Medical Physiology Block 5 Week 2 Flashcards
What is the equation for the amount of gas dissolved in a solution?
partial pressure of the gas multiplied by a solubility constant (linked to cardiac output)
What is the partial pressure of oxygen in alveolar air? mixed-venous blood? arterial blood? dry inspired air? wet inspired air?
100; 40; 100; 160; 150 ((760-47) x 0.21)
What is the partial pressure of carbon dioxide in alveolar air? mixed-venous blood? arterial blood? dry inspired air? wet inspired air?
40; 46; 40; 0; 0
How is oxygen carried in blood?
dissolved in the blood or bound to hemoglobin
Describe adult hemoglobin.
4 subunits each with a heme moiety (iron-containing porphyrin ring); 2 alpha chains and 2 beta chains
What is the oxidation state of iron in adult hemoglobin?
+2 (ferrous)
What is the oxidation state of iron in methemoglobin? Can it bind oxygen? what enzyme converts it back to hemoglobin?
+3; NO; methemoglobin reductase (NADH-dependent)
How is the structure of fetal hemoglobin different from adult hemoglobin?
2 alpha chains and 2 gamma chains
What is the oxygen binding capacity of blood?
maximum amount of oxygen that can be bound to hemoglobin in blood (measured at 100% saturation)
What is the oxygen content of blood?
total amount of oxygen carried by blood, including bound and dissolved; equals (oxygen binding capacity x % saturation) + dissolved oxygen
What is the oxygen saturation of arterial blood?
about 97-98% (at a partial pressure of about 100 mm Hg)
What is the oxygen saturation of mixed-venous blood?
about 75% (at a partial pressure of 40 mm Hg); meaning three of the four heme groups are bound by oxygen
Why is the hemoglobin-oxygen dissociation curve sigmoidal?
positive cooperativity
Is the optimal saturation of blood (arterial) resistant to changes in oxygen partial pressure (as low as 60 mm Hg at high altitudes)?
Yes; the Hb-O2 dissociation curve is relatively flat in this segment of the plot
What factors help to unload oxygen from hemoglobin (in peripheral tissues)?
high temperature, low pH & high carbon dioxide partial pressure (Bohr effect), and 2,3 di-phosphoglycerate (2,3 DPG)
What is one reason for the increased affinity for oxygen of fetal hemoglobin?
2,3 DPG can be formed but does not bind to the molecule (normally binds between beta chains)
What happens in carbon monoxide poisoning?
carbon monoxide competes with oxygen for hemoglobin binding (has a higher affinity decreasing the oxygen content of blood); binding of oxygen to remaining hemoglobin binding sites with a higher affinity (difficulty unloading the oxygen to peripheral tissues)
How is carbon dioxide carried in the blood?
primarily as bicarbonate inside the red blood cell (carbonic anhydrase speeds up reaction to form carbonic acid followed by bicarbonate and a proton; bicarbonate gets shuttled out of the cell in exchange for chloride and proton is buffered by hemoglobin); some may be dissolved and some may bind to hemoglobin to form a carbaminohemoglobin molecule (plus a proton)
What channels allow bicarbonate to diffuse through the red blood cell membrane?
aquaporin 1 and Rh complex
How is carbon dioxide released in the lungs?
Bicarbonate that was in the plasma enters the RBC (exchange with chloride), recombines with a proton to form carbon dioxide and water (first carbonic acid) and diffuses across the RBC membrane and endothelial cell membrane
What is the Haldane effect?
For a given pCO2, carbon dioxide content of blood increases as pO2 falls (or saturation decreases)
What is Fick’s law? Is this a general concept for the entire lung?
net flux of a gas = diffusing capacity of the lung multiplied by pressure difference between alveolar partial pressure and capillary partial pressure; NO: at one instance in time and at a discrete piece of alveoli
What is the path of an oxygen from inside the airway to the capillary?
Must first diffuse through an aqueous layer (Henry’s law) and then through type I pneumocyte membreane (epithelial cell), ECM, endothelial cell membrane + basement membrane, and finally across the red blood cell membrane
What defines the diffusing capacity of the lung?
surface area and thickness of the diffusion barrier, solubility of the gas, and size of the gas multiplied by a constant
What is another way to describe the diffusing capacity of the lung?
membrane diffusing capacity (through all the various barriers) multiplied by rate at which oxygen binds hemoglobin and the volume of pulmonary capillary blood; resistance is the reciprocal of diffusing capacity
How is diffusing capacity measured?
through carbon monoxide testing
What is Fick’s principle?
net flow equals content at the end of the capillary multiplied by cardiac output
What does increasing cardiac output do to the time an individual hemoglobin molecule is in contact with the alveolar barrier?
decreases the time and results in less uptake of oxygen (or carbon monoxide)
Describe perfusion-limited exchange. Which gases are perfusion limited?
gas reaches diffusion equilibrium (diffusion of the gas can only be increased if blood flow increases; net flux is proportional to cardiac output); carbon dioxide, oxygen, and N20
Describe diffusion-limited exchange. Which gases are diffusion limited?
gas does not reach diffusion equilibrium at the end of the capillary (diffusion of the gas is proportional to the diffusing capacity of the lung); carbon monoxide (binds to hemoglobin slower than oxygen) and has a low driving force that prevents it from reaching diffusion equilibrium
What diseases decrease the diffusing capacity of the lung? What other complication do these diseases cause?
pulmonary fibrosis, pneumonia, and pulmonary edema (increases aqueous layer); hypoxemia resulting from perfusion ventilation mismatch
