Gas Exchange and Transport (PDF)

Chapter 12 Gas Exchange and Transport Copyright © 2017 Elsevier Inc. All Rights Reserved.  Learning Objectives  Describe how oxygen and carbon dioxide move between the atmosphere and tissues.  Identify what determines alveolar oxygen and carbon dioxide pre...

Chapter 12 Gas Exchange and Transport Copyright © 2017 Elsevier Inc. All Rights Reserved.  Learning Objectives  Describe how oxygen and carbon dioxide move between the atmosphere and tissues.  Identify what determines alveolar oxygen and carbon dioxide pressures.  Calculate the alveolar partial pressure of oxygen at any given barometric pressure and fraction of inspired oxygen.  State the effects that normal regional variations in ventilation and perfusion have on gas exchange. Copyright © 2017 Elsevier Inc. All Rights Reserved. 2  Learning Objectives (Cont.)  Describe how to compute total oxygen content for arterial blood.  State the factors that cause the arteriovenous oxygen content difference to change.  Identify the factors that affect oxygen loading and unloading from hemoglobin.  Describe how carbon dioxide is carried in the blood. Copyright © 2017 Elsevier Inc. All Rights Reserved. 3  Learning Objectives (Cont.)  Describe how oxygen and carbon dioxide transport are interrelated.  Describe the factors that impair oxygen delivery to the tissues and how to distinguish among them.  State the factors that impair carbon dioxide removal. Copyright © 2017 Elsevier Inc. All Rights Reserved. 4  Recall  Respiration: process of moving oxygen to tissues for aerobic metabolism and removal of carbon dioxide  Involves gas exchange at lungs and tissues O2 from atmosphere to tissues for aerobic metabolism Removal of CO2 from tissues to atmosphere Copyright © 2017 Elsevier Inc. All Rights Reserved. 5  Clinical Conditions that Cause Diffusion Problems Clinical conditions that decrease rate of gas diffusion— diffusion- limited problems Figure 4-11.  Diffusion (Cont.) page 248 O2 shows a downward “cascade” of partial pressures from atmospheric to the cellular level CO2 is moving from the opposite direction where it is higher in the cells and cascades downward Copyright © 2017 Elsevier Inc. All Rights Reserved. 7  Determinants of Alveolar Gas Tensions  Alveolar carbon dioxide  PACO2 varies directly with the body’s production of carbon dioxide (CO2) and inversely with alveolar ventilation (VA). Under normal conditions it is maintained at approximately 35 to 45 mm Hg.  The PACO2 will increase above normal if carbon dioxide production increases while alveolar ventilation remains constant.  An increase in dead space (gas not participating in gas exchange), can also lead to an increased PACO2  PACO2 decreases if CO2 production decreases or alveolar ventilation increases  If CO2 production increases (exercise or fever), ventilation automatically increases in order to maintain the PACO2 within a normal range. Copyright © 2017 Elsevier Inc. All Rights Reserved. 8  Alveolar air equation Alveolar oxygen tension (PAO2)  In lungs the air is diluted by water vapor and CO2 Gas fully saturated with water vapor at BTPS is 47 mmHg Moving into the alveoli our PO2 is less because it contains water vapor and CO2 Healthy PCO2 is 40 mmHg (range is 35-45) Since the sum of all gases must equal PB (Duh Dalton) the PO2 falls by 40 mmHg when it enters the alveoli O2 diffuses out of the alveoli faster than CO2 diffuses into it 9  Determinants of Alveolar Gas Tensions (Cont.)  Changes in alveolar gas partial tensions  O2, CO2, H2O, and N2 normally compose alveolar gas N2 is inert but occupies space and exerts pressure Partial pressure of alveolar nitrogen (PAN2) is determined by Dalton’s law PAN2 = PB – (PAO2 + PACO2 + PH2O)  Only changes seen will be in O2 and CO2 Constant FiO2, PAO2 varies inversely with PACO2 Prime determinant of PACO2 is VA Copyright © 2017 Elsevier Inc. All Rights Reserved. 11  Mechanisms of Diffusion  Diffusion occurs along pressure gradients  Barriers to diffusion  A/C membrane has three main barriers Alveolar epithelium Interstitial space and its structures Capillary endothelium  RBC membrane  Fick’s law: The greater the surface area, diffusion constant, and pressure gradient, the more diffusion will occur Copyright © 2017 Elsevier Inc. All Rights Reserved. 12  Mechanism of gas diffusion  Given that the area of and distance across the alveolar-capillary membrane are relatively constant in healthy people, diffusion in the normal lung mainly depends on gas pressure gradients. Copyright © 2017 Elsevier Inc. All Rights Reserved. 13 Mechanisms of Diffusion (Cont.)  Pulmonary diffusion gradients  Diffusion occurs along pressure gradients  Time limits to diffusion: Pulmonary blood is normally exposed to alveolar gas for 0.75 second, during exercise may fall 0.25 second Normally equilibration occurs in 0.25 second With diffusion limitation or blood exposure time of less than 0.25 seconds, there may be inadequate time for equilibration Copyright © 2017 Elsevier Inc. All Rights Reserved. 14 Mechanisms of Diffusion (Cont.) Copyright © 2017 Elsevier Inc. All Rights Reserved. 15  Diffusion Across AC-Membrane Under Normal Conditions Under normal resting conditions, blood moves through alveolar capillary membrane in approximately 0.75 seconds  Diffusion Across AC-Membrane During Exercise During exercise or stress, total transit time for blood through alveolar capillary membrane is less than normal  Diffusion Across AC-Membrane with Alveolar Thickening When rate of diffusion is decreased because of alveolar thickening, oxygen equilibrium likely will not occur  Shunting  Ventilation & Perfusion is not perfect in the normal lungs  PaO2 is normally 5-10 mmHg less than PAO2 because of shunts  Anatomical (right to left shunts) Bronchial venous drainage Thebesian venous drainage PAO2 – PaO2 or P(A-a)O2 or A-a gradient  Measures the difference between Alveolar and arterial PO2  Indicates the efficiency of gas exchange  Estimates the degree of hypoxemia and shunting  5-10 mmHg on 21% = normal  25-65 mmHg on 100% FIO2 = normal  66-300 mmHg = V/Q mismatch  >300 mmHg = shunt  On the normal values sheet Copyright © 2017 Elsevier Inc. All Rights Reserved. 20  PAO2 –PaO2  PAO2 –PaO2: If increased then there is an abnormal O2 exchange  A small difference between alveolar and arterial O2 is due to small number of veins carrying deoxygenaged blood that bypasses the lungs and empties into arterial circulation  Thebesian vessels of the left ventricular myocardium drain directly into the left ventricle  Some bronchial veins and mediastinal veins drain into pulmonary veins and decreases arterial PaO2 Copyright © 2017 Elsevier Inc. All Rights Reserved. 21  Normal Variations From Ideal Gas Exchange (Cont.)  Ventilation/Perfusion ratio  Ideal ratio is 1, where / is in perfect balance  Areas with ventilation and no blood flow is called deadspace  Alveolar deadspace. Causes of increases: PE, partial obstruction of the pulmonary vasculature, destroyed pulmonary vasculature (like in COPD) and reduced cardiac output  Anatomic deadspace which is the portion of VT that never reaches the alveoli for gas exchange Copyright © 2017 Elsevier Inc. All Rights Reserved. 22  Normal Variations From Ideal Gas Exchange (Cont.)  If ventilation and blood flow are mismatched, impairment of both O2 and CO2 transfer occurs  If ventilation exceeds perfusion the V/Q is greater than 1  If perfusion exceed ventilation the V/Q is less than 1  Pneumonia -ventilation is decreased to the affected lobe. If perfusion is unchanged then perfusion is in excess of ventilation. The V/Q is less than 1. Because of the decreased ventilation, hypoxic vasoconstriction happens in the pulmonary capillary that supply the lobe. So then there is a decrease in perfusion. Copyright © 2017 Elsevier Inc. All Rights Reserved. 23  Normal Variations From Ideal Gas Exchange (Cont.)  Ventilation with zero blood flow = alveolar dead space (increases PO2 and lowers alveolar PCO2)  Lower alveolar PO2 increases PaCO2; perfusion but no ventilation  In an upright person the V/Q at the top of the lung is increased which means increase ventilation relative to little blood flow in pulmonary circulation because of gravity Copyright © 2017 Elsevier Inc. All Rights Reserved. 24 Copyright © 2017 Elsevier Inc. All Rights Reserved. 25  Oxygen Transport  Transported in two forms: dissolved and bound  Physically dissolved in plasma  Gaseous oxygen enters blood and dissolves.  Henry’s law allows calculation of amount dissolved Dissolved O2 (ml/dl) = PO2  0.003  Chemically bound to hemoglobin (Hb) – a Majority is carried here  Each gram of Hb can bind 1.34 ml of oxygen.  [Hb g]  1.34 ml O2 provides capacity.  70 times more O2 transported bound than dissolved. Copyright © 2017 Elsevier Inc. All Rights Reserved. 26  Oxygen Transport (Cont.)  Hemoglobin saturation  Saturation is % of Hb that is carrying oxygen compared to total Hb SaO2 = [HbO2/total Hb]  100 Normal SaO2 is 95% to 100%  HbO2 dissociation curve  Relationship between PaO2 and SaO2 is S-shaped  Flat portion occurs with SaO2 >90% Facilitates O2 loading at lungs even with low PaO2  Steep portion (SaO2