Respiratory Physiology

The primary function of the lungs is to conduct gas exchange. Typically, a healthy adult breathes at a rate of approximately 12-18 breaths per minute, at rest. Fresh oxygen (O2) is brought in during inhalation and stale carbon dioxide (CO2) is brought out during exhalation. This process occurs deep within the lungs, in an area known as the blood-gas interface barrier.

Here, small air sacs called alveoli allow for proper diffusion of oxygen and carbon dioxide to and from the blood. This diffusion process, also known as pulmonary gas exchange, is made possible due to a difference in high and low partial pressure, similar to how a river flows downhill. The human lungs contain 300-500 million alveoli that are dedicated to this crucial gas exchange mechanism.

In the clinical setting, however, patients with respiratory diseases such as COPD (Chronic Obstructive Pulmonary Disease) experience inefficient pulmonary gas exchange due to structural damage within their lungs. This results in abnormal gas measurements (PO2, PCO2 typically measured in mmHg), and difficulty breathing.  Furthermore, oxygen and carbon dioxide levels change as they cascade downward (PO2) and upward (PCO2) through the respiratory tract.

This situation necessitates different gas measurements at various parts of the respiratory system for clinical applications.

Oxygen Cascades Downward from Mouth to Lungs to Tissue

Pulmonary gas exchange is a vast topic that requires an in-depth understanding of respiratory physiology to appreciate all the intricacies associated with it. As we age, it becomes crucial to measure the gas exchange process, especially in those with lung disease.

This process occurs autonomously in healthy lungs, but occurs inefficiently in diseased lungs.  It is crucial that this process is measured by calculations with precise gas values using advanced oxygen and carbon dioxide sensors. In clinical practice, gas-based indices are compared at different points in the body to source the origin of various causes of gas exchange impairment. The values of oxygen and carbon dioxide in healthy lung tissue (specifically in the alveoli) will be close to that of blood in the arteries.

Gas levels are named according to the phase of the respiratory cycle and the site of measurement. For example, “alveolar” gas values are referred to as “end-tidal” values, and refer to the amount of gases left in the lungs at the end of normal exhalation. This distinction in nomenclature provides an important understanding of respiratory medicine. These breath-based metrics can be obtained simultaneously by the non-invasive MediPines AGM100®   An advanced respirtory monitoring system that measures pulmonary gas exchange non-invasively.

The gas exchange process itself begins with oxygen and carbon dioxide moving from the air and into the human body.  At sea level, dry atmospheric air (760 mmHg pressure) is composed of 21% oxygen (159 mmHg), 79% nitrogen, and trace amounts of carbon dioxide. As air moves through the upper airways during inspiration, it is saturated and humidified with water vapor, the partial pressure of which at body temperature, pressure, saturated (BTPS) is 47 mmHg. As such, the oxygen partial pressure (PO2) in inspired air is 149 mmHg [21% of (760 – 47) mmHg] and represented as PiO2. In alveolar air, carbon dioxide from gas exchange mixes with the incoming air.1


The human airway is divided into two regions; the conducting zone and the respiratory zone. The conducting zone moves air in and out of the lungs. This consists of the upper and lower airways: oral and nasal cavity → large airways (trachea – bronchi) → small airways (bronchioles). The respiratory zone is the main region of gas exchange and consists of the following: respiratory bronchioles → alveolar ducts  → alveoli. Air in this zone portion is composed of oxygen, nitrogen, water vapor, and carbon dioxide.Therefore, precise measurement (mmHg) of Oxygen (PO2) and CO2 (PCO2) levels within the respiratory zone (the lungs and arterial blood vessels) are vital in assessing pulmonary gas exchange efficiency (or inefficiency).

1.West, J.B. “Respiratory Physiology: The Essentials”