Mechanics of Breathing II

Question 1

What are four ways in which lung function can be investigated?

Answer 1

1. LUNG VOLUMES: what total volume of air can an individual breathe in/out?
2. VENTILATION: what volume of fresh air reaches respiratory surfaces over a given time?
3. LUNG COMPLIANCE: how much force is required to overcome the recoil of the lungs?
4. AIR FLOW: at what rate can air be moved between the lungs and the atmosphere?

Question 2

Why is the fall in alveolar pressure large enough to be observed? (Why are any differences in pressure between the alveoli and atmosphere not instantly negated by the movement of air?)

Answer 2

There is a delay due to the time taken for air to move into the lungs through the air passages.

Question 3

What does the rate of airflow depend on?

Answer 3

The rate of airflow depends on the pressure gradient and level of airway resistance.

Ohms law :-

Airflow (V) = ΔPressure (P) / Resistance (R)

• As the ΔP increases = the airflow will increase.
• As the resistance increases = the airflow will decrease.

Hagen - Poiseuille equation :-

​Resistance (R) ∝ 1 / radius⁴​

R = 8μl / πr⁴

• As an airways radius decreases, the resistance increases (and the airflow decreases) dramatically.

Question 4

What happens in asthma?

Answer 4

• There is difficulty breathing as the smooth muscles in the airways constrict and shrinks the size of the lumen.
• This increases resistance and decreases the flow of air.
• However the airflow obstruction in asthma is reversible.
• It can also be characterised by bronchospasms (is a sudden constriction of the muscles in the walls of the bronchioles).

Question 5

What are the 2 different ways in which air flows in the airways?

Answer 5

1. Laminar flow :-
   
   • Occurs when a air flows in parallel layers to the walls, with the central layers moving faster than the outer ones, creating a dome shaped front.
   • There is little amounts of airway disruption between the layers.
   • It generates less resistance compared to the turbulent flow.
2. Turbulent flow :-
   
   • It is any pattern of fluid motion characterized by chaotic changes in pressure and flow velocity.
   • The air starts to move in multiple directions and generates greater resistance and as a result decreasing the level of airflow.
   • In the airways you get a turbulent flow when:-
     
     • the individual begins to breathe very hard (as the turbulence of the airflow increases the faster the air flows). E.g. forced expiration.
     • You can aslo get it when there are areas of obstruction in the airways (changes in the diameter of the airway).
   • When you have a turbulent flow you start to generate a noise due to the vibration of the air/blood (such as wheezing).

Question 6

Describe how loss of airway patency due to degradation of structure can cause airway obstruction.

Answer 6

• In healthy alveoli, elastin in the surrounding alveoli provides radial traction to splint bronchioles against positive Palv.
• In alveoli with COPD (Chronic Obstructive Pulmonary Disease), there isn’t radial traction (due to the absence of elastin). Without radial traction, the bronchioles collapse.

Patency meaning - It refers to the state of being open or unobstructed.

Radial traction meaning - The connective tissue that surrounds the airways of the lungs are called the parenchyma. This forms a sort of scaffold around the airways, keeping them open with a force known as “radial traction”.

Question 7

What is COPD?

Answer 7

Chronic Obstructive Pulmonary Disease (COPD) is an umbrella term used to describe progressive lung diseases including -

1. Emphysema (breathing tubes and air sacs in the lungs are damaged)
2. Chronic bronchitis (long-term inflammation of the airways)
3. Refractory (non-reversible) asthma.

Question 8

How can the level of airway obstruction be investigated?

Answer 8

• We can visualise it using a spirometry graph demonstrating forced expiratory volume (FEV₁) and forced vital capacity (FVC) in one-second values.
• 100 x FEV₁ / FVC = % of total lung capacity an individual can exhale in the first second.
• The normal FEV₁/FVC ratio is 70% (and 65% in persons older than age 65).

1. It is OBSTRUCTIVE:
   
   • People with obstructive lung disease have shortness of breath due to difficulty exhaling all the air from the lungs. Because of damage to the lungs or narrowing of the airways inside the lungs, exhaled air comes out more slowly than normal.
   • The FVC will be about the same, but the FEV₁ and the FEV₁ / FVC will be decreased.
   • The FEV₁ / FVC < 70% (for example, asthma, with increased resistance).
2. It is RESTRICTIVE:
   
   • If the lungs are unable to fully expand, so they limit the amount of oxygen taken in during inhalation. This limitation also restricts what can be exhaled when compared to an average person.
   • The FVC will decrease (< 80%) but as the airways arent affected the FEV₁ / FVC ratio wont be affected.
   • The FEV₁ / FVC can be normal or increased **> 70%**.

Question 9

What is transpulmonary pressure and how does it affect lung volume?

Answer 9

• Transpulmonary pressure is the difference between the alveolar pressure and the intrapleural pressure in the pleural cavity.
• Transpulmonary pressure (P_tp) = Alveolar pressure (P_alv) - Intrapleural pressure (P_ip)
• Transpulmonary pressure ≈ the level of force acting to expand the lung.

Question 10

What is lung compliance? What is static and dynamic compliance?

Answer 10

• Lung compliance is a measure of the lung’s ability to stretch and expand (distensibility of elastic tissue).
• Compliance (C_L) = ΔVolume / ΔPressure
• In clinical practice it is separated into two different measurements, static compliance and dynamic compliance.

1. Static lung compliance
   
   • It is the change in volume for any given applied pressure. (measurements taken whilst airflow = 0).
   • The steepest part of the curve is used.
2. Dynamic lung compliance
   
   • It is the compliance of the lung at any given time during actual movement of air. (measurements taken in the presence of airflow).
   • The gradient between the end tidal inspiratory and end tidal expiratory points is used.

Question 11

How is lung compliance affected by disease?

Answer 11

• LOWER COMPLIANCE CAUSED BY: -
  
  1. Scoliosis - It is a medical condition in which a person’s spine has a sideways curve.
  2. Muscular dystrophy - is a group of muscle diseases that results in increasing weakening and breakdown of skeletal muscles over time.
  3. Obesity
  4. NRDS (Neonatal respiratory distress syndrome) - When a newborn baby’s lungs aren’t fully developed and they cant provide enough oxygen.
  5. Pulmonary fibrosis - It is a chronic and progressive lung disease where the air sac in the lungs (alveoli) becomes scarred and stiff making it difficult to breathe and get enough oxygen into the bloodstream.
• HIGHER COMPLIANCE CAUSED BY: -
  
  1. COPD
  2. Emphysema - It occurs when the breathing tubes are narrowed and the air sacs are damaged. These changes lead to shortness of breath with daily activities. The major cause of emphysema is smoking cigarettes.

Question 12

When does lung compliance increase (in presence of air or saline)?

Answer 12

• Lung compliance decrease when lungs are inflated with saline (as opposed to air), an dcompiance increases if inflated with air after lung lavage (washing out of a body cavity with water or medicated solution).
• Saline-filled lungs are much easier to distend (increased compliance after they were filled with saline and then lavaged).

Question 13

What is air - liquid surface tension?

Answer 13

• Air - liquid interfaces (e.g. alveoli) generate surface tension, which resist inflation.
• Alveoli are lined with fluid to enable gas exchange (the gas molecules dissole into water before diffusing).
• Within the bubble formed by the water - air interface, surface tension arises due to H-bonds between the water molecules, this generates a inward collapsing force toward the centre of the bubble.
• This increases the pressure within the alveoli.

Question 14

What law describes the pressure generated by the surface tension within a bubble?

Answer 14

• The collapsing force generates pressure.
• The amount wihtin a specific bubble is described by the law of laplace:-
  
  • P = 2T / r
    
    • P - pressure
    • T - surface tension (e.g. water = 0.075N/m)
    • r - radius of bubble (i.e. alveoli)
• Therefore if T remains constant:-
  
  • P ∝ 1 / r
    
    • The smaller the alveoli, the larger the pressure generated.
• The pressure move from a high region to a low region.

Question 15

What reduces the alveolar surface tension? And what is it?

Answer 15

• It is reduced by the presence of pulmonary surfactant, secreted by type II pneumocytes.
• Pulmonary surfactant is a mixture of lipids and proteins.
• They act to equalise pressure and volume across varying alveoli.
• As the alveoli expand the concentration of surfactant molecules decreases, increasing surface tension.
• Now larger alveoli tend to collapse ito smaller ones, helping consistent inflation of the lungs.

Question 16

How does pulmonary surfactant help to prevent alveolar oedema?

Answer 16

• Surface tension produced at the air-liquid interface also reduces hydrostatic pressure.
• Fluid is then pulled out of surrounding capillaries and into the alveoli.
• By reducing surface tension, pulmonary surfactant helps to prevent alveolar oedema, as observed inpatients with insufficent surfactant.

Question 17

What disease is caused by insufficient production of pulmonary surfactant?

Answer 17

Neonatal respiratory distress syndrome

• The condition makes it hard for the baby to breathe.
• Occurs when the baby is born prematurely and before their lungs have fully developed.
• Leads to the following problems.

Ways to treat -

1. In certain pregnant mothers that are at risk you can supplement them with glucocorticoid to stimulate the production of pulmonary surfactant.
2. Now we have a artificial surfactant supplementation therapy that you give to the infant to give them an artificial administrated surfactant.