Gas Exchange/Diffusion

Question 1

Gas exchange across alveolar membrane is by?

Answer 1

diffusion down partial press gradient

Question 2

Fick’s Law says diffusion rate affected by? (3)

Answer 2

membrane area and thickness

solubility of molecule

Question 3

2 Partial Pressure Gradients in respiration are?

Answer 3

P_A - P_V

(P_A = alveoli, P_V = venous)

Pa - Ptissue

(Pa = arterial, Ptissue = cell tissue)

Question 4

Pressure gradient for O2 compared to CO2?

Answer 4

O2 is 10x CO2

Question 5

Rate of diffusion for O2 compared to CO2?

Answer 5

CO2 diffuses 2x faster than O2

(CO2 more soluble)

Question 6

P_Aco₂ compared to Paco₂?

Answer 6

about equal due to rapid diffusion

Question 7

P_Ao₂ compared to Pao₂?

Answer 7

P_Ao₂ > Pao₂

(5-20 mmHg)

Question 8

Po₂ in mitochondria =

Answer 8

1-2 mmHg

Question 9

Oxygen Cascade is?

Answer 9

pressure ∆ of O2 from atmosphere to cells

Question 10

What happens to Po₂ during inspiration?

Answer 10

drops

Question 11

Step 1 of O2 Cascade?

Answer 11

Humidification: air saturated w/ H2O

Question 12

Partial pressure of humidified air eqn?

Answer 12

P_Io₂ = (P_B - Ph₂o) F_Io2

_I = inspiration

Question 13

Water vapor pressure at body temp =

Answer 13

Ph₂o = 47 mmHg

Question 14

P_B=

Answer 14

760 mmHg

Question 15

Fgas =

Answer 15

0.21

Question 16

Step 2 of O2 Cascade?

Answer 16

O2 depletion of alveolar gas: O2 extracted from inspired gas in alveoli

Question 17

Estimate P_Ao₂ how?

Answer 17

Alveolar Gas Eqn:

relationship b/w P_Ao₂ and P_Aco₂, O2 removed from alveolar air, CO2 added

**P_Aco₂ is about = to P_aco₂. If P_aco₂ is given, use it.

Question 18

Alveolar Gas Eqn

Answer 18

P_Ao₂ = P_Io₂ - (P_aco₂/RQ)

P_Io₂ = (P_B - Ph₂0) x F_Io₂

P_B = 760

Ph₂0 = 47

F_Io₂ = 0.21

RQ = 0.8

BOTTOM LINE: P_Ao₂ = 150 - (P_aco₂/0.8)

Question 19

Purpose of Alveolar Gas Eqn? (4)

Answer 19

1) gives average for whole lung
2) tells us P_B and F_Io₂ affect P_Ao₂
3) use to calculate A-a gradient
4) shows P_Ao₂ and P_Aco₂ inversely related

Question 20

Step 3 of O2 Cascade?

Answer 20

Alveolar - arterial O2 Gradient

(A-a gradient)

Question 21

Factors contributing to A-a Gradient? (2)

Answer 21

1) diffusion of O2 across _A membranes
2) shunting of blood to respiratory areas of lungs

Question 22

A-a calculated how?

Answer 22

measure P_ao₂, calc P_Ao₂

Question 23

Fick’s Law in lungs?

Answer 23

Vo₂ = (AD/T) (P1 - P2)

A = area of diffusion (75 m2)

D = diffusivity of molecule

T = memb thickness (0.5 μm)

P1 -P2 = pressure gradient

Question 24

D (diffusitivity of molecule) eqn

Answer 24

solubility/√molecular weight

Question 25

Diffusion Capacity (D_L) =

Answer 25

AD/T

Question 26

What happens to D_L during exercise? How? (2)

Answer 26

↑ during exercise 1) surface area ↑ (more capillaries open and more stretch on alveoli) 2) thickness ↓ (stretch thins membrane)

Question 27

What does emphysema do to D_L ? How?

Answer 27

↓ DL breaks down _A wall -\> ↓ surface area (A)

Question 28

What does Pulm Fibrosis do to D_L ? How?

Answer 28

↓ DL ↑ fibrous tissue -\> ↑ diffusion distance (T)

Question 29

What does Pneumonia do to D_L ? How?

Answer 29

↓ D_L fluid in _A -\> ↑ diffusion distance (T)

Question 30

What does CHF do to D_L ? How?

Answer 30

↓ D_L edema -\> ↑ diffusion distance (T)

Question 31

D_L estimated w/ what test?

Answer 31

single breath CO test

Question 32

What ultimately limits the O2 take-up in the lungs?

Answer 32

perfusion: blood Po₂ quickly equalizes P_Ao₂ at start of capillary (0.25 sec), happens well before diffusion comes into play (0.75 sec)

Question 33

When would O2 become diffusion limited instead of perfusion limited? (2)

Answer 33

1) insufficient time for equilibrium 2) low D_L