Introduction and overview
The respiratory organs are the site of gas exchange between the organism and the atmosphere (external respiration vs. internal respiration = cellular respiration). Additionally, respiratory organs contribute to voice production.
Inhaled air reaches the lung alveoli through a network of finely branched tubes (the trachea, bronchi and bronchioles). Gas exchange takes place in the alveoli. In the air passages, incoming air is warmed, moistened and filtered. Blood is transported to the lungs through a similarly finely branched network, the pulmonary arteries and their branches. Carbon dioxide, an end product of cellular metabolism, is carried with the blood to the lungs. During respiration, oxygen is absorbed from the air, and then binds with hemoglobin. At the same time, carbon dioxide is excreted. Carbon dioxide in the blood is a component of the bicarbonate buffering system. Thus, respiration influences the body‘s acid-base balance by releasing CO2. The gas exchange between air and blood occurs by diffusion, driven by the differences in partial pressure of the two gases (the difference in the pressure of the gas between the blood and air). Blood does not come into direct contact with the air; they are separated by the blood-air barrier. From the lungs, blood is pumped through the pulmonary veins back to the heart, and from there it reenters the systemic circulation.
A Structure of the air passages
The respiratory system is divided into an upper and lower respiratory tract:
• The upper air passages include
– the external nose and nasal cavity,
– the paranasal sinuses,
– the pharynx (only the upper portion, the nasopharynx, is exclusively a part of the respiratory tract. In the middle portion of the pharynx the respiratory and digestive tracts cross each other).
• The lower air passages include
– the larynx, which serves to temporarily close the air passages during swallowing, and also contributes to voice production;
– the trachea, which divides into the two main bronchi;
– the two main bronchi, which then progressively subdivide;
– the alveoli, located at the end of the network of progressively narrowing tubes. They are the site of gas exchange.
The histology of the different parts of the respiratory tract will be further discussed in the organ chapters.
B Upper air passages: nose, nasal cavity, and pharynx
a Main nasal cavity and pharynx viewed from the right side with the head turned left; b Bony skull, anterior view of the paranasal sinuses. Air is inhaled through the nostrils (nares) into the nasal cavity. It then passes through the posterior aperture of the nose (choana) into the pharynx, and then to the larynx. Narrow openings connect the paranasal sinuses to the main nasal cavity.
Note: In addition to conducting air, the main nasal cavity is also involved in odor perception.
C Lower air passages: larynx and trachea
a Larynx viewed from the right side; b anterior view of the trachea. The larynx marks the entrance to the lower air passages. The epiglottis, which is part of the larynx, can temporarily close the entrance to the airways during swallowing. This helps to prevent food from entering the lower respiratory passageways (which could lead to choking). Additionally, the larynx contributes to voice production. The trachea is the continuation of the larynx. It is located in the neck and thorax and divides into the two main bronchi, which carry air to each lung. Cartilage is an important structural component of the larynx and trachea.
D Lower air passages: bronchial tree and lungs
Anterior view of the bronchial tree and lungs. On the right side, the two main bronchi divide into three lobar bronchi, and on the left side into two lobar bronchi. They further subdivide over several more steps with the final respiratory bronchioles ending in alveoli, where gas exchange takes place. The bronchial tree provides the structural framework of the lungs. Each lung is located in a separate pleural cavity, which is lined by a pleural membrane. The function of the bronchial tree is to conduct air to and from the lungs.
E Breathing mechanics
Anterior view of the lungs (schematic frontal section). The rhythmic activity of the respiratory muscles causes the thoracic cavity to expand (upward, downward, and laterally) and contract. The change in thoracic volume also causes the lungs to rhythmically expand, and then retract due to their elasticity. Thus, the bony and muscular structures of the thoracic wall and diaphragm, which surround the lungs, function like a pair of bellows.
A Development of the respiratory tract from the three germ layers
All three germ layers are involved in the embryonic development of the larynx, trachea, and bronchial tree. A protrusion from the foregut in the area around the esophagus gives rise to the trachea and bronchial tree. While the cartilage, muscle, vessels, and nerves of the larynx are mostly derived from the 4th—6th pharyngeal arches, the laryngeal epithelium is derived from the foregut.