Mechanics of breathing

Question 1

Describe the process of inhalation

Answer 1

Inhalation is an active process that occurs when the chest wall expands, creating a negative pressure gradient between the atmospheric air and air within the lungs

• Diaphragm increases thorax height
• External intercostals increase anterior-posterior and left-right dimensions

Air flows into the lungs through bulk flow down its pressure gradient.

Question 2

Describe the process of exhalation

Answer 2

Passive process (usually).

As inhalation ceases, the lungs revert back to their original size due to the elastic recoil of lung tissue.

This creates a negative pressure gradient between the lungs and atmospheric pressure, causing air to flow via bulk flow out of the lungs.

Question 3

What provides resistance to flow?

Answer 3

Airway diameter and contents provide resistance to flow

Question 4

What muscles are used to facilitate forced exhalation?

Answer 4

• Abdominal wall muscles
• Extracartilagenous internal intercostals
• Pectoral girdle muscles

Question 5

What is the functional residual capacity?

Answer 5

The volume in the lungs remaining at end passive exhalation. Caused by equilibrium between thoracic wall and the lungs.

Question 6

Describe quiet respiration

Answer 6

Inhalation requires muscular work

Exhalation is passive process

Functional residual capacity always present in lungs.

Question 7

What is the pleural space?

What is its role?

Answer 7

The potential space between the pleural membranes covering the lung (visceral pleura) and the inside of the thoracic wall (parietal pleura).

• Provides lubrication during chest wall expansion and retraction.
• Transfers expansile force from the chest wall to the lung
• Lung expands with the chest wall due to the pleural seal which keeps the pleura held tightly together.
  
  • Drop in pressure in the pleural space forces lungs to expand with the chest wall.
  • Suction from the pleural seal between the two pleura forces the lungs to expand with the chest wall
  • Expansive force of chest wall exceeds elastic recoil of the lung.

Question 8

What nerve innervates the diaphragm?

Answer 8

Phrenic nerve: C3, 4 & 5

Question 9

Which muscles stabilise the upper part of the chest wall (first rib)?

Answer 9

Sternocleidomastoid

Anterior, middle and posterior scalenes.

Question 10

What is the work of breathing?

What are its components?

Answer 10

The energy expended during respiration:

1. The energy required to expand the lungs against the lung and chest elastic forces - compliance work/ elastic work
2. The energy required to overcome the viscosity of the lung and chest wall structures- tissue resistance work
3. The energy required to overcome airway resistance to movement of air into the lungs- airway resistance work

Question 11

What are the accessory muscles of respiration?

When may they be needed?

Answer 11

• Scalenes: elevate first and second ribs
• Sternocleidomastoid: elevates sternum

Needed in extremes of respiratory effort to aid inhalation.

Question 12

What lung volume is represented when the forces expanding and recoiling the lungs are equal?

At what phase of the respiratory cycle does this occur?

Answer 12

Functional residual capacity

Occurs at the end of passive exhalation

Question 13

What is a pneumothorax?

Answer 13

Air into the pleural space causing increase in pleural space pressure.

• Removes the suction effect of the pleural seal
  
  • Allows the lung to collapse away from the chest wall under the unopposed force of its elastic recoil.
• Occurs when the chest wall or lung surface are punctured.

Question 14

Define compliance

Answer 14

Volume change of the lung per unit force applied

• A measure of the disposition of the lungs to expand under traction or pressure

Question 15

Define elastance

Answer 15

A measure of the disposition of the lungs to return to resting position due to their intrinsic elasticity.

Question 16

Explain Hooke’s Law

How do the lungs differ from this law?

Answer 16

The force required to expand a spring (aka the lungs) by some distance is proportional to that distance.

BUT the lung is stiffer at high and low volumes.

Question 17

Explain the hysteresis curve

Answer 17

Caused by variations in compliance at different lung volumes.

The lung is stiffer at high and low lung volumes:

• Needs higher pressure change at low volumes to initially expand (due to surface tension in alveoli)
• Needs higher pressure change at high volumes to complete inhalation

Question 18

What are the effects of emphysema and fibrosis on lung compliance and FRC?

Answer 18

Emphysema:

• Increased compliance (tissue destruction = less elastic recoil = hyperinflation)
• Increased FRC (less resistance and elastic recoil of the lung)
• Reduced elastance

Fibrosis:

• Decreased compliance
• Decreased FRC (increased resistance and elastic recoil of the lung)
• Increased elastance

Question 19

Explain the effect of surface tension on compliance

Answer 19

Surface tension: at a surface (e.g. alveoli) attraction between water molecules are much higher than between water and air due to hydrogen bonds.

• Surface becomes as small as possible, decreasing compliance

Water lines alveoli, therefore a substance is needed to overcome the surface tension to allow alveoli expansion and increase compliance.

Question 20

Explain Laplace’s Law

Answer 20

P = 4T /r

Pressure = 4x surface tension / radius

The higher the surface tension, the smaller the radius and the higher the pressure in the alveoli.

All of the air in the smaller alveoli will flow into the larger alveoli due to the pressure gradient.

Question 21

How is surface tension reduced?

Answer 21

Surfactant is produced by type II pneumocytes.

Reduces the surface tension of the fluid in the alveoli.

Allows the lungs to expand in the early stage of inhalation by reducing surface tension to the point where muscular power can exceed it.

Question 22

What is surfactant?

What is it made from and secreted from?

What are its roles?

Answer 22

• Secreted by type II pneumocytes
• Main ingredient is phospholipid

Roles:

• Increases compliance by reducing surface tension
• Reduces likelihood of tissue fluid transudation (
• Bind pathogens- opsonisation (makes pathogens more attractive to immune cells)
• Activates macrophages and neutrophils

Question 23

How is resistance to air flow expressed?

Answer 23

Assuming flow is laminar:

Pouiselle’s Law R = 8nl/πr⁴

For individual airways, resistance rises by the fourth power of the radius.

Question 24

Where does the greatest resistance to airflow lie?

Answer 24

In the medium sized bronchi

Not in the bronchioles as they are many in number so have a low combined resistance (arranged in parallel)

Question 25

Which bedside test is used to assess airway resistance?

Answer 25

Peak Expiratory Flow Rate

Question 26

How does lung volume affect airway resistance?

Answer 26

As lung volume decreases, airways become smaller. Around 10% will close. This 10% can rise to 40% in older age and disease states