Respiratory System 4 Flashcards
What is alveolar pressure immediately before inspiration?
Equal to atmospheric pressure, so there’s no air flow , because there is no driving pressure from the atmosphere to the lungs (intrapleural pressure is about 4mmHg below atmospheric pressure)
When does inspiration start?
When contraction of the inspiratory muscles (primarily the diaphragm) enlarges the thoracic cavity
What happens as thoracic volume increases?
The intrapleural pressure becomes more sub-atmospheric and the lungs expand
What does the increase in lung volume cause?
Alveolar pressure to decrease and air moves into the lungs
When does air-flow cease?
When alveolar pressure returns to atmospheric pressure
What happens when inspiration terminates?
The diaphragm relaxes, intrapleural pressure rises and the lungs recoil. The gases in the alveoli are compressed which elevates alveolar pressure above atmospheric pressure, driving air from the lungs to the atmosphere
What do the negative values of air-flow during inspiration indicate?
That air is flowing from atmosphere into the lungs
What do the positive values of air-flow during inspiration indicate?
That air flows in the opposite direction during expiration
What is tidal volume (Vt)?
The total volume of air moving into (and out of) the lungs in a single breath
What factors affect the exchange of air?
Muscular effort
Lung characteristics;
- compliance (change of V per change of P)
- resistance (change of P per change of flow)
- dead space (Vd)
- diffusion
What is the inverse of compliance?
Elastance (reflecting the stiffness of the lung tissue)
What is the inverse of resistance?
Conductance
What is compliance?
A measure of the distensibility of the lungs and the chest wall and is defined as the change in volume that accompanies a small change in pressure
What is the equation for compliance?
Change in volume/change in pressure
When is compliance high (or elastance low)?
If a small change of pressure brings about a large change of volume
When is compliance low?
If it requires a large change of pressure to achieve a small change of volume
What is compliance given by?
The slope of a graph of volume as a function of pressure
At FRC, what would an increase in filling pressure by 5mmHg cause?
An increase of a excised lung by about 25% of total lung capacity
What characterises emphysema?
Greatly increased compliance (loss of elastic fibres)
What are lungs like at low lung volume?
Very incompliant
Why are low lung volumes incompliant?
Because most alveoli have collapsed so that relatively large pressure is required to overcome surface tension in order to reopen them
What happens once the lungs are open?
They distend relatively easily until they are near full inflation
Why does the deflation curve differ from the inflation curve?
Due to surface tension
Is there less effort to inflate lungs filled with saline or air?
Saline (hysteresis is much less evident)
What does air flow into and out of the lungs depend on?
The pressure difference between the atmosphere and alveoli and in the resistance to flow through the airways
Where is resistance greatest?
In the medium sized tertiary (segmental) bronchi
What else contributes significant resistance to air flow?
Mouth and nose
When does resistance decrease?
As total cross-sectional area increases in smaller respiratory pathways
What is asthma characterised by?
Sporadic bronchi-constriction that increases airway resistance
What is an anatomic deadspace?
The volume of conducting airways (no exchange of respiratory gases occurs in this volume)
What is dead space volume an obligatory consequence of?
Tidal ventilation
What does the presence of an anatomic deadspace do?
Dilutes each tidal inspiration with alveolar air remaining from the previous expiration
At rest, what does Vd (150ml in a healthy adult) comprise of the Vt?
One third, about 500ml
Vt= 1/3 alveolar air (exhaust) + 2/3 fresh air
What is diffusion described by?
Fick’s law
What is the process of diffusion?
The movement of oxygen and CO2 between the alveoli and pulmonary capillary and from systemic capillary blood to the cells by the process of diffusion
What is the volume of gas transported across a membrane, such as the alveolar-pulmonary capillary interface per unit time directly related to?
- Driving pressure or difference in partial pressure of the gas across the membrane
- The area of the membrane
What is the volume of gas transported across a membrane, such as the alveolar-pulmonary capillary interface per unit time inversely related to?
- The length of the diffusion pathway (thickness of the membrane)
- The square root of the molecular weight of the gas
Who does respiratory distress syndrome occur in?
Premature babies
What happens in respiratory distress syndrome?
In the alveoli, the air-water interface creates surface tension, in order to inflate an alveolus, sufficient pressure must be developed to overcome both the elastic recoil of lung tissue and the air-water surface tension
-pulmonary surfactant reduces surface tension (its absence in premature babies causes respiratory distress syndrome)
What is emphysema?
It’s characterised by the destruction of alveolar and peribronchial tissue, so, upon expiration, the bronchioles collapse, trapping air in downstream alveoli and, as a consequence, FRC increases (barrel-chest), thereby disadvantaging the muscles of inspiration
What is asthma?
Characterised by bronchiolar constriction (smaller r), therefore there’s increased R, and thus, increased P to achieve Vt, and therefore more muscular effort
What is pulmonary edema characterised by?
Increases diffusion distance
