The Respiratory System: Structure, Function, and Regulation

The Respiratory System

Overview and Functions

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. It involves several processes and specialized structures to ensure efficient ventilation and respiration.

• Pulmonary Ventilation: The movement of air into and out of the lungs (breathing).

• External Respiration: Gas exchange between the air in the alveoli and the blood in pulmonary capillaries.

• Internal Respiration: Gas exchange between systemic blood and tissues.

• Blood Gas Transport: The movement of O2 and CO2 in the blood to and from tissues.

Respiratory Tract Zones

The respiratory tract is divided into two main zones based on function:

• Conducting Zone: Passages that carry air to the respiratory zone; includes the nose, pharynx, larynx, trachea, bronchi, and most bronchioles.

• Respiratory Zone: Sites of gas exchange; includes respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

Structure and Function of Respiratory Organs

• Nose: Filters, warms, and moistens incoming air; houses olfactory receptors.

• Nasopharynx: Upper part of the pharynx; passageway for air only.

• Larynx: Voice box; routes air and food, houses vocal cords.

• Trachea: Windpipe; conducts air to bronchi, supported by C-shaped cartilage rings.

• Bronchi: Main airways branching from trachea into lungs.

• Bronchioles: Smaller branches of bronchi; regulate airflow with smooth muscle.

• Alveolar Ducts and Sacs: Terminal passages leading to clusters of alveoli.

• Lungs: Paired organs containing bronchial tree and alveoli; site of gas exchange.

• Pleurae: Double-layered serous membranes (parietal and visceral) surrounding lungs, reducing friction.

Respiratory Membrane

The respiratory membrane is a thin barrier (0.5 μm) facilitating rapid gas diffusion between alveolar air and blood.

• Composed of alveolar epithelium, fused basement membranes, and capillary endothelium.

• Thinness and large surface area enhance gas exchange efficiency.

Mechanics of Ventilation and Boyle’s Law

Ventilation depends on pressure changes in the thoracic cavity, governed by Boyle’s Law:

• Boyle’s Law: The pressure of a gas is inversely proportional to its volume at constant temperature.

$P_1 V_1 = P_2 V_2$

• Inspiration: Thoracic volume increases, intrapulmonary pressure drops below atmospheric pressure, air flows in.

• Expiration: Thoracic volume decreases, intrapulmonary pressure rises above atmospheric pressure, air flows out.

• Intrapleural Pressure: Always less than intrapulmonary and atmospheric pressure to prevent lung collapse.

Physical Factors Influencing Ventilation

• Pressure Gradients: Drive airflow; greater gradients increase ventilation.

• Airway Resistance: Resistance in airways; increased by constriction or obstruction.

• Lung Compliance: Ease of lung expansion; decreased by fibrosis or surfactant deficiency.

• Lung Elasticity: Ability to recoil after stretching; essential for expiration.

• Alveolar Surface Tension: Reduced by surfactant to prevent alveolar collapse.

Lung Volumes and Capacities

Lung volumes and capacities are measured to assess respiratory function.

Term	Definition
Tidal Volume (TV)	Amount of air inhaled or exhaled in a normal breath (~500 mL)
Inspiratory Reserve Volume (IRV)	Additional air inhaled after a normal inspiration
Expiratory Reserve Volume (ERV)	Additional air exhaled after a normal expiration
Residual Volume (RV)	Air remaining in lungs after maximal exhalation
Inspiratory Capacity (IC)	TV + IRV
Functional Residual Capacity (FRC)	ERV + RV
Vital Capacity (VC)	TV + IRV + ERV
Total Lung Capacity (TLC)	TV + IRV + ERV + RV
Dead Space	Air not involved in gas exchange (anatomic and physiologic)
Minute Respiratory Volume (MRV)	Total air moved per minute (TV × respiratory rate)
Forced Vital Capacity (FVC)	Maximal exhalation after maximal inhalation
Forced Expiratory Volume (FEV)	Volume exhaled in a specific time during FVC

Gas Exchange and Gas Laws

• External Respiration: O2 diffuses from alveoli to blood; CO2 diffuses from blood to alveoli.

• Internal Respiration: O2 diffuses from blood to tissues; CO2 from tissues to blood.

• Factors Affecting Diffusion: Partial pressure gradients, membrane thickness, surface area, and solubility.

• Dalton’s Law: Total pressure of a gas mixture equals the sum of partial pressures of each gas.

$P_{total} = P_1 + P_2 + P_3 + \ldots$

• Henry’s Law: The amount of gas dissolved in a liquid is proportional to its partial pressure and solubility.

$C = kP$

• Where C = concentration of dissolved gas, k = solubility constant, P = partial pressure.

Oxygen Transport and Hemoglobin Affinity

• O2 Transport: Mostly bound to hemoglobin (Hb) in red blood cells; small amount dissolved in plasma.

• Hemoglobin Affinity: Affected by pO2, pCO2, pH, temperature, and 2,3-BPG.

• Bohr Effect: Increased CO2 or decreased pH reduces Hb affinity for O2, enhancing O2 release to tissues.

Carbon Dioxide Transport and Plasma pH

• CO2 Transport: Dissolved in plasma, bound to Hb (carbaminohemoglobin), or as bicarbonate ions (HCO3-).

• Plasma CO2 and pH: CO2 reacts with water to form carbonic acid, which dissociates to H+ and HCO3-, affecting pH.

• Haldane Effect: Deoxygenated blood can carry more CO2 as carbaminohemoglobin.

$\mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \leftrightarrow \mathrm{H}_2\mathrm{CO}_3 \leftrightarrow \mathrm{H}^+ + \mathrm{HCO}_3^-$

Regulation of Ventilation

• Medullary Respiratory Centers: Control basic rhythm of breathing.

• Pons: Modifies and smooths respiratory rhythm.

• Pulmonary Reflexes: Include stretch and irritant reflexes affecting breathing pattern.

• Hypothalamus: Alters breathing in response to emotions and temperature.

• Chemical Factors: pH, pO2, and pCO2 levels detected by chemoreceptors regulate ventilation rate.

• Cerebral Cortex: Allows voluntary control of breathing.

Term	Definition
Eupnea	Normal, quiet breathing
Tachypnea	Rapid breathing
Dyspnea	Labored or difficult breathing
Apnea	Temporary cessation of breathing
Hyperpnea	Increased depth and rate of breathing (usually during exercise)

Respiratory System Disorders

• Emphysema: Destruction of alveolar walls, loss of elasticity, reduced gas exchange.

• Asthma: Chronic inflammation and constriction of airways, reversible airflow obstruction.

• Tuberculosis: Infectious disease causing granuloma formation in lungs.

• Lung Cancer: Malignant growths in lung tissue, often linked to smoking.

• Cystic Fibrosis: Genetic disorder causing thick, sticky mucus, leading to airway obstruction and infections.