Diffusion of Gases Review of the Physics and Physiology RET 1613C Mini Lecture 1 Dr. J. B. Elsberry Special thanks to: Barbara L. Kenney, RRT LSU, USC and UEL websites

Review of Important Gas Laws Ideal Gas Law PV = nRT Charles’ Law V1 /T1 = V2 /T2 Gay-Lussac’s P1 /T1 = P2 /T2 Boyle’s Law P1 V1 = P2 V2 *Examples: Ventilation Body plethysmograpy.. remember this from last semester A gaseous cylinder A Hot Air Balloon

Barometric Pressure Gases in the atmosphere exert a force on the earth’s surface called the Barometric pressure Pb at sea level is 760 mmHg (torr) Pb decreases with an increase in altitude Pb increases with a decrease in altitude

Gas Diffusion Diffusion Gradients random molecular activity net movement of gas particles from an area of high concentration to an area of lower concentration http://www.goldiesroom.org/Note%20Packets/06%20Transport/00%20Transport--WHOLE.htm Diffusion Gradients the individual gas partial pressure differences across a membrane

How do Fish “breathe” ?

Human (Mammalian) External Respiration http://www.youtube.com/watch?v=DoSTehS7iq8&eurl=http://people.eku.edu/ritchisong/301notes6.htm

Dalton’s Law of Partial Pressure In a mixture of gases, the total pressure is equal to the sum of the partial pressure of each separate gas In the Alveolus: PO2 + PH2O + PCO2 + PN2 = PB e.g.: add 5 mmHg of another gas to the alveolus and other gases are displaced…. If 10 molecules of a gas is in a container - total eesure is

Atmospheric Gases by apx. % Nitrogen (N2) 78% Oxygen (O2) 21% Trace Gases 1% ( argon, CO2, radon, CO etc.) density of gases change with changes in Barometric Pressure Individual Gas Percentages stay the same, regardless of elevation

Partial Pressure Partial Pressure is determined by the Barometric Pressure times the % of the gas in the atmosphere O2 = 21% or .21 at sea level 760 x .21 = 159; so the partial pressure of Oxygen in this room is 159 mmHg This is referred to as PIO2

This is what happens to Barometric Pressure as you ascend in the atmosphere…..

Internal Respiration: Gas exchange at the Cellular Level External Respiration: Ventilation and Gaseous Diffusion within the Lung Internal Respiration: Gas exchange at the Cellular Level http://www.youtube.com/watch?v=EFCj9STCvdI&NR=1&feature=endscreen

What about Water?

Water Vapor Pressure Water can exist as a gas, liquid or solid When in the molecular form it exerts a partial pressure just as oxygen does

Saturated Alveolar gas PIO2 & PAO2 Gas in the alveoli is assumed to be fully saturated when at body temperature 37o C A water vapor pressure of 47 mmHg is exerted in the human lung H2O mass, fully humidified is 44mg/L

The Partial Pressure of Oxygen (PAO2)in the Alveoli is 100 mmHg O2 must mix or compete with carbon dioxide and water vapor by the time the O2 reaches the alveolus it is diluted by the CO2 and H2O FIO2 = Fraction of Inspired Oxygen % PIO2 = PP of Inspired Oxygen PAO2 = PP of Alveolar Oxygen PaO2 = PP of Arterial Oxygen

The Balance between CO2 Excreted and O2 Uptake Every Cell in the body “bleeds” CO2 as part of Cellular Metabolism Every Cell in the body requires O2 for aerobic metabolism O2 CO2

Factors that Affect the Diffusion Rate: R = Respiratory Exchange Ratio VCO2 = 200 ml/min of CO2 removal VO2 = 250 ml/min of O2 uptake R = 200/250 = 0.8 Factors that Affect the Diffusion Rate: membrane surface area diffusivity of the gas or diffusion coefficient molecular weight and solubility membrane thickness pressure gradients

Alveolar Capillary Membrane alveolar fluid alveolar epithelium basal alveolar membrane interstitial space basal capillary membrane capillary epithelium Red Blood Cell

Dynamics of Gas Exchange in the Respiratory Exchange Zone:

The Oxygen Cascade

Alveolar Air Equation PAO2 = (Pb - H2O)FiO2 - PaCO2(1.25) .8 In Fort Myers Pb is 760 mmHg & room air is 21% oxygen PAO2 = (760 - 47).21 - 40(1.25) (713) .21 - 50 149 - 50 99

A-aDo2 = Difference between Alveolar and Arterial Oxygen Tensions Figure PAO2; then subtract the PaO2 Calculates the gradient b/w the alveoli and the blood to help identify problems associated with the A-C capillary membrane PAO2 = 99 PaO2 = 95 difference is 5 mm Hg normal gradient is 5 to 15 mmHg

Diffusion Rate Fick’s Law = A x D x (P1 - P2) T A= membrane surface area D= diffusion coefficient molecular & solubility inversely proportional to weight; > weight the < rate directly proportional to solubility; > solubility the > rate T = membrane thickness P1 - P2 = pressure gradient

Law of Mass Action Increasing surface area, diffusivity or partial pressure increases the diffusion rate Increasing membrane thickness decreases diffusion rate Graham’s Law - gas diffusion rate is inversely proportional to the square root of its gram molecular weight Oxygen is lighter (smaller molecular mass); thereby it diffuses 1.7 x faster than CO2

Gases dissolve due to Driving Pressures Henry’s Law - amount of gas dissolving in a liquid is directly proportional to the partial pressure of that gas; however, CO2 is 24 x more soluble than O2 Combined Henry’s and Graham’s CO2 diffuses about 20 x faster than O2

Capillary Transit Time At resting Cardiac Output, a RBC is exposed to the alveoli for .75 seconds Normal gas equilibrium is met after .25 seconds during exercise, transit time is shortened doesn’t affect healthy individuals with vascular compromise, gases don’t reach an equilibrium so diffusion rate is decreased

Diffusion during Exercise Diffusion at Rest Diffusion during Exercise

Hypobaric Conditions Hypoxia 2o Hypobaric exposure: Altitude and PO2 Hypoxia 2o Hypobaric exposure: http://www.youtube.com/watch?v=HPzL1iKPLIM&feature=related

Dramatic Movie Clip Airport 1970 http://www.youtube.com/watch?v=rRee5Z5fkYc&feature=related

Perfusion and Diffusion Limitations Oxygen diffusion is normally perfusion limited: dependent on the amt and speed of blood flow increases in blood flow decrease transit time decreased transit time can prevent an equilibrium of gases thus thus decreasing diffusion Diffusion limited impairments occur when: A/C capillary membrane is thickened preventing an equilibrium of gases regardless of transit time thus decreasing diffusion rate

Summary of Factors Determining Diffusion Rates Reduced Diffusion Surface Area Emphysema - destruction of alveoli Tumors (distortion of lung parenchyma) Pulmonary embolus V/Q mismatch (COPD/Atelectasis/Pneumonia) Increased Membrane Thickness Fibrotic thickening of alveoli, capillary or interstitial space Interstitial edema fluid Dilated, engorged capillaries (CHF)

Summary of Factors Determining Diffusion Rates (cont.) Blood Flow exercise low pulmonary capillary volume low cardiac output (heart failure & blood loss) Decreased Uptake by RBC Anemia & Hb pathologies High carbon monoxide levels (smokers) Diffusivity predetermined by gas molecule physics Pressure Gradient as the pressure gradient ↓ the diffusion rate↓

Remember Diffusion occurs at many levels Preview to Next Week: http://www.youtube.com/watch?v=5LjLFrmKTSA&feature=related

Let’s break… O 2 Transport Discussion is next

O2 transport can take 2 distinct pathways: RBCTransport Dissolved in Plasma

Whole Blood Transport components

Oxygen Transport due to Diffusion Dissolved in Plasma Henry’s Law - the amount of oxygen dissolved in plasma is directly proportional to the partial pressure or PO2 that it is exposed to. As PO2 increases - dissolved O2 increases .003 ml/dl of dissolved oxygen per 1 mmHg of PO2 At a PO2 of 100 the blood has .3 ml/dl of dissolved oxygen (100 x .003 = .3 ml/dl) ml/dl can also be expressed as vols %

Henry’s law regulates dissolved gases

How Much O2 can be transported by the Plasma? Let’s do the math…

Red Blood Cell— Gas Transport Workhorse

The O2 Journey at a Glance

How Much O2 can be transported by the Hemoglobin? Let’s do the math…

Next Week Top Ten Review O2 Transport Mechanisms Hypoxemia & Hypoxia Types Clinical Implications of Hypoxia