Skip to Main Content


The Respiratory System


Overview of the Respiratory System


The primary function of the respiratory (pulmonary) system is to maintain systemic arterial blood gas levels within normal range. To achieve this, the rates of O2 uptake and CO2 excretion at the lungs must match the respective rates of O2 use and CO2 production by cellular respiration. The main components of the respiratory system are the lungs, the chest wall, and the pulmonary blood vessels. Muscles of the chest wall power the movement of air into the lungs during inspiration. Distribution of the pulmonary blood flow, to match ventilation, ensures the proper gas exchange. The levels of systemic O2 and CO2 are monitored by chemoreceptors, allowing the pulmonary system to respond to changes in cellular respiration. Treatment of respiratory disorders requires an understanding of factors that govern ventilation (gas flow), diffusion of gases, and perfusion (blood flow) in the lungs.


Blood-Gas Interface


The lung is specialized for gas diffusion and has an internal surface area of 50–100 m2. The large surface area is produced by repeated branching of the airways, which begins at the trachea and terminates in over 300 million closed air sacs called alveoli. Ventilation is the process whereby air enters the lungs and comes into contact with alveoli, which are the sites of gas exchange. Each alveolus is surrounded by a dense network of pulmonary capillaries. The blood gas interface is less than 1-μm thick and consists of the following four elements in series (Figure 5-1):


  1. Thin layer of surface liquid.

  2. Alveolar lining cells (type 1 pneumocytes), plus associated basement membrane.

  3. Thin layer of interstitial fluid.

  4. Pulmonary capillary endothelial cells, plus associated basement membrane.


Gas Laws


The volume, pressure, and temperature of gases are all closely related by physical laws (Table 5-1). These laws can be applied to help explain the mechanics of air movement into and out of alveoli as well as the diffusion of gases across the blood-gas interface.

Table Graphic Jump Location
Table 5-1Gas Laws and Applications in Respiratory Physiology

Want remote access to your institution's subscription?

Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.


Create a Free MyAccess Profile

* Required Fields

Note: If you have registered for a MyAccess profile on any of the Access sites, you can use the same MyAccess login credentials across all sites.

Passwords must be between 6 and 40 characters long (no whitespace), cannot contain characters #, &, and must contain:
  • at least one lowercase letter
  • at least one uppercase letter
  • at least one digit

Benefits of a MyAccess Profile:

  • Remote access to the site off-campus on any device
  • Notification of new content via custom alerts
  • Bookmark your favorite content such as chapters, figures, tables, videos, cases and more
  • Save and download images to PowerPoint
  • Self-Assessment quizzes saved for quick review
  • Custom Curriculum access for both instructors and learners

Subscription Options

AccessPhysiotherapy Full Site: One-Year Subscription

Connect to the full suite of AccessPhysiotherapy content and resources including interactive NPTE review, more than 500 videos, Anatomy & Physiology Revealed, 20+ leading textbooks, and more.

$595 USD
Buy Now

Pay Per View: Timed Access to all of AccessPhysiotherapy

24 Hour Subscription $34.95

Buy Now

48 Hour Subscription $54.95

Buy Now

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.