Gas Exchange and Transport
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Questions and Answers

What determines gas transfer between different phases?

  • Concentration gradient (ml/l or mmol/l)
  • Partial pressure gradient (correct)
  • Surface area of alveolar-capillary membrane
  • Temperature difference

    • Why does CO2 equilibrate rapidly across the alveolar-capillary exchange surface?

  • Due to the larger surface area of the alveolar-capillary membrane
  • Due to its lower molecular weight
  • Due to its lower partial pressure gradient
  • Due to its higher solubility coefficient in plasma (correct)

    • What happens to the partial pressure of O2 in the pulmonary capillary blood at rest?

  • It decreases along the length of the pulmonary capillary
  • It increases along the length of the pulmonary capillary
  • It equals the alveolar PO2 by about 1/3rd of the way along the pulmonary capillary (correct)
  • It remains constant along the length of the pulmonary capillary

    • What is the consequence of an disrupted alveolar-capillary unit in respiratory disease?

    <p>Diffusion limitation, especially during exercise</p> Signup and view all the answers

    Why does CO2 diffuse more rapidly than O2 despite its higher molecular weight?

    <p>Due to its higher solubility coefficient in plasma</p> Signup and view all the answers

    What is the direction of CO2 movement in the alveolar-capillary exchange surface?

    <p>From the pulmonary capillary blood to the alveolar air</p> Signup and view all the answers

    What is the characteristic of gas transfer in a normal lung?

    <p>More than adequate diffusion reserve</p> Signup and view all the answers

    What is the relationship between the diffusion rates of O2 and CO2?

    <p>CO2 diffuses at approximately 85% of the rate of O2</p> Signup and view all the answers

    What is the unit of pressure equivalent to 7.5 mmHg?

    <p>1 kPa</p> Signup and view all the answers

    What is the rate of transfer of gas through a sheet of tissue dependent on?

    <p>the area and thickness of the tissue, as well as the pressure gradient</p> Signup and view all the answers

    What is the pressure gradient driving diffusion across the alveolar-capillary membrane?

    <p>alveolar pressure - mean pulmonary capillary pressure</p> Signup and view all the answers

    What does the diffusing capacity of the lungs (DLg) represent?

    <p>the combination of area and thickness of the alveolar-capillary membrane</p> Signup and view all the answers

    What is the formula to calculate oxygen diffusing capacity (DLO2)?

    <p>oxygen uptake from the lungs / (PAO2 - PCO2)</p> Signup and view all the answers

    Why can mean pulmonary capillary PCO be assumed to be zero in the measurement of carbon monoxide diffusing capacity (DLCO)?

    <p>because almost all CO transferred across the alveolar-capillary membrane enters chemical combination with haemoglobin</p> Signup and view all the answers

    What is the advantage of using carbon monoxide in the measurement of diffusing capacity?

    <p>it can be used to estimate mean pulmonary capillary PCO</p> Signup and view all the answers

    What is the purpose of including a tracer gas in the breath inhaled by the patient during the measurement of DLCO?

    <p>to examine the composition of exhaled gas</p> Signup and view all the answers

    What is the effect of an acute ascent to altitude on the oxygen capacity of arterial blood?

    <p>It remains normal despite the decrease in inspired PO2</p> Signup and view all the answers

    What is the effect of a decrease in pH on the oxygen-haemoglobin dissociation curve?

    <p>It shifts the curve to the right</p> Signup and view all the answers

    What is the effect of an increase in 2,3-DPG on the oxygen-haemoglobin dissociation curve?

    <p>It shifts the curve to the right</p> Signup and view all the answers

    What is the effect of anaemia on the oxygen capacity of arterial blood?

    <p>It decreases due to the reduction in haemoglobin levels</p> Signup and view all the answers

    What is the PO2 at the base camp of Mt Aconcagua, given a barometric pressure of 60kPa?

    <p>11.2 kPa</p> Signup and view all the answers

    What is the effect of an increase in temperature on the oxygen-haemoglobin dissociation curve?

    <p>It shifts the curve to the right</p> Signup and view all the answers

    What is the effect of an increase in PCO2 on the oxygen-haemoglobin dissociation curve?

    <p>It shifts the curve to the right</p> Signup and view all the answers

    What is the oxygen saturation level of resting mixed venous blood?

    <p>75%</p> Signup and view all the answers

    What is the primary reason for reduced arterial oxygen content in iron deficiency anemia?

    <p>Low oxygen-carrying capacity due to low Hb</p> Signup and view all the answers

    At high altitude, what is the primary reason for improved physical activity after breathing 100% oxygen for short periods?

    <p>Increased oxygen-carrying capacity of blood</p> Signup and view all the answers

    What is the approximate fraction of oxygen in dry air at sea level?

    <p>0.209</p> Signup and view all the answers

    What is the term for the rate at which oxygen diffuses through the alveolar-capillary membrane?

    <p>Transfer factor</p> Signup and view all the answers

    Why can CO transfer factor be measured but O2 transfer factor cannot?

    <p>CO has a higher affinity for hemoglobin than O2</p> Signup and view all the answers

    What would be the approximate partial pressure of oxygen on a mountain where the barometric pressure is 50.5 kPa (380 mmHg)?

    <p>10.5 kPa (79 mmHg)</p> Signup and view all the answers

    Which of the following would NOT cause a low CO transfer factor?

    <p>Increased pulmonary blood volume</p> Signup and view all the answers

    What would be the primary effect of hypoventilation on gas exchange in the lungs?

    <p>Decreased oxygen diffusion</p> Signup and view all the answers

    What is the effect of polycythaemia on the oxygen reservoir in severe chronic respiratory failure?

    <p>It increases the oxygen reservoir</p> Signup and view all the answers

    In severe chronic respiratory failure, why is the patient vulnerable to a small further fall in arterial PO2?

    <p>Because a small further fall in arterial PO2 will have a large effect on arterial O2 content</p> Signup and view all the answers

    What is the effect of anaemia on average tissue capillary PO2 compared to a person with a normal Hb concentration?

    <p>It reduces the average tissue capillary PO2</p> Signup and view all the answers

    Why is the fall in PO2 along the tissue capillary less steep in polycythaemia?

    <p>Because polycythaemia increases the oxygen reservoir and the amount of bound oxygen available</p> Signup and view all the answers

    What is the consequence of a small further fall in arterial PO2 in a patient with severe chronic respiratory failure?

    <p>A large effect on arterial O2 content</p> Signup and view all the answers

    Why is the patient in severe chronic respiratory failure still vulnerable despite having adequate O2 delivery?

    <p>Because of a large effect of a small further fall in arterial PO2 on arterial O2 content</p> Signup and view all the answers

    What is the relationship between the average tissue capillary PO2 and the venous PO2 in severe chronic respiratory failure?

    <p>The average tissue capillary PO2 is close to the venous PO2</p> Signup and view all the answers

    What is the effect of polycythaemia on the PO2 gradient driving diffusion in severe chronic respiratory failure?

    <p>It decreases the PO2 gradient driving diffusion</p> Signup and view all the answers

    Study Notes

    Gas Exchange and Transport

    • Diffusion of O2 and CO2 across the alveolar-capillary membrane is driven by partial pressure gradients.
    • O2 diffuses from the alveoli into the pulmonary capillaries, while CO2 diffuses from the pulmonary capillaries into the alveoli.
    • The rate of diffusion is affected by the partial pressure gradient, solubility coefficient, and molecular weight.
    • CO2 has a higher solubility coefficient than O2, making it 23 times more soluble and allowing it to diffuse 20 times faster.

    Factors Affecting Diffusion

    • Partial pressure gradient: gas moves down its partial pressure gradient from high to low.
    • Area and thickness of the alveolar-capillary membrane: increased surface area and decreased thickness facilitate diffusion.
    • Solubility coefficient: gases with higher solubility coefficients diffuse faster.

    Measurement of Diffusing Capacity

    • Oxygen diffusing capacity (DLO2): measures the rate of O2 transfer from the alveoli to the pulmonary capillaries.
    • Carbon monoxide diffusing capacity (DLCO): measures the rate of CO transfer from the alveoli to the pulmonary capillaries.
    • DLCO is commonly used in clinical investigations to assess lung function.

    Oxygen-Haemoglobin Dissociation Curve

    • The curve shows the relationship between oxygen saturation and partial pressure.
    • Factors affecting the curve include pH, PCO2, temperature, and 2,3-DPG levels.

    Acute Ascent to Altitude

    • Arterial oxygen content and saturation decrease due to decreased inspired PO2.
    • Oxygen capacity of arterial blood is normal, but oxygen-carrying capacity is reduced.

    Anaemia

    • Oxygen capacity of arterial blood is reduced due to low haemoglobin levels.
    • Arterial oxygen content and saturation are reduced due to decreased oxygen-carrying capacity.

    Gaseous Diffusion and Transport Quiz

    • Questions to consider:
      • What factors affect water vapour pressure in the air we breathe?
      • How do changes in barometric pressure affect PO2 and fractional concentration of oxygen in dry air?
      • What is meant by "transfer factor" of a gas in the lungs?
      • Why can CO transfer factor be measured but O2 transfer factor cannot?
      • What causes a low CO transfer factor?

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    Description

    Learn about the diffusion of oxygen and carbon dioxide across the alveolar-capillary membrane, and how the rate of diffusion is affected by various factors. Quiz yourself on the principles of gas exchange and transport!

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