Lung Ventilation and Perfusion Flashcards
alveolar ventilation
Rate at which new air reaches alveoli
Alveoli in top of lung
are more expanded than those at the bottom
• Pleural pressure is less (more negative) at apex than base of lung, with inspiration pleural pressure decreases further
• As inspiration begins alveoli in lungs are at different lung volumes
• Underinflated (smaller) alveoli at base of lung are more compliant so receive more of tidal volume
• Overinflated (expanded) alveoli at top have a lower compliance and receive less of tidal volume
• Lung has 2 blood supplies
– Pulmonary arteries
– Bronchial arteries
Bronchial arteries
branch from aorta and supply oxygenated blood to conducting airways
Bronchial veins exist
but majority of blood drains into pulmonary veins - Venous admixture
how are alveolar walls ruptured
Alveolar walls extremely thin and alveolar epithelium is weak and can be ruptured by a positive pressure
• Why do alveoli not fill with fluid?
– Normally pulmonary capillaries and lymphatics maintain a slight negative pressure in interstitial spaces
– Excess fluid will be sucked back into interstitial space from alveoli
• Zone 1
– P>P >P A PA PV – Apex of lung under specific conditions – No blood flow during all portions of the cardiac cycle – Doesn’t occur normally
• Zone 2
PA A PV – Apex to mid lung – Intermittent blood flow only during the pulmonary arterial pressure peaks • Systolic Ppc > Palv • Diastolic Ppc < Palv
• Zone 3
– P >P >P
PA PV A
– Mid to lower lung
– Continuous blood flow during entire cardiac output
• Ppc » Palv
– Get distension of pulmonary capillaries
• Zone 4
– P >P >P PA PV A – Extreme base of lung – Constriction of extra-alveolar vessels – Peak flow decreases
Pulmonary Blood Flow and its Regulation
• Regulated by PO2 and PCO2
– areas of low PO2 (hypoxia) or high PCO2 (hypercapnia)
– Arteries constrict so that blood is diverted to better oxygenated areas
– Mechanism thought to involve inhibition of K channels on smooth muscle cells
• This is a local response as remains even after section of autonomic nerves
Bronchial Circulation
• Arteries supply oxygenated blood to smooth muscle of airways, intrapulmonary nerves and interstitial lung tissue
• Venous blood returns to heart from bronchial circulation via
– true bronchial veins
– or drains into bronchopulmonary veins where it mixes with oxygenated blood from alveoli (venous admixture)
Ventilation and perfusion are matched when
pulmonary blood flow is proportionally matched to the pulmonary ventilation - greatest efficiency for gas exchange
• Ventilation-perfusion ratio (V/Q)
– Single alveolus defined as alveolar
ventilation/capillary blood flow
– Lung defined as total alveolar ventilation/cardiac output
• If ventilation exceeds perfusion
(V/Q ratio > 1)
• If perfusion exceeds ventilation
(V/Q ratio < 1)
Normal V/Q ratio
0.85 (4.2L/min /5L/min)
• Arterial hypoxemia
abnormal PaO2 (adult at sea level is PaO2 less than 80 mmHg)
• Hypoxia
insufficient O2 to carry out normal metabolic functions (PaO2 less than 60 mmHg)
• 4 major causes of hypoxemia
– Anatomical shunt (perfusion that bypasses
lung)
– Physiological shunt (absent ventilation to areas being perfused)
– V/Q mismatching (low ventilation to areas being perfused)
– Hypoventilation (underventilation of lung units)
Anatomical shunts
– Alveolar ventilation, distribution of alveolar gas and composition of alveolar gas are normal
– Distribution of CO changed as some blood now bypasses gas exchange unit
– Right-to-left shunt (as blood is deoxygenated)
– Hypoxemia cannot be abolished by giving 100% O2
– Cyanotic congenital heart diseases most common
• Shunt occurs when deoxygenated blood from RA or RV crosses septum to LA or LV
• Physiological Shunts
– If airway completely blocked alveoli supplied by that airway will receive no ventilation
– All ventilation goes to other lung units
– Perfusion will be equally distributed to both ventilated and non-ventilated lung units
– Lung unit without ventilation but with perfusion has a V/Q = 0
– Atelectasis most common cause of physiological shunt
• May be due to obstruction by mucous plug, airway oedema, foreign body or tumour
• V/Q mismatching (low V/Q)
– Most respiratory diseases produce global changes of varying extent in lungs (e.g. chronic bronchitis, asthma)
– So individual airways will have varying degrees of abnormal ventilation, but perfusion will be normally distributed
– Results in V/Q mismatching or low V/Q (V/Q < 1)
– Alveolar and end capillary gas compositions will vary according to degree of obstruction
– Supplemental O2 will correct hypoxemia as poorly ventilated units will get enriched O2
• Hypoventilation
– Underventilation will bring less
fresh gas to alveoli
– O2 levels in alveoli will decrease, CO2 levels will increase
– If ventilation halved, arterial CO2 will double
– Patients with respiratory muscle weakness (e.g. muscular dystrophy or diaphragmatic paralysis) are at risk of hypoventilation
• Results in both hypercapnia and hypoxemia
