Overview

The conducting portion or upper airway consists of the oral airway, nasal cavities, pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles.

The conducting portion cleans and humidifies inspired air and provides conduits for air movement to and from alveoli. A combination of cartilage, collagen and elastic fibers, and smooth muscle provides the conducting portion with rigid structural support and the necessary

The respiratory portion, where the system’s main function of gas exchange occurs, consisting of respiratory bronchioles, alveolar ducts, and alveoli.

Function

+++++++++++++++++++++++++++++

The airways have three main functions:

The primary function of the respiratory system is the uptake of O₂ and removal of CO₂ from the body.

Other functions:

- To protect the lungs from the entry of any foreign matter;

- To control the heat and humidity of the gases.

+++++++++++++++++

Oxygen Transport

There are five major mechanisms of oxygen transport from the outside world to its final destination in end-organ tissues that utilize oxygen for fuel:

Ventilation is the mechanical process of drawing air into the airways and alveoli via negative pressure created by active expansion of the chest wall.
Alveolar diffusion is the transfer of oxygen molecules across the alveolar cell membrane, interstitium, and pulmonary capillary membrane into the pulmonary circulation.
Binding is a function carried out by hemoglobin in red blood cells whereby four oxygen molecules are bound to a hemoglobin moiety.
Circulation refers to cardiac output that propels oxygenated blood to the peripheral capillary tissue beds.
Peripheral diffusion occurs in oxygen-utilizing tissues as oxygen molecules are off-loaded from saturated hemoglobin molecules across capillary membranes and interstitial tissue into cells that utilize oxygen.

There is a limit to oxygen utilization and thus a maximum workload achievable before oxygen utilization in peripheral tissues surpasses oxygen delivery. At this point, anaerobic metabolism comes in to play and its byproduct lactic acid begins to accumulate in peripheral tissues. This is referred to as the anaerobic threshold and corresponds with maximal oxygen consumption. Of the five steps of oxygen transport, cardiac output is the rate-limiting factor that determines maximal oxygen consumption in a healthy person.