On these “Gold Standard Step 1 Facts” pages you will find Free:

- USMLE Audio Review files from our “Gold Standard USMLE Reviews”
- Transcriptions of those files
- And videos (as they become available)

The idea is that you can review for the USMLE online by:

- Listening to the Audio
- Following along with the transcription
- Or by watching the video (if available)

If you like what you here, you can purchase the entire Gold Standard Step 1 MP3 audio USMLE review for your iPhone, iPod, or computer here.

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

### Play USMLE Audio MP3 61 06 Diffusion 1_3 Below

### Begin 61 06 Diffusion 1_3 Transcription

Alright, moving now to gas exchange. In the respiratory zone of the lungs, gases pass through the respiratory membrane and are exchanged between the alveolar air and the blood in the pulmonary capillaries.

What is the term for the movement of ions or molecules from areas of higher concentration to areas of lower concentration? It starts with the letter D.

**Diffusion**

Define pulmonary gas diffusion rate.

**The pulmonary gas diffusion rate is the permanent volume of gas that crosses the blood gas barrier in the lungs.**

For a given gas, the rate of diffusion across the respiratory membrane is determined by several factors. The first of these is the partial pressure difference between the gas and two lung structures. List the two lung structures

**The alveoli and the pulmonary capillaries.**

Let’s call this partial pressure difference Δ P. Are ΔP and diffusion rate inversely or directly proportional?

**Δ P and diffusion rate are directly proportional.**

Two additional factors that determine the rate of gas diffusion in the lungs are both geometric properties of the respiratory membrane.

Which dimension of the respiratory membrane is inversely proportional to the rate of diffusion of gas across the barrier?

**The thickness of the membrane is inversely proportional to the rate of diffusion. This is also called the diffusion distance.**

What is the approximate diffusion distance in micrometers?

**Anywhere from .5 to 2 micrometers.**

The other geometric property of the respiratory membrane that affects diffusion rate is its surface area. How is the surface area of the membrane related to the rate of diffusion?

**They are directly proportional.**

What is the approximate surface area of the respiratory membrane?

**50 to 100 square meters.**

What is it that makes the surface area of the respiratory membrane so large?

**The surface area of each alveolus contributes to the total surface area of the respiratory membrane.**

About how many alveoli are there in the lungs?

**Together, the lungs contain about 300 million.**

Finally, the diffusion rate is also determined by something called the diffusion coefficient. The diffusion coefficient for a particular gas can be looked up in a table.

What are two physical constants of a gas that determine its diffusion coefficient?

**Molecular weight and solubility.**

How could all this be expressed in equation form according to Fick’s law of diffusion?

**Volume of gas equals ΔP times surface area times diffusion coefficient over thickness.**

Student doctor, please pause the tape and summarize what we’ve discussed about the pulmonary gas diffusion rate.

**In the respiratory zone of the lungs, gases pass through the respiratory membrane and are exchanged between the alveolar air and the blood in the pulmonary capillaries. Diffusion is the movement of ions or molecules from areas of higher concentration to areas of lower concentration. The pulmonary gas diffusion rate is the permanent volume of gas that crosses the blood gas barrier in the lungs. For a given gas, the rate of diffusion across the respiratory membrane is determined by several factors. One of these is the partial pressure difference between gas and the alveoli and gas in the pulmonary capillaries. ΔP and diffusion rate are directly proportional. Two geometric properties of respiratory membrane also determine the diffusion rate. The thickness of the membrane is inversely proportional to the rate of diffusion. Approximate diffusion distance is anywhere from .5 to 2 micrometers. The surface area of the membrane is about 50 to 100 square meters and is directly proportional to the rate of diffusion. The surface area of each of the 300 million alveoli in the lungs contributes to the total surface area of the respiratory membrane. Finally, the diffusion rate is also determined by the diffusion coefficient, which can be looked up in a table. Two physical constants of a gas that determine its diffusion coefficient are molecular weight and solubility. According to Fick’s law of diffusion all of this can be expressed by the following equation: Volume of gas equals ΔP times surface area times diffusion coefficient over thickness.**

****END OF TRANSCRIPTION****

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