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Respiratory System > Gas Exchange in the Lungs > Flashcards

Gas Exchange in the Lungs Flashcards

1
Q

How is O₂ transported from the atmosphere to cells?

A
  1. O₂ inhaled from atmosphere into alveoli within lungs
  2. O₂ diffuses from alveoli into blood within pulmonary
    capillaries
  3. O₂ transported in blood predominantly bound to
    haemoglobin
  4. O₂ diffuses into cells/tissues for use in aerobic
    respiration
  5. CO₂ diffuses from respiring tissues to blood -
    exchanged at lungs
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2
Q

How does gas exchange occur?

A

Gas exchange involves diffusion of blood gases through multiple structures and mediums

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3
Q

What structures does O₂ diffuse through to reach red blood cells?

A 
Airspace 
↓
Alveolar Lining fluid 
↓
Alveolar epithelial layer
↓
Basement membrane and interstitial fluid 
↓ 
Capillary endothelial layer
↓
Dissolves in blood plasma
↓
RBCs - O₂ binds to Hb molecules
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4
Q

When does blood oxygenation occur?

A

Oxygen of blood must occur in the brief time taken for RBCs to flow through pulmonary capillaries

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5
Q

Outline the equation used to determine rate of diffusion

A

Rate of diffusion ∝ SA / d² x (Pa - Pc)

SA - alveolar surface area

d² - endothelial + epithelial cell, basement membrane
thickness and fluid layer depth

(PA - Pc) - Partial pressure gradient between alveolar air
and capillary blood

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6
Q

How can we alter factors to ensure maximum diffusion?

A
  • Increased SA
  • Increased Partial pressure gradient
  • Decreased distance (barrier thickness)
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7
Q

What defects can occur at gas exchange surfaces?

A

Hypoventilation
- Type II respiratory failure

Emphysema
- decreased SA

Fibrosis
- increased basement membrane thickness

Pulmonary Oedema

  • (e.g. pneumonia)
  • increased fluid layer thickness
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8
Q

What is the role of ventilation in gas exchange?

A

Adequate ventilation maintains the pressure gradient between alveoli and blood

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9
Q

Describe the relationship between pP(O₂) and ventilation?

A

The partial pressure of O₂ within alveoli increases with increased ventilation

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10
Q

Describe the relationship between CO₂ and ventilation

A

The partial pressure of CO₂ in alveoli decreases as ventilation increases

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11
Q

What is the significance of good perfusion in gas exchange?

A

Maintaining pressure gradients for diffusion also requires adequate perfusion

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12
Q

Describe the partial pressure of O₂ and CO₂ during hyperventilation.

A

O₂ levels remain the same - once PA(O₂) is 100%, no more oxygen can be obtained (can’t exceed 100%) so levels remain constant

PA(CO₂) decreases, as hyperventilating (breathing in and out v. quickly) so blood CO₂ decreases rapidly

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13
Q

Describe the partial pressure of CO₂ and O₂ during hypoventilation

A

Insufficient ventilation so decreased PA(O₂) and not enough CO₂ removed so PA(CO₂) increases

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14
Q

How do we ensure 100% oxygen saturation isn’t constantly reached to cause hyperventilation?

A

V/Q ratio:
Blood flow through pulmonary capillaries (Q - perfusion), needs to be matched to alveolar ventilation (Va) to enable efficient gas exchange as there is a maximum amount of O₂ each unit of blood can carry

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15
Q

What is the ideal V/Q ratio?

A

Ideally V/Q should = 1
At rest, ventilation and perfusion both = 5 L/min
so V/Q = 1 : 0.8

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16
Q

How much O₂ can 1 L of blood carry?

A

1 L of blood can carry 200 ml of O₂

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17
Q

How much O₂ does dry air carry?

A

1 L of dry air can carry 200 ml of O₂

18
Q

How is ventilation-perfusion ratio maintained?

A

Ventilation-Perfusion coupling is maintained by hypoxic vasoconstriction

19
Q

How is ventilation-perfusion mismatching prevented?

A

V/Q mismatching is prevented by homeostatic mechanisms

20
Q

How does hypoxic vasoconstriction maintain V/Q ratio?

A

Hypoxic vasoconstriction of capillaries diverts blood from poor to well ventilated alveoli

21
Q

What is the consequence of V/Q mismatching?

A

In situations where ventilation and perfusion to individual alveolar units aren’t matched, gas exchange will be reduced

22
Q

How does V/Q inequality affect partial pressure of CO₂ and O₂?

A

V/Q mismatch affects both O₂ and CO₂

Increased PA(CO₂) induces reflex hyperventilation to wash out excess CO₂, but doesn’t increase PA(O₂)

23
Q

What is the effect of a pulmonary embolism on the V/Q ratio?

A

Embolism occludes pulmonary artery supplying a region of the lung

Where blood flow is blocked:
- Unperfused alveoli = ↑V/Q

Where excess perfusion
- Perfusion to these vessels / alveoli increases as CO
is diverted = ↓V/Q
- unless ventilation of these alveoli increases to match
perfusion, hypoxaemia and hypercapnia will occur

24
Q

What is hypoxaemia?

A

Abnormally low [O₂] in the blood

25
Q

What is hypercapnia?

A

Abnormally high [CO₂] in the blood

26
Q

What is the consequence of increased V/Q ratio?

A
  • Heart failure (cardiac arrest)
  • Blocked vessels (pulmonary embolism)
  • Loss / damage to capillaries (emphysema)
27
Q

What is the consequence of unperfused alveoli?

A

Physiological dead space as there is little or no gas exchange occurring

28
Q

What is a pulmonary shunt?

A

Perfusion without ventilation

Blood travels from Right heart -> left heart without taking part in gas exchange

29
Q

What causes a pulmonary shunt to occur?

A

Reduced ventilation of alveoli or limited diffusion causes decreases in V/Q ratio which may be due to:

  • Pneumonia
  • Acute lung injury
  • Respiratory Distress syndrome
  • Atelectasis
30
Q

How effective is supplemental O₂ therapy to shunt induced hypoxaemia?

A

Shunt induced hypoxaemia responds poorly to supplemental O₂ therapy

31
Q

Describe the features of healthy lung function

A

Pulmonary capillaries innervate alveoli before returning to systemic circulation

  • Blood saturation ~60%
  • Blood saturation post Oxygenation ~95%
    when they mix total saturation ~90% (lower than 95%)
32
Q

Describe the features of shunt lungs

A
  • Not enough ventilation occurs; no gas exchange
  • Capillaries innervating alveoli are still perfused
  • Blood oxygenation enters + leaves at 60% as no
    ventilation is occurring
  • Blood innervating working alveoli still ventilated like
    normal
    => when blood mixes % saturation falls to 75%
33
Q

What is the effect of ventilation during shunt lungs?

A

Can get O₂ into working part of lung

  • increased ventilation of functional airways / alveoli
  • supplies additional O₂
  • removal of CO₂

BUT
shunt affected part still not ventilated
blood mixes with systemic circulation
(De)oxygenated blood mix - healthy saturation is never reached

34
Q

What affect does pulmonary shunt have on CO₂?

A

Shunt only affects O₂ loading, CO₂ is still able to be removed

35
Q

How can we determine the cause of hypoxaemia?

A

Calculate Alveolar O₂ pressure to determine if respiratory failure is due to hypoventilation or poor oxygenation

36
Q

Outline the equation used to calculate alveolar O₂ pressure

A

PAO₂ = FiO₂ x (Pb - PH₂O) - (PaCO₂/RER)

PAO₂ - alveolar O₂ pressure 
FiO₂ - O₂ friction in inspired gas
Pb - barometric pressure 
PH₂O - H₂O vapour pressure 
PaCO₂ - arterial CO₂ pressure
RER - respiratory exchange ratio
37
Q

What is RER?

A

Respiratory exchange ratio determines the relationship between CO₂ elimination and O₂ consumption

RER = VCO₂ produced / VO₂ consumed

measures difference between O₂ and CO₂ in inspired and expired air

38
Q

What determines RER value?

A

The main determinant of RER is the particular substrate being used (e.g. fat / carbohydrate)

RER for modern diet = 0.8

39
Q

How can we investigate hypoxaemia?

A

Alveolar gas equation and alveolar-arterial pressure gradient (PAO₂ - PaO₂) is used to investigate hypoxaemia

ABG - arterial blood gases
AGE - alveolar gas equation
A-aO₂ - pressure gradient

40
Q

How do we interpret ABG readings using AGE and A-aO₂?

A
  1. See if hypoventilation is contributing to hypoxaemia?
    • if PA(CO₂) > 6kPa
  2. Is O₂ reaching alveoli diffusing into blood?
    • use AGE and ABG readings to calculate A-a gradient
    • should be > 2kPa

Respiratory System (10 decks)

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