Session 1 ILOs - Intro to unit and Ventilation and lung mechanics Flashcards
Explain the relevance of Boyle’s law in ventilation of the lung
Boyle’s law - Pressure is inversely proportional to the volume (as one goes up, the other one goes down)
Boyle’s law is relevant as the human body brings air into the lungs by negative pressure:
- During inspiration, there is a contraction of inspiratory muscles causes expansion of the thoracic cavity which increases intra-thoracic volume
- As the intrapleural volume increases, the intrapleural pressure decreases, which draws air IN
- During expiration, the inspiratory muscles relax and the volume within the thorax decreases
- Therefore the pressure increases and forces out air back OUT
Define anatomical, alveolar and physiological dead space
Anatomical dead space:
- Volume of air located in the respiratory tract segments (responsible for conducting air) but are NOT involved in gas exchange itself
- Upper airways, trachea, bronchi and terminal bronchi
Alveolar dead space:
- Volume of air in the alveoli which does not take part in gas exchange
- Either are not perfused or are damaged
Physiological dead space:
- Anatomical dead space PLUS alveolar dead space
Define pulmonary ventilation rate (the minute volume) and alveolar ventilation rate
Total Pulmonary ventilation (also known as minute volume) = Tidal volume x respiratory rate
Alveolar ventilation = (Tidal volume – dead space) x respiratory rate
Define the resting expiratory level
Resting expiratory level - occurs at rest, at the end of quiet expiration, before inspiration has started, and when the respiratory muscles are relaxed.
At the REL, the lung is subject to two equal and opposing forces (one is directed inwards and the other outwards)
Affects the functional residual capacity
Explain the forces acting on the lung and chest wall at the equilibrium position at the end of a quiet expiration (resting expiratory level) and how these forces result in the intrapleural pressure being lower than atmospheric pressure
At the REL, the lung is subject to two equal and opposing forces (one is directed inwards and the other outwards)
Inward force:
- Generated by lung’s inward elastic recoil and surface tension
Outward force:
- Generated by outward elastic recoil of muscles and various connective tissues associated with the rib cage
Net effect:
- At rest the two opposing forces balance each other, and also create a negative pressure within the intrapleural space relative to atmospheric pressure
Describe the mechanism of normal quiet inspiration and the role of inspiratory muscles
Quiet inspiration:
- Active process
- Contraction of both the diaphragm (>70%) and external intercostal muscles
Describe the mechanism of quiet expiration and the role of lung elastic recoil
Quiet expiration:
- Passive process
- Due to elastic recoil
- No muscles used
Describe the mechanism of forced inspiration and forced expiration and the accessory muscles of inspiration and expiration (and the muscles involved in each)
Forced inspiration:
- Still active
- Requires 4 additional muscles (in addition to diaphragm and external intercostals)
1. Sternocleidomastoid
2. Scalenes
3. Serratus anterior
4. Pectoralis major
Forced expiration:
- Now active (no longer passive)
- Requires 2 muscles
1. Internal intercostals
2. Abdominal wall muscles
Explain the pleural seal in respiration and its importance
Pleural seal:
- A small amount of fluid fills the potential space between the parietal and visceral pleura of the intrapleural space
- The surface tension between the molecules of this fluid forms a seal, holding the outer surface of the lungs to the inner surface of the chest wall
- This pleural seal ensures that the chest wall and lungs move together
Define the term ‘compliance of the lungs’ and describe the factors which affect the compliance of the lungs
Compliance = Distensability (stretchiness)
- Compliance is defined as the volume change per unit pressure change
- Need to overcome the elastic recoil of the lungs in order to expand which is affected by 2 factors:
- Elastic tissue in the lungs
- Surface tension forces of the fluid lining the alveoli
Define the term ‘elastic recoil of the lungs’ and describe the factors which affect the elastance/elastic of the lungs
Elastic recoil - the ability of a stretched elastic object or organ to return to its resting position
Factors affecting elastic recoil:
- Elastic recoil increased by increased connective tissue (more elastin and collagen within the lung parenchyma)
- Elastic recoil increased by increased alveolar fluid tension (surface tension)
- Elastic recoil DEcreased by increased lung compliance (inversely related)
Explain the effect of surface tension in the alveoli and the role surfactant
- Airways and alveoli of the lungs are lined with a film of fluid which has to be stretched as the lungs expands (increases surface tension)
- Surfactant (secreted by type II pneumocytes) is a complex mixture of phospholipids & proteins with detergent properties
- Surfactant disrupts the interaction between fluid molecules on the surface thereby reducing the surface tension
- As an alveolus expands, surfactant molecules are spread further apart, making them less efficient
- Hence, as the alveolus expands the surface tension increases
- As an alveolus shrinks, the surfactant molecules come closer together increasing their concentration on the surface and act more efficiently to reduce the surface tension
- Hence, surfactant reduces surface tension as area of the alveolus decreases
Describe the factors which influence airway resistance in the normal lung, and in airway diseases, and how airway resistance changes over the breathing cycle
Poiseuille’s Law - vessel resistance is directly proportional to the length of the vessel and the viscosity of the blood but inversely proportional to the radius to the fourth power
Therefore, the resistance can be affected by:
- Length of the vessel
- Blood viscosity
- Most importantly, the vessel radius
In healthy lungs, most of the resistance to breathing resides in the upper respiratory tract, except when the small airways are compressed during forced expiration
Resistance to flow can often be increased in diseased lungs
Explain the pathophysiological basis of Diffuse Lung Fibrosis (aka Diffuse Pulmonary Fibrosis) - describe the ‘interstitial space’ and explain why fibrous tissue deposition in it causes a restrictive type of lung disease
Diffuse Lung Fibrosis:
- Interstitial lung disease refers to a large group of lung disorders that affect the interstitium, which is the connective tissue that forms the support structure of the alveoli (air sacs) of the lungs
- Diffuse Lung Fibrosis can be an end point of untreated interstitial lung diseases to could have an idiopathic cause
- Stiff lungs from increased collagen deposition in interstitial space
- Decreased compliance and increased elastic recoil
- Restrictive ventilation
Interstitial space:
- A potential space between alveolar cells and capillary basement membrane
- Only apparent in disease states where it might contain cells, fluid or fibrous tissue
Explain the pathophysiological basis of Respiratory Distress Syndrome in the Newborn (in terms of altered lung mechanics)
Respiratory Distress Syndrome in the Newborn:
- Reduced/no surfactant production in the newborn (often due to prematurity) results in highly increased surface tension
- Decreased compliance and increased elastic recoil
- Makes lungs harder to expand at birth and some alveoli remain collapsed (never open)
- Increased effort to breathe and impaired ventilation
Treatment:
- Synthetic surfactant replacement via endotracheal tube
- Supportive treatment with ventilation and oxygen
