Ventilation Matters

One of the students has posted on the Blackboard discussion pages!

“In Dr Khogali’s lecture, he mentioned that the lungs were better ventilated at the bottom. But in the tutorial, it says the upper lobes of the lungs are more ventilated…”

A fellow student replies:

“There is greater ventalation at the lung bases but because there is also greater perfusion at the bases. V/Q isn’t the same at the top and bottom of the lungs but the top has relatively higher ventilation, ie. a higher V/Q even though the actual ventilation is less. I reckon that what the tutorial meant by realatively high ventalation.”

Let me throw something in here.

Lung ventilation is simply a measure of available increase in lung volume.  Because the lungs have mass, the bases of the lung are compressed by the lung above, so their resting volume is slightly smaller than that of the apices (In the upright position).  When the intraplural pressure is lowered (during inspiration) the bases of the lung have more potential for expansion than the already slightly expanded apices, so the ventilation of the bases is higher than that of the apices.  When the patient is supine, however, the weight of the lungs is from anterior to posterior, so the posterior lung ventilates better than the anterior.  If you stand on your head the apices ventilate better than the bases.

There is an exception to this rule, of course (isn’t there always).  At very low lung volumes, close to residual volume (that is, after a full expiration), the pressure at the bases of the lung actually exceeds atmospheric pressure, due to the weight of the lung above it.  So during small inspirations there is airway closure and the bases ventilate poorly, whereas the apices will ventilate normally.  In this situation, the apices have superior ventilation to the bases. Once lung volume increases, the airway closure is overcome, and the normal pattern of ventilation occurs.

Perfusion is pretty much the same pattern, only more so.  Blood flows due to pressure gradients, and prefers to go inferiorly, rather than superiorly.  Blood flow is markedly superior in the bases than in the apices, the change being far more marked than for ventilation.

Blood Flow and Ventilation Relationships According to Rib Space

Blood Flow and Ventilation Relationships According to Rib Space

If you plot out Blood Flow, Ventilation, and V/Q ratio, the blood flow and ventilation lines cross at about rib 4/5 in the diagram (ratio = 1) and then towards the apices the v/q ratio rapidly increases (>3).

The tutorial doesn’t make this particularly clear, so we can certainly go in and change it.

So thanks to the students for pointing out this discrepancy, and for the insightful response.

Keep feeding back, and we’ll keep updating things – as Dennis Hopper said in Speed – Interactive TV, Jack; Wave of the Future!

Anyway – remember everyone:

Impossible Is Nothing

Impossible Is Nothing

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One Comment on “Ventilation Matters”

  1. dundeechest Says:

    Dr Khogali has written a response for you:

    Slide no 11 in “Respiratory Physiology Lecture 2” shows a similar graph to that provided by Dr Fardon.

    The graph shows the gravitational effects on ventilation, perfusion, and ventilation perfusion ratios (V/Q Ratio) at the top and bottom of the lungs during normal breathing in the upright position.

    The essence is that under these circumstances:
    1. The bottom of the lungs receives more airflow and blood flow than the top of the lungs,
    2. But the bottom of the lungs receives relatively less airflow (ventilation) than blood flow (perfusion), and
    3. The top of the lungs receives relatively more airflow (ventilation) than blood flow (perfusion).

    Hence, as Dr Fardon said, If you plot out airflow (Ventilation), blood flow (Perfusion), and Ventilation Perfusion Ratio (V/Q Ratio), the airflow (Ventilation) and the blood flow (Perfusion) lines cross where the V/Q Ratio = 1; and the V/Q Ratio increases rapidly towards the top of the lungs.

    As explained during the lecture, this mismatch between airflow (Ventilation) and blood flow (Perfusion) have a negligable effects in overall O2 uptake and CO2 elimination in the healthy lungs. Local controls normally work to achieve as much as possible match between airflow (Ventilation) and blood flow (Perfusion) at a particular alveolar interface in the healthy lungs.

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