleural pressure>than inward force generated by lung coil?
When the force generated by negative PIP becomes greater than the inward force generated by lung recoil, the visceral pleura (and the attached lungs) will be pulled outwards, expanding the lung.
What happens to the parietal pleura during expiration and what does it increase?
During (passive) expiration, relaxation of previously contracted respiratory muscles, reduces the outward force acting on the parietal pleura, reducing the degree to which the cavity is stretched, increasing PIP
What happens to the visceral pleura when the increased (less negative) intrapleural pressure no longer generates sufficient force to overcome the elastic recoil of the lung?
When the increased (less negative) intrapleural pressure no longer generates sufficient force to overcome the elastic recoil of the lung, the visceral pleura will be pulled inward (along with the pleural cavity and parietal pleura) decreasing lung volume.
What happens during forced expiration and what happens to the lung volume?
During a forced expiration, contraction of other respiratory muscles (such as the abdominals and internal intercostals) acts to provide further inward force on the parietal pleura, compressing the pleural cavity (further increasing PIP), forcing an increased and more rapid decline in lung volume.
Steps involved in inspiration
Respiratory muscles (e.g. diaphragm) contract
↓
Volume of thoracic cavity increases
↓
Intrapleural pressure becomes more negative
↓
Outward force exerted on visceral pleura becomes greater than inward recoil force
↓
Lungs expand, increasing volume
↓
PAlv (alveolar pressure) decreases below PAtm (atmospheric pressure)
↓
Air moves down pressure gradient, through airways into alveoli, expanding the lungs
Steps involved in expiration
Respiratory muscles (e.g. diaphragm) relax, lungs recoil due to elastic fibres ↓ Volume of thoracic cavity decreases ↓ Intrapleural pressure increases ↓ Lungs compressed*, volume decreases ↓ PAlv increases above PAtm ↓ Air moves down pressure gradient, into atmosphere, deflating lungs
What does piercing of either pleura cause?
cause air to enter the space (pneumothorax) from either the lungs or atmosphere, depending on the injury
What does a pneumothorax cause to the negative intrapleural pressure?
Negative intrapleural pressure is lost – as the chest wall and lung are no longer indirectly attached, they will recoil in opposite directions, causing the lungs to collapse (atelectasis)
What happens if either pleural membrane is ruptured in terms of pressure?
If either pleural membrane is ruptured, the pressure gradient between the pleural cavity and surrounding environment will cause air to enter (pneumothorax) until intrapleural pressure = atmospheric pressure.
