The ins and outs of respiratory physiology
Ventilation
- Ventilation = movement of air in & out of the lungs
- Alveolar ventilation (VA) = movement of air in & out of the alveoli
- ~4 L of air ventilate the alveoli at rest per minute
o Thus: VA = 4L/min
Perfusion
- Alveolar perfusion (Q) = movement of blood in & out of the alveolar capillaries
o = Pulmonary blood flow
- ~5 L of blood pass through the lungs each minute at rest
o Thus Q = 5L/min
Importance of perfusion and ventilation
- Normally alveolar ventilation matches alveolar perfusion
o → efficient exchange of O2 & CO2
Ventilation/Perfusion ratio
- Alveolar ventilation must match alveolar perfusion for efficient exchange of O2 & CO2
- ~4 L of air ventilate the alveoli (VA) per min
- ~5 L of blood pass through the lungs (Q) per min
- Mean ventilation-perfusion ratio (VA/Q) = 4/5 = 0.8
- Lung blood flow is uneven
- Blood flow highest at the base of the lung
- Decreases towards the apex
- In the upright position:
o Results in 7-10-fold ↑perfusion at the base in comparison to the apex
o And causes variations in hydrostatic pressure
Pressures acting on pulmonary capillaries
- Hydrostatic pressure:
- Pulmonary arterial pressure ~11-15mmHg
- Pulmonary venous pressure ~5mmHg
Ventilation/perfusion ratio
- Distribution of inspired air is also uneven
- In the upright position:
o 1.5-3-fold ↑ ventilation of the base of the lung vs apex
- Due to variations in airway:
o Resistance & compliance influence the alveolar filling time
in each region
, Local control of perfusion
- Normally in areas of poor ventilation:
o Local hypoxia (low O2)
o → vasoconstriction
o → blood diverted to better perfused regions of the lung
o = Hypoxic pulmonary vasoconstriction
CONTROL OF BREATHING
- Production of CO2 and requirement O2 will vary, but will need to keep the pressures of each
the same
- Would need to regulate this
- Regulation is done by receptors which sense changes in O2 and CO2 levels and in the pH
- The receptors send information to the control centres in the brain which alters and regulates
the pH
Receptors
- Majority of control is done by chemoreceptors
Respiratory control centres
- Within the brain
Respiratory muscles (Effectors)
- INSPIRATION
- Primary caused by diaphragm
- External intercostals
- Accessory:
o Scalenes
o Sternocleidomastoid
- EXPIRATION
- Rectus abdominus
- Transversus abdominus
- Internal oblique
- External oblique
- Internal intercostals
Ventilation
- Ventilation = movement of air in & out of the lungs
- Alveolar ventilation (VA) = movement of air in & out of the alveoli
- ~4 L of air ventilate the alveoli at rest per minute
o Thus: VA = 4L/min
Perfusion
- Alveolar perfusion (Q) = movement of blood in & out of the alveolar capillaries
o = Pulmonary blood flow
- ~5 L of blood pass through the lungs each minute at rest
o Thus Q = 5L/min
Importance of perfusion and ventilation
- Normally alveolar ventilation matches alveolar perfusion
o → efficient exchange of O2 & CO2
Ventilation/Perfusion ratio
- Alveolar ventilation must match alveolar perfusion for efficient exchange of O2 & CO2
- ~4 L of air ventilate the alveoli (VA) per min
- ~5 L of blood pass through the lungs (Q) per min
- Mean ventilation-perfusion ratio (VA/Q) = 4/5 = 0.8
- Lung blood flow is uneven
- Blood flow highest at the base of the lung
- Decreases towards the apex
- In the upright position:
o Results in 7-10-fold ↑perfusion at the base in comparison to the apex
o And causes variations in hydrostatic pressure
Pressures acting on pulmonary capillaries
- Hydrostatic pressure:
- Pulmonary arterial pressure ~11-15mmHg
- Pulmonary venous pressure ~5mmHg
Ventilation/perfusion ratio
- Distribution of inspired air is also uneven
- In the upright position:
o 1.5-3-fold ↑ ventilation of the base of the lung vs apex
- Due to variations in airway:
o Resistance & compliance influence the alveolar filling time
in each region
, Local control of perfusion
- Normally in areas of poor ventilation:
o Local hypoxia (low O2)
o → vasoconstriction
o → blood diverted to better perfused regions of the lung
o = Hypoxic pulmonary vasoconstriction
CONTROL OF BREATHING
- Production of CO2 and requirement O2 will vary, but will need to keep the pressures of each
the same
- Would need to regulate this
- Regulation is done by receptors which sense changes in O2 and CO2 levels and in the pH
- The receptors send information to the control centres in the brain which alters and regulates
the pH
Receptors
- Majority of control is done by chemoreceptors
Respiratory control centres
- Within the brain
Respiratory muscles (Effectors)
- INSPIRATION
- Primary caused by diaphragm
- External intercostals
- Accessory:
o Scalenes
o Sternocleidomastoid
- EXPIRATION
- Rectus abdominus
- Transversus abdominus
- Internal oblique
- External oblique
- Internal intercostals
