3. Structure & Function of the Respiratory System Flashcards
Upper respiratory tract
Consists of the nose, nasal passages, paranasal sinuses, pharynx & portion of larynx above the vocal cords
NOTE: Tracheo-bronchial treee has VOLUME but no gas exchange takes place - ANATOMICAL DEAD SPACE
Tidal volume (VT_ ~500 mL
Alveolar volume: (VT – VD) ~350 mL
Function of the respiratory system
- Gaseous exchange (diffusion):
- Oxygen exchange at alveolar-capillary interface
- Carbon dioxide exchange at alveolar-capillary interface
- Oxygen exchange at cells
- Carbon dioxide exchange at cells - Transport (conventional process):
- Oxygen transport
- Carbon dioxide transport
Diffusion distance overcome by coupling diffusion with transport
Lung volumes:
- Functional Residual Capacity (FRC)
- Tidal volume (VT)
- Inspiratory reserve volume
- Expiratory reserve volume
- Inspiratory capacity
- Vital capacity
- Residual volume
- Total lung capacity
- Functional Residual Capacity (FRC): 3L
- Tidal volume (VT): 0.5 L
- Inspiratory reserve volume: ~2 L
- Expiratory reserve volume: ~2 L
- Inspiratory capacity: ~3 L
- Vital capacity: ~5 L
- Residual volume: ~1 L
- Total lung capacity: ~6 L
Pleural membranes & Liquid layer of pleural membranes
Plural membranes:
- Cover the outer surface of the lungs (visceral) & thoracic cavities (parietal)
- Coupled together with a thin layer of liquid (~20 µm)
Liquid layer:
- Lubricant which allows the lung movement relative to chest wall
Thoracic movements are transmitted to the lungs by pressure changes in the intra-pleural space
Static mechanics of the lungs:
- Fcw
- FL
- Pip
- PA
- Fcw: Force exerted by chest wall
- FL: Force exerted by lungs
- Pip: Intrapleural pressure (pressure between visceral & parietal pleural membranes)
- PA: Atmospheric pressure
Lungs are highly elastic & tend to collapse in which reduces Pip & pulls the chest inwards
PA = PB when airways are open and no air flow
FRC & Compliance of lung & chest wall
- FRC is determined by Compliance of the lung & chest wall
- Compliance is inversely proportional to elastance - how easily the lung deflates
Compliance = ∆V / ∆P
How is compliance reduced?
- Reduced outwards mobility of the chest wall e.g. severe obesity, constrictive bandages
- Reduced lung volumes e.g. pulmonary oedema
Relaxation Pressure-Volume Relationship
When flow is zero: Ptotal = Pelastic = V / C
Ventilation - Definition & how it occurs
Movement of air into & out of the lungs
Occurs via bulk flow down the pressure gradient (convection):
Convection is the bulk movement of a fluid (gas/liquid) driven by a pressure gradient
Respiratory muscles: Inspiratory & Expiratory
Respiratory muscles (intercostals & others) are utilised to change thoracic volume & create pressure gradients
Inspiratory muscles:
- Diaphragm
- External intercostals
- +/- Accessory muscles (strenuous exercise)
Expiratory muscles:
- Abdominal wall muscles
- Internal intercostals
Ventilator settings/requirements
- VT required (10 - 15 mL/kg)
- Respiratory rate
- Set concentration of oxygen inspired to maintain normal PaO2 (80 - 100 mmHg)
Peak inspiratory pressure (5 - 10 cm H2O)
Minute ventilation:
Volume of air shifted in & out of the lungs per minute
At rest: VT x fR = 0.5 L x 12 min-1 = 6 L min-1
Exercise: VT x fR = 3 L x 40 min-1 = 120 L min-1
Alveolar ventilation
VT: ~ 500 mL
VD: ~ 150 mL
Pulmonary blood volume: ~ 70 mL
Pulmonary blood flow: ~5000 mL/min
V̇A = fR x (VT – VD) V̇A = 15 x (500 – 150) = ~5250 mL min-1
Factors affecting ventilation
A pressure gradient is required to change lung volume & generate air flow V̇
3 other factors that affect the flow rate & ease of ventilation:
- Surface tension of alveolar fluid
- Compliance of the lungs
- Airway resistance
Ptotal = Pelastic + Presistive
Ptotal = V/C + V̇R
where C is compliance & R is resistance
Effect of surface tension: Saline vs Air filled lungs
Saline-filled lungs:
- Lungs inflated with saline have much larger compliance
Air-filled lungs:
- Effects of elasticity & surface tension are seen
- Larger pressure is required during inflation (hysteresis)
Alveoli - features of cells present
Type I cells: Gaseous exchange
Type II cells: Secrete surfactant
Many elastic fibres & capillaries
Large volume of blood in pulmonary circulation (entire RV output)
Pulmonary resistance
Ptotal = Pelastic + Presistive = V/C + V̇R
Composed of:
- Frictional resistance of lung & chest wall tissue (~20%)
- Airway resistance (~80%)
Movement of fluid (air) through a rigid, smooth bore tube is governed by Poiseuille’s Law
Airway flow: Laminar vs Turbulant
Laminar flow has orderly layers & low resistance
Turbulent flow has disorganised layers & high resistance
Airway resistance
Total airflow resistance: All the resistances of the nose & mouth (substantial portion of the total) + 23 generation of the tracheobronchial tree
Airway resistance is from the friction between gas molecules & between gas molecules & the airway walls
Airway resistance is important & makes the sliding of lung tissue over each other (viscous tissue resistance) a minor issue
Poiseuille’s Law
V̇ = (∆P πr^4) / 8nl r = radius of cylinder l = length of cylinder n = viscosity
The rate of flow is due to the difference in pressure between the 2 ends i.e. Flow is proportional to ∆P
Turbulent flow requires a higher driving pressure
Airways - Bronchioles
Bronchioles are small passages surrounded by bands of muscles
They divide into smaller & smaller units until they reach the alveoli
Distribution of airway resistance
Airway resistance is proportional to the length of the air way i.e. doubling length doubles airway resistance
Halving the radius increases the resistance 16-fold
R = (8nl) / πr^4
Features of airway resistance
- Most of the resistance is observed in the intermediate sized airways
- Total cross sectional area increases towards periphery where total airflow is constant
- Flow is more laminar in small airways
Flow volume curves
Flow is:
- Effort dependent at high lung volume
- Effort independent at low lung volumes
Work of breathing
Work of inspiration consists of:
- Compliance work/Elastic work: Work required to expand lungs against elastic forces
- Tissue resistance work: Work required to overcome the viscosity of the lungs & chest wall structures
- Airway resistance work: Work required to move air through the airway into the lungs
Work is proportional to ∆P x ∆V
Breathing rates that can affect the work of breathing
Rapid shallow breathing: Decrease elastic work but increase frictional (viscous) work
Slow deep breathing: Decrease frictional work but increase elastic work
Ventilation can increase to levels of ~ 50L/min
Control of breathing
- Central control:
- Pons, medulla & other parts of the brain
- Controls frequency & duration - Sensors:
- Chemoreceptor, lungs & other receptors - Effectors:
- Respiratory muscles
All work in sync
Refer to diagram on lecture slides
