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05. Respiratory System in Human Body
The respiratory system consists of the set of organs involved in the uptake of oxygen from
the atmosphere and the release of carbon dioxide generated
during aerobic respiration. This gas exchange is also called
breathing or external respiration. Organs specialized for
breathing usually contain moist structures with large surface
areas to allow the diffusion of gases. They are also adapted to
protect the organism from the invasion of pathogens along
those surfaces.
In fish, this gas exchange occurs through gills. Some
invertebrates, like cockroaches, have simple respiratory
systems made of interconnecting tubules directly delivering
oxygen to tissues. In humans and other mammals, there is an
Figure 5.1 (Respiratory System)
extensive, highly vascularized organ system specialized for
gas exchange. It begins in the nose, continues into the pharynx
and larynx, leads to the trachea which branches to create bronchi, and bronchioles. This
respiratory tree ends in puffy structures called alveoli that are made of a single layer of
squamous cells, surrounded by a network of capillaries. Gas exchange occurs at alveoli.
Since external respiration in many vertebrates involves lungs, it is also called pulmonary
ventilation. Changes to the volume and pressure in lungs are the primary driving forces for
breathing.
Functions of the Respiratory System
While the primary function of the respiratory system is gas exchange, this extensive organ
system also has some other roles. In humans and other mammals, the respiratory system is
integral towards phonation. Structures of the upper respiratory tract, especially the larynx,
are involved in the production of sound, and can modulate pitch, volume and clarity.
The nose plays an important role in respiration, but the olfactory nerves and their associated
structures are also involved in sensing smell. This has functions ranging from digestion (the
cephalic phase of digestion) to hunting, recognition and mating.
The cells of the respiratory tract also protect the body from the invasion of pathogens
through the nasal passages. They have an important role in the immune system since the
respiratory tract is one of the organ systems with intensive and repeated interaction with the
environment (the other is the digestive system). Epithelial cells in the airway can secrete
antibodies, defenses and various enzymes and peptides, as well as small oxidative molecules
that hamper pathogenic colonization. In addition, some of these epithelial cells also secrete
mucus to trap larger dust particles. The respiratory system plays host to a specialized
lymphoid tissue that can produce lymphocytes as a first-line of defense. Coughing and
sneezing are other important mechanisms used to fight infections, removing large quantities
of bacteria or viruses trapped in mucus.