Oxygen Diffusion

Question 1

What is the Warburg equation?

Answer 1

U = C - a/2D (Hx-x^2). Used to calculate PO2 at x cm into the tissue, based on the diffusion coefficient D of whatever gas in Krogh units.

Question 2

What assumption does the Warburg equation make?

Answer 2

It assumes that the Km (affinity) for oxygen of cytochrome enzymes is infinite. Ignores the weakly binding flavoproteins which also contribute to respiration rate.

Question 3

What is Fick’s first law?

Answer 3

Flux (diffusion) is proportional to the concentration gradient. The steeper the gradient, the greater the flux.

Question 4

What is Fick’s second law?

Answer 4

Diffusion time increases greatly with distance: T = x^2/2D. As a consequence, cardiovascular systems and hemoglobin is essential in large animals.

Question 5

What is the oxygen diffusion coefficient of cell membranes?

Answer 5

It varies between species and organs. For RBC membranes it is, 0.84 in sheep, 1.29 in humans, 1.49 in rat, and 1.92 x 10^-7 cm2/sec in rabbit. Probably depends on oxygen consumption, since within frog, it is highest in myocytes (2.64 x 10-7 cm2/sec). (Koyama 1986)

Question 6

What is one inadequacy of the Krogh model?

Answer 6

The Krogh model does not take into account the resistance to the passage of oxygen flow from blood into capillary tissue (Kassissia 1995).

Question 7

(Kassissia 1995)

Answer 7

Oxygen flow is one way. Convection of oxygen back into blood is limited. Used 18O tracer. Capillary is somewhat resistant to bloodflow, may be beneficial for insuring that at least some oxygen is present in blood by the time it reaches distal brain tissues.

Question 8

(Kuschinsky 1992)

Answer 8

At least 10-20% of blood vessels have no erythrocytes in them at one time in rat brain.