Respiratory System Overview

Key Homeostatic Variable

• Primary: Oxygen (O2) levels
• Main concern: Adequate O2 in the blood
• Lungs exhale CO2, but primary task is maintaining PaO2

Components of Respiratory System

• Lungs, chest wall, pulmonary blood vessels
• Chemoreceptors: Sense concentrations of O2 and CO2
• Processes: Ventilation (air flow), diffusion (gas exchange), perfusion (blood flow)

Ventilation (VE)

• Minute ventilation: Volume of air moved in and out per minute (liters/min)
• Formula: Tidal volume (Vt) x Respiration rate (RR)

Gas Exchange

• Thin membrane (~1 micrometer)
• Layers: Surfactant, alveolar lining, interstitial fluid, pulmonary capillaries
• Increased diffusion time can indicate disease (e.g., chronic bronchitis)

Gas Laws

• Boyle's Law: Gas pressure inversely proportional to volume
• Dalton’s Law: Total pressure = Sum of partial pressures of each gas
• Henry’s Law: Gas dissolved in liquid proportional to partial pressure

Breathing Mechanics

• Involves pressure gradients: High to low pressure
• Requires compliant (elastic) chest wall
• Diseases: Emphysema (loss of elasticity), asthma (high resistance)

Lung Volumes and Capacities

• Tidal Volume (TV): Regular inhalation/exhalation
• Inspiratory Reserve Volume (IRV): Max additional air inhaled
• Expiratory Reserve Volume (ERV): Max additional air exhaled
• Residual Volume (RV): Air remaining after maximal exhalation
• Vital Capacity (VC): TV + IRV + ERV
• Total Lung Capacity (TLC): VC + RV

Pulmonary Function Tests (PFT)

• Forced Expiratory Volume (FEV1): Amount of air expelled in 1 second
• Indicators for obstructive (COPD, asthma) and restrictive diseases

Airway Anatomy

• Conducting Zone: Trachea, bronchi, terminal bronchioles
• Respiratory Zone: Alveolar ducts, alveolar sacs for gas exchange

Effects of pH and CO2 on Breathing

• Hypoventilation: Increased CO2, respiratory acidosis
• Hyperventilation: Decreased CO2, respiratory alkalosis
• Regulation of breathing maintains constant CO2 and blood pH

Brain Control of Breathing

• Medulla oblongata: Main control center
• Voluntary control: Cerebral cortex
• Involuntary control: Medullary respiratory centers

Receptor Types

• Stretch receptors: Monitor lung expansion
• Irritant receptors: Trigger coughing
• Chemoreceptors: Monitor O2, CO2, pH levels

Pulmonary Blood Flow

• Right ventricle pumps to pulmonary system
• Low pulmonary pressure (MAP ~10 mm Hg)
• Blood flow greater at lung bases due to gravity

Pulmonary Vascular Resistance (PVR)

• Low resistance allows easy blood flow
• Affected by lung volumes – higher resistance at low/high volumes

Regional Differences in Perfusion

• Zone 1 (Apex): Ventilation without perfusion (dead space)
• Zone 2 (Middle): Limited blood flow
• Zone 3 (Base): Ventilated and perfused (best gas exchange)

Oxygen and Carbon Dioxide Transport

• Hemoglobin: Main carrier for O2, can carry 4 O2 molecules
• O2 saturation (pulse oximetry): Measures oxygen bound to hemoglobin

Physiological Effects

• Temperature: Increased temperature -> increased O2 unloading
• pH (Bohr Effect): Lower pH -> increased O2 unloading

CO2 Transport

• Dissolved in plasma (5%)
• Bound to hemoglobin (5%)
• Bicarbonate ion (90%)
• Chloride shift: CO2 conversion to bicarb in tissues, reverse shift in lungs for exhalation