Chest – expands outwards
Lungs – collapse
Alveoli – act as bubbles (liquid-air interface)
P = (2 x surface tension)/radius
Collapse more likely when surface tension increases or alveolar size decrease
Surfactant made from Type II pneumocytes decreases surface tension
Compliance – change in volume/change in distending pressure – can be measured for lung, chest wall
Measures elastic recoil
Total system compliance = 1/Ctotal = 1/Cw +1/CL
FRC – volume at end of normal exhalation (Vt)
Inward recoil of lung approx equal to outward recoil of chest – elastic properties determine normal breathing – can be measure by nitrogen washout, helium wash-in, or body plethysmography
Factors affecting FRC: 1) habitus – directly proportional to height. Obesity decreases frc d/t dec chest compliance.
2) Sex – females 10% less; 3) posture – decreases from upright to sitting to prone – d/t dec chest compliance from abd contents – Greatest change from 0-60 deg of inclination
4)lung dz, 5) diaphragmatic tone
Decreases 15-20% with induction of anesthesia
Vt – normal breath
IRV – max inspiration above Vt; ERV – max exp below Vt
RV – volume remaining after max exp
TLC – RV+ERV+Vt+IRV
FRC – RV + ERV
VC – max volume expired after max inspiration – dependent on habitus, respiratory muscle strength, chest-lung compliance
Closing capacity – volume at which small airways begin to close
small airways depend on elastic recoil of surrounding tissues for patency – dependent on lung volume, especially at bases
Normally below FRC, but rises with age, leading to shunts
(perfused but not ventilated), probably responsible for agerelated decline in PaO2
At avg 44, FRC=CC while supine, at 66, CC = or > FRC while upright
Flow = pressure gradient/Raw
Raw = (8 x length x viscosity)/(pi x radius^4)
Turbulent flow – high gas flows, branching points – sensitive to airway caliber
Reynolds number <1000 – laminar flow = (linear velocity x diameter x gas density)/gas viscosity
Airway resistance highest in medium airways
Forced expiration of VC to evaluate airway resistance
FEV1 – forced expiratory volume in 1 sec
FEV1/FVC - proportional to degree of obstruction – normally >80%
FEV25-75% - less effort dependent
Dead space – anatomic + alveolar
Approx 150cc in adults (2cc/kg)
Shunt vs Dead Space
Hypoxic pulmonary vasoconstriction – shunts blood away from underventilated areas, preventing hypoxemia(alveolar hypoxia more powerful stimulus than pulmonary arterial hypoxia)
Volatile agents can inhibit this in high doses
Hypercapnia and acidosis – constrict
Hypocapnia – pulmonary vasodilation
Alveolar Oxygen Tension
PAO2 = ((Pb – PH20) x FiO2) – (PaCO2/RQ)
Pb – barometric pressure – 760
PH20 – vapor pressure of water 47 mmHg at 37 deg
RQ – usually 0.8
(760-47) x .21 = 149.7
PCO2 of 75, RA
PAO2 for PCO2 of 40, FiO2 100%
PCO2 of 40, FiO2 50%
Binding of first 3 O2 to Hgb greatly facilitates binding of last O2 – responsible for linear portion of curve
Rightward shift – decreases O2 affinity, increases availability
Leftward shift – opposite
2,3 DPG – byproduct of glycolysis, builds up in anaerobic metabolism
Important points on O2 curve: P50 – 26.6 mmHG PO2
SpO2 90% - 60mmHg O2; SpO2 80% - 50mmHg O2; SpO2 70% -
40mmHg O2
Carbon Monoxide – higher affinity for Hgb, keeps
SpO2 at 100%
Methemoglobin – also displaces O2, sats 85%-90%; treatment is methylene blue
Hurricane Spray
Oxygen Carrying Capacity
CaO2 = (Hb x 1.36 x SpO2/100) + (PaO2 x .003)
Central Receptors – medulla – responds to changes in CSF H+ concentration
Regulates PCO2, BBB permeable to dissolved Co2, not bicarb
Increase in H+ concentration increases ventilation to a point (CO2 narcosis)
Peripheral Receptors – carotid, aortic bodies
Carotid bodies – main receptors – sensitive to PaO2, PaCO2, pH, arterial perfusion pressure (most sensitive to PO2 – activate when PO2<50)
Communicate with central respiratory centers via glossopharyngeal nerves
Also stimulated by cyanide, doxpram, large doses of nicotine
Dopaminergic neurons – ventilatory response abolished by anti-dopaminergic drugs, b/l carotid surgery
Lung Receptors – transmitted by vagus
Stretch receptors in bronchial smooth muscle – inhibit inspiration when lung volumes high; shorten exhalation when volumes low
Irritant receptors – in mucosa – noxious stimuli – increases respiratory rate, bronchoconstriction, coughing
J receptors – interstitial space – induce dyspnea in response to expansion of interstitial space volume and chemical mediators after tissue damage