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SGUL - Physiology 1 > Gas Exchange In The Lung > Flashcards

Gas Exchange In The Lung Flashcards

1
Q

Gas exchange involves diffusion of blood gases through multiple structures and mediums.
Describe how oxygen gets from the lungs to the blood. PART 1

A
  • O2 enters the alveolar airspace from the atmosphere.
  • O2 dissolves into the alveolar fluid lining.
  • O2 diffuses through the alveolar epithelium, the basement membrane and the capillary endothelial cells.
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2
Q

Gas exchange involves diffusion of blood gases through multiple structures and mediums.
Describe how oxygen gets from the lungs to the blood. PART 2

A
  • O2 dissolves in blood plasma.
  • O2 binds to the haemoglobin molecule.
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3
Q

What is the rate of diffusion determined by?

A
  • Partial pressure gradient between alveolar air and capillary blood
  • Diffusion distance e.g epithelial/endothelial thickness or basement membrane thickness
  • Alveolar surface area.
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4
Q

List some diseases that affect the rate of diffusion, and how they affect it.

A
  • hypoventilation (type II respiratory failure) = lowers partial pressure gradient
  • hypoperfusion (type I respiratory failure) = lowers Pc
  • emphysema = lowers surface area
  • fibrosis = increase basement membrane thickness
  • pulmonary oedema (eg. pneumonia) = increases thickness of fluid layer/ oedema
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5
Q

Alveoli have intricate structures and multiple adaptations to maximise the rate of gas exchange.
List a few of them.

A
  • Large surface area (lungs have a high surface-area to volume ratio due to their 3D structure)
  • Walls are only one cell thick, and the basement membrane is fused with the blood vessel, which decreases the distance for diffusion
  • Innervated by capillaries (given an adequate blood supply to maintain the gradient)
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6
Q

How are the pressure gradients between the alveoli and the blood maintained?

A

Adequate ventilation and perfusion.

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7
Q

What are hypoventilation and hyperventilation defined by (in terms of CO2 levels)?

A
  • Hyperventilation is defined by excessive levels of CO2 within arterial blood
  • Hypoventilation is defined by reduced levels of CO2 within arterial blood
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8
Q

What is the effect of increasing rate of ventilation?

A
  • Increases alveolar oxygen partial pressure and decreases alveolar carbon dioxide partial pressure
  • Leads to decreased oxygenation of blood flowing through innervating capillaries
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9
Q

What is the effect of decreasing rate of ventilation?

A
  • Decreases alveolar oxygen partial pressure and increases alveolar carbon dioxide partial pressure
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10
Q

Explain how adequate perfusion is required for maintaining the pressure gradients for diffusion.

A
  • Blood flow through the pulmonary capillaries (perfusion, Q) needs to be matched to alveolar ventilation (VA) to enable efficient gas exchange.
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11
Q

What relationship does V/Q describe?

A

V/Q ratio describes the relationship between pulmonary perfusion (Q) and alveolar ventilation (V).

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12
Q

How is ventilation-perfusion coupling maintained?

A

Hypoxic vasoconstriction - diverts blood flow from poor to well-ventilated alveoli

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13
Q

Outline the mechanism by which hypoxic vasoconstriction maintains ventilation-perfusion coupling. PART 1

A
  • Under normal conditions, blood flow and ventilation are matched.
  • If ventilation of alveoli decreases, PACO2 will rise and PAO2 will fall.
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14
Q

Outline the mechanism by which hypoxic vasoconstriction maintains ventilation-perfusion coupling. PART 2

A
  • Therefore, decreased oxygenation of blood flowing through the innervating capillaries.
  • The decreased PaO2 induces vasoconstriction, which decreases blood flow.
  • The blood flow is then diverted to alveoli with increased ventilation.
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15
Q

What happens when there is a ventilation-perfusion mismatch (with reduced perfusion)?

A
  • Increase in V/Q ratio.

CAN LEAD TO:
- heart failure (cardiac arrest)
- blocked vessels (pulmonary embolism)
- loss/ damage to capillaries (emphysema)
- reduced gas exchange

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16
Q

What happens when there is a ventilation-perfusion mismatch (with reduced ventilation)?

A
  • Decrease in V/Q ratio.

CAN LEAD TO:
- asthma (COPD)
- pneumonia (fibrosis)
- Blood returns to the left part of the heart from the right without taking part in gas exchange.

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17
Q

What is the formula for partial pressure of oxygen?

A

PAO2 = FIO2 x (P B - P H2O) - PaCO2/RER

FIO2 is the fraction of oxygen present in inspired gas, P B is the barometric pressure, P H2O is the water vapour pressure, PaCO2 is the arterial CO2 pressure and RER is the respiratory exchange ratio.

18
Q

What is the RER (respiratory exchange ratio)?

A

RER = VCO2 produced/ VO2 consumed
- Describes relationship between the CO2 elimination and O2 consumption.
- Measures difference between O2 and CO2 in inspired and expired air.

19
Q

What is the RER for the oxidation of carbohydrates, and for the oxidation of fatty acids?

A

Oxidation of CARBOHYDRATES = 1

Oxidation of FATTY ACIDS = 0.7

20
Q

How is partial pressure calculated?

A

Multiplying total pressure by mole fraction

21
Q

What is concentration of a gas dissolved within a liquid determined by?

A

Partial pressure and solubility of a gas

22
Q

What does partial pressure of a gas dissolved in a liquid mean?

A

Volume of gas that would dissolve at equilibrium if the liquid was placed in contact with a gas phase of equivalent partial pressure

23
Q

What features do you want for maximum diffusion?

A

High partial pressure gradient
High surface area
Low distance

24
Q

What happens if the VQ ratio is more than 1?

A

Hypoperfusion (dead space effect)

25
Q

What happens if the VQ ratio is less than 1?

A

Hypoventilation (shunt)

26
Q

What happens when an embolism blocks the pulmonary artery supplying a region of the lung?

A

Perfusion to the other alveoli increases as cardiac output is diverted
Unless ventilation of alveoli is increased, hypoxia and hypercapnia will occur

27
Q

What causes reduced perfusion to lungs?

A

Heart failure
Blocked vessels
Loss/damage to capillaries

28
Q

What is a shunt in the lungs?

A

Perfusion without ventilation

29
Q

What causes shunt in the lungs?

A

Pneumonia, acute lung injury, respiratory distress syndrome

30
Q

What does reduced ventilation do to the VQ ratio?

A

Decrease it

31
Q

What is the equation for concentration of dissolved gas in a liquid?

A

Concentration = Partial pressure x Solubility

32
Q

Why does efficient gas exchange need V/Q coupling?

A
  • For effective oxygenation to take place ensure any blood that flows through pulmonary capillaries are perfusing an alveoli which is ventilated properly
33
Q

What causes VQ to go above 1?

A

Perfusion (Q) decreases in a particular part of the alveoli but ventilation (V) stays the same

34
Q

What causes VQ go below 1?

A

Ventilation (V) decreases but perfusion (Q) stays the same

35
Q

Why does PaCO2 stay the same during VQ mismatch?

A

As partial pressure of CO2 increases hyperventilation induced within the lung which clears the excess CO2

36
Q

How does a pulmonary embolism lead to V/Q mismatch? PART 1

A
  • Occludes blood vessels - blood is unable to flow to the alveoli (called physiological dead space)
  • As a result the alveoli has an increase in the V/Q (as perfusion has decreased).
  • Since cardiac output remains the same the blood will now flow through another alveoli
37
Q

How does a pulmonary embolism lead to V/Q mismatch? PART 2

A
  • In this secondary alveoli V/Q decreases (as perfusion increases)
  • Excessive perfusion for the amount of ventilation
  • Blood flows through the alveoli which are not fully oxygenated before it returns back to the heart
38
Q

What is a physiological dead space?

A
  • Alveoli within the lung that do not carry out gas exchange due to reduced perfusion
  • There is ventilation without perfusion and hence occurs when there is an increase in V/Q
  • Responds well to oxygen theraphy
39
Q

What is a pulmonary shunt effect?

A
  • Movement of blood across the alveoli without taking part in gas exchange
  • Occurs when there is a decrease in ventilation but perfusion stays the same and hence V/Q will decrease
  • Poor response to oxygen therapy
40
Q

Why does oxygen therapy not produce a good response during a shunt? PART 1

A
  • Upon perfusion of blood past the inactive alveoli they will return with the same partial pressure of oxygen they entered
  • Upon pumping oxygen, oxygen will flow into a secondary alveoli and improve the ventilation here
  • As a result it will cause a higher increase in partial pressure in the blood
41
Q

Why does oxygen therapy not produce a good response during a shunt? PART 2

A
  • Finite amount of haemoglobin in the blood
  • Only a finite amount of oxygen can be carried and so the extra oxygen that has been taken in cannot compensate for the oxygen lost by the non-functional alveoli
42
Q

For gases, why is pressure used rather than concentration?

A

Gases react, dissolve, and diffuse in accordance with their pressure

SGUL - Physiology 1 (68 decks)

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