S1: introduction to the unit & lung mechanics

Question 1

Explain the relevance of Boyle’s law in ventilation of the lung

Answer 1

Boyle’s law – the volume of gas is inversely proportional to pressure (when temperature is constant)

• when the volume of the thoracic cavity increases: the volume of the lungs increases and pressure within the lungs decreases
• when the volume of the thoracic cavity decreases: the volume of the lungs decreases and the pressure within the lungs increases

Question 2

Define tidal volume

Answer 2

Volume of air during quiet breathing that enters and leaves the lungs with each breath

Question 3

Define inspiratory reserve volume

Answer 3

The amount of air a person can inhale forcefully after normal tidal volume inspiration

Question 4

Define expiratory reserve volume

Answer 4

The maximal amount of air that can be expired beyond the normal TV expiration

Question 5

Define residual volume

Answer 5

Volume air that remains after a forced expiration

Question 6

Define inspiratory capacity

Answer 6

From the end of quiet expiration to maximum inspiration

inspiratory reserve volume + tidal volume

Question 7

Define functional residual capacity

Answer 7

Volume of air in the lungs at the end of a quiet expiration

expiratory reserve volume + residual volume

Question 8

Define vital capacity

Answer 8

Inspiratory capacity + expiratory reserve volume

OR inspiratory reserve volume + TV + expiratory reserve volume

Question 9

Define forced vital capacity

Answer 9

Represents the volume of air that can be exhaled following a deep inhalation

Question 10

Define total lung capacity

Answer 10

Vital capacity + residual volume

Question 11

Define anatomical dead space

Answer 11

Volume of air located in the respiratory tract segments that are responsible for conducting air to the alveoli & respiratory bronchioles, but do not take part in the process of gas exchange itself
-upper airways, trachea, bronchi & terminal bronchioles

Question 12

Define alveolar dead space

Answer 12

Air in alveoli which are not perfused, or are damaged, so not take part in gas exchange

Question 13

Define physiological dead space

Answer 13

Anatomical dead space + alveolar dead space

Question 14

Define pulmonary ventilation rate/minute volume and alveolar ventilation rate

Answer 14

Total pulmonary ventilation = tidal volume x respiratory rate (breaths per minute)
Alveolar ventilation = (tidal volume – dead space) x respiratory rate

Question 15

What is significance of the resting expiratory level?

Answer 15

Occurs at rest at the end of quiet expiration before inspiration has started & when the respiratory muscles are relaxed
The lung is subject to two equal and opposing forces:
-inward: the lung’s elastic recoil and surface tension generate an inwardly directed force
-outward: the muscles and various connective tissues associated with the ribcage also have elastic recoil
The two opposing forces balance each other and create a negative pressure within the intrapleural space relate to atmospheric pressure

Question 16

Describe the mechanism of normal quiet inspiration and the role of inspiratory muscles

Answer 16

Contraction of the diaphragm and the external intercoastal muscles expands the thoracic cavity outward from this equilibrium position
Pleural seal ensures that the lungs expand along with the thorax
As the lung volume increases, the air pressure within the lungs falls below atmospheric pressure and air flows into the lungs

Question 17

Describe the mechanism of quiet expiration and the role of lung elastic recoil

Answer 17

When the muscle contraction ceases, the elastic recoil of the lung results in the thoracic cavity and lung returning to the original equilibrium position -> a smaller volume in the lungs, increased intrapulmonary pressure (relative to the atmosphere) & air flowing out of the lungs
Quiet expiration is a passive process not requiring muscle contraction

Question 18

Explain the changes in intrapulmonary and intrapleural pressure during respiratory cycle

Answer 18

The intrapleural pressure that is negative at rest becomes more negative during the inspiratory phase due to expansion of the thorax, and then returns to the resting pressure at the end of quiet expiration
The intrapulmonary pressure is negative relative to the atmosphere during inspiration, whilst in expiration the intrapulmonary pressure is positive relative to the atmosphere
Difference between intrapulmonary and intrapleural = transpulmonary pressure

Question 19

Describe the mechanism of forced inspiration and the accessory muscles of inspiration

Answer 19

Involves the contraction of the accessory muscles of breathing -> these muscles act to increase the volume of the thoracic cavity
Scalenes – elevates the upper ribs
Sternocleidomastoid – elevates the sternum
Pectoralis major & minor – pulls ribs outwards
Serratus anterior – elevates the ribs
Latissimus dorsi – elevates the lower ribs

Question 20

Describe the mechanism of forced expiration and the accessory muscles of expiration

Answer 20

Utilises the contraction of several thoracic and abdominal muscles -> act to decrease the volume of the thoracic cavity
Anterolateral abdominal wall – increases the intra-abdominal pressure, pushing the diaphragm further upwards into the thoracic cavity
Internal intercoastal – depresses the ribs
Innermost intercoastal – depresses the ribs

Question 21

Explain the importance of the pleural seal in respiration

Answer 21

The pleural seal ensures that the lungs expand along with the thorax
The parietal pleura lines the inside of each hemi-thorax and is continuous at the hilum of the lung with the visceral pleura which lines the outside of the lungs

Question 22

Define the term ‘compliance of the lungs’ and describe the factors which affect the compliance of the lungs

Answer 22

Compliance = distensibility of the lungs, volume change per unit pressure change
To stretch the lungs the elastic recoil of the lung must be overcome
Inversely related to connective tissue surrounding alveoli
Inversely related to alveolar fluid surface tension

Question 23

Define the term ‘elastance/elastic recoil of the lungs’ and describe the factors which affect the elastance/elastic of the lungs

Answer 23

Elastance = measure of elastic recoil (tendency of something that has been distended to return to its original size)
Directly related to connective tissue surrounding alveoli
Directly related to alveolar fluid surface tension
Inversely related to lung compliance

Question 24

Explain the effect of surface tension in the alveoli

Answer 24

Airways & alveoli of the lungs are lined with a film of fluid which must be stretched as the lungs expands
Increase in surface area is opposed by surface tension of the lining fluid
Surfactant counter-acts alveolar surface tension

Question 25

What is the role of surfactant?

Answer 25

Secreted by type II pneumocytes in the lung The hydrophilic ends of these molecules lie in the alveolar with their hydrophobic ends projecting into the alveolar gas -> interspersed between fluid molecules & disrupt the interaction between fluid molecules on the surface thereby reducing the surface tension

Question 26

How does surface tension of the alveolar fluid vary with the surface area of the alveolus?

Answer 26

As an alveolus expands -> surfactant molecules are spread further apart, making them less efficient -surface tension increases As an alveolus shrinks -> surfactant molecules come closer to together -surfactant in higher concentration, therefore act more efficiently to reduce their surface tension

Question 27

Describe the factors which influence airway resistance in the normal lung, and in airway diseases, and how airway resistance changes over the breathing cycle

Answer 27

Vessel resistance is directly proportional to the length of the vessel and the viscosity of blood & inversely proportional to the radius (Poiseuille’s equation, flow is laminar) Combined resistance of small airways is normally low -> connected in parallel over a branching structure Resistance to flow can often be affected by disease

Question 28

Describe diffuse lung fibrosis

Answer 28

The end result of various interstitial lung diseases if left untreated, or may occur without a known underlying cause, in which it is called idiopathic pulmonary fibrosis

Question 29

What is the interstitial space?

Answer 29

Potential space between alveolar cells and the capillary basement membrane, which is only apparent in disease states when it may contain fibrous tissue, cells or fluid

Question 30

Explain the pathophysiology of interstitial lung disease

Answer 30

The deposition of fibrous tissue has the following effects: 1) Lungs are stiffer & harder (collage less flexible than elastin) 2) Lung compliance is reduced 3) Elastic recoil of the lungs is increased 4) Lungs are smaller than usual due to the increased elastic recoil & thus lung volumes are also smaller NB: airways are NOT narrowed in ILD -> fibrous tissue exerts an outward pull

Question 31

What are the signs and symptoms of ILD?

Answer 31

Symptoms – reduced exercise tolerance with dyspnoea on exertion, dry cough, malaise Signs – tachypnoea, tachycardia, reduced chest movement & diffuse fine crackles

Question 32

What are the causes of ILD?

Answer 32

``` Occupational Treatment related Connective tissue disease Immunological Idiopathic ```

Question 33

Describe respiratory distress syndrome in the newborn

Answer 33

Surfactant is produced by type II alveolar cells in increasing quantities from 32 weeks’ gestation RDS = deficiency in surfactant in premature babies, particularly those less than 30 weeks old -surface tension is high -> lungs are harder to expand Treatment: surfactant replacement

Question 34

Describe emphysema

Answer 34

Emphysema = loss of elastin and breakdown of alveolar walls causing increased lung compliance, decreased elastic recoil, narrowing of small airways & loss of alveolar surface area Feature of COPD Many effects are opposite to those of lung fibrosis On examination causes the appearance of a ‘barrel chest’ Much rarer cause = alpha-1 antitrypsin deficiency

Question 35

Describe asthma

Answer 35

Reversible airways obstruction Chronic inflammation process -> causes airway narrowing due to bronchial smooth muscle contraction, thickening of airway walls by mucosal oedema & excess mucous production which can partially block the lumen

Question 36

Describe pneumothorax

Answer 36

A disorder where air enters the pleural space with loss of pleural seal and lung collapse Air flows into the intrapleural space due the pressure gradient until the pressure in the intrapleural space reaches the atmospheric pressure When the pleural seal is broken, the elastic recoil of the lung causes the lung to collapse towards the hilum

Question 37

Describe atelectasis

Answer 37

Incomplete expansion of the lungs (neonatal atelectasis) Partial/full collapse of previously inflated lung, producing areas of relatively airless pulmonary parenchyma Main types: -compression atelectasis: secondary to increased pressure exerted on the lung causing the alveoli to collapse -resorption atelectasis: complete obstruction of an airway Most common setting – post operatively, within 24 hours of surgical intervention

Question 38

Describe hypoventilation

Answer 38

Poor expansion of the thoracic cavity or lungs Respiratory muscle weakness of any cause or severe thoracic wall deformities can cause hypoventilation and respiratory failure Defining feature is hypercapnia (elevated CO2)

Question 39

List conditions which increase and decrease lung compliance

Answer 39

Increase: emphysema Decrease: diffuse lung fibrosis & respiratory distress syndrome of newborn