Ch 12 respiration

Functions of the Respiratory System

• The respiratory system allows gases from the environment to enter the bronchial tree through inspiration by expanding the thoracic volume.
• It allows gas exchange to occur at the respiratory membrane, so that oxygen diffuses into the blood while carbon dioxide diffuses into the bronchial tree.
• It permits gases in the lungs to be eliminated through expiration by decreasing the thoracic volume.

General Anatomy of the Respiratory System

• The respiratory system consists of a tube that divides into small branching tubes in the lungs.
• The pathway from the external nares to the alveoli is: external nares → nasal cavity → nasopharynx → laryngopharynx → larynx → trachea → primary bronchi → lungs (secondary bronchi → tertiary bronchi → bronchioles → alveolar sacs → alveoli).

Lungs

• The lungs are cone-shaped organs located in the thoracic cavity.
• The thoracic cavity is lined with a body membrane called parietal pleura, while the surface of the lungs is covered with visceral pleura.
• The thin space between the two pleural membranes is called the pleural cavity, which is filled with a clear fluid called plural fluid to minimize friction between the tissues and to provide surface tension in the pleural cavity.
• A chemical substance called surfactant secreted by the lungs also facilitates the surface tension.

Inspiration (Inhalation)

• Inspiration is an active process where nerve impulses from the medulla oblongata cause the contraction of the diaphragm and external intercostals muscles.
• As these muscles contract, thoracic volume increases, which decreases the pressure within the lung (intraalveolar pressure) due to Boyle's Law.
• When intraalveolar pressure falls below atmospheric pressure, gases move from the environment into the lungs.

Expiration (Exhalation)

• Expiration is a passive process where elastic tissues of the lungs and diaphragm recoil to their original position.
• As the diaphragm and external intercostals muscles relax and recoil, thoracic volume decreases, which raises the intraalveolar pressure due to Boyle's Law.
• When intraalveolar pressure rises above atmospheric pressure, gases move from the lungs into the environment.

Pulmonary Ventilation

• Pulmonary ventilation is the mechanism by which air is exchanged between the atmosphere and the alveoli.
• Air is exchanged due to the expansion and contraction of the lungs.
• Contraction of the diaphragm pulls down, enlarging the intrapleural cavity, and elevation of the ribs also expands the intrapleural cavity.
• These factors decrease the intrapleural cavity pressure, thus air flows into the lungs (inspiration).

Lung Capacities

• Lung capacities can be measured by a spirometer, providing valuable information regarding the normal function of the respiratory system.

Control of Breathing

• Four major factors affect normal breathing: stretching in the lungs and thoracic walls, O2 level in the blood, CO2 level in the blood, and H+ level in the blood.
• Normal breathing is inhibited by stretching of the lungs and thoracic walls, a rise in O2 level, and a decrease in CO2 and H+ levels; while normal breathing is stimulated by relaxing of the lungs and thoracic walls, a decrease in O2 level, and a rise in CO2 and H+ levels.
• Chemicals and emotional state also affect breathing.

Respiratory Centers

• Normal breathing is a rhythmic, involuntary action regulated by the respiratory centers in the pons and medulla oblongata of the brain stem.
• Rhythmicity area in the medulla oblongata sets the basic rhythm of inspiration and expiration, and is subdivided into the dorsal respiratory group and the ventral respiratory group.

Chemoreceptors

• Central chemoreceptors are associated with the respiratory centers.
• CO2 combines with water to form carbonic acid, which in turn, releases H+ ions in CSF.
• Stimulation of these areas increases alveolar ventilation.
• Peripheral chemoreceptors are in the carotid bodies and aortic bodies, sensing low O2 concentration, and when O2 concentration is low, alveolar ventilation increases.