Respiratory system

• The respiratory system consists of the respiratory and conducting zones.
• The respiratory zone is the site of gas exchange and consists of bronchioles, alveolar ducts, and alveoli.
• The conducting zone provides rigid conduits for air to reach the sites of gas exchange and includes all other respiratory structures such as the nose, nasal cavity, pharynx, and trachea.
• Respiratory muscles include the diaphragm and other muscles that promote ventilation.
• The major functions of the respiratory system are to supply the body with oxygen and dispose of carbon dioxide.
• Respiration consists of four distinct processes: pulmonary ventilation, external respiration, transport, and internal respiration.
• Pulmonary ventilation is the process of moving air into and out of the lungs.
• Olfactory placodes invaginate into olfactory pits by the 4th week.
• Laryngotracheal buds are present by the 5th week.
• Mucosae of the bronchi and lung alveoli are present by the 8th week.
• By the 28th week, a baby born prematurely can breathe on its own.
• During fetal life, the lungs are filled with fluid and blood bypasses the lungs.
• Gas exchange takes place via the placenta.
• At birth, respiratory centers are activated, alveoli inflate, and lungs begin to function.
• Respiratory rate is highest in newborns and slows until adulthood.
• Lungs continue to mature and more alveoli are formed until young adulthood.
• Respiratory efficiency decreases in old age.
• External respiration is the process of gas exchange between the lungs and the blood.
• Transport is the transport of oxygen and carbon dioxide between the lungs and tissues.
• Internal respiration is the process of gas exchange between systemic blood vessels and tissues.
• The nose is the only externally visible part of the respiratory system that functions by providing an airway for respiration, moistening (humidifying) and warming the entering air, filtering inspired air and cleaning it of foreign matter, serving as a resonating chamber for speech, and housing the olfactory receptors.
• The structure of the nose includes the external nose, which includes the root, bridge, dorsum nasi, and apex, and the internal nasal cavity.
• The nasal cavity lies in and posterior to the external nose, is divided by a midline nasal septum, opens posteriorly into the nasal pharynx via internal nares, and is lined with respiratory mucosa.
• The functions of the nasal mucosa and conchae are to filter, heat, and moisten air during inhalation and reclaim heat and moisture during exhalation.
• Paranasal sinuses are sinuses in bones that surround the nasal cavity.
• The pharynx is a funnel-shaped tube of skeletal muscle that connects to the nasal cavity and mouth superiorly, the larynx and esophagus inferiorly, and extends from the base of the skull to the level of the sixth cervical vertebra.
• Control of Respiration: Medullary Respiratory Centers include the dorsal respiratory group (DRG), or inspiratory center, located near the root of nerve IX, which appears to be the pacesetting respiratory center, exciting the inspiratory muscles and setting eupnea (12-15 breaths/minute), and the ventral respiratory group (VRG) involved in forced inspiration and expiration.
• Pons Centers influence and modify activity of the medullary centers, smooth out inspiration and expiration transitions and vice versa.
• The pontine respiratory group (PRG) continuously inhibits the inspiration center.
• Respiratory Rhythm is a result of reciprocal inhibition of the interconnected neuronal networks in the medulla.
• Inspiratory depth is determined by how actively the respiratory center stimulates the respiratory muscles, and rate of respiration is determined by how long the inspiratory center is active.
• Respiratory centers in the pons and medulla are sensitive to both excitatory and inhibitory stimuli.
• Pulmonary irritant reflexes promote reflexive constriction of air passages in response to irritants, and the inflation reflex (Hering-Breuer) is stimulated by lung inflation, causing inhibitory signals to be sent to the medullary inspiration center to end inhalation and allow expiration.
• Hypothalamic controls act through the limbic system to modify rate and depth of respiration, for example, breath holding that occurs in anger, and a rise in body temperature acts to increase respiratory rate.
• Cortical controls are direct signals from the cerebral motor cortex that bypass medullary controls, for example, voluntary breath holding, taking a deep breath.
• Changing PCO2 levels are monitored by chemoreceptors of the brain stem, and carbon dioxide in the blood diffuses into the cerebrospinal fluid where it is hydrated, resulting in increased depth and rate of breathing (hypercapnia).
• Hyperventilation is an increased depth and rate of breathing that quickly flushes carbon dioxide from the blood and occurs in response to hypercapnia.
• Hypoventilation is slow and shallow breathing due to abnormally low PCO2 levels, and apnea (breathing cessation) may occur until PCO2 levels rise.
• Arterial oxygen levels are monitored by the aortic and carotid bodies, and substantial drops in arterial PO2 (to 60 mm Hg) are needed before oxygen levels become a major stimulus for increased ventilation.
• If carbon dioxide is not removed (e.g., as in emphysema and chronic bronchitis), chemoreceptors become unresponsive to PCO2 chemical stimuli, and in such cases, PO2 levels become the principal respiratory stimulus (hypoxic drive).