USMLE Step 1 Respiratory Physiology Review 61 10 Ventilation and Blood Flow

# USMLE Step 1 Respiratory Physiology Review 61 10 Ventilation and Blood Flow

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### Begin   61 10 Ventilation and Blood Flow  Transcription

Now lets talk about ventilation and blood flow.

Is alveolar ventilation greater at the apex or the base of the vertical lung?

• Alveolar ventilation is greater at the base of the vertical lung.

Where in the lung is pulmonary capillary blood flow greater?

• Also at the base

What is the term for the ratio of alveolar ventilation to pulmonary blood flow?

• The ventilation perfusion ratio or the VQ ratio

What is the ideal relationship between ventilation and perfusion?

• They are equal

In liters per minute, what is normal alveolar ventilation?

• Normal alveolar ventilation is 4.5 to 5 liters per minute

In liters per minute, what is normal cardiac output at rest?

• About 5 liters per minute

In liters per minute, what is normal pulmonary blood flow at rest?

• Also about 5 liters per minute

What is the ventilation perfusion ratio equal to when the most efficient gas exchange occurs?

• Ideally, the ventilation perfusion ratio equals 1 or close to 1.

If the ventilation perfusion ratio is greater than one, what is happening?

• If the ventilation perfusion ratio is greater than 1, there is less perfusion than ventilation which means wasted ventilation.

What is happening when the ventilation perfusion ratio is less than 1?

• Perfusion is greater than ventilation, thus, wasted perfusion.

Student doctor, please pause the tape and summarize what has been discussed about the ventilation perfusion ratio.

• Alveolar ventilation is greater at the base of the vertical lung. This is also true for pulmonary capillary blood flow. The ratio of alveolar ventilation to pulmonary blood flow is called the ventilation perfusion ratio or the VQ ratio. The ideal relationship between ventilation and perfusion is when they are equal. Normal alveolar ventilation is 4.5 to 5 liters per minute. Both cardiac output and pulmonary blood flow at rest are about 5 liters per minute. Ideally, the ventilation perfusion ratio equals 1 or close to 1. If the ventilation perfusion ratio is greater than 1, there is less perfusion than ventilation which means wasted ventilation. When the ventilation perfusion ratio is less than 1, perfusion is greater than ventilation, thus, wasted perfusion.

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