Respiratory Physiology

The respiratory system is best known for breathing, or ventilation. This is not as simple as it sounds as the act of ventilation depends on many other factors, processes, and body systems. Respiration, as many think, is not ventilation. For respiration to take place, there are three separate, but related functions that need to take place: 1) ventilation (breathing); 2) gas exchange (the exchange of oxygen and carbon dioxide), and; 3) oxygen utilization (by cells and tissues of the body). Respiration is further broken down into external and internal respiration. External respiration refers to ventilation and gas exchange, while internal respiration includes gas exchange and oxygen utilization. In this Wiki, however, we are going to focus on ventilation. Ventilation is the first stage of respiration and consists of inhalation and expiration.

Ventilation is the mechanical process that moves air in and out of the lungs. Since the oxygen concentration of air is higher than in the lungs than in the blood, oxygen diffuses from the air to the blood, while carbon dioxide does the reverse by moving from the blood to the air in the lungs, also by diffusion due to the concentration. Ventilation is influenced by physical properties of the lungs, including their compliance, elasticity, surface tension, pressure differences inside the lungs, and by changes in lung volumes. Compliance can be defined as the change in lung volume per change in transpulmonary pressure or more simply, the stretchability or level of distention of the lung. Elasticity refers to the tendency of the lung to return to its initial size after being distended. Surface tension is exerted by fluid in the alveoli (air sacs where gas exchange occurs). Pressure differences inside the thoracic cavity control the movement of air from areas of higher pressure to areas of lower pressure. The act of inhalation results from the contraction of the diaphram, which increases thoracic volume and with the assistance of accessory muscles like the parasternal and external intercostals, scalenes, and pectoralis minor, decreases intrapulmonary pressure. This decrease causes air to flow into the lungs. For expiration, the diaphram, thoracic muscles, thorax, and lungs recoil back into their relaxed positions, thus reducing lung volume and increasing intrapulmonary pressure to force air out of the lungs. The intercostal and abdominal muscles assist with exhalation by depressing the rib cage and forcing the diaphram up.

Notice on the following video how the diaphram and rib cage moves with each breath.

Another important factor in ventilation is Boyle's Law. Boyle's Law states that the pressure of a given quality of gas is inversely proportional to its volume. An increase in lung volume during inspiration decreases intapulmonary pressure to subatmospheric levels, causing air to go in. A decrease in lung volume, conversly, raises the intrapulmonary pressure above that of the atmosphere, expelling the air from the lungs.

As a student nurse I have already seen the importance of effective ventilation and the disease and injury processes that affect it. One of the most common methods used is an incentive spirometer. The incentive spirometer is a simple way of exercising the respiratory system and monitoring the patients progress or lack of one. It also helps to prevent some of the complications that may arise from being bedridden, inactive, and/or ill by ensuring proper oxygenation and keeping things working as they should. Post-operatively it is recommended that a patient do ten sets of inhalations with the spirometer every hour or more often as needed. The patient needs to inhale through the mouth piece in amooth, controlled motion and to monitor how high they can move the plunger. The goal is to see an improvement each time they use the device. Consistent use may reduce the incident of pneumonia, pulmonary embolism, or need for oxygen therapy, among other possibilities.


Fox, Stuart Ira. Human Physiology, Tenth Edition