Respiratory Physiology


Respiratory Physiology


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



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

Airway Resistance

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

Calculate PAO2 for RA, PCO2 of 40

 PCO2 of 75, RA

 PAO2 for PCO2 of 40, FiO2 100%

 PCO2 of 40, FiO2 50%

Hemoglobin Dissociation Curve

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)

Control of Breathing

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