Respiratory Mechanics Flashcards
What are the 2 meanings of “respiration”?
Tissue respiration: the aerobic metabolism in cells
Breathing: gas exchange and the associated processes
What does ‘respiratory mechanics’ refer to?
- The study of the mechanical properties of the lung and the chest wall
- The process by which air enters and leaves the lungs
What do the lungs do (broadly)?
- Bring in fresh air rich in oxygen needed to fuel the body, and
- Expel waste gas (CO2) produced from cells in the body
The movement of air into and out of the lungs is coupled to what?
Cellular respiration
Death from lung disease is usually due to what?
Inability to overcome changes in lung and/or chest mechanics
What is the typical value of PO2 at rest?
100 ± 2 mmHg
What is the typical value of PCO2 at rest?
40 ± 2 mmHg
What is the exchange of O2 and CO2 at rest?
~250ml O2/min, ~200ml CO2/min
What is the exchange of O2 and CO2 when walking slowly (~3mph)?
~800ml O2/min, ~750ml CO2/min
What is the exchange of O2 and CO2 during severe exercise?
~5000ml O2/min, ~6000ml CO2/min
What is the function of the alveoli?
Act as sites of gas exchange between air and blood
What is the role of the upper airways?
Conduct air to lung
Major functions: humidify air (saturate with water), warm air (to body temperature), filter air
How do the upper airways filter air?
Upper airways to bronchioles lined by pseudo-stratified, ciliated columnar epithelium
Inhaled particles stick to mucus. Mucus moves towards mouth by beating cilia
What volume of air is moved into the lungs at rest?
~6L/min
What volume of air is moved into the lungs during severe exercise (max ventilation)?
~160L/min
What is cardiac output value at rest?
5L/min
What is the max cardiac output?
25L/min
During quiet breathing, which process is active/passive?
Inspiration is active
Expiration is passive
During quiet breathing, which process is active/passive?
Both inspiration and expiration are active
Describe the active process of inspiration during quiet breathing
Diaphragm contracts downwards pushing abdominal contents outwards
External intercostals pull ribs outwards and upwards
Describe the passive process of expiration during quiet breathing
Elastic recoil of the lungs and chest wall
Describe the active process of inspiration during strenuous breathing
Greater contraction of the diaphragm and external intercostals
Inspiratory accessory muscles active e.g. sternocleidomastoid, alae nasi, pectoralis major muscles, etc.
Describe the active process of expiration during strenuous breathing
Abdominal muscles (rectus abdominus, internal oblique, external oblique and transverse abdominus) Internal intercostal muscles appose external intercostals by pushing ribs downwards and inwards
What occurs during inspiration and expiration in terms of pressure?
Changes in pressure within the lung - intrapulmonary pressure
What is the intrapleural pressure?
Pressure in the space between lung and chest wall
Pressure within this space is always negative
What does ‘tidal volume’ refer to?
The volume of air that comes into the lung during each breath, typically 0.5L
What is the pressure within the lung called?
Intrapulmonary pressure/alveolar pressure
Why is the intrapleural pressure always negative?
Because the lung is always trying to pull inwards while the chest wall is always wanting to push outwards
The 2 linings (parietal and visceral) are always trying to pull away from each other so the space between them is always negative
At the beginning of inspiration, what is the value of PA (alveolar pressure i.e. pressure within the lungs)?
PA = 0
Because there is no air flow
Describe the pressure changes during inspiration
Beginning of inspiration, no flow -> PA (alveolar pressure) = 0
Inspiratiory muscles contract -> increase in thoracic volume
Intrapleural pressure becomes more negative
Increase in transpulmonary pressure
Lungs expand and alveolar volume increases
Alveolar pressure becomes negative (PA<0)
Air moves into lungs from higher pressure to lower pressure
Inspiration ends when PA = PB
PB = barometric pressure
What does transpulmonary pressure refer to?
The difference between alveolar pressure and the intrapleural pressure
Describe the pressure changes during expiration
Beginning of expiration: thoracic volume decreases
Intrapleural pressure and transpulmonary pressure return to pre-inspiration values
Thorax and lungs recoil (elastic recoil pressure)
Air in alveoli compressed
Alveolar pressure becomes greater than barometric pressure
Air flows out of lungs
Describe low level breathing in a healthy individual
Low level breathing is very efficient: small muscle effort
1ml of O2 for each litre of air breathed, 2% of resting energy expenditure
What is the functional residual capacity (FRC)?
Functional residual capacity is the volume of air in the lung at the end of expiration during quiet breathing
What effect do elastic recoil forces have on lung volume?
Decrease lung volume
What effect does the outward recoil of chest all have on lung volume?
Increases lung volume
At functional residual capacity, which forces are opposite and equal to one another?
Elastic recoil forces
Outward recoil forces
When might FRC be decreased?
Functional residual capacity might be decreased when chest muscle walls are weak (neuromuscular disease) - elastic recoil is greater
Even if an individual expires maximally, why does the lung not collapse?
Functional residual capacity within the lung
Residual volume maintained within lung at all time, keeps the alveoli open
What can be used to measure lung volume?
Vitalograph/spirogram
Displays volume of gas exhaled against time
What is FVC?
Forced vital capacity - total amount of air exhaled during the FEV test
What is FEV1?
Forced expiratory volume in 1 second
What is FEF25-75?
Average mid-maximal expiratory flow
What is the FEV1/FVC ratio in a healthy individual?
> 70%
i.e. >70% of lung volume expired in 1 second
What is the FEV1/FVC ratio in patients with obstructive lung diseases?
<70% - less than 70% of lung volume expired in 1 second
What is the FEV1/FVC ratio in patients with restrictive lung diseases?
> 70%
Lung volume is smaller (FVC), usually 80% or less of healthy individual
What are 2 examples of obstructive lung diseases?
Emphysema and asthma
What are 2 examples of restrictive lung diseases?
Pulmonary fibrosis
Neuromuscular diseases
