Module 3 Section 3 (Dynamics of Flow)

Question 1

Describe how changes in bronchiolar radius affect air flow.

Answer 1

Broncho constriction

Smooth muscle contraction in the bronchioles occurs b/c ventilator demand is low.
- These fibres have a tonic activity to maintain a resting level of contraction or “tone”.

Bronchoconstriction also occurs under the influence of local chemical control.
- Ex: decreased CO2 causes constriction to decrease ventilation and maintain a physiological level of CO2.

Pathological factors also play a role:

1) Histamine release
2) Excess mucus
3) Airway collapse
4) Oedema of the airway walls
5) Allergy-induced spasm of the airways caused by slow-reactive substance of anaphylaxis

Bronchodilation

Sympathetic activity is mediated by both direct and indirect innervation.

1) Direct innervation: nerve terminals release norepinephrine, which activates β2-receptors on the bronchial smooth muscle cells.
2) Indirect innervation: epinephrine released from the adrenal medulla circulates through the pulmonary circulation to the airway smooth muscle.

Increased CO2 concentrations can also cause bronchodilation in an effort to increase ventilation and
remove the excess CO2.

Interestingly, there are no pathological conditions that cause bronchodilation.

Question 2

Compare and contrast how asthma, chronic bronchitis, and emphysema affect air flow.

Answer 2

Asthma

• In asthmatic ppl, the airways can be impaired in three different ways:
  1) The airway walls are thickened due to histamine-induced oedema.
  2) Thick mucus secretion physically blocks the airways.
  3) Airway hyper-responsiveness causes spasms of smooth muscles in smaller airways, resulting in their constriction.
• The triggers for these impairments is varied but frequently involves repeated exposure to allergens, irritants, or infection. Severe asthma attacks can narrow airways to the point that all airflow is blocked, leading to death.

Chronic Bronchitis

• This is a long-term inflammatory condition of the lower airways.
• It is usually caused by chronic exposure to cigarette smoke, allergens, or air pollution.
• The airways become narrowed due to oedema of the airway walls and secretion of a thick mucus.

Emphysema
- This irreversible condition is characterized by the collapse of the smaller airways and breakdown of alveolar tissues.
- In response to chronic exposure to cigarette smoke, alveolar macrophages release
substances like trypsin as a defensive mechanism.
- Excess trypsin and other destructive enzymes destroy the lung tissue.

Question 3

Describe what is meant by a “restrictive” airway disease.

Answer 3

Restrictive lung disease, a decrease in the total volume of air that the lungs are able to hold, is often due to a decrease in the elasticity of the lungs themselves or caused by a problem related to the expansion of the chest wall during inhalation.

• This is the problem w/ COPD (look at last slide).

Question 4

Airways, like blood vessels, are tubes. As such, there is resistance to movement of fluids through the airways. What is resistance to flow determined by?

Answer 4

Poiseuille’s Law.

Question 5

Flow rate (Q) = 𝝅△Pr^4 / 8ηL

Looking at this formula, can you predict what would be the primary determinant of resistance?

a) The length of the airway
b) The change in pressure across the airway
c) The radius of the airway
d) The volume of air

Answer 5

c) The radius of the airway

The primary determinant of resistance is the radius of the airway. In the previous section, we didn’t mention the impact of resistance b/c in healthy airways, the radii of the airways in the conductive zone are large enough that the effective resistance is extremely low. B/c of this, only a small pressure gradient of only 1-2cm H2O is required to generate flow in either direction. This isn’t to say that airway resistance is a negligible factor, just that airway resistance becomes more important when not at rest of during pathological conditions.

Question 6

What is bronchoconstriction?

Answer 6

When airway radii become smaller and cause an incr in airway resistance.

Question 7

True or false: the activities of both sympathetic and parasympathetic inputs plays a role in determining airway resistance.

Answer 7

True

Smooth muscle contraction in the bronchioles occurs b/c ventilator demand is low. These fibres have a tonic activity to maintain a resting level of contraction or “tone”.

Question 8

What is bronchodilation?

Answer 8

Increased radius of the bronchioles.

When not at rest, or during periods of sympathetic domination when body O2 demands are increased, sympathetic activity causes bronchodilation to allow max flow rates with min resistance.

Question 9

Select whether the following statements are true or false.

1) Oedema of the airway walls leads to bronchodilation of airways.
2) Norepinephrine release activates β2-receptors on the bronchial smooth muscle cells.
3) Bronchodilation primarily occurs under the influence of local chemical control.
4) There are no pathological conditions that cause bronchodilation.
5) When body O2 demands are increased, sympathetic activity causes bronchodilation.

Answer 9

1) Oedema of the airway walls leads to bronchodilation of airways.
- False

2) Norepinephrine release activates β2-receptors on the bronchial smooth muscle cells.
- True

3) Bronchodilation primarily occurs under the influence of local chemical control.
- False

4) There are no pathological conditions that cause bronchodilation.
- True

5) When body O2 demands are increased, sympathetic activity causes bronchodilation
- True

Question 10

True or false: chronic lung diseases are often characterized by a narrowing of the lower airways, which results in an increased resistance.

Answer 10

True

Because of this, a larger pressure gradient is necessary to maintain flow. This can cause people with pulmonary diseases to need to work harder to breathe.

Question 11

What is asthma?

Answer 11

Asthma is a chronic inflammatory disease of the airways that causes difficulty breathing.

Symptoms: SOB, chest tightness, coughing, or wheezing.

Question 12

What is COPD?

Answer 12

Chronic obstructive pulmonary disease (COPD) is a term used to cover both emphysema and chronic bronchitis and is usually caused by long-term cigarette smoking. It is currently the 4th leading cause of death in Canada.

Question 13

Using what you have learned in this section, match the correct disease to their
corresponding description: 
- Asthma
- Bronchitis
- COPD
- Emphysema

1) Long-term inflammatory disease of the airways, characterized by variable/recurring symptoms.
2) Inflammation of the lining of your lower airways (bronchioles), which carry air to and from your lungs.
3) An umbrella term used to describe progressive pulmonary diseases.
4) A progressive disease of the lungs that primarily causes shortness of breath due to breakdown of alveolar tissue

Answer 13

1) Long-term inflammatory disease of the airways, characterized by variable/recurring symptoms.
= COPD

2) Inflammation of the lining of your lower airways (bronchioles), which carry air to and from your lungs.
= Asthma

3) An umbrella term used to describe progressive pulmonary diseases.
= Emphysema

4) A progressive disease of the lungs that primarily causes shortness of breath due to breakdown of alveolar tissue
= Bronchitis

Question 14

When pulmonary disease increases airway resistance, expiration is more difficult than inspiration. During inspiration, the larger airways are kept open by cartilaginous rings whereas the smaller airways are held open by the positive transpulmonary pressure gradient (negative pleural pressure). During
expiration, the increased intrathoracic pressure places additional pressure on the already constricted airways, decreasing flow of air out of the lungs.

Considering what you just learned, you might find it surprising that COPD patients often complain that they cannot get enough air in!

Why do you think patients experience this sensation?

Answer 14

The initial symptom pulmonary disease patients experience is often obstructive breathing (“can’t breathe out”), however, this symptom can quickly become restrictive (“can’t breathe in”). When a patient has
problem breathing out, they do not return to the usual end expiratory lung volume. Consequently, their next inhalation starts at a higher lung volume. This is good b/c higher lung volumes have lower resistance. But at this elevated lung volume, the person cannot breathe out completely, so any future breaths will only
be able to draw a limited amount of air in.

This breath stacking can continue until their lungs hyperinflate (dynamic hyperinflation) to the point that it becomes very difficult to breathe either in or out. At this point, patients will complain of dyspnea, a discomfort during inhalation.