POISEUILLE'S LAW
Constant Pressure
(1) Decreasing the radius of a tube by 1/2 reduces the gas flow to 1/16 of its original
flow.
ex. If the radius of a bronchial tube through which gas flows at a rate of 50 L/min is
reduced to one half its original size, the flow through the bronchial tube would change to:
50 = 3.13 L/min
16
(2) Decreasing the radius of a tube by 1/16 (16%) reduces the gas flow to 1/2 of its
original flow.
ex. If the radius of a bronchial tube through which gas flows at a rate of 50 L/min is
reduced by 16%, the flow through the bronchial tube would change to:
50 = 25 L/min
2
Constant Flow
(1) If the radius of a tube is reduced by 1/2, the driving pressure would have to increase
16 times its original level to maintain the same flow.
ex. If the radius of a bronchial tube with a driving pressure of 10 cmH20 is reduced to 1/2
its original size, the driving pressure would have to change to:
10 x 16 = 160 cmH20
(2) If the radius of a tube is reduced by 1/16 (16%), the driving pressure would have to
increase 2 times its original level to maintain the same flow.
ex. If the radius of a bronchial tube with a driving pressure of 10 cmH20 is reduced by
16% , the driving pressure would have to change to
10 x 2 = 20 cmH20
Practice Problems: Poiseuille's Law
1. Assuming the pressure remains constant, if the radius of a bronchial tube through
which gas flows at a rate of 400 L/min is reduced to one half of its original size,
the flow through the bronchial tube would change to:
a. 800 L/min
b. 200 L/min
c. 50 L/min
d. 25 L/min
2. Assuming the pressure remains constant, if the radius of a bronchial tube through
which gas flows at a rate of 40 L/min is reduced by 16%, the flow through the
bronchial tube would change to:
a. 2.5 L/min
b. 20 L/min
c. 80 L/min
d. 640 L/min
3. Assuming the flow remains constant, if the radius of a bronchial tube with a
driving pressure of 15 cmH20 is reduced to 1/2 its original size, the driving
pressure would have to change to:
a. 240 cmH20
b. 30 cmH20
c. 7.5 cmH20
d. 0.94 cmH20
4. Assuming the flow remains constant, if the radius of a bronchial tube with a
driving pressure of 20 cmH20 is reduced by 16%, the driving pressure would have
to change to:
a. 1.25 cmH20
b. 10 cmH20
c. 40 cmH20
d. 320 cmH20
5. Assuming the pressure remains constant, if the radius of a bronchial tube through
which gas flows at a rate of 35 L/min is reduced to one half of its original size, the
flow through the bronchial tube would change to:
a. 560 L/min
b. 70 L/min
c. 17.5L/min
d. 2.19 L/min
6. Assuming the pressure remains constant, if the radius of a bronchial tube through
which gas flows at a rate of 70 L/min is reduced by 16%, the flow through the
bronchial tube would change to:
a. 4.4 L/min
b. 35 L/min
c. 140 L/min
d. 1120 L/min
7. Assuming the flow remains constant, if the radius of a bronchial tube with a
driving pressure of 30 cmH20 is reduced to 1/2 its original size, the driving
pressure would have to change to:
a. 480 cmH20
b. 60 cmH20
c. 15 cmH20
d. 1.88 cmH20
8. Assuming the flow remains constant, if the radius of a bronchial tube with a
driving pressure of 40 cmH20 is reduced by 16%, the driving pressure would have
to change to:
a. 2.5 cmH20
b. 20 cmH20
c. 80 cmH20
d. 640 cmH20
9. Assuming the pressure remains constant, if the radius of a bronchial tube through
which gas flows at a rate of 200 L/min is reduced to one half of its original size,
the flow through the bronchial tube would change to:
a. 400 L/min
b. 100 L/min
c. 12.5L/min
d. 3200 L/min
10. Assuming the flow remains constant, if the radius of a bronchial tube with a
driving pressure of 25 cmH20 is reduced to 1/2 its original size, the driving
pressure would have to change to:
a. 400 cmH20
b. 25 cmH20
c. 12.5 cmH20
d. 1.56 cmH20
Poiseuille's Law
Answers to practice problems:
1. d
2. b
3. a
4. c
5. d
6. b
7. a
8. c
9. c
10. a
400 / 16 = 25 L/min
40 / 2 = 20 L/min
15 x 16 = 240 cmH2O
20 x 2 = 40 cmH2O
35 / 16 = 2.19 L/min
70 / 2 = 35 L/min
30x 16 = 480 cmH2O
40 x 2 = 80 cmH2O
200 / 16 = 2.5 L/min
25 x 16 = 400 cmH2O