Respiratory System

Functions of the Respiratory System

The respiratory system is essential for gas exchange and maintaining homeostasis. It performs several key functions:

• Gas Exchange: Provides a large surface area for the exchange of oxygen (O2) and carbon dioxide (CO2) between air and blood.

• Air Movement: Moves air to and from the exchange surfaces of the lungs via respiratory passageways.

• Protection: Protects respiratory surfaces from dehydration, temperature changes, and pathogens.

• Sound Production: Produces sounds for communication (speaking, singing).

• Olfaction: Detects odors via olfactory receptors in the nasal cavity.

Upper vs. Lower Respiratory System

The respiratory system is divided into upper and lower regions, each with distinct structures and functions.

• Upper Respiratory System: Filters, warms, and humidifies inhaled air. Includes:

  • Nose: Airway for breathing; moistens, warms, filters, and cleans air; houses olfactory receptors; serves as a resonating chamber for speech.

  • Nasal Cavity: Contains paranasal sinuses; meatuses (narrow grooves) swirl air; vascularization warms and humidifies air; nosebleeds are common due to extensive blood supply.

  • Pharynx: Shared by digestive and respiratory systems; divided into:

    • Nasopharynx: Connected to nasal cavity; separated from oral cavity by soft palate; contains pharyngeal tonsil; auditory tube opens here.

    • Oropharynx: Between soft palate and base of tongue; connects to oral cavity.

    • Laryngopharynx: Inferior part; between hyoid bone and entrance to larynx/esophagus.

• Lower Respiratory System: Conducts air and facilitates gas exchange. Includes:

  • Larynx: Voice box; glottis produces sound; thyroid and cricoid cartilages protect glottis; epiglottis prevents food/liquids from entering airway during swallowing; corniculate and arytenoid cartilages control glottis; laryngitis is inflammation/infection.

  • Trachea: Windpipe; tough, flexible tube (~1 inch diameter, ~4.3 inches length); 15-20 tracheal cartilages prevent collapse; anular ligament and trachealis muscle connect cartilage ends; sympathetic stimulation increases diameter for greater airflow.

  • Bronchi & Bronchioles: Air-conducting passageways leading to lungs; bronchial tree branches into primary, secondary, and segmental bronchi; right lung has three lobes, left has two; bronchopulmonary segments (right: 10, left: 8-9); cartilage decreases and smooth muscle increases as airways branch; bronchitis is inflammation; bronchioles control resistance to airflow (sympathetic: bronchodilation, parasympathetic: bronchoconstriction); asthma is excessive bronchoconstriction.

  • Alveoli: Air-filled pockets for gas exchange; respiratory bronchioles connect to alveoli via alveolar ducts; alveolar sacs contain ~150 million alveoli per lung; type I pneumocytes (squamous epithelial cells) are sites of gas exchange; type II pneumocytes produce surfactant (reduces surface tension, prevents alveolar collapse); alveolar macrophages engulf debris; respiratory distress syndrome is caused by insufficient surfactant; blood-air barrier consists of alveolar cell layer, capillary endothelial layer, and fused basement membrane; pneumonia compromises blood-air barrier.

Conducting vs. Respiratory Portions

The respiratory tract is divided into conducting and respiratory portions:

• Conducting Portion: Entrance of nasal cavity through pharynx, larynx, trachea, bronchi, and larger bronchioles; moves air but does not participate in gas exchange.

• Respiratory Portion: Smallest, thinnest respiratory bronchioles and associated alveoli; site of gas exchange.

Respiratory Mucosa

The respiratory mucosa lines the conducting portion and provides filtration and protection.

• Mucosa: Filters debris and pathogens from inhaled air.

• Cilia: Move mucus toward pharynx, cleaning surfaces.

• Nasal Cavity Filtration: Removes nearly all particles from inhaled air.

• Mucus Production: Increases with exposure to dust, allergens, or pathogens.

Anatomy of the Lungs

The lungs are housed in pleural cavities and covered by serous membranes.

• Pleural Cavities: Separated by mediastinum; each contains a lung; lined with pleura (parietal: lines thoracic wall, visceral: covers lungs); pleural fluid lubricates space between layers.

• Right Lung: Wider, displaced upward by liver.

• Left Lung: Longer, indented medially (cardiac notch).

External and Internal Respiration

Respiration involves both external and internal processes for gas exchange.

• External Respiration: Exchange of O2 and CO2 with the external environment; abnormalities include hypoxia (low O2 levels) and anoxia (complete lack of O2).

• Internal Respiration: Uptake of O2 and release of CO2 by cells; result of cellular respiration.

• Boyle’s Law: Relationship between gas pressure and volume:

• Air flows from higher to lower pressure.

• Respiratory cycle: inspiration (muscles contract), expiration (muscles relax).

• Muscles used in inhalation: diaphragm (75% of air movement), external intercostals (25%), accessory muscles (sternocleidomastoid, scalenes, pectoralis minor, serratus anterior).

• Muscles used in exhalation: internal intercostals, transversus thoracis, abdominal muscles.

• Pneumothorax: air enters pleural cavity, causing atelectasis (collapsed lung).

Pulmonary Volumes and Respiratory Capacities

Several volumes and capacities describe lung function:

• Tidal Volume (VT): Air moved in/out per breath.

• Expiratory Reserve Volume (ERV): Additional air exhaled after normal exhalation.

• Residual Volume: Air remaining after maximal exhalation.

• Inspiratory Reserve Volume (IRV): Additional air inhaled after normal inhalation.

Gas Laws: Dalton’s Law and Henry’s Law

Gas exchange is governed by physical laws:

• Dalton’s Law: Each gas in a mixture contributes to total pressure proportionally to its abundance.

• Partial Pressure: Pressure of a single gas in a mixture.

• Henry’s Law: At a given temperature, the amount of gas dissolved in a liquid is proportional to its partial pressure.

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

Gas Exchange: External and Internal Respiration

• External Respiration: Blood in pulmonary arteries has low PO2 and high PCO2; O2 enters blood, CO2 leaves; rapid exchange reaches equilibrium.

• Internal Respiration: Oxygenated blood mixes with deoxygenated blood; PO2 entering systemic circuit ~95 mm Hg; interstitial fluid PO2 40 mm Hg, PCO2 45 mm Hg; CO2 diffuses into blood, O2 diffuses out.

Oxygen–Hemoglobin Saturation Curve

Hemoglobin’s affinity for oxygen changes with pH and temperature.

• Standardized for pH 7.4, 37ºC.

• Lower pH or higher temperature: more O2 released (curve shifts right).

• Higher pH or lower temperature: less O2 released (curve shifts left).

Hemoglobin and pH: The Bohr Effect

The Bohr effect describes how pH affects hemoglobin’s oxygen binding.

• CO2 diffuses into RBCs; carbonic anhydrase catalyzes reaction with H2O:

• Hydrogen ions lower pH, causing hemoglobin to release more O2.

Hemoglobin and Temperature

• Increased temperature: hemoglobin releases more O2 (important in active tissues).

• Decreased temperature: hemoglobin holds O2 more tightly.

• Example: Active skeletal muscles generate heat, promoting O2 release.

Carbon Dioxide Transport

CO2 is transported in blood by three mechanisms:

• Converted to carbonic acid.

• Bound to hemoglobin within RBCs.

• Dissolved in plasma.

Respiratory Centers of the Brain and SIDS

Respiratory control is managed by centers in the brain.

• Voluntary Centers: Located in cerebral cortex; affect respiratory centers in pons and medulla oblongata; control motor neurons for respiratory muscles.

• Sudden Infant Death Syndrome (SIDS): Disruption in normal respiratory reflex pattern; may result from connection problems between pacemaker complex and respiratory centers.