USMLE Step 1 Respiratory Physiology Review 60 13 PO2

USMLE Step 1 Respiratory Physiology Review 60 13 PO2

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Let’s start our USMLE Review with Anatomy from the Gold Standard USMLE Step 1 Audio Review program.

Play USMLE Audio MP3 60 13 PO2 Below

Begin 60 13 PO2 Transcription

As air passes through the conducting zone of the respiratory system, it becomes humidified. How does increased humidity affect the various partial pressures of ambient gases?

  • As the air becomes humidified, the partial pressures of the gases decrease.

Keeping  this in mind, what is the partial pressure of oxygen in tracheal air?

  • PO2 of tracheal air is 150 mm Hg.

What is oxygen tension in alveolar air?

  •  PO2 of alveolar air is about 100.

What is the upper limit for oxygen tension in the body?

  •  The same as alveolar PO2, about 100 mm Hg.   

Where is the PO2 about equal to alveolar PO2?

  • Arterial PO2 is about equal to alveolar PO2.

So what is arterial PO2?

  • 95 to 100 mm Hg.

What is oxygen tension in venous blood?

  • PO2 of venous blood is 40 mm Hg. 

Why is the PO2 of venous blood so low?

  • Because the blood has unloaded its oxygen at the tissues. So when the blood first arrives at the pulmonary capillaries from the body, its PO2 is much lower than the PO2 of the air in the alveoli.

What is it, then, that causes oxygen to diffuse across the blood gas barrier into the pulmonary capillary?

  •  The difference between alveolar PO2 and capillary PO2 causes oxygen to diffuse across the blood gas barrier into the pulmonary capillary.

And why does the oxygen diffuse from the alveolar air into the blood instead of the other way around?

  • Gas tensions want to be equal and gas always flows from the area of higher concentration to the area of lower concentration.

If the blood remains in the pulmonary capillary long enough, what will the oxygen tension of capillary blood tend to equal?

  • Oxygen tension in the capillaries will tend to equal oxygen tension in the alveolar air.

And what is this state called? The answer begins with an E.

  • The gas and the liquid are said to be in equilibrium.

Student doctor, please pause the tape and summarize what we’ve discussed about oxygen tension in the various structures of the respiratory system.

  • As air passes through the conducting zone of the respiratory system, it becomes humidified. As the air becomes humidified, the partial pressures of the gases decrease. Oxygen tension in tracheal air is 150 mm Hg. PO2 of alveolar air is about 100. The upper limit for oxygen tension in the body is 100 mm Hg. Alveolar and arterial oxygen tensions are about equal. Arterial PO2 is 95 to 100 mm Hg. Because the blood has unloaded it’s oxygen at the tissues, the partial pressure of oxygen in venous blood is reduced. PO2 of venous blood is 40 mm Hg. So when the blood first arrives at the pulmonary capillaries from the body, its oxygen tension is much lower than the PO2 of the air in the alveoli. It is this pressure difference that causes oxygen to diffuse across the blood gas barrier in the lungs. It diffuses from alveolus to capillary because gas tensions want to be equal and gases always flow from the area of higher concentration to the area of lower concentration. If the blood remains at the respiratory membrane long enough, oxygen tension in the capillaries will tend to equal oxygen tension in the alveolar air. And the gas and the liquid are said to be in equilibrium.

 

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