Session 2 - The gas exchange Flashcards

1
Q

What three factors determine rate of gas exchange?

A
  • Area available for exchange
    • Resistance to diffusion

Gradient of partial pressure

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2
Q

How does diffusion resistance change?

A

• Depends on the nature of the barrier
○ Cells, water
• What gas is moving through it
○ O2/CO2

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3
Q

Is area a limiting factor in the lungs?

A

• No, lungs have a gas exchange area of 80m2 - huge area

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4
Q

Outline the structures which provide diffusion resistance in the lungs

A
  • The alveolar epithelial cell
    • Interstitial fluid
    • Capillary endothelial cell
    • Plasma
    • RBC membrane
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5
Q

Overall, what do gases have to diffuse through from the alveoli to reach the blood cells?

A
  • 5 cell membranes
    • 3 layers of intracellular fluid
    • 2 layers of extra cellular fluid
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6
Q

How fast do gases move through other gases? How does this relate to O2 and CO2?

A

• At rate inversely proportional to molecular weight

○ CO2 moves slower than O2

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7
Q

How fast do gases move through liquids? How does this relate to O2 and CO2?

A

• At rate proportional to solubility

○ CO2 much more soluble than O2, moves 21 times faster

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8
Q

Is the rate of diffusion of CO2 a limiting factor?

A

• No, O2

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9
Q

How fast is O2 exchange

A

• 0.5 seconds

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10
Q

How long do blood cells stay in capillary?

A

How long do blood cells stay in capillary?

• 1 second

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11
Q

What is the pO2 and pCO2 in the blood leaving capillaries in a normal lung?

A

• Same as in alveolar air

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12
Q

Where does exchange occur in lungs?

A

• Across the alveolar membrane

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13
Q

How is alveolar air different to atomspheric air?

A
  • Less oxygen

* More carbon dioxide

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14
Q

What is the partial pressure of O2 in the alveolar air?

A

• 13.3 kPa

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15
Q

What is the partial pressure of pCO2 in the alveolar air?

A

• 5.3 kPa

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16
Q

What is the pO2 in the venous blood which returns to the lungs from the body?

A

• 6.0 kPa

17
Q

What is the pCO2 in the venous blood which returns to the lungs from the body?

A

• 6.5 kPa

18
Q

Why does O2 move from alveoli to blood?

A
  • Diffusion gradient

* 13.3 - 6.0

19
Q

Why does CO2 move from blood to alveoli

A
  • Diffusion gradient

* 6.5 kPa –> 5.3 kPa

20
Q

What does alveolar ventilation determine?

A
  • Gas composition of arterial blood

* And therefore oxygen supply to tissues

21
Q

What is ventilation of the lungs?

A
• Expansion of lungs
	• Increases volume of
		○ Respiratory bronchioles
		○ Alveolar ducts
	• So air flows down airways to them
22
Q

What parts of the lungs expand with ventilation?

A
  • Respiratory bronchioles

* Alveolar ducts

23
Q

Does fresh air enter the alveoli in the lungs?

A

• No, reaches as far as the respiratory and terminal bronchioles

24
Q

Why would it be bad to have air going directly into alveoli?

A

• Make the blood impossibly alkaline on way in, very acidic on way out

25
How can we measure ventilation?
• Using a spirometer
26
What is a spirometer?
• A closed chamber is which gas is held at constant pressure, but the volume of which can change with ventilation
27
What is anatomical/serial dead space?
* Volume of the airways | * Measured by nitrogen washout (typically 0.15l)
28
Outline the nitrogen washout test
* Patient takes a maximum inspiration of 100% oxygen * Mixes with nitrogen naturally present in alveolar air * Air in conducting pathway will still be filled with pure O2 * Person exhales through one way valve - nitrogen content is measured * A graph can be drawn plotting Nitrogen% against expired volume
29
What is distributive/a;veolar dead space?
• Some parts of the lung are not airways and do not support gas exchange ○ Dead or damaged alveoli ○ Alveoli with poor perfusion (ventilation/perfusion ratio)
30
What is physiological dead space?
• Serial dead space + distributive dead space (0.17l)
31
What is alveolar ventilation rate?
• Dead space must be completely filled with air at each breath • Dead space ventilation rate ○ Dead space volume x respiratory rate • Subtracted from pulmonary ventilation rate to get AVR
32
How is physiological dead space measured?
* pCO2 of expired alveolar air | * Alveolar air is diluted by dead space air
33
Give the calculation for alveolar ventilation rate
• Pulmonary ventilation rate (tidal volume x RR) - Dead space ventilation rate (dead space volume x RR)