The Respiratory System: Structure, Function, and Physiology

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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 that ensure efficient ventilation and gas transport.

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 between the lungs and tissues.

Anatomy of the Respiratory System

The respiratory tract is divided into the conducting zone and the respiratory zone.

Conducting Zone: Passages that serve as conduits for air movement but do not participate in gas exchange. Includes the nose, pharynx, larynx, trachea, bronchi, and terminal bronchioles.
Respiratory Zone: Structures where gas exchange occurs. Includes respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

Major Respiratory Organs

Nose: Filters, warms, and moistens incoming air; contains olfactory receptors.
Nasopharynx: Upper part of the pharynx, serves as an air passageway.
Larynx: Voice box; routes air and food into proper channels and houses vocal cords.
Trachea: Windpipe; conducts air to the bronchi.
Bronchi and Bronchioles: Branching airways leading to the lungs; bronchioles are smaller branches lacking cartilage.
Alveolar Ducts and Sacs: Terminal ends of the respiratory tree; sites of gas exchange.
Lungs: Paired organs containing alveoli for gas exchange.
Pleurae: Double-layered membranes surrounding each lung, reducing friction during breathing.

The 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 diffusion efficiency.

Mechanics of Ventilation and Boyle’s Law

Ventilation depends on pressure gradients created by changes in thoracic volume, as described by Boyle’s Law.

Boyle’s Law: (at constant temperature, pressure and volume are inversely related).
Atmospheric Pressure (Patm): Pressure exerted by air outside the body (~760 mmHg at sea level).
Intrapulmonary Pressure (Ppul): Pressure within alveoli; equalizes with atmospheric pressure between breaths.
Intrapleural Pressure (Pip): Pressure within pleural cavity; always negative relative to Ppul to prevent lung collapse.
Air flows into lungs when Ppul < Patm; out when Ppul > Patm.

Physical Factors Influencing Ventilation

Pressure Gradients: Drive airflow; created by changes in thoracic volume.
Airway Resistance: Opposition to airflow; increased by constriction or obstruction.
Lung Compliance: Ease with which lungs expand; decreased in fibrosis, increased in emphysema.
Lung Elasticity: Ability of lungs to recoil after stretching.
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
Minute Respiratory Volume (MRV)	TV × respiratory rate
Forced Vital Capacity (FVC)	Maximal air exhaled forcefully after maximal inhalation
Forced Expiratory Volume (FEV)	Volume exhaled in a specific time during FVC test

Gas Exchange and Gas Laws

External Respiration: Occurs in lungs; O2 diffuses into blood, CO2 diffuses into alveoli.
Internal Respiration: Occurs in tissues; O2 diffuses into cells, CO2 into blood.
Factors Affecting Diffusion: Partial pressure gradients, membrane thickness, surface area, and gas solubility.
Dalton’s Law: (total pressure is sum of partial pressures).
Henry’s Law: (amount of gas dissolved in liquid is proportional to its partial pressure and solubility).

Oxygen Transport and Hemoglobin Affinity

O2 Transport: Mostly bound to hemoglobin (Hb) in red blood cells; small amount dissolved in plasma.
Hemoglobin Affinity: Influenced by pO2, pCO2, pH, temperature, and 2,3-BPG.
Bohr Effect: Increased CO2 or decreased pH reduces Hb affinity for O2, enhancing O2 unloading in tissues.

Carbon Dioxide Transport and the Haldane Effect

CO2 Transport: Dissolved in plasma (7%), bound to Hb as carbaminohemoglobin (23%), or as bicarbonate ions (70%).
Effect on pH: CO2 + H2O → H2CO3 → H+ + HCO3-; increased CO2 lowers plasma pH.
Haldane Effect: Deoxygenated blood can carry more CO2 as carbaminohemoglobin.

Control of Ventilation

Medullary Respiratory Centers: Set basic rhythm of breathing.
Pons: Modifies and smooths respiratory rhythm.
Pulmonary Reflexes: Include stretch and irritant reflexes.
Hypothalamus: Alters breathing in response to emotions and temperature.
Chemical Factors: pH, pO2, and pCO2 detected by chemoreceptors regulate ventilation rate.
Cerebral Cortex: Allows voluntary control of breathing.

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

Respiratory System Disorders

Emphysema: Destruction of alveolar walls, loss of elasticity, reduced surface area for gas exchange.
Asthma: Chronic inflammation and constriction of airways, leading to reversible airflow obstruction.
Tuberculosis: Infectious disease caused by Mycobacterium tuberculosis; forms nodules in lungs.
Lung Cancer: Uncontrolled growth of abnormal cells in lung tissue; leading cause of cancer death.
Cystic Fibrosis: Genetic disorder causing thick, sticky mucus that obstructs airways and increases infection risk.

Example: In emphysema, decreased lung elasticity and increased compliance make exhalation difficult, leading to air trapping and hyperinflation of the lungs.

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