Transport of Oxygen and Carbon Dioxide II Matthew L. Fowler, Ph.D. Cell Biology and Physiology Block 4

Reading Guyton Chapters 39 and 40 Important – pp

Learning Objectives Define P 50 for oxygen. Describe Grams Law and Ficks’ Law in regard to oxygen diffusion Describe the kinetics of oxygen transfer from alveolus to capillary and the concept of capillary reserve time (i.e., the portion of the erythrocyte transit time in which no further diffusion of oxygen occurs). Describe the difference between perfsion and diffusion limited disorders and detail how 02 is perfusion limited and CO is diffusion limited and how we can use this clinically. Define oxygen diffusing capacity, and describe the rationale and technique for the use of carbon monoxide to determine diffusing capacity. Describe how anemia and carbon monoxide poisoning affect the shape of the oxyhemoglobin dissociation curve, PaO 2, and SaO 2. List the forms in which carbon dioxide is carried in the blood. Identify the percentage of total CO 2 transported as each form. Describe the forms of oxygen that are transported in the blood, how much oxygen is soluble and how binding to hemoglobin increase the amount of 02 in the blood substantially. Describe how anemia, polycythemia and C0 poisoning influence Hb and blood flow.

Molecular Basis of Diffusion of Gases Net diffusion of a gas will occur down a concentration gradient from an area of high concentration to low concentration…until an equilibrium is established. In the body, an equilibrium is never established. The goal of the body is to keep an equilibrium from being established. Diffusion requires energy –Provided by the kinetic motion of the molecules themselves

Gas Pressure and Force Pressure is caused by total sum of moving molecules against a surface at any given instant. Respiratory tract –Surface = Alveoli –Alveolar surface = unit area Mass is a component of force Higher the mass = greater the force Pressure α Concentration of Gas Molecules

Partial Pressure of Gases Respiratory physiology deals with mixtures of gases –O 2, CO 2, N 2 Rate of diffusion α Single Gas Pressure Ex. Rate of O 2 diffusion α O 2 Gas Pressure Portion of pressure caused by that gas is the partial pressure –O 2 gas pressure alone in a mixture indicated by PO 2 –21% of air is O2. Total air pressure is 760 mm Hg –PO 2 of air is mm Hg

Pressure of Dissolved Gases Most gases in the body are dissolved in fluid; i.e. the blood Partial pressure of a gas depends on: 1.Concentration 2.Solubility coefficient Why? –Some molecules are physically/chemically attracted to water molecules; others not. –This forms the basis for Henry’s Law of Partial Pressure.

Partial Pressure of Dissolved Gases Henry’s Law Fewer dissolved molecules means lower solubility; therefore… Lower solubility = higher partial pressure Note: A higher solubility coefficient means a gas is more soluble. Partial Pressure = _____________________ Concentration of dissolved gas Solubility coefficient Oxygen0.024 Carbon dioxide0.57 Carbon monoxide0.018 Nitrogen0.012 Helium0.008 CO 2 is more than 20X more soluble than O 2

Overview of Oxygen Transport Alveolar Exchange dPO 2 /CO 2 = dissolved bPO 2 = bound P A = alveolar P a = arterial O 2 is not soluble in plasma Only about 2% is found in serum (unbound) Gases bound to Hb do not exert a partial pressure! (dP a O 2 ) (dP a CO 2 ) HbO 2 (bP a O 2 ) (P A O 2 ) (P A CO 2 )

Gas Movement Respiratory Exchange Ratio (R) The CO 2 production to O 2 consumption R = 0.8 (at rest) For obvious reasons you need to take in more oxygen than you produce carbon dioxide.

How Gases Diffuse Graham’s Law The rate of diffusion is directly proportional to the solubility coefficient of the gas and inversely proportional to the square root of it molecular weight. Oxygen0.024 Carbon dioxide0.57 Carbon monoxide0.018 Nitrogen0.012 Helium0.008 CO 2 is more than 20X more soluble than O 2

How Gases Diffuse Fick’s Law The diffusion of a gas across a membrane to tissue is directly related to the surface area, diffusion constant of the specific gas, partial pressure difference, and thickness of the membrane.

Diffusion of Gases from the Alveoli Gas Uptake is Determined by (3) Factors: 1.Diffusion properties of the alveolar-capillary membrane; 2.Partial pressure gradient of oxygen, 3.Pulmonary capillary blood flow Diffusion of gases is a function of the partial pressure difference of the individual gases –The partial pressure difference for oxygen is called the oxygen diffusion gradient Examples of Diffusion Gradients In the normal lung, the initial oxygen diffusion gradient, alveolar PO 2 (102 mm Hg) minus venous PO 2 (40 mm Hg) is 62 mm Hg The initial diffusion gradient across the alveolar-capillary membrane for CO 2 (venous PCO 2 ) minus alveolar PCO 2 is about 6 mm. Take Home: O 2 diffuses rapidly compared to CO 2

Control of Diffusion in the Alveoli The average male functional residual capacity of the lungs (volume remaining in the lungs at the end of normal expiration) is about 2300 mL 350 mL new air inspired 350 mL old air expired Volume replaced with each breath is 1/7 total Result is a slow rate of air renewal. The slow replacement of alveolar air prevents sudden changes in gas concentrations in the blood – the makes the respiratory control mechanism much more stable. –Allows for achievement of steady state. Diffusion Flashback Net diffusion of a gas will occur down a concentration gradient from an area of high concentration to low concentration.

Alveolar Structure Capillary network with large surface area –1 μm barrier Pneumocytes Type I Gas exchange surface of the alveoli Type II Proliferate and differentiate into Type I cells to restore the damaged barrier, Produce surfactant

Alveolar Diffusion Rate of gas diffusion in inversely related to its membrane thickness. If the membrane thickness is doubled, the rate of diffusion is halved Remember, Fick’s Law says that rate of gas diffusion is is directly proportional to the surface area. If two lungs have the same oxygen gradient and membrane thickness but one has 2X alveolar capillary surface area, the rate of diffusion will differ by 2-fold What does this mean in emphysema? What happens if an entire lung is removed from a patient? Fick’s Law Flashback The diffusion of a gas across a sheet to tissue is directly related to the surface area, diffusion constant of the specific gas, and the partial pressure difference.

Limitations of Gas Movement Perfusion Limitations O 2 and CO 2 exchange in the lungs is perfusion limited Partial pressures of gas leaving the capillary have reached equilibrium with alveolar gas and is limited by the amount of blood perfusion the alveolus. Blood enters the arterial end of the pulmonary capillary at a PaO 2 of 40 mm Hg, whereas alveolar P A O 2 is approximately 100 mm Hg. As the blood passes along the capillary, O 2 diffuses across the blood gas barrier and dissolves in the plasma, increasing the plasma PaO 2 until it equals P A O 2. Under normal conditions, equilibration occurs before the midpoint of the capillary. Direction of flow

Limitations of Gas Movement Diffusion Limitations Movement of CO in the lungs is diffusion limited CO has low solubility in the alveolar capillary membrane but high solubility in the blood because of its high affinity for hemoglobin. This prevents the equilibration of CO between alveolar gas and blood during the red blood cell transit time. The high affinity for CO with Hb enables large amounts of CO to be taken up in the blood with little or nor appreciable increase in its partial pressure. Take Home: High affinity = diffusion limited Direction of flow

Perfusion vs. Diffusion Limitations Solubility determines the limitation to the rate of gas diffusion. Pressure (ex. via cardiac output) determines the limitation to the rate of gas perfusion. Perfusion = Pressure

Partial Pressure of O 2 in Blood Blood gets oxygenated in pulmonary capillaries. 98% of blood entering the left atrium is oxygenated to a PaO 2 of 104 mm Hg 2% is shunted - passed from the aorta through the bronchial circulation – not exposed to lung air PaO 2 remains high till systemic capillaries The PaO 2 of shunt blood is about 40 mm Hg or that of normal venous blood Shunt blood combines in the pulmonary vein with oxygenated blood from alveolar capillaries (venous admixture of blood) Result: PaO 2 of blood entering the left heart and pumped into the aorta to fall to around 95 mm Hg

Partial Pressure of O 2 in Blood Arterial blood reaching peripheral tissues has a PO 2 of 95 mm Hg. The PO 2 of interstitial fluid surrounding cells is around 40 mm Hg The pressure difference causes O 2 to diffuse rapidly from capillary blood into tissues. The PO 2 of capillaries falls almost to equal the 40 mm Hg pressure in the interstitium. Why is the drop from 104 mm Hg to 95 mm Hg needed? What would be the consequence of blood with a PO 2 of 104 mm Hg reaching tissues?

Partial Pressure Affect Blood Flow Rate If blood flow is increased, greater quantities of O 2 are transported into the tissue and tissue PO 2 increases. Note in the figure that and increase in blood flow of 400% of normal increases the PO 2 from 40 mm Hg to 66mm Hg The upper limit the which PO 2 can rise even with maximal blood flow cannot exceed 95 mm Hg or the PO 2 in arterial blood. –If cells use more oxygen for metabolism than normally, interstitial fluid PO 2 is reduced –Conversely, with lower PO 2 consumption or reduced metabolism, there is increase interstitial fluid PO 2. Tissue PO 2 depends upon: –The rate of oxygen transport to the tissues in blood –The rate at which O 2 is used by tissues.

Diffusion of Gases from Tissues to Circulation Carbon Dioxide Virtually all oxygen used by the cell is converted to CO 2 PCO 2 is high and it diffuses from tissues to tissue capillaries CO 2 is carried to the lungs where it diffuses into pulmonary capillaries It then diffuses from pulmonary capillaries to alveoli where it is expired. There is one major difference – CO 2 diffuses 20X faster than oxygen Pressure differences required to cause carbon dioxide diffusion are far less than pressure differences required for O2 diffusion. Why is that?

Diffusion of Gases from Tissues to Circulation Carbon Dioxide PCO 2 of blood entering pulmonary capillaries is 45 mm Hg PCO 2 of the alveolar end is 40 mm Hg. Thus only a 5 mm Hg difference is required for CO 2 diffusion out of the pulmonary capillaries into alveoli. Because of the solubility, this is why only a small difference of CO 2 pressure is required to drive diffusion out of the cell.

Diffusing Capacity Ability of the respiratory membrane to exchange a gas between alveoli and the pulmonary blood can be expressed quantitatively by the diffusing capacity. Defined as the volume of a gas that will diffuse through the membrane each minute for a partial pressure difference of 1 mm Hg. CO 2 really diffuses easily Why? Very soluble compared to CO and O 2. Indicated by the higher solubility coefficient.

Uptake of Gases Gases that are perfusion limited have their partial pressures equilibrated with the alveolar pressure before exiting the capillary. Includes O 2, CO 2, and N 2 (N 2 O) N 2 (N 2 O) does not bind Hb and therefore equilibrates rapidly CO (diffusion limited) does not reach equilibrium

Terminology for O 2 Transport O 2 content (or concentration) – total amount (or concentration) of O 2 in the blood, including free and bound forms Dissolved O 2 –amount of O 2 in solution as defined by the P O2 and O 2 solubility Hb or O 2 saturation –amount of O 2 bound to Hb as a percentage of the total O 2 binding capacity of Hb in a blood sample Arterial O 2 saturation (SaO 2 ) –Relative measurement of Hb saturation by a colorimetric transcutaneous measurement

Oxygen Transport About 20% volume (20 ml of O 2 per 100 ml of blood (exits in two forms) Form 1 0.3% dissolved in blood is related to PO 2 and solubility Form 2 Amount carried by Hb –Depends on concentration and saturation –Normal Hb concentration g Is increased polycythemia (does not affect PO 2 ) Is decreased anemia

Forms of Oxygen in Blood Dissolved (Unbound) The amount of dissolved O 2 in the blood is a linear function of the PO 2 in the blood. The amount of O 2 that can be dissolved in aqueous solution is related to the partial pressure of O 2 in the compartment, and solubility of O 2 in the liquid. The amount of dissolved O 2 in the blood is therefore a linear function of the PO 2 in the blood. Dissolved O 2 account for only about 3 ml of O 2 per liter of blood at 100 mm Hg PO 2. O 2 dissolved = PO 2 x O 2 solubility O 2 dissolved = PO 2 x 0.03 mL/(L x mm Hg)

Forms of Oxygen in Blood Bound to Hemoglobin The amount of O 2 bound to Hb can be calculated by multiplying the capacity of Hb to bind O 2 (1.39 mL/g Hb). O 2 bound = PO 2 x O 2 solubility O 2 dissolved = PO 2 x 0.03 mL/(L x mm Hg)

Summary of O 2 Binding O 2 content = Dissolved + Bound –~21 ml/dl or 210 ml/L blood Dissolved O 2 –O 2 dissolved = PO 2 x solubility –O 2 dissolved = PO 2 x ml/(L x mm Hg) –~0.3 mL O 2 /dL or 3 ml O 2 /L plasma at PO 2 = 100 mm Hg Bound O 2 –O 2 bound to Hb =O 2 capacity x g of Hb x % sat. of Hb 1.39 mL/mL x g of Hb x % sat. of Hb 1.39 mL/g x 15 g Hb/dL x 0.98 –~20.4 ml O 2 /dL or 204 mL O 2 /L

Oxygen Saturation Curve Summary of O 2 Binding At 100 mm Hg, a liter of normal blood contains approximately 195 ml of O 2 bound to Hb. Total O 2 content of blood is the sum of the dissolved O 2 and the O 2 bound to Hb. Binding of O 2 to Hb is dependent on the partial pressure of O 2, but the relationship is not linear. The binding of O 2 to the 4 heme groups on a Hb molecule is cooperative. That is, as O 2 binds to the heme groups, the conformation of the Hb molecule changes and the affinity of the other heme groups for O 2 changes. Binding of 1 O 2 molecule to Hb facilitates the binding of the second and third O 2 molecules to that Hb molecule. Conversely, loss of 1 O 2 molecule from a fully saturated Hb molecule, facilitates the release of the 2 nd and 3 rd. Thus, the amount of O 2 bound to Hb is related to P O2 in a curvilinear fashion. Hb (g) = 15 g/dL Hb binding capacity = 1.39 O 2 /g O 2 Bound to Hb = HbO 2 Binding Capacity x Hb (g) x % Saturation of Hb

Oxygen Saturation Curve Effect of Carbon Monoxide CO Poisoning Curve shifted to left, Hb has super-high affinity for the last 2 molecules of oxygen already bound. Actual amount of Hb is not reduced, but functionally there is less O 2 binding sites, so the O 2 carrying capacity of the blood is reduced. Note that CO participates in cooperative binding of O 2. Carboxyhemoglobin causes the Hb to have an increased affinity for O 2. The O 2 bound to carboxyhemoglobin is more difficult to release in the tissue. The saturation of Hb would be nearly 100%, but you do not unload any of your O 2 so it is the same as having no oxygen delivered to your tissues. Normal blood (HbO 2 ) has a P 50 of 27 mmHg. Even with only 50% of sites bound with oxygen (50% HbO 2 ) the P 50 remains the same. HbCO has a P 50 that is lower (green dashed vertical line). 50% HBCO50%HbO 2 PaO 2 Normal SaO 2 Normal O 2 ContentLow P 50 LowNormal

O 2 and Hb in Disease States Polycythemia Increase in the size and number of RBCs (and thereby increase in Hb) Total O 2 will increase but dissolved O 2 will remain the same –Therefore PO 2 is not affected Anemia Decrease in the number of RBCs (and thereby decrease in Hb) In anemia total O 2 will decrease but dissolved O 2 will remain the same (so PO 2 will not be effected) –Therefore PO 2 is not affected

Summary Summary of Disease States PO 2 [Hb]Saturation O 2 /Hb (g) Total O 2 (Free + Bound) Anemia N  N  Polycythemia N  N  CO Toxicity NN  Summary of O 2 Stores Dissolved O 2 2% of blood - Minimal Alveoli500 mL O 2 – 1-2 minutes basal metabolism Blood (HbO 2 )750 mL O 2 – 3 minutes basal metabolism MbO seconds of cardiac metabolism