7 The Respiratory System

The respiratory system

The respiratory system brings oxygen $(O_2)$ into the body and removes carbon dioxide $(CO_2)$, a waste product of cellular metabolism. Its airways conduct and condition inhaled air, the lungs provide surfaces for gas exchange, and the blood transports respiratory gases between the lungs and body tissues.

The respiratory tract

Air travels through a branching passageway from the nose or mouth through the pharynx, larynx, trachea, main bronchi, smaller bronchi, and bronchioles, ending at the alveoli.

The conducting zone extends from the nose and mouth through most bronchioles. It moves air but does not itself exchange gases; it also warms, humidifies, and filters incoming air. Mucus traps particles, and cilia move the mucus toward the throat, where it can be swallowed or expelled.

The respiratory zone includes respiratory bronchioles and alveoli. Gas exchange occurs in this region.

Lungs and alveoli

The lungs are paired organs in the chest, protected by the rib cage and resting on the diaphragm. The right lung has three lobes; the left has two, leaving room for the heart.

Each lung is covered by the visceral pleura, while the parietal pleura lines the chest wall. A small amount of fluid between these membranes reduces friction as the lungs move.

The airways branch into millions of tiny alveoli. Their thin walls lie next to pulmonary capillaries, creating the respiratory membrane. Its large surface area and short diffusion distance support rapid gas exchange.

Pulmonary ventilation

Pulmonary ventilation is the movement of air into and out of the lungs. Air flows from higher pressure to lower pressure, so changing the volume of the chest changes pressure in the lungs.

During inhalation, the diaphragm contracts and flattens, and external intercostal muscles lift the ribs. The chest cavity expands, lung pressure falls below atmospheric pressure, and air flows in.

During quiet exhalation, the diaphragm and external intercostals relax. Elastic recoil reduces lung volume, pressure rises, and air flows out. Exhalation is usually passive at rest; forceful breathing recruits additional muscles.

Not all inhaled air reaches gas-exchanging surfaces. Air remaining in the conducting passages is anatomical dead space. Consequently, the volume of air reaching the alveoli each minute, called alveolar ventilation, is lower than total ventilation.

Gas exchange

Gas exchange occurs by diffusion: each gas moves down its own partial-pressure gradient. In the lungs, oxygen partial pressure is higher in alveolar air than in incoming pulmonary blood, so $O_2$ diffuses into the blood. Carbon dioxide partial pressure is higher in that blood than in the alveoli, so $CO_2$ diffuses into the alveoli and is exhaled. This exchange between alveoli and blood is external respiration.

In body tissues, the gradients are reversed. Cells use oxygen and produce carbon dioxide, so $O_2$ diffuses from blood into tissues while $CO_2$ diffuses from tissues into blood. This exchange is internal respiration.

Efficient gas exchange depends on both adequate ventilation, meaning air reaches the alveoli, and perfusion, meaning blood reaches their capillaries.

Transport of respiratory gases

Most oxygen travels in red blood cells bound reversibly to hemoglobin, a protein containing iron. A small fraction is dissolved in plasma. In the lungs, oxygen binds to hemoglobin; in tissues, where oxygen pressure is lower, it dissociates from hemoglobin and diffuses into cells.

Carbon dioxide travels in blood in three forms: mostly as bicarbonate ions in plasma, with smaller amounts bound to hemoglobin and dissolved in plasma. In red blood cells, much of the $CO_2$ is converted to bicarbonate through reactions accelerated by the enzyme carbonic anhydrase. In the lungs, these reactions reverse: bicarbonate is converted back to $CO_2$, which diffuses into the alveoli and is exhaled.