Some Basics of Pulmonary Physiology Douglas Wangensteen, Ph.D wange001@umn.edu

The function of the lungs is to change venous blood into arterial blood, keeping PO2 and PCO2 relatively constant.

Where We’re Going Introduction Gas Exchange Mechanics of Breathing - Diffusion - Ventilation - Causes of arterial hypoxemia Mechanics of Breathing - Airways resistance - Elastic recoil - Pulmonary function tests

Lung Anatomy Overview

How the Lungs Work

Where We’re Going Introduction Gas Exchange Mechanics of Breathing - Diffusion - Ventilation - Causes of arterial hypoxemia Mechanics of Breathing - Airways resistance - Elastic recoil - Pulmonary function tests

About Diffusion

Diffusion is Efficient in Healthy Lungs

Measuring Alveolar - Capillary Diffusion: the Diffusion Capacity

Marie Krogh (~1910): Measure DLCO, not DLO2

About Ventilation

PACO2 Depends on VA and Metabolism

VA Determines PO2 and PCO2

Causes of Arterial Hypoxemia in Lung Injury and Disease Hypoventilation Slow diffusion between blood and alveolar gas VA/Q mismatching (Q = blood flow) True shunts

For gas exchange to be efficient, ventilation and blood flow should be “matched” throughout the lungs.

When ventilation and blood flow are “mismatched” arterial PO2 will always be lower than normal.

A True Shunt is a Big Problem Arterial PO2 is always reduced; breathing O2 won’t help much.

Important Points about Gas Exchange VA should be at a level to provide normal arterial PO2 and PCO2. Slow diffusion is generally not a problem. Gas exchange is most efficient if VA and Q are matched within the lungs. If not, arterial PO2 will be reduced.

Where We’re Going Introduction Gas Exchange Mechanics of Breathing - Diffusion - Ventilation - Causes of arterial hypoxemia Mechanics of Breathing - Airways resistance - Elastic recoil - Pulmonary function tests

How We Breathe at Rest

Lung-Thorax System

Overview

Airways Resistance

The Importance of Airway Radius

Lung Elastic Recoil Compliance: CL = DV/DPtp ~ 200 ml/cm H2O

What determines lung elastic recoil? Tissue Forces and Surface Tension Forces

Surface Tension Forces The alveolar surface is covered with a thin aqueous layer. Alveoli are small (r = 10-2 cm); if the interface was pure air:water (g = 70 dyne cm-1) inflation would take 10x the pressure actually needed. The lungs produce “Alveolar Surfactant” to reduce surface tension on the alveolar surface, so inflation requires less effort.

Alveolar Surfactant DPPC

Production, Recycling by Type II Cells

Importance of Alveolar Surfactant: Reduces surface tension on the alveolar surface, therefore reduces lung elastic recoil Promotes alveolar stability because surface tension changes with alveolar area When lung disease reduces or inactivates alveolar surfactant, breathing is more difficult and alveoli collapse.

Pulmonary Function Tests A disease that increases airways resistance is called an OBSTRUCTIVE disease. A disease that increases lung elastic recoil is called a RESTRICTIVE disease. Both types of disease can have similar symptoms, so how can a diagnosis be made? Measure lung volumes and maximum expiratory flow.

Lung Volumes

The Body Plethysmograph

Forced Vital Capacity Examples

Obstructive Disease Example

Restrictive Disease Example

Questions?

Because of alveolar surfactant, alveolar surface tension changes with area.

If alveolar surface tension was the same in every alveolus, gas would flow from small alveoli into large alveoli. Small alveoli would collapse.