List 5 defence mechanisms of the respiratory system.
- Mucociliary defenses - mucous traps microbes, cilia beat to move them away from lungs.
- Sneeze and cough reflex - stimulation of irritant receptors.
- IgA - antibody found in respiratory secretions
- Alveolar macrophages - engulf small particles
- pulmonary microbiome
What are the 3 processes involved in gas exchange (conditions that affect these factors impair gas exchange)
- alveolar-capillary diffusion
- ventilation
- pulmonary perfusion
Define ventilation and what it depends on.
The physical movement of air into and out of the lungs (=inspiration and expiration)
Depends on:
1. pressure gradients
2. airway resistance
3. lung and chest compliance
Explain the physical and pressure related process associated with inspiration.
- Diaphragm and external intercostals contract
- Increases thoracic volume
- lungs are pulled open
- decreased pressure in the airways and alveoli pressure (Pa)
- Pa<Patm = air enters the lungs down a pressure gradient
Explain the physical and pressure related process associated with exhalation.
- Inspiratory muscles relax
- Decreased thoracic volume
- elastic components recoil and the lungs are compressed
- Increased alveolar pressure
- Pa>Patm = air flows out down a pressure gradient
What is the effect of bronchiole diameter on respiration?
- Constriction of the bronchioles - parasympathetic stimulation (vagus nerve) = bronchoconstriction which increases airway resistance
- Stimulation of the sympathetic system (B2 receptors) = bronchodilation which decreases airway resistance
What is the role of elastic fibres in airway resistance?
Elastic fibres within bronchiole walls - stretch during inspiration (allow air flow in) and recoil during expiration (push air out).
NOTE: radiating elastic fibres pull outwards on bronchiole wall preventing complete collapse between breaths.
How does chronic bronchitis contribute to airway resistance?
Thickening of airway walls with inflammation.
How does cystic fibrosis contribute to airway resistance?
Airway occlusion from mucus accumulation.
How does emphysema contribute to airway resistance?
Loss of elastic fibres need to keep airways open.
Define lung compliance.
The effort required to expand the lungs and chest wall.
High compliance = the lungs expand easily
Low compliance = greater force is required to expand the lungs during inhalation.
What are mechanisms causing decreased lung compliance (5)?
Conditions that decrease elasticity:
- scarring of the lungs (collagen is inelastic)
- e.g. pulmonary fibrosis.
Conditions that increase the water content of the lungs:
- filling of lung tissue with fluid makes the lungs stiff
- e.g. pulmonary edema
Conditions that impair diaphragm flattening or rib cage expansion:
- e.g. obesity, pregnancy, ascites, kyphosis
Conditions that cause lung collapse:
- e.g. pneumothorax (air enters the pleural cavity causing an increase in Pip)
Conditions that increase alveolar surface tension:
- causes alveolar collapse and difficulty expanding the lungs
- e.g., surfactant deficency
What are some causes of neuromuscular dysfunction that impact ventilation?
- CNS depression (opioid overdose)
- Injury or lesion affecting the brainstem (stroke)
- Disease of the motor neurons of the spinal cord (spinal cord injury, ALS)
- Disease of the neuromuscular junction (myasthesia gravis)
- disease of respiratory muscles (muscular dystrophy)
What are 3 causes of impaired ventilation leading to restrictive or obstructive lung disease?
- decrease in lung or chest compliance
- neuromuscular dysfunction
(1and 2 decrease lung expansion and inhalation) = RESTRICTIVE lung disease - increase in airway resistance
(impaired lung emptying and exhalation) = OBSTRUCTIVE lung disease
Describe alveolar-capillary diffusion.
O2 and CO2 move by diffusion across a pressure gradient between the alveoli and capillaries.
Efficiency of alveolar-capillary diffusion depends on:
- differences in partial pressure across the membrane
- the diffusion distance (thickness of the membrane)
- surface area available
What are 3 causes of impaired alveolar-capillary diffusion?
- loss of alveolar surface area (e.g. emphysema)
- increased thickness of the respiratory membrane (e.g. pulmonary edema or fibrosis)
- decreased pressure gradient (e.g. altitude)
What is pulmonary perfusion?
The blood flow through the pulmonary capillary bed that is available for gas exchange.
What are 3 causes of impaired pulmonary perfusion?
- Occlusion/obstructed blood flow (e.g. PE or valve stenosis)
- Hypotension (e.g. shock)
- right-to-left-shunt (e.g. AVM or VSD shunt reversal
What is ventilation/perfusion matching?
Optimal gas exchange requires that the areas of the lungs that are ventilated (V) are equally perfused (Q).
What is a V/Q mismatch?
When ventilation and perfusion are mismatched = hypoxemia
Low V/Q (shunt) occurs in airway obstruction
High V/Q (wasted ventilation) occurs in pulmonary obstruction
What is hypoxic vasoconstriction?
A compensatory mechanism in V/Q mismatch.
- A decrease in PaO2 (alveolar O2) stimulates the constriction of arterioles supplying the affected alveoli
- redirects blood to better ventilated areas
Describe respiratory control.of br
- the medullary rhythmicity area (medulla oblongata) controls the basic rhythm of breathing. It receives inputs from the pons and sensory neurons. These inputs modify rate and depth of breathing.
- contains motor neurons that control respiratory muscles
- resting adult rhythm = inhale for 2 sec, exhale for 3 sec (=12 breaths/min)
What are inputs that trigger signals from pons and sensory neurons to the medullary rhythmicity area to affect rate and depth of breathing?
- Chemoreceptors (PO2, PCO2, pH)
- baroreceptors (BP)
- Lung stretch receptors (lung volume)
- airway irritant receptors
- proprioceptors (joints)
- limbic system (emotions)
- hypothalamus (body temp)
- cerebral cortex (voluntary control)
