Lecture 3 - Lung mechanisms Flashcards
Ventilation
Process of inspiration and expiration
Lung volumes
Tidal volume Residual volume Inspiratory reserve volume Expiratory reserve volume Total lung volume
Tidal volume
Volume of air that enters and leaves the lungs at each breath during normal respiration
IRV
Max inspired volume of air
ERV
Max expired volume of air
Residual volume
Volume of air retained after forced expiration
Total lung volume
Vital capacity + RV
Lung capacities
Inspiratory capacity
Functional residual capacity
Vital capacity
Inspiratory capacity
End of quiet expiration to max inspiration
The volume of air that can be inhaled
Functional residual capacity
ERV + RV
Volume of air in lungs after quiet expiration
Vital capacity
IC + ERV
(IRV+ TV) + ERV
Volume of air breathed in and out of lungs during forced respiration
Anatomical dead space
Air that fills the conducting airways not available for gas exchange
Physiological dead space
Anatomical dead space + alveolar dead space
Alveolar dead space
Air in alveoli that are not perfused or are damaged. Therefore gas exchange does not occur
Tidal volume in terms of anatomical dead space
Anatomical dead space + alveolar ventilation
Total pulmonary ventilation (minute volume)
TV x RR
Alveolar ventilation
(TV- anatomical dead space) x RR
Describe quiet inspiration
- At the end of quiet expiration the atmospheric pressure = intrathoracic pressure
- The ribs move laterally and superiorly as the chest expands (30% external intercostal and 70% diaphragm) Muscles contract overcome elastic recoil
- The pleural fluid ensures that the lungs expand with the thorax
- Intrapulmonary volume increases and pressure decreases
- The intrapulmonary pressure is lower than the atmospheric pressure which creates a negative pressure
- Air is drawn in
Boyle’s law
An inverse relationship between pressure and volume of a gas
Describe quiet expiration
Air is expelled passively
- Relaxation of inspiratory muscles so they no longer overcome elastic recoil, reducing the intrathoracic volume and reduces lung volume.
- The intrapulmonary pressure is greater than the atmospheric pressure so air is drawn out
Resting expiratory level
State of equilibrium where all forces are equal and opposite so balance out resulting in no chest wall movement at rest
Creates negative pressure within the intrapleural space
There is a tendency to always want to return to the resting state
Forces exerted in the chest wall
Lung elasticity and surface tension - lungs pull in and up
Muscles and fascia - Chest wall pulls out
Passive stretch - Diaphragm pulls down
Why is intrapleural space negative?
The elastic recoil of the lung pulls the visceral pleura inwards and the chest wall pulls the parietal pleura outward
Breach in pleural seal
Pneumothorax- air is drawn in due to negative pressure
Lung collapse due elastic recoil
Accessory muscles of inspiration
SCM
Pectoralis major
Serratous anterior
Scalene muscles
Accessory muscles of expiration
Internal intercostal muscles
Innermost intercostal muscles
Abdominal wall muscles - rectus abdominis + EO + IO
Compliance
The extent to which the lungs can stretch
Volume change per unit volume change
Elastic recoil
The tendency to return to a resting state
What determines compliance?
Elasticity of lung tissue
Surface tension of fluid lining the alveoli
Surfactant
Surface tension
During respiration, the fluid lining the airways and alveoli must stretch due to the increasing SA
Surface tension resist this as want to achieve minimal SA. Reduces compliance
Surfactant
Secreted by type II pneumocytes
Mixture of phospholipids and proteins with detergent properties which floats on fluid
Surfactant molecules between the fluid molecules disrupt the interactions between the fluid molecules
Reduces surface tension
Increases compliance
Ensures that pressure is the same in all alveoli regardless of size
Prevents collapse of small alveoli into big alveoli
How does SA affect surface tension
As SA increases, surface tension increases
Surfactant effects decrease therefore surface tension increases as alveoli size increases.
When does surfactant have the most effect?
When there is a smaller SA as more molecules are present in a given area
Pressure
P= 2T/r
T - Surface tension
r - radius
Respiratory distress syndrome
Premature babies under 30 weeks
Do not produce surfactant therefore:
- Decrease in compliance - stiff
- Hard to expand
Resistance to flow
Tubes of smaller diameter have a higher resistance to flow
Energy needed to overcome resistance
Small decrease in radius - large increase in resistance (r^4)
Parallel lower airways
Numerous small airways running in parallel compensate for the increase in individual resistance
Upper respiratory system
Highest resistance as have a serial arrangement
Wider tubes
Cartilaginous rings - reduce compression
Lowest resistance in lower airways except when compressed during forced expiration
