"Medical Physiology Diffusion of Oxygen and Carbon Dioxide Huaiyu Hu" GABY

Question 1

Henry’s law states that when a liquid and gas phase are in __1__ at a given temperature, the concentration of gas in solution is __2__ proportional to the partial pressure of the gas

Answer 1

1. equilibrium

2. directly

Question 2

At equilibrium, the ___ of the gas is identical in the gas and liquid phases

Answer 2

partial pressure

Question 3

Henry’s law

Answer 3

Describes the equilibrium between gaseous and liquid phases

Question 4

What does the following equation represent?

[C]=aP

Answer 4

Henry’s law

[C] –> gas concentration in solution
a –> solubility coefficient
P –> partial pressure of gas

Question 5

The values of the solubility coefficient depend on:
(1)
(2)
(3)

Answer 5

(1) the specific gas
(2) the liquid in which the gas is dissolved
(3) temperature

Question 6

The concentration of a gas dissolved in blood is expressed in ___, or as ___ that would be in equilibrium with the solution in accordance with Henry’s law if a gas phase were present

Answer 6

vol% (or molar); the partial pressure in mm Hg of gas

Question 7

Since the partial pressure of oxygen in alveolar gas (PA02) is __1__ than in pulmonary artery blood (PVO2), and since PAC02 is __2__ than PVCO2, these gases are transferred by __3__ across the alveolar-capillary membrane

Answer 7

1. greater
2. less
3. diffusion

Question 8

Fick’s law

Answer 8

Describes the rate of movement of gas between two compartments containing gases of differing partial pressures

Question 9

In solutions chemical potentials are determined by the __1__; in gases chemical potentials are determined by __2__

Answer 9

1. concentration of molecules

2. partial pressures

Question 10

In the lung, diffusion occurs across the ___ membrane

Answer 10

alveolar-capillary

Question 11

The ___ barrier separates the alveolar gas from the liquid blood

Answer 11

alveolar-capillary membrane

Question 12

What does the following equation represent?

V=(A/T)•D•(PA -PC)

Answer 12

Fick’s law

V –> the flow (ml/min) of a gas
A –> the alveolar surface area available for diffusion (about 50 - 100 m2)
T –> the thickness of the alveolar membrane (< 0.5 micron)
(PA - PC) –> the difference in partial pressure (mm Hg) of the gas between the alveoli
and the pulmonary capillary blood
D –> the diffusion coefficient (cm2/min/mm Hg) of the gas

Question 13

The diffusion coefficient of a gas (D) is proportional to its __1__ and inversely proportional to the square root of its __2__

Answer 13

(1) solubility

(2) molecular weight

Question 14

Why do patients develop hypoxemia before arterial hypercapnia when impairment to diffusion occurs?

Answer 14

Because carbon dioxide diffuses about 20 times more rapidly than oxygen across the alveolar-capillary membrane

Question 15

Red blood cells in their plasma spend about ___ sec in the pulmonary capillaries at resting cardiac outputs

Answer 15

0.75 - 1.2

Question 16

At time zero, the partial pressure is that of __1__; at 0.75 sec the partial pressure is that of __2__

Answer 16

(1) mixed venous blood

(2) end- capillary blood

Question 17

Alveolar partial pressure is different for each gas and depends on:
(1)
(2)
(3)

Answer 17

(1) the inspired gas composition
(2) the rates of inspiration and expiration
(3) the rate of removal of the gas by capillary blood

Question 18

Perfusion-limited uptake

illustrated by N20

Answer 18

• rapid equilibration
• uptake from lung can be increased by increasing cardiac output –> reduces amount of time the blood stays in the capillary after equilibration of alveolar partial pressure with its capillaries

Question 19

Diffusion-limited uptake

illustrated by CO

Answer 19

*higher affinity for Hb –> prevents equilibration of partial pressure between alveolar gas and capillary plasma

Question 20

Uptake of oxygen from the lung by the blood is ___ limited

Answer 20

perfusion

Question 21

Why would an individual with a diffusion problem would have exercise-induced hypoxemia?

Answer 21

Because transit time in the pulmonary capillaries is reduced by 2/3 to 0.25.

Question 22

Why would an individual with a diffusion problem (compared to a healthy person) will be even more hypoxemic?

Answer 22

At high altitude, the partial pressure of inspired oxygen is reduced –> decreasing the partial pressure of oxygen in the alveolar gas

A normal individual will equilibrate their end-capillary blood with alveolar gas

In an individual with a diffusion problem the end- capillary partial pressure of oxygen will be even lower than the alveolar partial pressure of oxygen

Question 23

Why is DLCO rather than DLO2 measured at steady state?

Answer 23

Because binding of CO to hemoglobin maintains a virtually zero PCO in pulmonary capillaries when a nonlethal alveolar PCO is used, thus maintaining the partial pressure gradient between alveoli and capillaries

Question 24

What equation relates DL02 and DLCO?

Answer 24

DLO2=1.23DLCO

Question 25

Factors that influence DLCO

Answer 25

Body position
-DLCO greater when supine than upright

Exercise
-increases DLCO

Lung diseases and dysfunction that decrease DLCO

• loss of lung tissue from surgery
• mismatch of ventilation to perfusion
• pulmonary hypertension with edema