Ventilation: Physics of breathing Flashcards
How does air move?
From high to low pressure
How is the pressure inside the lungs lowered?
By expanding the chest and lungs
What is the intrapulmonary pressure?
The pressure within the alveoli
What is the intrapleural pressure?
- The elastic nature of the lung tissue versus ribcage and thorax trying to pull apart visceral from parietal pleura.
- Always more negative than intrapulmonary
- -4 mmHg (756)
What is more negative the intapleural or intrapulmonary pressure?
The intrapleural pressure
What is normal atmospheric pressure?
760 mmHg (0 mmHg)
What is the collapsing force of lung?
4 mmHg
What is the role of the diaphragm?
- Main muscle of respiration
- Contraction flattens domes
- Allows lungs to increase in size
- Abdominal wall relaxes to allow abdominal contents to move downwards
What is the role of the intercostals (externals) in respiration?
- With first rib fixed, two movements, forward movement of lower end of the sternum, and upward and outward movement of ribs
What is the value of normal tidal volume?
~500ml
What is the intrapleural pressure in quiet inspiration?
~ -6 mmHg (drops by another 2 mmHg)
How much does the intrapulmonary pressure decrease by during inspiration?
~ 1 mmHg
What is quiet expiration?
- PAssive movement of lungs
- Elastic recoil drives air out of lungs
- Thoracic volume decreases by 500ml
- Intrapulmonary pressure increases
- air moves down pressure gradient
What is forced expiration?
- Expiration where abdominal walls are contracted, forces abdominal contents up against diaphragm, and internal intercostals - pull ribs downwards
How long on average is the respiratory cycle?
4 seconds
What is the trans-pulmonary pressure?
The difference between the intrapulmonary and intrapleural pressure
What is the resting intrapleurl pressure?
756 mmHg
What percentage of energy expenditure in quiet breathing is due to contraction of the diaphragm?
75%
What is energy used for in breathing?
- Contract the muscles of inspiration (e.g diaphragm)
- Stretch elastic elements
- Overcome airway resistance
- Overcome frictional forces arising from the viscosity of the lung and chest wall
- Overcome inertia of the air and tissues
What is airway resistance?
- The most significant non-elastic source of resistance
- F = DiffP/R
Amount of air that flows is determined by the change of pressure divided by the resistance
When is turbulent flow of air more likely?
- High velocities
- Large diameter airways
What percentage of airway resistance is due to the upper airways?
1/3
Where is the greatest resistance to airflow and why?
- Segmental bronchi
- Cross sectional area is relatively low and airflow is high and turbulent
What is flow like in the smallest airways?
Laminar
What is the resistance like in the smallest airways?
Small
What is an over-inflated ‘barrel-chest’ usually a result of?
COPD
What is the effect of inspiration on airway resistance?
Decreases
What is the definition of compliance?
The ease of stretch of the lung tissue when external force applied, or the ease with which the lungs expand under pressure
What pressure change causes a change in volume of the chest?
Change in intrapleural pressure
How does a high compliance influence volume?
High compliance means there is a large change in volume for a given change in pressure
What are the major determinants of compliance?
- Elastic components
- Alveolar surface tension
What is compliance in a normal healthy individual?
~ 1 L per kPa (1 L per 7.5 mmHg)
What can compliance be reduced by?
- Replacing elastic tissue with non-elastic tissue - pulmonary fibrosis - lungs become stiffer
- Blocking smaller respiratory passages
- Increasing alveolar surface tension
- Decreasing the flexibility of the thoracic cage or its ability to expand
What can compliance can be increased by?
Pulmonary emphysema. Due to alveolar rupture, creating larger air space and thus reducing surface area of lung. Impaired elastic recoil leads to poor deflation, trapping more air
Why is compliance greater at the base of the lung compared with the apex?
The base of the lung is more compressed than the apex. For the same change in intrapleural pressure at inspiration the base of the lung expands more than the apex
What does surfactant do?
- Lines alveoli
- Increases lung compliance by reducing surface tension - allows greater expansion for a given change in pressure
What is surfactant produced by?
Type II alveolar cells
What is surfactant made up of?
phospholipids
What is a capacity?
Volumes added together in various combinations
What is the definition of tidal volume (TV)?
Volume of air breathed in and out in a single breath (500ml)
What is the definition of inspiratory reserve volume (IRV)?
Volume breathed in by max inspiration at end of normal inspiration
What is the value of inspiratory reserve volume (IRV)?
3.3L
What is the defintion of expiratory reserve volume (ERV)?
Volume of air expelled by max effort at the end of normal expiration
What is the value of expiratory reserve volume (ERV)?
1 L
What is the definition of residual volume (RV)?
The volume of air in the lungs at the end of maximum expiration
What is the value of residual volume (RV)?
1.2 L
What is the inspiratory capacity (IC)?
- TV + RV
- Volume of air breathed in by max inspiration at the end of a normal expiration
- 3.8 L
What is the functional residual capacity (FRC)?
- ERV + RV
Vol air left in lungs at the end of normal expiration. Buffer against extreme change in alveolar gas levels in each breath - 2.2 - 2.4 L
What is the vital capacity (VC)?
- IRV + TV + ERV
- Vol of air that can be breathes by max inspiration following max expiration
- 4.8 L
What is the total lung capacity (TLC)?
- VC + RV
- Only a fraction of TLC used in normal breathing
- 6.1 L
What is a spirometer?
Used to measure and record volumes of inspired and expired air. The graph produced is called a spirogram
What does a vitalograph measure?
Expired air in 1 second FEV1
What would the affect of asthma be on FEV1?
Decreased FEV1
What would the affect of pulmonary emphysema be on FEV1?
Increased FEV1
What is the dead space?
The volume of ventilated air not involved in gas exchange
What happens to the alveolar dead space in certain lung conditions?
Increases
What is the anatomical dead space?
- Areas of airway not involved in gas exchange
- 150 ml
What is the alveolar dead space?
- The volume of air in alveoli that is not perfused
- 5 ml
What is the physiological dead space?
- Anatomical plus alveolar
- Almost the same as anatomical in a healthy individual
