Respiratory Physiology Lecture 5 Part 23: More on the Ventilation-perfusion Relationship

Question 1

What is the V/Q ratio of the lung?

Answer 1

There are regional differences in both lung perfusion and lung ventilation

Question 2

What determines local local alveolar PO2 and PCO2?

Answer 2

The local ventilation-perfusion ratio (V_A/Q) determines the local alveolar PO2 and PCO2

Question 3

What causes regional differences in lung perfusion?

Answer 3

• The weight of the lungs increases pressure in regions near bottom (Makes PIP less negative) therefore less pressure pulling it open than regions at top of lung
• Since alveoli at bottom are starting more deflated, they are able to expand more. Thus, bottom regions of lung receive a larger portion of the inspired air.

Question 4

Where is the lowest ventilation and greatest ventilation in the lung?

Answer 4

Question 5

How can regional differences in lung perfusion be measured?

Answer 5

Inject patient with 133Xe into vein, which then travels to right heart and pulmonary circulation

• If the patient holds breath as soon as Xe is injected we can measure radioactivity in different regions of the lungs. Perfusion is higher at base of the lung per unit volume and falls toward the apex. Associated with gravity and posture so if you change postures it will effect ventilation and perfusion

Question 6

Where is the basal and apex of the lung?

Answer 6

• basal → bottom/ base of lung
• Apex → top of lung

Question 7

What does perfusion depend on?

Answer 7

• gravity
• posture

Question 8

Ideal V_A/Q of apical and basal of lung

Answer 8

• Basal VA/Q = 0.6x ideal VA/Q
• Apical VA/Q = 3x ideal VA/Q

Question 9

Ventilation-perfusion relationship diagram

Answer 9

Both decrease as we move from bottom to top of lung but slope is different which means that if we calculate the ventilator to perfusion ratio there will be lower volume at bottom of lung and higher at top

Question 10

What helps to keep ventilation and perfusion matched?

Answer 10

Homeostatic mechanisms exist to limit the mismatch

Question 11

What homeostatic mechanisms exist to limit the mismatch of ventilation-perfusion?

Answer 11

Most important is the unique response of pulmonary capillaries to low O2: pulmonary hypoxic vasoconstriction

Question 12

Hypoxic pulmonary vasoconstriction

Answer 12

HPV

is a homeostatic mechanism that is intrinsic to the pulmonary vasculature. Intrapulmonary arteries constrict in response to alveolar hypoxia, diverting blood to better-oxygenated lung segments, thereby optimizing ventilation/perfusion matching and systemic oxygen delivery.

Question 13

Hypoxia

Answer 13

Hypoxia is a state in which oxygen is not available in sufficient amounts at the tissue level to maintain adequate homeostasis; this can result from inadequate oxygen delivery to the tissues either due to low blood supply or low oxygen content in the blood (hypoxemia)

Question 14

What happens with decreased airflow to regions of the lung?

Answer 14

⇣ PO2

⇡ PCO2

Question 15

What happens with decreased bloodflow to regions of the lung?

Answer 15

⇡ PO2

⇣PCO2