Lecture 18- Oxygen and carbon dioxide exchange

Question 1

What is respiratory minute volume? What is the unit?

Answer 1

• How much air is passing in and out of the lungs

- L/min

Question 2

What is dead space? How much is this in healthy individuals?

Answer 2

• Some of the inhaled air never gets to the alveoli so cannot be invovled in gas exchange
• This air is known as DEAD SPACE ( VD ) and just fills up the airways
• The amount is about 150ml with each breath in a normal healthy individual

Question 3

What is alveolar ventilation? How is it calculated?

Answer 3

• The amount of air that actually gets into the alveoli, and therefore the amount available for gas exchange
• VA = (VT - VD) x f (tidal volume- dead space) x rate

Question 4

What is the equation for respiratory minute volume?

Answer 4

VE = VT x f
(Note E and T are in subscript)

• VT= tidal volume in L/breath
• f= Respiratory rate in breaths/min

Question 5

How does the equation for respiratory minute volume relate to that for cardiac output?

Answer 5

CO = SV x HR
or in other words contraction strength x contraction speed

For VE its the exact same. VT is breathing strength and f is the breathing speed

Question 6

Capital A means…

Lowercase a means…

Answer 6

• alveolar

- arteries

Question 7

Why are tiny rapid breaths not helpful?

Answer 7

• Means that tidal volume will decrease
• However, dead space is fixed/will remain the same
• Therefore, tiny rapid breaths will mean that the amount of gas entering the alveoli will decrease

Question 8

What is Dalton’s law?

Answer 8

-In a gas mixture (air), each gas exerts its own individual
pressure, called a partial pressure (P)
-The pressure of a mixture of gases = the sum total of the pressures of each individual gas

Question 9

Why do you need to consider the partial pressure of water when calculating the pressure of a mixture of gases?

Answer 9

Air is moistened and warmed as it comes in to the lungs

Question 10

What is the goal in terms of the movement of gases across the blood air boundary?

Answer 10

-Move gas back and forth between the alveoli
and the capillaries
-Transport inhaled oxygen into the capillaries
-Transport carbon dioxide into the alveoli to be exhaled

Question 11

What is Fick’s law of diffusion? What do the letters stand for?

Answer 11

-Describes the determinants in the rate of diffusion for gases across the blood-air boundary

F = A/T D (P1-P2)

F = Flux (Amount flowing)
A = surface area
T = thickness
D = Diffusion constant
P1-P2 = pressure difference

Question 12

Specifically what are the three things that determine the rate of diffusion?

Answer 12

• Surface area of the membranes
• Thickness of the membranes
• Pressure difference between the two sides

Question 13

Which of the following statements about alveolar ventilation is NOT correct?
A. Alveolar ventilation is measured as a volume of air
per unit time.
B. Alveolar ventilation is affected by the amount of dead
space in the respiratory system.
C. The frequency of breaths affects alveolar ventilation.
D. Alveolar ventilation is usually a bit higher than
respiratory minute volume.
E. Taking deep breaths will increase tidal volume and
alveolar ventilation.

Answer 13

D. Alveolar ventilation is usually a bit higher than

respiratory minute volume.

Question 14

What does D of the fick’s law depend on?

Answer 14

Diffusion constant or coefficient of a substance depends on gas solubility and its molecular weight

Question 15

What diffuses faster carbon dioxide or oxygen? Why?

Answer 15

Carbon dioxide diffuses about 20x faster than oxygen on a per molecule basis due to carbon dioxide having higher solubility

Question 16

Does D play a major role in determining gas exchange?

Answer 16

No, not really

Question 17

How does A for fick’s law influence that movement of gases across membranes?

Answer 17

• Is the surface area of the alveoli
• Normally the bulbous structure of the alveoli and the high density of the capillaries creates lots of surface area for gas exchange
• Big area= more efficient gas exchange

Question 18

What is emphysema? What is the consequence?

Answer 18

• A disease characterized by dilation of the alveolar spaces and destruction of the alveolar walls
• Reduction in surface area means less contact between the air and the capillaries, so the rate of oxygen exchange is greatly reduced.

Question 19

How does T in fick’s law influence the movement of gases across the blood-air membrane?

Answer 19

The blood air membrane is comprised of the alveolar and capillary walls. This means the distance between the alveolar air and blood is very small allowing for efficient diffusion.

Question 20

What happens when the walls of the alveoli get thick? What is this condition called?

Answer 20

• Called pulmonary fibrosis where the thickening and scarring of the alveoli membranes occurs
• Can arise from chronic inflammation or exposure to industrial chemicals
• means less efficient diffusion

Question 21

What is the main factor (included in Fick’s law) that influences diffusion across the blood/air barrier?

Answer 21

• Pressure difference (P1-P2)

- This drives diffusion as air moves from high to low pressure

Question 22

What is the partial pressure of oxygen in the alveolar dependant on?

Answer 22

1. Partial pressure of oxygen in inspired air
2. Alveolar ventilation
3. Oxygen consumption- blood oxygen

However, atmospheric levels of oxygen are relatively constant so its the balance between oxygen consumption and alveolar ventilation that is actually important.

Question 23

What happens to oxygen use in low activity states versus high activity states?

Answer 23

• Low activity means reduced oxygen consumption and so high levels of oxygen in the venous blood
• High activity means increased oxygen consumption and so low levels of oxygen in the venous blood

Question 24

What does the partial pressure of carbon dioxide in the alveolar dependant on?

Answer 24

1. Partial pressure of carbon dioxide in the inspired air
2. Alveolar ventilation
3. Carbon dioxide production (by tissue)

Atmospheric carbon dioxide levels is negligible therefore, partial pressure of carbon dioxide is usually determined by only the balance in carbon dioxide production and alveolar ventilation

Question 25

Explain the pressure differences resulting in the movement of oxygen at the blood/ air boundary…

Answer 25

• Blood in veins coming back via the alveoli has a low partial pressure of oxygen (been depleted by the tissues) compared to the alveoli.
• Gradient therefore causes the gas to move from alveoli to capillaries
• Blood continues to move so this happens continuously/ gradient maintained

Question 26

Explain the pressure differences resulting in the movement of carbon dioxide at the blood/ air boundary…

Answer 26

• Blood coming in past the capillaries via the veins has higher partial pressure of carbon dioxide compared to
  alveoli.
• The gradient is therefore opposite to that of oxygen and so diffusion occurs in opposite direction with carbon dioxide being moved into the alveoli to be exhaled by the lungs. Again blood keeps moving so this happening continuously/ gradient maintained

Question 27

What is external respiration?

Answer 27

This is the gas exchange occurring between the capillary and alveoli (at level of the lungs)

Question 28

What is internal respiration?

Answer 28

• At the level of the tissues
• High oxygen level in blood stream low in tissues means it will diffuse in
• In contrast, low level of carbon dioxide in the bloodstream and high in the tissues (due to metabolism) mean it will diffuse into the blood stream to be taken back to the lungs ( will be exhaled)

Question 29

A person with pulmonary fibrosis has increased thickness of the Blood Air Barrier. Which of the following statements is
most likely to be CORRECT for this person?
A. His breathing frequency will be reduced.
B. His blood oxygen will be reduced.
C. His blood carbon dioxide will be reduced.
D. His hematocrit will be reduced.
E. His heart rate will be reduced.

Answer 29

B. His blood oxygen will be reduced.