Respiratory Physiology II

Question 1

What is pulmonary ventilation?

Answer 1

• It is the process of air flowing into the lungs during inspiration (inhalation) and out of the lungs during expiration (exhalation).
• Air flows because of pressure differences between the atmosphere and the gases inside the lungs.
• Air, like other gases, flows from a region with higher pressure to a region with lower pressure.
• Muscular breathing movements and recoil of elastic tissues that create the changes in pressure that result in ventilation.

Question 2

How does differences in pressure cause airflow in and out of the lungs?

Answer 2

• Movement of air (in and out of the lungs) due to pressure differences.
• Pressure at beginning of the respiratory tract is atmospheric (Patm)
• Pressure inside the lungs is alveolar pressure (PA)
• If Patm = PA no airflow
• If PA < Patm Air flows into the lungs
• If PA > Patm Air flows out of the lungs

Question 3

What is the law that defines the relationship between expansion and gas flow? How does this contribute to breathing?

Answer 3

Boyle’s law: if the volume of a gas is made t increase, the pressure exerted by the gas decreases

• As the alveoli are forced to expand, the pressure inside them decreases and gas flows in from the conducting airways.
• Pressure differences are created by changes in lung volume.

Question 4

What are the elastic recoil properties of the lungs?

Answer 4

• Lungs are elastic.
• They return to their original shape if a force that is distorting them is removed (like an elastic band).
• If you inflate a balloon and prevent the air escaping by blocking the neck the elastic recoil of the balloon will produce a recoil pressure.

Question 5

How does the balloon model of the lungs relate to inflation and deflation?

Answer 5

• For the balloon model – blowing into the balloon means subjecting it to a force (creating a pressure) to inflate it.
• Rather than blowing into the balloon the lungs are inflated by reducing pressure outside (like a plunger in a syringe).

Question 6

How are the lungs inflated by reduction of pressure?

Answer 6

• Lowering the plunger (diaphragm) reduces the pressure around the balloon (lungs) and generates inspiration

Question 7

What are the elastic properties of the chest wall?

Answer 7

• The thoracic cage is also elastic.
• Under normal conditions the chest wall has a tendency to pull outwards and the lung to pull inwards thus balancing themselves.

Question 8

How does intrapleural pressure work?

Answer 8

• The lungs and chest wall are “locked together” by the intrapleural fluid in the intrapleural space.
• The intrapleural fluid functions as a lubricant allowing the pleura to glide smoothly during exhalation and inhalation.
• At the end of an expiration (complete relaxation), there is a tension between the lungs, whose elasticity is causing them to collapse and the chest wall whose elasticity is causing it to spring outwards.
• This generates a pressure in the intrapleural space known as the Intrapleural Pressure (Ppl).
• Intrapleural pressure is usually negative with respect to the atmosphere (and the air pressure in the alveoli which is connected to atmosphere).

Question 9

What cohesive forces are present in the intrapleural space? How do they affect lung inflation?

Answer 9

• The intrapleural cohesive forces resemble those present when a water droplet is placed between two glass slides. While the two glass slides move over one another very easily, they are difficult to separate perpendicular to their adjoining surfaces.
• Thus, as the chest wall expands during inspiration, the lung is obligated to follow, so the two structures expand as a single unit.
• If the pleural cavity is damaged/ruptured air will enter the pleural space - the pleural pressure is lower than the atmospheric pressure.
• The intrapleural pressure becomes equal to (or sometimes exceeds) the atmospheric pressure and the pressure surrounding the lungs will increase which may cause the lungs to collapse.
• E.g. pneumothorax

Question 10

What is lung compliance?

Answer 10

Lung compliance CL

• Lungs are elastic structures and return to original shape and size when distorting forces are removed.
• Elasticity can be measured (indicator as to how easily the lungs can be stretched) and is conventionally expressed as compliance.
• Compliance is the ease at which the lungs expand under pressure.
• The compliance of the lungs is changed by most lung diseases.

In respiratory physiology, compliance can be defined as the change in volume produced by a change in pressure
Compliance = (change in volume)/(change in pressure)
Across the wall of the structure being investigated: e.g. lungs (CL), chest wall (CW) or lungs and chest wall (total compliance CTOT)

Question 11

How can you measure the pressure-volume curve of the lung?

Answer 11

Excised animal lung, with trachea cannulated placed within a jar
Change the pressure inside the jar using a pump
Measure the volume using a spirometer

Question 12

What is the normal pressure-volume curve of the lung like?

Answer 12

In normal range the lung is very compliant

However, at high expanding pressure the lung is stiffer and compliance is smaller (flattened slope of the curve)

Question 13

How are the pressure-volume curves different for inflation and deflation?

Answer 13

Curves during inflation and deflation are different
At any given pressure lung volume during inhalation is less than the lung volume during exhalation (red line)
Even without any expanding pressure the lung always has some air in it
This is due to airway close trapping gas in alveoli
Airway closure increases in certain conditions, such as age and lung disease

Question 14

Which conditions can alter compliance?

Answer 14

Diseases that affect either the chest wall or lung structure will affect compliance
-	Reduced compliance: 
o	Increase of fibrous tissue in the lung (pulmonary fibrosis)
o	Collapse/closure of lung (Atelactasis)
o	Increase in pulmonary venous pressure
-	Increased compliance: 
o	Age
o	Emphysema

Question 15

How can emphysema affect lung compliance?

Answer 15

• In emphysema there is a destruction of the normal lung architecture which includes the elastic fibres and collagen.
• There is also impaired elastic recoil and lungs do not deflate as easily.
• The lung is more easily distended, and the compliance of the lung is increased (more compliant).

Question 16

Mathematically, what is the effect of disease on lung compliance?

Answer 16

• Normal C = V/P
• Emphysema: same amount of pressure, easier to inflate.
• Fibrosis: same amount of pressure harder to inflate

Question 17

What is the effect of disease on compliance of chest wall?

Answer 17

• Structural change in the thorax (e.g. kyphoscoliosis: a disorder characterized by progressive deformity of spine) will affect compliance
• However, more usual that lung compliance is affected.

Question 18

What two main components generate the elastic properties of the lungs?

Answer 18

1. Elastic fibres and collagen

2. Surface tension forces

Question 19

What elastic fibres are present in the lungs?

Answer 19

• Elastic fibres form the bulk of the connective tissue present in the walls of the alveoli.
• Elastin fibres act like a stocking when stretched

Question 20

How is inflating an air-filled lung different to a saline-lung lung?

Answer 20

• For an air-filled lung:
  o Inflation of the lung follows a different pressure/volume curve from deflation.
  o This is known as hysteresis which literally means “to lag behind”.
  o A greater pressure is required to reach a specific lung volume when you are inflating it rather than deflating it.
• For an saline-filled lung:
  o Hysteresis is abolished.
  o Much easier (less pressure) to expand fluid filled lungs.
  o In fluid filled lungs only the elastic forces are working - there must be another component that also contributes.
  o In the fluid filled lungs the air-fluid interface has been abolished – surface tension has been removed.

Question 21

What is surface tension?

Answer 21

• The cohesive forces between liquid molecules at the surface are responsible for the phenomenon known as surface tension.
• The molecules at the surface of the liquid do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface.
• This forms a surface “film” which makes it more difficult to move an object through the surface than to move it when it is completely submersed.
• The water molecules on the boundary have an especially strong attraction for one another and as a result the water surface is always trying to contract (e.g drop of liquid).

Question 22

What is the surface tension in alveoli?

Answer 22

• On the inner surface of the alveoli, the water surface is also trying to contract
• This will result in the alveoli trying to collapse (forcing air out through bronchi).
• The net effect is to generate an elastic contractile force throughout the entire lungs, this known as the surface tension elastic force.

Question 23

What is Laplace’s law?

Answer 23

• The pressure in a bubble is equal to twice the surface tension divided by the radius
  o P = 2T/r
   P=Pressure within the bubble
   T= Surface tension
   r=radius
• The smaller the bubble the greater the internal pressure that is required to keep it inflated

Question 24

How do the lungs compensate with problem of pressure differences arising from having alveoli of different sizes?

Answer 24

Surfactant stabilises alveoli by lowering surface tension more in smaller alveoli

Question 25

What is the action of surfactant on surface tension?

Answer 25

• Pulmonary surfactant is a complex mixture of lipids and proteins.
• A major component (approx. 50%) of surfactant is the phospholipid Dipalmitoylphosphatidylcholine (DPPtdCho)
• Amphipathic character (hydrophilic and hydrophobic parts).
• Surfactants greatly reduce the surface tension and thus reduce the surface tension elastic forces.

Question 26

What is surfactant secreted by?

Answer 26

Type II alveolar epithelial cells
- Assembly of surfactant occurs in the lamellar bodies and it is secreted into alveolar fluid to form a surfactant layer at water-air interface.

Question 27

What is infant respiratory distress syndrome?

Answer 27

• Caused by developmental insufficiency of surfactant production and structural immaturity in the lungs
• A baby normally begins producing surfactant between weeks 24 and 28 of pregnancy.
• Most babies produce enough surfactant to breathe normally by week 34.
• In babies born prematurely not enough surfactant produced which may cause lungs to collapse.

Question 28

What is airway resistance and why diseases could affect it?

Answer 28

• Air must be moved into the lungs and then removed (breathing).
• Airway resistance (Raw) is defined as the resistance to the flow of gas within the airways of the lung.
• Name a disease that might affect the resistance of the airways?
  o Asthma - Increases in airway resistance due to a reduction in airway diameter because of smooth muscle contraction, swelling due to inflammation and/or mucus production.

Question 29

What is airway flow and what are the two types?

Answer 29

• The pattern of fluid flowing through a tube (e.g. airway or blood vessel) varies with the velocity and physical properties of the fluid.
• Two types of flow: laminar and turbulent.
• In laminar flow the movement is orderly and streamlined whereas in turbulent flow movement is chaotic.
• In most circumstances flow can be considered laminar as a first approximation.

Question 30

How does Poiseuille’s law relate to respiration?

Answer 30

• Laminar flow is described by Poiseuille’s law: relationship between driving pressure and flow.
• The equation can be roughly applied to breathing.
• Note: most important factor is radius of the tube (r4).
• Small changes in the diameter of airways leads to relatively big changes in flow.

(complicated equation - see word doc notes)

• Example: Airway radius of 4 units dilates to a radius of 5units.
  o What is the percentage increase in the radius of the airway?
  o (5-4/4)100=25%
  o What is the percentage increase in the airflow?
  o r=4, r4=256 and r=5, r4=625
  o (625-256/256)100=144%
• Relatively small changes in the diameter of airways leads to big changes in flow.

Question 31

What are the sites of airway resistance and how much resistance happens in each?

Answer 31

Sites of airway resistance: upper respiratory tract

• Just under half of resistance to airflow resides in the upper respiratory tract (nose, pharynx and larynx).
• Significant resistance in nose (e.g.inflammation and cold)
• Reduced resistance when breathing through mouth e.g. shift during exercise.

Sites of airway resistance: lower respiratory tract

• Half of resistance to airflow resides in the lower respiratory tract.
• Assuming laminar flow, Poiseuille’s law would predicts that major resistance to air flow would occur in airways with smaller radius.
• Why is this not so?
• Because the total cross-sectional area increases as you go down the tracheobronchial tree - although the diameter of each airway is small there are a larger number of them.

Sites of airway resistance: small bronchi and bronchioles

• The most important part of the bronchial tree in terms of physiological control of airway resistance are small bronchi and bronchioles.
• Found at the level in bronchial tree where the increase in the number of airways has not yet exerted its effects and the cross-sectional area is relatively small.
• Virtually no cartilage but innervated smooth muscle.
• Resistance of small bronchi and bronchioles is variable and under the influence of neuronal and hormonal factors.

Question 32

How is the bronchial smooth muscle tone controlled by autonomic nervous system?

Answer 32

• Parasympathetic: stimulate muscarinic receptors and cause smooth muscle constriction and bronchoconstriction.
• Sympathetic: activate adrenergic receptors and cause smooth muscle relaxation and bronchodilation.

Question 33

What other factors contribute to Bronchomotor tone?

Answer 33

• Non-adrenergic non cholinergic systems, NANC: includes bronchodilators (and possibly constrictors).
• Mediator release (e.g histamine etc): mast cell degranulation, neutrophils and eosinophils important in various stages of asthma
• Rapidly adapting pulmonary receptors: also known as irritant or cough receptors
• Slowly adapting/stretch pulmonary receptors: activity reduces bronchomotor tone
• Carbon dioxide: causes bronchodilation in underventilated areas where the gas builds up

Question 34

What kind of work is done in breathing? What factors may affect this?

Answer 34

• As performed by the respiratory muscles to over come:
  1) resistance to airflow
  2) elastic recoil of the lungs
• Relationship between work (W) done to change a volume (∆V) of gas at constant pressure.
• W* = P . ∆V
  o *measured in joules
• Normally respiration is very efficient and represents a small fraction of the total cost of metabolism; however this changes in disease.
• Changes in compliance and airway resistance may lead to increased work load.
• Note: in extreme conditions, such as severe COPD, energy requirements increase in order to breath. A situation may be reached in which the increased oxygen supplied from increasing ventilation is all consumed by respiratory muscles.