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SA respiratory > S3) Gas Exchange in the Lungs > Flashcards

S3) Gas Exchange in the Lungs Flashcards

1
Q

Identify the 6 factors affecting the diffusion rate of a gas in a fluid

A
  • Pressure difference (ΔP)
  • Solubility of a gas in solution (S)
  • Cross-sectional area of the fluid (A)
  • Distance the gas molecules must diffuse (d)
  • Molecular weight of the gas (MW)
  • Temperature of the fluid (assume 37o)
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2
Q

Identify the equation for Fick’s first law of Diffusion

A
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3
Q

Which is more soluble between CO2 and O2?

A

Carbon dioxide (20x)

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4
Q

What is the result of CO2’s greater solubility?

A

Greater diffusion coefficient – rate at which a substance diffuses

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5
Q

What role does ΔP have in the diffusion of O2

A

Diffusion coefficient is compensated by differences in partial pressures i.e. larger ΔP compensates for slower diffusion of O2

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6
Q

In a diseased lung, the diffusion of which substance is predominantly impaired?

A

Oxygen gas exchange is more impaired than CO2 because of its slower diffusion rate

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7
Q

Identify the 3 components of the diffusion barrier

A
  • 5 cell membranes
  • 3 layers of cytoplasm
  • 2 layers of tissue fluid
    0. 6 micron
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8
Q

How does the distance in the blood gas barrier change during inhalation?

A
  • Distance between blood and air (in alveoli) = 0.6 mm
  • Distance decreases during inhalation as lung distends
  • Allows fast and efficient diffusion
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9
Q

Diffusion resistance depends on 2 factors.

Identify them

A
  • Nature of barrier
  • Nature of gas
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10
Q

Describe how the nature of the barrier affects the diffusion resistance

A
  • Permeability = (D x S)/thickness of membrane
  • Larger molecules have small diffusion coefficients
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11
Q

Which substance diffuses faster, CO2 or O2?

Why?

A
  • O2 is smaller so greater diffusion coefficient
  • CO2 is more soluble

CO2 diffuses faster because gas exchange of O2 is a limiting step

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12
Q

Describe the gradients of partial pressure of O2 in the returning blood and alveoli

A

PO2 in alveolar gas > PO2 in returning blood

So, oxygen diffuses into blood

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13
Q

Describe the gradients of partial pressure of CO2 in the returning blood and alveoli

A

PCO2 in alveolar gas < PCO2 in returning blood

So, carbon dioxide diffuses out of blood

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14
Q

Identify 3 conditions which impair diffusion

A
  • Fibrotic lung disease
  • Pulmonary oedema
  • Emphysema
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15
Q

Explain how fibrotic/interstitial lung disease impairs diffusion

A

Fibrotic lung disease: thickened alveolar membrane slows gas exchange

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16
Q

Explain how pulmonary oedema impairs diffusion

A

Pulmonary oedema: fluid in the interstitial space increases diffusion distance

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17
Q

Explain how emphysema impairs diffusion

A

Emphysema: destruction of alveoli reduces surface area for gas exchange

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18
Q

What is anatomical dead space?

A

- Anatomical deadspace is the volume of air which is inhaled that does not take part in the gas exchange because it remains in the conducting airways

  • Normally 0.15 L in adults
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19
Q

What is alveolar dead space?

A

Distributive/alveolar deadspace involves air reaching the lungs that is not perfused or poorly perfused due to dead/damaged alveoli (0.12 L)

20
Q

What is physiological deadspace?

A

Dead space is the volume of a breath that does not participate in gas exchange (ventilation without perfusion)

Physiological dead space = anatomical + alveolar

ventilation but no perfusion

21
Q

How can one calculate the dead space ventilation rate?

A

DSVR = Dead space volume x respiration rate

E.g. 0.15 x 15 = 2.25 L

22
Q

How can one calculate alveolar ventilation rate?

A

AVR = (tidal volume - dead space volume) x respiration rate

E.g. (0.5 - 0.15) x 15 = 5.25L

23
Q

How can one calculate lung perfusion?

A
  • Lung perfusion (Q) = RV output
  • It is the same as cardiac output (approx. 5 litres/min)
24
Q

What is the ideal value for V/Q ratio in the lungs?

A

V = Alveolar ventilation rate (approx. 5 l/min)

Q = Lung perfusion (approx. 5 l/min)

V/Q = 1 (ideally)

25
Q

An imbalance between alveolar ventilation and alveolar blood flow is described by ventilation-perfusion ratio.

Outline this

A
  • If VA is 0, but there is still perfusion, VA/Q = 0
  • If VA is normal, but perfusion is 0, VA/Q = infinity
26
Q

Define hypoxia and hypoxaemia

A

Hypoxia – low oxygen levels in body or tissues

Hypoxaemia – low pO2 in arterial blood, most commonly caused due to a VQ mismatch

27
Q

What is cyanosis?

A

Cyanosis is bluish discolouration due to unsaturated haemoglobin

28
Q

Which parts of the body can be affected by cyanosis?

A
  • Can be peripheral (hands or feet) due to poor local circulation
  • Can be central (mouth, tongue, lips, oral mucosa) due to poorly saturated blood in systemic circulation
29
Q

Why can it be difficult to detect cyanosis?

A
  • Poor lighting
  • Skin colouration
30
Q

What does pulse oximetry do?

A

Detects level of Hb saturation by detecting difference in absorption of light between oxygenated and deoxygenated Hb

31
Q

What are the limitations with pulse oximetry?

A
  • Only detects pulsatile arterial blood
  • Ignores levels in tissues and non-pulsatile venous blood
  • Doesn’t say how much Hb present
32
Q

rate of gas exchange is determined by 3 factors

A
  1. surface area
  2. resistance to diffusion
  3. gradient of partial pressure
33
Q

definition of perfusion

A
  • flow
  • somewhat gravity dependent
34
Q

ventilation

A
  • circulation of gasses
35
Q

what is a pulmonary shunt

A
  • blood is directed into an alveoli that is working and healthy and diverts it from one that is damaged 3
  • shunt = perfusion but no ventilation
36
Q

what dies it mean if the V:Q<1

A

ventilating a poorly perfused alveoli

37
Q

V:Q during exercise

A
  • during exercise it improves to 1 due to increased blood flow and perfusion
38
Q

what does increased ventilation do to partial pressure of oxygen

A
  • it increases partial pressure of oxygen in blood as it increases alveolar oxygen partial pressure
  • more oxygen gets dissolved
39
Q

how does hyper ventilation lower co2 levels

A
  • as you are breathing out more and you are maintaining the diffusion distance
    you are expiring more and so you remove more c02 faster
40
Q

how does the Body avoid V/Q mismatch

A
  • arterial vasoconstriction so blood is diverted away from poorly ventilated alveoli to well ventilated alveoli
41
Q

limitations of lung hypoxic vasoconstriction

A
  1. diverted blood can only carry a limited amount of oxygen
  2. people with severe lung disease have fewer well ventilated alveoli so compensation is incomplete
  3. people with severe lung disease will have so much vasoconstriction it causes high blood pressure → heart failure
  4. can worsen V/Q mismatch in patients with lung disease
  5. some lung diseases disable lungs ability to vasoconstrict so increases V/Q mismatch and shunt
42
Q

Bohrs Curve

A
43
Q

where will bohrs curve shift when affinity os reduced for oxygen and when its increases

A
  • shifts right when reduced affinity ( increased DPG)
  • shifts left when the affinity increases
44
Q

how to calculate the content of 02 in the blood?

A

total content of 02 in blood = oxygen dissolved and oxygen on haemoglobin

  1. Find the amount of 02 dissolved:
  2. Partial Pressure of 02 (13.5 Kpa) x solubility coefficient of 02 in plasma (0.01) = 0.135mmol/L
  3. find the amount of 02 on haemoglobin:
  4. normal heam conc = 2.2mmol/L but multiply by 4 (4 02) 2.2x4 = 8.8mmol/L oxygen
  5. 0.135 + 8.8 = 8.935
45
Q

how to measure 02 saturation on a person

A
  • use a pulse oximeter
46
Q

name a medical condition where haemoglobin is fully saturated with oxygen by the patient Is hypoxic?

A
  • anemia
  • total content of oxygen in the blood is low so there will be tissue hypoxia despite haemoglobin being low
47
Q

in what situation would a someone have a low haemoglobin but are not hypoxic?

A
  • high altitude
  • people have acclimatised by increasing haemoglobin and 2- DPG

SA respiratory (23 decks)

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