Module 3 Section 2

Question 1

explain respiratory mechanics

Answer 1

• the pressure gradient is the driving force of air flow
• this pressure gradient is used to overcome the stiffness of the respiratory system, the resistance to flow, and the inertia of the system
• for air to flow into the alveoli, the pressure in the alveoli must be lower than the pressure in the nose
• for air to flow out of the alveoli, the pressure in the alveoli must be greater than the pressure in the nose

Question 2

what is atmospheric pressure (PB)

Answer 2

• also called barometric pressure and is the pressure exerted by the weight of the air in the atmosphere on earths surface
• at sea level it is 760 mmHg and this value decreases as you gain altitude
• even when standing, there is not enough different in height between the lungs and the nose/mouth, PB is the same in both places so we effectively treat it as if it was 0

Question 3

what is alveolar pressure (PA)

Answer 3

• this is the pressure in the alveoli, also called intrapulmonary pressure
• at the end of inspiration, alveolar pressure is the same as atmospheric pressure at 0 cmH2O

Question 4

what is pleural pressure

Answer 4

• this is the pressure in the pleural space
• it is also called the intrapleural pressure and it closely approximates the intrathoracic pressure
• pleural pressure is negative to atmospheric pressure and is normally around -5 cmH2O
• it is negative because the lungs want to collapse yet the chest wall wants to expand

Question 5

what is transpulmonary pressure (Ptp)

Answer 5

• this pressure is the difference between the alveolar pressure and the pleural pressure
• it is also referred to as lung recoil pressure (Pl) or transmural pressure

Question 6

what are different expressions of units

Answer 6

• traditional units based on the height of a column of mercury (mmHg) or water (cmH2O)
  1. mmHG for the partial pressures of gases when discussing diffusion
  2. cmH2O when discussing bulk flow (convection)
• for the latter, we use cmH2O rather than mmHg because the pressures needed to generate flow are typically small, only a few cmH2O
• pressures related to convective flow are expressed relative to atmospheric pressure

Question 7

explain elastic recoil in the lung

Answer 7

• a very important property of the lungs is that they have an intrinsic tendency to deflate following inflation
• this is because of elastin fibres and surface tension

Question 8

what is elastin fibres

Answer 8

• the connective tissues within the lung contain lots of elastin fibres that are arranged in a meshwork that enhances their elastic behaviour
• when the lung is stretched, as happens during inhalation, this elastic recoil causes the lung to deflate

Question 9

what is surface tension

Answer 9

• the force exerted by the liquid lining the inside of the alveoli and accounts for about 70% of the elastic recoil properties of the lung
• surface tension has a two-fold effect on elastic recoil
  1. the liquid layer resists any forces that try to increase its surface area. this is due to the water molecules resisting being pulled apart
  2. the surface area of the liquid shrinks as much as it possibly can. this is due to the water molecules being so strongly attracted to each other
• because of surface tension of the liquid lining the alveoli, in the absence of expanding forces, the alveoli shrink as much as possible and expel alveolar gas

Question 10

explain alveolar stability

Answer 10

• if the surface liquid was water alone, they actually would collapse and the forces, or pressure, required to open them upon inspiration would be much greater
• the alveoli do not collapse because of the pulmonary surfactant and alveolar interdependence

Question 11

explain pulmonary surfactant

Answer 11

• this is a complex mixture if lipids and proteins secreted by type II alveolar cells
• these secretions help disperse the water molecules on the surface of the alveoli
• by dispersing the water molecules, the water-water attractions are decreased, which causes alveolar surface tension to be decreased
• this reduced surface tension is very important for respiratory mechanics as it decreases the effort needed for inflation (increasing compliance) and it reduced the surface tension of smaller alveoli more so than it does larger alveoli

Question 12

explain alveolar interdependence

Answer 12

• each alveolus is connected to its surrounding alveoli by connective tissues
• if one alveolus starts to collapse, it is supported by its neighbours

1. when an alveolus (in link) in a group of interconnected alveoli starts to collapse, the surrounding alveoli are stretched by the collapsing alveolus
2. as the neighbouring alveoli recoil, they pull outward on the collapsing alveolus. this helps prevent the alveolus from collapsing

Question 13

the law of LaPlace

Answer 13

states that the magnitude of this collapsing pressure is directly proportional to the surface tension and inversely proportional to the radius of the alveoli

Question 14

explain alveolar pressure

Answer 14

• for air to flow into the alveoli, alveolar pressure must be less than atmospheric pressure and for air to flow out of the alveoli, alveolar pressure must be greater than atmospheric pressure
• atmospheric pressure is generally fixed at 0mmHg os alveolar pressure must change in order to generate the pressure gradient necessary for air flow

Question 15

how to change alveolar pressure

Answer 15

• need to change either pleural pressure or lung recoil pressure, or both
• recoil pressure of the lung depends on lung volume
• can not change lung volume by changing a pressure that reflects lung volume (to change Pa you have to change Ppl

Question 16

explain inhalation and exhalation

Answer 16

• the result of changes in pleural and alveolar pressure
• before inspiration (at the end of the preceding expiration), there is no flow as the alveolar pressure equals atmospheric pressure
• during inspiration, a pressure gradient is established from atmosphere to alveoli which results in inspiratory flow
• inspiration ends as the contraction of inspiratory muscles decreases, allowing lung recoil pressure to “catch up to” and equal pleural pressure
• expiration starts when inspiratory muscles have stopped contracting
• lung recoil pressure is now greater than pleural pressure, resulting in a positive alveolar pressure and, therefore expiratory flow

Question 17

explain the onset of inhalation

Answer 17

• immediately before inhalation, alveolar pressure equals atmospheric pressure
• air flows neither in nor out of the lungs
• contraction of the inspiratory muscles causes the pleural pressure to decrease and the thoracic cavity enlarges
• this decreases alveolar pressure and air flows down its pressure gradient into the lungs and inflates the alveoli
• this continues until the increasing alveolar pressure again equals that of atmospheric pressure
• changes in alveolar pressure are very small and not linear

Question 18

explain the onset of exhalation

Answer 18

• at the end of inspiration, the inspiratory muscles relax
• this increases pleural pressure and therefore increases alveolar pressure
• air flows from the lungs until alveolar pressure equals atmospheric pressure
• activation of expiratory muscles is not necessary for normal expiration due to the strong recoil forces

Question 19

explain active exhalation

Answer 19

• in a healthy person at rest, expiration is passive
• it is possible to empty the lungs faster, or more forcefully, using active expiration
• activation of expiratory muscles reduces the end-expiratory lung volume, which increases the tidal volume (volume of air inhaled and exhaled) independent of the inspiratory muscles

Question 20

explain active exhalation during routine exercise

Answer 20

• to forcefully breath out, alveolar pressure must be increased by more than is accomplished by decreased excitation of inspiratory muscles and elastic recoil
• this is accomplished by the expiratory muscles of the abdominal wall
• upon contraction, the increased abdominal pressure is transferred to the pleural space, increasing pleural pressure
• contraction of the internal intercostals helps by pulling the ribs downward and inward to decrease the size of the thoracic cavity

Question 21

explain active exhalation during forced expiration

Answer 21

• during forced expiration, the expiratory muscles contract, generating a high pleural pressure
• as air flows toward the mouth, pressure within the airways progressively decreases due to energy loss from airway resistance
• at a specific point along the airway, the pressure inside the airway becomes equal to the pleural pressure; this is the equal pressure point (EPP)
• transpulmonary pressure becomes negative (airways pressure is less than pleural pressure), causing compression of the airways
• once the EPP is established at a given lung volume, increasing expiratory effort does not increase airflow because any rise in pressure is offset by increased airway resistance due to dynamic airway compression
• under these conditions, airflow is determined solely by the elastic recoil pressure of the lungs, expressed as (Pa - Pi)

Question 22

explain pressure-volume relationship (PI)

Answer 22

as lung volume increases, its PI increases from about 0mH2O at residual volume to about 30 cmH2O at total lung capacity

Question 23

explain pressure-volume relationship (Pw)

Answer 23

the pressure of the chest wall (Pw) functions more like a spring in that bellow about 65% of total vital capacity, the compressed spring exerts negative (inflating) pressures yet at 100% total vital capacity, the chest wall, now a stretched spring, wants to collapse

Question 24

explain pressure-volume relationship (Prs)

Answer 24

if we combine PI and Pw, we get Prs, which represents the pressure-volume relationship of the respiratory system

Question 25

explain compliance

Answer 25

- from the pressure-volume curve we can derive compliance, which is the slope of the curve - it is the greatest at functional residual capacity, which means the amount of work, or pressure needed, to either inhale or exhale is at its minimum

Question 26

explain low compliance

Answer 26

- means more pressure is required to move air in or out - compliance is affected by lung diseases