S1: introduction to the unit & lung mechanics Flashcards
Explain the relevance of Boyle’s law in ventilation of the lung
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
Define tidal volume
Volume of air during quiet breathing that enters and leaves the lungs with each breath
Define inspiratory reserve volume
The amount of air a person can inhale forcefully after normal tidal volume inspiration
Define expiratory reserve volume
The maximal amount of air that can be expired beyond the normal TV expiration
Define residual volume
Volume air that remains after a forced expiration
Define inspiratory capacity
From the end of quiet expiration to maximum inspiration
inspiratory reserve volume + tidal volume
Define functional residual capacity
Volume of air in the lungs at the end of a quiet expiration
expiratory reserve volume + residual volume
Define vital capacity
Inspiratory capacity + expiratory reserve volume
OR inspiratory reserve volume + TV + expiratory reserve volume
Define forced vital capacity
Represents the volume of air that can be exhaled following a deep inhalation
Define total lung capacity
Vital capacity + residual volume
Define anatomical dead space
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
Define alveolar dead space
Air in alveoli which are not perfused, or are damaged, so not take part in gas exchange
Define physiological dead space
Anatomical dead space + alveolar dead space
Define pulmonary ventilation rate/minute volume and alveolar ventilation rate
Total pulmonary ventilation = tidal volume x respiratory rate (breaths per minute)
Alveolar ventilation = (tidal volume – dead space) x respiratory rate
What is significance of the resting expiratory level?
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
Describe the mechanism of normal quiet inspiration and the role of inspiratory muscles
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
Describe the mechanism of quiet expiration and the role of lung elastic recoil
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
Explain the changes in intrapulmonary and intrapleural pressure during respiratory cycle
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
Describe the mechanism of forced inspiration and the accessory muscles of inspiration
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
Describe the mechanism of forced expiration and the accessory muscles of expiration
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
Explain the importance of the pleural seal in respiration
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
Define the term ‘compliance of the lungs’ and describe the factors which affect the compliance of the lungs
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
Define the term ‘elastance/elastic recoil of the lungs’ and describe the factors which affect the elastance/elastic of the lungs
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
Explain the effect of surface tension in the alveoli
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
What is the role of surfactant?
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
How does surface tension of the alveolar fluid vary with the surface area of the alveolus?
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
Describe the factors which influence airway resistance in the normal lung, and in airway diseases, and how airway resistance changes over the breathing cycle
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
Describe diffuse lung fibrosis
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
What is the interstitial space?
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
Explain the pathophysiology of interstitial lung disease
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
What are the signs and symptoms of ILD?
Symptoms – reduced exercise tolerance with dyspnoea on exertion, dry cough, malaise
Signs – tachypnoea, tachycardia, reduced chest movement & diffuse fine crackles
What are the causes of ILD?
Occupational Treatment related Connective tissue disease Immunological Idiopathic
Describe respiratory distress syndrome in the newborn
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
Describe emphysema
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
Describe asthma
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
Describe pneumothorax
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
Describe atelectasis
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
Describe hypoventilation
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)
List conditions which increase and decrease lung compliance
Increase: emphysema
Decrease: diffuse lung fibrosis & respiratory distress syndrome of newborn
