Physical principles of gas exchange. O2 and CO2 Molecules move randomly & rapidly in relation to each other Net diffusion is from [high] to [low] Partial pr. of the gas is proportional to [gas]  nitrogen  79% 600 mmHg  Oxygen  21% 160 mmHg According to Henry's law the partial pr. of a gas in sln. depend on: 1- concentration 2- solubility coefficient P gas = _concentration of dissolved gas______ solubility coefficient gas Sol. Co. O2 0.024 CO2 0.57 CO 0.018

Solubility coefficient : Molecules dissolved in water if they are attracted to water more can dissolved without build up excess partial pressure within the solution as CO2

Physical Principles of Gas Exchange Diffusion in response to concentration gradient Pressure proportional to concentration Gas contributes to total pressure in direct proportion to concentration CO2 20 times as soluble as O2 Diffusion depends on partial pressure of gas Air is humidified yielding a vapor pressure of 47 mmHg.

Determinants of Diffusion Ficks Law Diffusion = (P1-P2 ) * Area * Solubility Distance * MW Pressure Gradient Area Distance Solubility and MW are fixed

Determinants of Diffusion Diffusion coefficient proportional to Solubility Different gases at the same partial pressure Will diffuse proportional to their diffusion coefficient MW

Composition of Alveolar Air Pn2 = (760 - 47) * 0.79 = 713 * 0.79 = 563 Questions: What is the effect of humidification on the partial pressures? Explain the expired air partial pressures? Calculate Po2 in alveoli

Diffusion between gas phase & dissolved phase Net diffusion is determined by gradient Vapor pr. of H2O is the partial pressure that water excretes to escape through the surface at normal body temperature 47 mmHg the greater the temperature the greater kinetic activity higher PH2O Temerature PH2O 0 ̊C 5 mmHg 100 ̊C 760 mmHg

VA VT F I E A VD Expired air has alveolar and dead space air

Po2 IN THE ALVEOLI PAlvO2= PIO2 - (PCO2/R) PO2 = 149 - (40/0.8) = 99 R is respiratory exchange ratio ~0.8 Remember in a normal person alveolar PO2 = arterial PO2, and alveolar PCO2 = arterial PCO2 .

Pco2 IN THE ALVEOLI PCO2 = CO2 production * K Alveolar Ventilation K is constant If ventilation is doubled then Pco2 is ½ If ventilation is halved then Pco2 is doubled

Question A person is breathing from a gas tank containing 45% oxygen. What is the alveolar PO2? A. 149 mmHg B. 250 mmHg C. 270 mmHg D. 320 mmHg E. 340 mmHg

Answer 760 – 47 = 713 713 * 0.45 = 321 mmHg = inspired PO2 Alveolar PO2 = 321 - (40/0.8) = 321 - 50 = 271 mmHg

Question An alveoli that has normal ventilation and no blood flow (V/Q=0) has an alveolar PO2 of A. 40 mmHg B. 100 mmHg C. 149 mmHg D. 159 mmHg O2=?

V/Q = 0 V/Q = normal V/Q =  O2 = 150 CO2 = 0 O2 = 150 CO2 = 0 O2 = 40

Ventilation/perfusion Physiologic shunt Va/Q < normal low ventilation Physiologic dead space Va/Q > normal wasted ventilation Abnormalities Upper lung Va/Q 3 x normal Lower lung Va/Q .5 x normal

Diffusion rate (D) proportional to P x AxS d x MW S : diffusion coefficient of gas. √MW A-cross- sectional area S-solubility of the gas d-distance P- pressure gradient Gas Diffusion co. O2 1 CO2 20.3 CO 0.81 N 0.53

Most gasses are lipid soluble so the diffusion in tissue is similar to diff. in water because these gases can pass easily through the cell membrane. Rate at which alveolar air is renewed by atmospheric air FRC 2.3 L only 350 ml of new air each breath. One seventh of the total, so many breaths are required to exchange most of the alveolar air. Half of gas will be removed in 17sec. Why this graduate clearance: - to prevent sudden change in [gas] in the blood. - to make respiratory control mechanism much more stable.

Concentration of gasses in alveoli O2 is supplied by inspiration and removed by diffusion PO2 is controlled by: a- rate of diffusion into blood (250ml/min) b- rate of O2 entry by ventilation normally Po2= 104 mmHg in alveoli  if alv. Ventilation 4.2L/min

Concentration of gasses in alveoli CO2 in alveoli depends on: a- rate of CO2 excretion b- ventilation rate If vent. Rate= 4.2 l/min, and rate of excretion = 200ml/min PCO2 40 mmHg Expired air= alveolar air+ air in dead space

Diffusion through respiratory membrane 300 million alveoli, each alveolus with the diameter of 0.2 mm. Respiratory membrane: 1-fluid layer with surfactant 2-epithelium of alveoli 3-basement membrane of epithelium 4-interstitial space 5-capillary basement membrane 6-endothelial cells of the capillary

Diffusion through respiratory membrane Respiratory membrane specifications: 1- 0.2 – 0.6 μm 2- 70m2 surface area 3- total volume of blood 60-140ml 4- capillary diameter is 5 μm so RBCs squeeze through

Diffusion through respiratory membrane Diffusion rate depends on: 1- thickness 2- surface area 3- Pgas gradient 4-Diffusion coefficient Diffusion capacity: the volume of a gas that will diffuse through the res. membrane each minute for a partial pressure difference of 1mmHg

Diffusion through respiratory membrane O2 21 ml/min/mmHg . 11 mmHg Mean O2 part. pr. In all lungs 230 ml/min “at rest” . 65 ml/min/mmHg “exercise” CO2 400 - 450 ml/min/mmHg “at rest”. 1200-1500 ml/min/mmHg “exercise” average of P CO2 gradient is 1mmHg