Respiratory Physiology: Lung Volumes and Airflow

Module 9 Learning Objectives

• Students learn to define lung volumes/capacities.
• Students will learn how pressures/lung volumes change during normal breathing, and how it affects airflow in the respiratory system.
• Students will be able to compare/contrast compliance and elastance in respiratory physiology, and provide examples of disease states.
• Students will learn the role of surface tension and surfactants in respiratory physiology.

Alveoli

• Type I alveolar cells enable gas exchange.
• Type II alveolar cells produce surfactant.
• Connective tissue contains elastin and collagen and is present is alveoli.
• Pulmonary circulation has high flow and low pressure.
• Blood flows from the right ventricle to pulmonary arteries, then to the lungs, then to the pulmonary veins, and finally to the left atrium.
• Blood flow through the lungs equals blood flow through the rest of the body.
• Lungs have low resistance.

Airway Epithelium

• Epithelial cells lining the airways and submucosal glands secrete saline and mucus.
• Cilia moves mucus layer towards the pharynx, removing trapped pathogens and particulate matter.
• Watery saline layer helps cilia push mucus towards the pharynx.

Ventilation

• A respiratory cycle consists of one inspiration followed by one expiration.
• Lung volumes change during ventilation.
• Spirometers are used for pulmonary function tests.

Lung Volumes

• Tidal Volume (VT) is a lung volume.
• Inspiratory Reserve Volume (IRV) is a lung volume.
• Expiratory Reserve Volume (ERV) is a lung volume.
• Residual Volume (RV) is a lung volume.

Lung Capacity

• It's the sum of 2 or more lung volumes.
• Vital Capacity (VC) = IRV + ERV + VT.
• Total Lung Capacity (TLC) = IRV + ERV + VT + RV.
• Inspiratory Capacity is VT + IRV.
• Functional Reserve Capacity is ERV + RV.

Pressure in Pleural Cavity

• In a normal lung at rest, pleural fluid keeps the lung adhered to the chest wall.
• Intrapleural pressure is subatmospheric at -3 mm Hg.
• Pneumothorax occurs if the sealed pleural cavity opens to the atmosphere, allowing air to flow in.
• Pneumothorax breaks the bond holding the lung to the chest wall, causing the lung to collapse.

Lung Compliance/Elastance

• High or low compliance can indicate disease.
• Restrictive lung diseases impact are a type of low complinace, and includ fibrotic lung diseases (fibrosis) and inadequate surfactant production (NRDS)
• Elastance is a related lung characteristic.
• NRDS stands for Newborn Respiratory Distress Syndrome.

Surfactants

• Surfactants decrease the work of breathing.
• LaPlace's Law states P = 2T/r, where P is pressure, T is surface tension, and r is the radius.
• Surfactants are surface-active agents that disrupt the cohesive force of water.
• Proteins and phospholipids are surfactants.
• Surfactants are more concentrated in smaller alveoli.
• Premature babies may experience inadequate surfactant concentrations, leading to Newborn Respiratory Distress Syndrome (NRDS).

Clinical Applications

• Auscultation and spirometry are used to assess pulmonary function.
• Obstructive lung diseases increase airway resistance, including asthma, obstructive sleep apnea, and COPD.
• Restrictive lung diseases reduce lung compliance, leading to pulmonary fibrosis.
• Forced Vital Capacity (FVC) measurements include:
  • Forced Expiratory Volume in 1 second (FEV1)
  • FEV1/FVC ratio helps distinguish between obstructive and restrictive lung diseases.
• COPD is Chronic Obstructive Pulmonary Disease.