5 The Respiratory System
Learn how ventilation, gas exchange, gas transport, and breathing control work together to supply oxygen, remove carbon dioxide, and support homeostasis.
The respiratory system's role
The respiratory system supports by bringing oxygen into the body and removing carbon dioxide, a waste product of cellular metabolism. The heart and blood vessels carry these gases between the lungs and tissues.
Respiratory function involves four linked processes: ventilation, gas exchange, gas transport, and control of breathing.
Ventilation moves air
moves air into and out of the lungs. Air flows because changes in chest volume create pressure differences.
During inhalation, the diaphragm contracts and flattens, and the external intercostal muscles lift the ribs. The chest cavity and lungs expand, causing pressure inside the lungs to fall below atmospheric pressure, so air flows in. During quiet exhalation, these muscles relax, the lungs recoil, and pressure inside them rises, moving air out. Forced breathing, such as during exercise, also uses accessory muscles to move air more rapidly.
Not all inhaled air reaches gas-exchanging surfaces. Air remaining in the conducting airways is . —the amount of fresh air reaching the alveoli each minute—is therefore more directly relevant to gas exchange than the total air moved in and out.
Gas exchange in lungs and tissues
Alveoli are tiny air sacs surrounded by pulmonary capillaries. Their thin, moist walls provide a large surface for gases to cross. Oxygen and carbon dioxide move by , from regions of higher partial pressure to regions of lower partial pressure.
In the lungs, oxygen partial pressure is higher in alveolar air than in incoming, oxygen-poor blood, so oxygen diffuses into the blood. Carbon dioxide partial pressure is higher in that blood than in the alveoli, so carbon dioxide diffuses into the alveoli and is exhaled. This exchange is .
At body tissues, the gradients run in the opposite direction. Cells use oxygen and produce carbon dioxide, so oxygen leaves the blood and diffuses into tissues, while carbon dioxide diffuses from tissues into the blood. This exchange is . Effective lung exchange also depends on adequate : air and blood must reach compatible regions of the lung.
How blood transports gases
Only a small amount of oxygen dissolves directly in blood plasma. Most oxygen travels attached to , a protein in red blood cells. In the lungs, where oxygen availability is high, binds oxygen. In tissues, where oxygen availability is lower and carbon dioxide, acidity, and temperature tend to be higher, releases more oxygen. This helps active tissues receive oxygen when their demand rises.
Carbon dioxide travels in blood in three main forms: dissolved in plasma, attached to , and, most commonly, as in plasma. In red blood cells, carbonic anhydrase speeds the reversible reaction:
In tissues, this reaction helps convert carbon dioxide into bicarbonate for transport. In the lungs, it runs in reverse, producing carbon dioxide that diffuses into the alveoli and is exhaled. Because this process affects hydrogen-ion concentration, breathing also contributes to regulation of blood pH.
How breathing is controlled
Breathing is usually automatic. Networks in the generate the basic breathing rhythm, while the helps adjust its timing and pattern. Signals from these centers travel to the diaphragm and other breathing muscles.
The strongest routine chemical influence on breathing is usually rising carbon dioxide. Carbon dioxide entering the fluid surrounding the brain changes its acidity; detect this change and signal the respiratory centers to increase breathing. Faster or deeper ventilation removes more carbon dioxide, helping bring its level and blood pH back toward normal.
in the carotid and aortic bodies respond to changes in blood chemistry, including acidity and oxygen. They become especially important when arterial oxygen falls substantially.
During exercise, working muscles produce more carbon dioxide. Breathing generally becomes faster and deeper, increasing gas exchange and helping meet the body's changing demands. Breathing can also be voluntarily altered for a time, but automatic control continues to respond to the body's needs.
How the processes work together
Ventilation moves air to and from the lungs. exchanges gases between alveoli and blood, and between blood and tissues. carries most oxygen, while most carbon dioxide travels as bicarbonate and is converted back to exhaled carbon dioxide in the lungs.
Brainstem centers adjust breathing chiefly in response to carbon-dioxide-related changes in acidity, while peripheral sensors also respond to low oxygen. Together, these linked processes supply tissues with oxygen, remove carbon dioxide, and help maintain a stable internal environment.