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 that 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 between the lungs and tissues.

Example: During exercise, increased tissue demand for O2 accelerates all these processes.

Anatomy of the Respiratory Tract

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

• Conducting Zone: Passages that transport air but do not participate in gas exchange (nose, pharynx, larynx, trachea, bronchi, bronchioles).

• Respiratory Zone: Sites of gas exchange (respiratory bronchioles, alveolar ducts, alveolar sacs, alveoli).

Major Respiratory Organs

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

• Nasopharynx: Upper part of the pharynx, passageway for air from the nasal cavity.

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

• Trachea: Windpipe; conducts air to bronchi, lined with ciliated mucosa.

• Bronchi and Bronchioles: Branching airways leading to alveoli; bronchioles are smaller and lack cartilage.

• Alveolar Ducts and Sacs: Terminal airways leading to clusters of alveoli where gas exchange occurs.

• Lungs: Paired organs containing bronchial tree and alveoli; right lung has three lobes, left has two.

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

The Respiratory Membrane

The respiratory membrane is the thin barrier between alveolar air and blood in pulmonary capillaries, facilitating rapid gas diffusion.

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

• Thinness and large surface area enhance diffusion efficiency.

Example: Pulmonary edema thickens the membrane, impairing gas exchange.

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, the pressure of a gas varies inversely with its volume.

$P_1 V_1 = P_2 V_2$

• 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.

• Air flows into lungs when Ppul < Patm, and out when Ppul > Patm.

Example: If intrapulmonary pressure drops to 758 mmHg, air enters the lungs.

Physical Factors Affecting Ventilation

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

• Airway Resistance: Resistance to airflow, mainly in bronchioles; increased by constriction (e.g., asthma).

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

• Lung Elasticity: Ability of lungs to recoil after stretching.

• Alveolar Surface Tension: Reduced by surfactant, preventing alveolar collapse.

Lung Volumes and Capacities

Lung volumes and capacities are measured to assess respiratory function.

Term	Definition
Tidal Volume (TV)	Volume 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)	TV × respiratory rate
Forced Vital Capacity (FVC)	Maximal air exhaled forcefully after maximal inspiration
Forced Expiratory Volume (FEV)	Volume exhaled in a specific time during FVC test

Gas Exchange and Gas Laws

Gas exchange occurs by diffusion, driven by partial pressure gradients and governed by physical laws.

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

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

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

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

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

$C = kP$

• Factors affecting diffusion: partial pressure gradients, membrane thickness, surface area, and gas solubility.

Oxygen Transport and Hemoglobin Affinity

• O2 Transport: Mostly bound to hemoglobin (Hb) in red blood cells; a 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 release to tissues.

Carbon Dioxide Transport and the Haldane Effect

• CO2 Transport: Dissolved in plasma (7%), as bicarbonate ions (70%), and bound to hemoglobin (23%).

• Plasma CO2 and pH: CO2 forms carbonic acid, lowering blood pH.

$\mathrm{CO}_2 + \mathrm{H}_2O \leftrightarrow \mathrm{H}_2CO_3 \leftrightarrow \mathrm{H}^+ + \mathrm{HCO}_3^-$

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

Regulation of Ventilation

• Medullary Respiratory Centers: Set basic rhythm of breathing.

• Pons: Modifies and smooths respiratory rhythm.

• Pulmonary Reflexes: Include stretch and irritant reflexes that adjust breathing.

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

• Chemical Factors: pCO2 (most potent), pO2, and pH detected by chemoreceptors.

• 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 (e.g., during exercise)

Respiratory System Disorders

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

• Asthma: Reversible airway constriction and inflammation, causing wheezing and dyspnea.

• Tuberculosis: Infectious disease causing granuloma formation and lung tissue destruction.

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

• Cystic Fibrosis: Genetic disorder causing thick, sticky mucus that obstructs airways and increases infection risk.