Cardiopulmonary 2: Gas Exchange MEDI221/EXSC221 PDF

Cardiopulmonary 2: Gas Exchange MEDI221/EXSC221 Lecture objectives Describe factors affecting diffusion of CO2 and O2 from lung to blood (Fick’s law) Describe pressure (gradients) for CO2 and O2 Describe how CO2 and O2 are transported Describe how exercise-related factors affect exchange...

Cardiopulmonary 2: Gas Exchange MEDI221/EXSC221 Lecture objectives Describe factors affecting diffusion of CO2 and O2 from lung to blood (Fick’s law) Describe pressure (gradients) for CO2 and O2 Describe how CO2 and O2 are transported Describe how exercise-related factors affect exchange and carriage of CO2 and O2 Describe other factors that do/don’t limit or improve gas exchange Reading Chapter 13. Gas exchange and transport McArdle, W.D., Katch, F.I. and Katch, V.L. Exercise Physiology: Nutrition, Energy and Human Performance. Lippingcott, Williams and Wilkins, Sydney, NSW, 2014. ISBN/ISSN: 9781451191554. Pressure of respiratory gases in air (at sea level) Pressure gradient for each gas determines its diffusion rate, and so its rate of gas exchange Dalton’s Law: [Gas] x Total pressure of gas mixture = Partial Pressure (i.e., barometric pressure) Partial Pressure Partial pressure is the amount of pressure each gas would exert if it were the only gas in the volume Revision: Fick’s law of diffusion Extent of gas dissolving into a fluid depends on: Pressure gradient, Solubility, Temperature How does exercise affect Time (if time constraint) these and thus diffusion??? Pressure gradient for oxygen: Alveolus to blood PAO2 (~100 mmHg) lower than in atmosphere (~159 mm Hg) Water has evaporated into it CO2 diffuses into it Pressure gradients for O2 and CO2: Blood – Muscle CO2 is very soluble, so small pressure gradient will allow equivalent exchange rate. Terminology The oxygen concentration (usually termed “oxygen content”) of systemic arterial blood depends on several factors, including the partial pressure of inspired oxygen, the adequacy of ventilation and gas exchange, the concentration of haemoglobin and the affinity of the haemoglobin molecule for oxygen. The content (or concentration) of oxygen in arterial blood (CaO2) is expressed in mL of oxygen per 100 mL or per L of blood, while the arterial oxygen saturation (SaO2) is expressed as a percentage which represents the overall percentage of binding sites on haemoglobin which are occupied by oxygen. The relationship in blood between oxygen saturation (SaO2) and partial pressure (PO2) is described graphically by the oxygen–haemoglobin dissociation curve Arterial blood can desaturate during exercise Typical in COPD = impaired gas exchange (obstruction) But also, Elite athletes What does this mean? saturation dips below 90% ↑ oxygen offloaded from the hemoglobin into the tissues WHY? Short transit time (high vascularization) More Ventilation: Perfusion mismatch at high intensities (superior CV function) – page 264 High oxygen delivery & large pressure = high offload of O2 from hemoglobin The arteriovenous oxygen difference, or a-vO2 diff, is the difference in the oxygen content of the blood between the arterial blood and the venous blood. Rest Whole-body during intense exercise Active muscle during intense exercise Figure 13.6 text book Several pulmonary factors can limit exercise Desaturation of O2 High airway resistance (COPD) Pulmonary pressure effects on cardiac output High work of breathing Resp. muscle fatigue ↑ sympathetic outflow Can limit blood flow to locomotive muscles O2 transport in the blood Arterial transport = 200 mL O2/L blood But, is poorly soluble in blood Just 3 mL O2/L Almost all (~99%) of O2 is bound to Hb. Hb can carry up to four O2 molecules 100% saturation Affinity for O2 binding ↑ as saturation ↑ Oxygen binding occurs in response to high PO2 in lungs 25% of O2 is offloaded to tissues at rest 80-90% O2 is offloaded during exercise Hemoglobin concentration is a primary determinant of O2 transport and fitness. ↑ hemoglobin→ ↑ oxygen delivery → ↑ VO2max. Mb Hb Fig 13.4 page 279 Text book Bohr effect Factors that help to offload O2 to metabolically active tissues: Bohr effect: Curve shift down to right (= favour unloading) If: ↑ Temp ↓ pH ↑ CO2 2,3 DPG Produced in RBC during glycolysis Similar to Bohr effect High levels during exercise, altitude, pregnancy, anaemia  CO2 transport 5% dissolved in plasma Establishes PCO2 Tightly regulated Drives ventilation (major at rest) 20% is bound to Hb 60-80% is as bicarbonate Buffer to maintain PH Acid-base buffering Buffers resist changes in pH Bicarbonate accepts an increase in H+ and releases carbon dioxide which lungs will expel. Most of CO2 is in this buffer system The arrows indicate that this system moves backwards and forwards all the time to maintain an acid-base balance. pH is important for many biological and physiological functions. Enzyme activity, transport etc CO2 and H+ both metabolites, are tightly correlated and stimulate breathing- both indicate inadequate breathing and/or perfusion. Three ‘buffering’ mechanisms: Chemical (bicarbonate, phosphate, protein (incl Hb)) Pulmonary ventilation Renal function In a practical sense: Exercise at altitude How is exercise at altitude going to effect exercise performance (VO2 max)? > ~2100m exercise performance impacted % oxygen (20.9%) is constant BUT the atmospheric pressure falls with increasing altitudes Atmospheric pressure and inspired PO2 fall ~ linearly with altitude. At 5500m → ~ 50% of the sea level value At 8900m (summit of Everest) → ~ 30% of the sea level value The Diffusion Gradient is in the right direction but the Pressure Gradient is not! Pressure gradient for each gas determines its diffusion rate, and so its rate of gas exchange Dalton’s Law: [Gas] x Total pressure of gas mixture = Partial Pressure (i.e., barometric pressure) Mb The Diffusion Hb Gradient is in the right direction but the Pressure Gradient is not PO2 Overview of responses to altitude Generally, level of response is proportional to level of altitude (curvilinear) Non-linear effect of altitude on SaO2 and VO2max Acute responses are cardiorespiratory ↑ HR and VE at rest and submax exercise Then begin to ↑ Hct as fluid is lost Adaptive responses also include peripheral changes Muscle oxidative changes (up or down, depending on altitude) ↑ RBC Early responses to altitude (seconds – hours) Pulmonary ↑ ventilation (↑ O2 into blood) (↑HVR) Acid/Base respiratory alkalosis ↓ buffering capacity Haematological/cardiovasc. ↑ HR, but HRmax may ↓ ↑ a-vO2 difference ↑ 2,3 DPG (help offload O2) ↓ Plasma volume (↑ O2 concentration) ↓ SV (due to ↓ PV) (rest & exercise) 7/29/2023 ↑ EPO; potentially lead to ↑ Hb & RCV How is exercise at altitude going to effect exercise performance (VO2 max)? - What physiological phenomena occur? ↓ PO2 (due to ↓ atmospheric pressure) = Smaller pressure gradient for gas exchange Shift oxygen dissociation curve to right to try to account for this (↑ 2,3 DPG) BUT depending on how long and how high still get decreased performance via: ↑ HR and VE (central command and chemoreceptors (high CO2 low O2) Impaired buffering capacity (so higher PH) Respiratory alkalosis Reduced cardiac output (fluid shifts) = less oxygen delivery to skeletal muscle VO2max 1 150 Lecture summary Several pulmonary factors can limit exercise performance Desaturation of Hb in arterial blood esp. elite females? And hypoxic environs and/or COPD Respiratory work and fatigue can be important Transport of O2 and CO2 differ markedly CO2 diffuses and dissolves easily, whereas O2 doesn’t. Multiple means to transport CO2 Lungs and HCO3 important in buffering H+ in exercise Hemoglobin is a remarkable molecule and is really important for exercise performance!