Mechanics of Breathing Flashcards
What is Ohm’s law?
The rate of airflow depends on the pressure gradient and level of airway resistance
Air flow= pressure/resistance
Increased pressure means increased airflow
Increased resistance= decreased airflow
What does Hagen-Poiseuille’s equation tell us?
As an airway’s radius decreases, the resistance increases (and the airflow decreases) dramatically
How does turbulent airflow affect the airways?
Airway resistance is further increased by turbulent airflow
e.g. in asthma
How does patency affect the airways?
Loss of airway patency due to degradation of structure can cause airway obstruction
Patency- the state of being open, so loss of patency is collapsing of airway
Healthy- elastin in surrounding alveoli provides radial traction to splint bronchioles against positive Palv
COPD- Without radial traction, bronchioles collapse
How does lung compliance affect gas exchange?
Lung compliance quantifies the relationship between the level of expansive force applied to the lung and the resulting change in lung volume
Transpulmonary pres. (Ptp)= alveolar pres. (Palv)- intrapleural pres. (Pip)
Transpulmonary pressure ≈ the level of force acting to expand the lung
How can you tell lung compliance from a graph??
Lung compliance is calculated by dividing a change in lung volume by the associated change in transpulmonary pressure
Compliance (CL)= D volume/ D Pressure
Compliance is expressed as the gradient of the curve
Steeper curve= greater level of lung compliance
‘Looser’/ easier to inflate lung= greater lung compliance
Stiffer/harder to inflate lung= lower lung compliance
What factors and diseases affect lung compliance?
Chest wall mechanics: Scoliosis Muscular dystrophy Obesity (decreases lung compliance) Alveolar surface tension: Neonatal respiratory distress syndrome (NRDS) decreased compliance Elastin fibres: Fibrosis decreases compliance COPD increases compliance
What is the purpose of air-liquid interfaces of alveoli?
Air-liquid interfaces (e.g. alveoli) generate surface tension, which resists inflation
Alveoli are lined with fluid to enable gas exchange (the gas molecules dissolve into water before diffusing
Within the bubble formed by the water-air interface, surface tension arises due to H-bonds between the water molecules, exerting a collapsing force toward the centre of the bubble
How does Laplace’s law describe the pressure created by the surface tension within an alveolar bubble?
The Law of Laplace describes the pressure generated by the surface tension within a bubble
The collapsing forces generates pressure
The amount within a specific bubble is described by the Law of Laplace:
P= 2T/r
P- pressure
T- surface tension
r- radius of bubble (i.e. alveoli)
Therefore if T remains constant:
P a 1/r (the smaller the alveoli, the larger the pressure generated)
What are the types of pulmonary surfactant?
Alveolar surface tension is reduced by the presence of pulmonary surfactant, secreted by type 2 pneumocytes The surface of alveoli primarily consists of two types of cells (pneumocytes): Type 1 (95% of alveolar surface) is responsible for gas exchange Type 2 secrete pulmonary surfactant
What is the purpose of pulmonary surfactant?
Pulmonary surfactant acts to equalise pressure and volume across varying alveoli
As alveoli expand, the concentration of surfactant molecules decreases, increasing surface tension
Now larger alveoli tend to collapse into smaller ones, helping consistent inflation of the lungs
Pulmonary surfactant helps to prevent alveolar oedema
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 in patients with insufficient surfactant
How is NRDS caused by insufficient production of pulmonary surfactant/
Neonatal respiratory distress syndrome is caused by insufficient production of pulmonary surfactant
1. Premature birth, maternal diabetes, congenital developmental issues 2. Insufficient surfactant production- artificial surfactant supplementation of infant, maternal glucocorticoid supplementation 3. Stiff (low compliance) lungs, alveolar collapse, oedema 4. Respiratory failure 5. Hypoxia 6. Pulmonary vasoconstriction, endothelial damage, acidosis, pulmonary + cerebral haemorrhage
What happens when ex vivo lungs are inflated with saline?
When ex vivo lungs are inflated with saline (rather than air), compliance increases
In classic experiments, physiologists compared compliance in ex vivo lungs inflated with air vs. lungs inflated with saline
Saline-filled lungs required less pressure to inflate (increasing compliance)
Washing lungs with saline before inflating with air, produced lungs that require more pressure to inflate (decreasing compliance)
