SH

Respiratory System and Fluids & Electrolytes

Respiratory System Functions

  • Gas Exchange: Transport O2 to cells for cellular respiration, remove waste CO2 (exhalation).

  • Air Passageway: Conduction of air.

  • Olfaction: Detection of odors (olfactory epithelium in nasal cavity).

  • Vocalization: Sound production (larynx).

Respiratory Mucosa

  • Epithelium: Primarily pseudostratified ciliated columnar epithelium (PCCE) from nasal cavity to lobar bronchi.

    • Goblet Cells: Secrete mucin, which becomes mucus when mixed with water.

    • Cilia: Propel mucus (mucus escalator) towards esophagus/stomach, trapping debris and microorganisms.

  • Alveoli: Simple squamous epithelium for thin gas exchange barrier; no mucus layer.

  • Mucus: Traps inhaled particles (pollen, allergens, microorganisms, dust); contains lysozymes, defensins, and IgA (immune defense).

    • Daily production: $1$ to $7$ tablespoons, increases with illness/allergies.

Air Conditioning (Nasal Cavity)

  • Warming: Superficial blood vessels in mucosa.

  • Cleansing: Mucus and cilia trap particles.

  • Humidifying: Mucus adds moisture.

  • Turbinates (Conchae): Swirl air, increasing surface area for conditioning.

Larynx Functions

  • Sound Production (Phonation): True vocal cords vibrate during exhalation across glottis.

  • Protection: Epiglottis closes airway during swallowing to prevent food/liquid entry.

  • Other: Increases abdominal-pelvic pressure, generates coughing/sneezing reflexes.

Vocalization in the Larynx

  • Vocal Folds: Vestibular (false, protection), True (sound production).

  • Epiglottis: Elastic cartilage, bends to cover glottis.

  • Voice Characteristics:

    • Length of Vocal Folds: Determines range (males post-puberty develop longer folds resulting in deeper voices).

    • Pitch: Determined by tension of vocal folds (increased tension = higher pitch).

    • Loudness: Determined by amount of air forced over vocal folds.

  • Laryngitis: Inflammation of vocal folds, impairing vibration (causes: overuse, infection). Epiglottitis is a serious, potentially lethal inflammation of the epiglottis.

Bronchioles & Alveoli

  • Bronchioles: Smaller diameter, no cartilage, possess more smooth muscle for bronchoconstriction/dilation (regulating airflow).

  • Alveoli: Primary site of gas exchange, thin simple squamous epithelium, covered by pulmonary capillaries.

  • Respiratory Membrane: Combined alveolar and capillary epithelia, must be thin for efficient gas exchange.

  • Surfactant: A lipid substance (phospholipid) produced by Type II alveolar cells. Reduces surface tension inside alveoli, preventing their collapse.

    • Begins production around fetal month 7; premature birth before this can lead to respiratory distress.

Pulmonary Ventilation (Breathing)

  • Definition: Movement of gas from one place to another; first step of respiration.

  • Types:

    • Eupneia: Normal, quiet breathing at rest.

    • Forced Breathing: Deep breaths (voluntary or involuntary, e.g., exercise).

  • Underlying Principle: Boyle's Law – inverse relationship between volume and pressure (when temperature is constant).

  • Mechanism:

    1. Nervous System Stimulus: Initiates muscle action.

    2. Skeletal Muscle Movement: Contraction/relaxation of respiratory muscles.

    3. Volume Change: Alters thoracic cavity volume.

    4. Pressure Change: Alters intrapulmonary pressure (due to Boyle's Law).

    5. Pressure Gradient: Creates difference between atmospheric and intrapulmonary pressure.

    6. Air Movement: Air flows down the pressure gradient (from high to low pressure).

Inhalation (Quiet)

  1. Muscle Contraction: Diaphragm contracts (moves inferiorly), external intercostals contract.

  2. Volume Change: Thoracic cavity volume increases.

  3. Pressure Change: Intrapulmonary pressure decreases (e.g., from 760 mmHg atmospheric to 758 mmHg).

  4. Air Movement: Air flows into lungs from atmosphere.

Exhalation (Quiet)

  1. Muscle Relaxation: Diaphragm and external intercostals relax.

  2. Volume Change: Thoracic cavity volume decreases (elastic recoil of lungs).

  3. Pressure Change: Intrapulmonary pressure increases (e.g., from 760 mmHg atmospheric to 762 mmHg).

  4. Air Movement: Air flows out of lungs to atmosphere.

Key Pressures

  • Atmospheric Pressure: Constant reference (e.g., 760 mmHg at sea level).

  • Intrapulmonary Pressure: Pressure within the alveoli; equalizes with atmospheric pressure at rest.

  • Intrapleural Pressure: Pressure within the pleural cavity; always approximately 4 mmHg lower than intrapulmonary pressure to prevent lung collapse (atelectasis). A puncture can change this, leading to lung collapse.