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Respiratory System: Structure,
Function, and Disorders
Physiology of the Respiratory System
Functions of the Respiratory System
Provides O2 to all body tissues Removes CO Regulates plasma pH by controlling [CO2] and [HCO3-] Enables communication through sounds and speech Defends against pathogens Supplies O2 for the metabolism of glucose and synthesis of ATP
Functional Organization of the Respiratory Tract
Conducting Zone: - Nasal cavity - Nose - Nasopharynx - Larynx - Trachea - Bronchi - Terminal bronchioles
Functions: - Saturate inspired air with water vapor - Remove bacteria and other particulate matter
Respiratory Zone: - Respiratory bronchioles - Alveoli
Components of the Respiratory Tract
Nasal Cavity: - Warms, moistens, and filters incoming air - Receives olfactory stimuli - Modifies speech sounds
Larynx: - Maintains a patent airway, but is closed during swallowing to prevent aspiration - Also closed when intrathoracic pressure increases during cough reflex and defecation
Gas Exchange
Occurs at the blood-air interface by simple diffusion of O2 and CO For maximum gas exchange, the respiratory surfaces must have: Large surface area (70-140 m2) Small diffusion distance (< 1 μm) Moist exchange surface (water and surfactant) The bronchial tree enables a large surface area due to approximately 23 bifurcations (trachea → bronchi → bronchioles) Gas exchange occurs from the respiratory bronchioles onwards (respiratory bronchioles → alveolar ducts → alveolar sacs) The alveolar organization enables a small diffusion distance (0.1-0. μm) between the capillary endothelium, basement membrane, and alveolar epithelium
Red blood cells spend approximately 1 second in the capillaries, passing through two-thirds of the alveoli Gas exchange is the diffusion of O2 and CO2 across the respiratory membrane, which is the external respiration
Pulmonary Ventilation
The physical movement of air in and out of the respiratory tract, ensuring adequate alveolar ventilation and preventing CO2 buildup A component of external respiration, involving the exchange of O2 and CO2 between the interstitial fluid and the external environment Internal respiration is the exchange of O2 and CO2 between the interstitial fluid and the cells, occurring between the systemic capillaries and tissues Relies on the physical principles governing air movement, as described by Boyle's Law
Inspiration: - Respiratory muscles contract, increasing thoracic volume - Intrathoracic pressure decreases (Pa > Pi), and air is drawn in
Expiration: - Respiratory muscles relax, decreasing thoracic volume - Intrathoracic pressure increases (Pa < Pi), and air is expelled
Tidal volume is the amount of air moved in and out of the lungs per breath, approximately 500 mL The respiratory system adapts to changing O2 demands by varying the number of breaths per minute and the volume of air moved per breath
Assessing Lung Function
Breath sounds can indicate the presence of mucus/fluid or the absence of breath sounds, which may suggest a collapsed lung Pulmonary function tests: Peak flow meter measures the speed at which a person can breathe out, used by chronic asthmatics Spirometer measures the amount of air entering and leaving the lungs, producing a spirogram that shows respiratory volumes and capacities Functional residual capacity (FRC) is the amount of air left in the lungs after normal tidal expiration, helping to stabilize the composition of alveolar air
Respiratory Volumes
Tidal Volume (TV): Volume of air inhaled/exhaled in one quiet breath Expiratory Reserve Volume (ERV): Amount of air forcibly exhaled after normal tidal expiration Inspiratory Reserve Volume (IRV): Amount of air forcibly inhaled after normal tidal inhalation Residual Volume (RV): Air remaining in the lungs after maximum expiration
Henry's Law: The amount of gas that dissolves in a liquid is proportional to the partial pressure of the gas in the gas phase
Hemoglobin
Hemoglobin (Hb) is a globular protein with 4 subunits (2α + 2β), each containing a heme group with a Fe2+ ion Hb binds O2 rapidly and reversibly, with each Hb molecule binding 4 O2 molecules Approximately 97% of O2 is transported in the blood bound to hemoglobin Hb saturation is the percentage of Hb molecules with bound O2, affected by PO2, pH, temperature, and the state of O2 binding Cooperative binding of O2 to Hb results in a sigmoidal O2-Hb dissociation curve O2 unloading in tissues is enhanced by decreased pH, increased CO2, increased temperature, and increased 2,3-DPG
Carbon Dioxide Transport
CO2 is generated by aerobic metabolism in peripheral tissues and transported in the blood in three ways: Carbonic acid formation (70%) Binding to hemoglobin as carbaminohemoglobin (23%) Dissolved in plasma (7%) The conversion of CO2 to HCO3- produces H+, which is buffered to minimize pH changes The Haldane effect aids the unloading of CO2 in the lungs and its transport from the tissues
Control of Breathing
Establishes an automatic rhythm and adjusts the rhythm to accommodate metabolic, mechanical, and episodic non-ventilatory demands Under normal conditions, O2 absorption and CO2 generation rates are matched by changes in blood flow and depth/rate of respiration No single pacemaker generates the basic rhythm of breathing, and no single muscle is devoted to pumping air
Local Control of Gas Transport
Increased blood flow to active tissues in response to decreased PO2 and increased PCO Vasoconstriction in the lungs in response to decreased PO2, directing blood to areas of higher PO Bronchodilation in response to increased PCO2, directing airflow to areas of higher PCO
Central Control of Ventilation
Aims to keep arterial PCO2 and PO2 as constant as possible Involves sensors (central and peripheral chemoreceptors, mechanoreceptors), a central controller (respiratory centers in the pons and medulla), and effectors (muscles of ventilation) Factors influencing the rate and depth of breathing include changing body demands, altitude, disease, and changing levels of CO2, H+, and O
Chemoreceptors
Central chemoreceptors in the medulla are stimulated by changes in CSF pH and PCO Peripheral chemoreceptors (carotid and aortic bodies) detect changes in PO2, PCO2, and pH in the arterial blood Peripheral chemoreceptors are vital for the response to a fall in PO
Respiratory Physiology and Diseases
Gas Exchange and pH Regulation
In emphysema, limited gas exchange leads to an increase in PCO2, which pushes the equation CO2 + H2O ➀ H2CO3 ➀ H+ + HCO3- to the right, resulting in a drop in pH (respiratory acidosis). Conversely, if PCO2 decreases (e.g., in hyperventilation), the equation shifts to the left, leading to an increase in pH (respiratory alkalosis). Increased PO2 (e.g., breathing O2-rich gas mixtures) can generate free radicals and lead to coma or death. Decreased PO2: Arterial PO2 must fall below 60 mmHg before ventilation increases, and central chemoreceptors switch off while peripheral chemoreceptors increase the breathing rate.
Mechanical Control of Respiration
Three types of mechanoreceptors in the lung tissue and airways: Slowly adapting (Hering-Breuer reflex) Rapidly adapting (cough reflex) C-fiber endings (defense mechanism) All three are innervated by the vagus nerve. Slowly adapting stretch receptors (bronchopulmonary stretch receptors) inhibit the respiratory centers when the lungs are over- inflated (Hering-Breuer reflex). Rapidly adapting stretch receptors (irritant receptors) in the airway epithelium respond to noxious stimuli, leading to bronchoconstriction and the cough reflex.
