Respiration During Exercise II

Transcription

1 Slide 1 Respiration During Exercise II Slide 2 Partial Pressure of Gases Computing the P I O 2 Each gas in a mixture exerts a portion of the total pressure of the gas (Dalton s Law) The partial pressure of oxygen (PO ) 2 Air is 20.93% oxygen Total pressure of air at sea level = 760 mmhg PO2 = 760* = 159 mmhg Water vapor: 100% saturation at body temp = 47 mmhg P I O 2 = x (760-47) = 150 mmhg Slide 3 Got P I O 2, Now computing the P A O 2 In healthy mammals, Alveolar PO 2 depends on: Atmospheric Pressure (sets P I O 2 ) P A CO 2 very similar to P a CO 2 - RER alveolar gas equation P A O 2 = P I O 2 P a CO 2 /RER Example: P A O 2 = /.8 = = 100 mmhg

2 Slide 4 Computing P a CO 2 (~P A CO 2 ) In healthy mammals, PaCO 2 depends on: M etabolic rate V CO2 Alveolar ventilation rate V A O2 PACO2 = VCO2/VA * K (K = constant) So, PaCO2 is proportional to the metabolic rate divided by the alveolar ventilation Slide 5 - hyperventilation - high altitude - hypoventilation Partial Pressure and Gas Exchange Slide 6 O 2 Transport in the Blood O 2 is bound to hemoglobin (Hb) for transport in the blood Oxyhemoglobin: O 2 bound to Hb Deoxyhemoglobin: O 2 not bound to Hb Carrying capacity: 4 O2 per Hb = 100% saturated (3/Hb = 75% etc.) 1.39 ml O2/g Hb Human males have about 15 g Hb/100ml blood Human females tend to have lower [Hb]

3 Slide 7 Oxyhemoglobin Dissociation Curve Slide 8 O 2 -Hb Dissociation Curve: Effect of ph -Exercise humans, dogs -High altitude short term, llam as -High altitude long term Slide 9 O 2 -Hb Dissociation Curve: Effect of Tº T

4 Slide 10 Why is PaO 2 slightly less than PAO 2? PAO 2 is always greater than PaO 2 Under normal conditions in a healthy mammal, this difference is < or = ~10 mmhg Contributors: Shunt thebian circulation from lungs (normal) Incomplete equilibrium (diffusion limitation) Mismatch of ventilation and perfusion Slide 11 Slide 12 Factors Affecting Gas Exchange at the Lung **The PAO2 defines the upper limit to PaO2** Factors that affect the A-a difference: 1. Shunt - in newborns, patent ductus arteriosus or foramen ovale - in adults, pathology 2. Diffusion Limitation - particularly during exercise in horses, but also in some people 3. V-Q inequality mismatch of perfusion and ventilation - can be caused by disease - worsens during intense exercise even in healthy people Right to Left Shunt Deoxygenated blood leaks through the shunt from the right side to the left (systemic) side Normal: bronchial supply for larger airways, can account for up to 4 mmhg reduction in PaO2 Abnormal: Can occur through a patent ductus arteriosus or septal defect (for examples) Rare in adults Causes dilution of oxygenated blood, lowers PaO2 Clinically diagnosed by 100% O2 breathing

5 Slide 13 Diffusion Limitation Equilibrium between PAO2 and PaO2 is not reached DOES NOT AFFECT PaCO2 (Why?) Can be caused by any factor that affects diffusion BGB area or thickness (loss of lung due to surgery, etc.) Pressure gradient (altitude) Can occur during exercise due to reduced time for diffusion Capillary transit time may decrease from ~0.75 sec to only 0.25 sec Extreme case in race horses PaO2 drops to 50 s or 60 s, A-a difference is mmhg! Can occur in humans, particularly in elite athletes with very high cardiac output Slide 14 More extreme Than usual (horse like) Slide 15 Ventilation/Perfusion Matching Review: Pulmonary circuit Same rate of flow (L/min) as systemic circuit Cardiac output systemic = Cardiac output pulmonary M inute ventilation, also measured in L/min When standing, most of the blood flow is to the base of the lung Due to gravitational force

6 Slide 16 Slide 17 Ventilation-Perfusion Relationships Ventilation/perfusion ratio Indicates matching of blood flow to ventilation Ideal: ~1.0 Base Overperfused (ratio <1.0) Extreme case ratio near 0 - perfusion with no ventilation = physiological shunt, as with collapsed alveoli or severe pulmonary edema Apex Underperfused (ratio >1.0) Extreme case ratio near infinity ventilation with no perfusion, as with blocked or collapsed blood vessel Ventilation/Perfusion Ratios in the Lung Slide 18 Chemoreceptors Peripheral chemoreceptors Aortic arch chemoreceptor Carotid bodies Carotid sinus nerve carries messages Detect PaO 2, PaCO 2, H +, and K + in arterial blood Central chemoreceptors Located in the medulla PCO 2 and H + concentration in cerebrospinal fluid

7 Slide 19 Effect of Increasing PaCO 2 on Minute Ventilation Slide 20 Effect of Decreasing PaO 2 on Minute M Ventilation