Medical Physiology Block 5 Week 2

Question 1

What is the equation for the amount of gas dissolved in a solution?

Answer 1

partial pressure of the gas multiplied by a solubility constant (linked to cardiac output)

Question 2

What is the partial pressure of oxygen in alveolar air? mixed-venous blood? arterial blood? dry inspired air? wet inspired air?

Answer 2

100; 40; 100; 160; 150 ((760-47) x 0.21)

Question 3

What is the partial pressure of carbon dioxide in alveolar air? mixed-venous blood? arterial blood? dry inspired air? wet inspired air?

Answer 3

40; 46; 40; 0; 0

Question 4

How is oxygen carried in blood?

Answer 4

dissolved in the blood or bound to hemoglobin

Question 5

Describe adult hemoglobin.

Answer 5

4 subunits each with a heme moiety (iron-containing porphyrin ring); 2 alpha chains and 2 beta chains

Question 6

What is the oxidation state of iron in adult hemoglobin?

Answer 6

+2 (ferrous)

Question 7

What is the oxidation state of iron in methemoglobin? Can it bind oxygen? what enzyme converts it back to hemoglobin?

Answer 7

+3; NO; methemoglobin reductase (NADH-dependent)

Question 8

How is the structure of fetal hemoglobin different from adult hemoglobin?

Answer 8

2 alpha chains and 2 gamma chains

Question 9

What is the oxygen binding capacity of blood?

Answer 9

maximum amount of oxygen that can be bound to hemoglobin in blood (measured at 100% saturation)

Question 10

What is the oxygen content of blood?

Answer 10

total amount of oxygen carried by blood, including bound and dissolved; equals (oxygen binding capacity x % saturation) + dissolved oxygen

Question 11

What is the oxygen saturation of arterial blood?

Answer 11

about 97-98% (at a partial pressure of about 100 mm Hg)

Question 12

What is the oxygen saturation of mixed-venous blood?

Answer 12

about 75% (at a partial pressure of 40 mm Hg); meaning three of the four heme groups are bound by oxygen

Question 13

Why is the hemoglobin-oxygen dissociation curve sigmoidal?

Answer 13

positive cooperativity

Question 14

Is the optimal saturation of blood (arterial) resistant to changes in oxygen partial pressure (as low as 60 mm Hg at high altitudes)?

Answer 14

Yes; the Hb-O2 dissociation curve is relatively flat in this segment of the plot

Question 15

What factors help to unload oxygen from hemoglobin (in peripheral tissues)?

Answer 15

high temperature, low pH & high carbon dioxide partial pressure (Bohr effect), and 2,3 di-phosphoglycerate (2,3 DPG)

Question 16

What is one reason for the increased affinity for oxygen of fetal hemoglobin?

Answer 16

2,3 DPG can be formed but does not bind to the molecule (normally binds between beta chains)

Question 17

What happens in carbon monoxide poisoning?

Answer 17

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)

Question 18

How is carbon dioxide carried in the blood?

Answer 18

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)

Question 19

What channels allow bicarbonate to diffuse through the red blood cell membrane?

Answer 19

aquaporin 1 and Rh complex

Question 20

How is carbon dioxide released in the lungs?

Answer 20

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

Question 21

What is the Haldane effect?

Answer 21

For a given pCO2, carbon dioxide content of blood increases as pO2 falls (or saturation decreases)

Question 22

What is Fick’s law? Is this a general concept for the entire lung?

Answer 22

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

Question 23

What is the path of an oxygen from inside the airway to the capillary?

Answer 23

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

Question 24

What defines the diffusing capacity of the lung?

Answer 24

surface area and thickness of the diffusion barrier, solubility of the gas, and size of the gas multiplied by a constant

Question 25

What is another way to describe the diffusing capacity of the lung?

Answer 25

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

Question 26

How is diffusing capacity measured?

Answer 26

through carbon monoxide testing

Question 27

What is Fick’s principle?

Answer 27

net flow equals content at the end of the capillary multiplied by cardiac output

Question 28

What does increasing cardiac output do to the time an individual hemoglobin molecule is in contact with the alveolar barrier?

Answer 28

decreases the time and results in less uptake of oxygen (or carbon monoxide)

Question 29

Describe perfusion-limited exchange. Which gases are perfusion limited?

Answer 29

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

Question 30

Describe diffusion-limited exchange. Which gases are diffusion limited?

Answer 30

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

Question 31

What diseases decrease the diffusing capacity of the lung? What other complication do these diseases cause?

Answer 31

pulmonary fibrosis, pneumonia, and pulmonary edema (increases aqueous layer); hypoxemia resulting from perfusion ventilation mismatch