S3) Gas Exchange in the Lungs Flashcards
Identify the 6 factors affecting the diffusion rate of a gas in a fluid
- 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)
Identify the equation for Fick’s first law of Diffusion
Which is more soluble between CO2 and O2?
Carbon dioxide (20x)
What is the result of CO2’s greater solubility?
Greater diffusion coefficient – rate at which a substance diffuses
What role does ΔP have in the diffusion of O2
Diffusion coefficient is compensated by differences in partial pressures i.e. larger ΔP compensates for slower diffusion of O2
In a diseased lung, the diffusion of which substance is predominantly impaired?
Oxygen gas exchange is more impaired than CO2 because of its slower diffusion rate
Identify the 3 components of the diffusion barrier
- 5 cell membranes
- 3 layers of cytoplasm
- 2 layers of tissue fluid
How does the distance in the blood gas barrier change during inhalation?
- Distance between blood and air (in alveoli) = 0.6 mm
- Distance decreases during inhalation as lung distends
- Allows fast and efficient diffusion
Diffusion resistance depends on 2 factors.
Identify them
- Nature of barrier
- Nature of gas
Describe how the nature of the barrier affects the diffusion resistance
- Permeability = (D x S)/thickness of membrane
- Larger molecules have small diffusion coefficients
Which substance diffuses faster, CO2 or O2?
Why?
- O2 is smaller so greater diffusion coefficient
- CO2 is more soluble
CO2 diffuses faster because gas exchange of O2 is a limiting step
Describe the gradients of partial pressure of O2 in the returning blood and alveoli
PO2 in alveolar gas > PO2 in returning blood
So, oxygen diffuses into blood
Describe the gradients of partial pressure of CO2 in the returning blood and alveoli
PCO2 in alveolar gas < PCO2 in returning blood
So, carbon dioxide diffuses out of blood
Identify 3 conditions which impair diffusion
- Fibrotic lung disease
- Pulmonary oedema
- Emphysema
Explain how fibrotic lung disease impairs diffusion
Fibrotic lung disease: thickened alveolar membrane slows gas exchange
Explain how pulmonary oedema impairs diffusion
Pulmonary oedema: fluid in the interstitial space increases diffusion distance
Explain how emphysema impairs diffusion
Emphysema: destruction of alveoli reduces surface area for gas exchange
What is anatomical dead space?
- 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
What is alveolar dead space?
Distributive/alveolar deadspace involves air reaching the lungs that is not perfused or poorly perfused due to dead/damaged alveoli (0.12 L)
What is physiological deadspace?
Dead space is the volume of a breath that does not participate in gas exchange (ventilation without perfusion)
Physiological dead space = anatomical + alveolar
How can one calculate the dead space ventilation rate?
DSVR = Dead space volume x respiration rate
E.g. 0.15 x 15 = 2.25 L
How can one calculate alveolar ventilation rate?
AVR = (tidal volume - dead space volume) x respiration rate
E.g. (0.5 - 0.15) x 15 = 5.25L
How can one calculate lung perfusion?
- Lung perfusion (Q) = RV output
- It is the same as cardiac output (approx. 5 litres/min)
What is the ideal value for V/Q ratio in the lungs?
V = Alveolar ventilation rate (approx. 5 l/min)
Q = Lung perfusion (approx. 5 l/min)
V/Q = 1 (ideally)
An imbalance between alveolar ventilation and alveolar blood flow is described by ventilation-perfusion ratio.
Outline this
- If VA is 0, but there is still perfusion, VA/Q = 0
- If VA is normal, but perfusion is 0, VA/Q = infinity
Define hypoxia and hypoxaemia
Hypoxia – low oxygen levels in body or tissues
Hypoxaemia – low pO2 in arterial blood
What is cyanosis?
Cyanosis is bluish discolouration due to unsaturated haemoglobin
Which parts of the body can be affected by cyanosis?
- 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
Why can it be difficult to detect cyanosis?
- Poor lighting
- Skin colouration
What does pulse oximetry do?
Detects level of Hb saturation by detecting difference in absorption of light between oxygenated and deoxygenated Hb
What are the limitations with pulse oximetry?
- Only detects pulsatile arterial blood
- Ignores levels in tissues and non-pulsatile venous blood
- Doesn’t say how much Hb present
