The Respiratory System: Physiology Overview

Introduction

The respiratory system is essential for gas exchange, providing oxygen for cellular respiration and removing carbon dioxide waste. This process is vital for ATP production, which powers cellular activities throughout the body.

Processes of Respiration

Main Steps in Respiration

• Pulmonary Ventilation (Breathing): The movement of air into and out of the lungs, driven by pressure gradients.

• External Respiration: Gas exchange between alveoli and blood in pulmonary capillaries; involves ventilation-perfusion (V/Q) matching.

• Transport of Respiratory Gases: The cardiovascular system transports oxygen (O2) and carbon dioxide (CO2) between the lungs and tissues, primarily via hemoglobin saturation.

• Internal Respiration: Gas exchange between systemic capillaries and body cells, facilitating oxygen delivery and carbon dioxide removal.

Gas Laws Relevant to Respiration

Key Physical Principles

• Boyle's Law: The pressure of a gas is inversely proportional to its volume at constant temperature. Equation:

• Dalton's Law: The total pressure of a gas mixture equals the sum of the partial pressures of each component gas. Equation:

• Henry's Law: The amount of gas dissolved in a liquid is proportional to its partial pressure and solubility. Equation: Where C = concentration, k = solubility constant, P = partial pressure

Mechanics of Breathing

Inspiration and Expiration

• Inspiration (Inhalation): An active process where the diaphragm and external intercostal muscles contract, increasing thoracic cavity volume and decreasing intrapulmonary pressure (), causing air to flow into the lungs.

• Expiration (Exhalation): Usually a passive process where the diaphragm and intercostals relax, decreasing thoracic volume and increasing intrapulmonary pressure (), pushing air out.

Pressure Relationships

• Atmospheric Pressure (Patm): Pressure exerted by air surrounding the body (760 mm Hg at sea level).

• Intrapulmonary Pressure (Ppul): Pressure within the alveoli; equalizes with atmospheric pressure between breaths.

• Intrapleural Pressure (Pip): Pressure within the pleural cavity; always negative relative to Ppul to prevent lung collapse.

• Transpulmonary Pressure: Difference between Ppul and Pip; keeps lungs inflated. Equation:

Clinical Note: Pneumothorax

• Occurs when air enters the pleural cavity, causing lung collapse due to loss of negative intrapleural pressure.

Nonrespiratory Air Movements

• Include coughing, sneezing, crying, laughing, hiccups, and yawning.

• Often reflexive, but can be voluntary.

Physical Factors Influencing Pulmonary Ventilation

• Airway Resistance: Primarily in medium-sized bronchi; increased resistance (e.g., asthma) makes breathing more difficult.

• Alveolar Surface Tension: Surfactant reduces surface tension, preventing alveolar collapse.

• Lung Compliance: Measure of lung "stretchability." Lower compliance increases the work of breathing (e.g., fibrosis, old age).

Respiratory Volumes and Capacities

Key Definitions

• Tidal Volume (TV): Air exchanged during normal breathing (~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 (~20% of total lung capacity).

• Vital Capacity (VC): Maximum air exhaled after maximal inhalation. Equation:

• Total Lung Capacity (TLC): Total volume of air in lungs after maximal inhalation. Equation:

• Functional Residual Capacity (FRC): Volume of air remaining after normal expiration; marker of lung and chest wall compliance.

Dead Space

• Anatomical Dead Space: Air in conducting passages (trachea, bronchi) not involved in gas exchange (~150 mL).

• Alveolar Dead Space: Nonfunctional alveoli due to collapse or obstruction.

• Total Dead Space: Sum of anatomical and alveolar dead space.

Measurement of Pulmonary Function

Spirometry

• Measures ventilation rate and lung volumes.

• Forced Expiratory Volume (FEV1): Volume exhaled in the first second of forced expiration.

• Forced Vital Capacity (FVC): Total volume exhaled forcefully after deep breath.

• FEV1/FVC Ratio: Normal > 0.7; lower values suggest obstructive disease (e.g., asthma, COPD).

Pulmonary Function Test Table

Alveolar Ventilation

• Minute Ventilation: Total gas flow in/out of respiratory tract per minute (~6 L/min at rest).

• Alveolar Ventilation Rate (AVR): Volume of air reaching alveoli per minute; better indicator of effective ventilation.

• AVR is affected by tidal volume and frequency; increasing TV is more effective than increasing frequency.

External Respiration: Gas Exchange

Factors Affecting Gas Exchange

• Surface area and structure of respiratory membrane (large in healthy lungs).

• Partial pressure gradients (Dalton's Law) and gas solubility (Henry's Law).

• Ventilation-perfusion (V/Q) coupling: Matching of alveolar ventilation and pulmonary blood flow for optimal exchange.

Dalton's Law Example

• Oxygen makes up 20.9% of air:

Transport of Gases

Oxygen Transport

• 98.5% of O2 is carried by hemoglobin (Hb); 1.5% dissolved in plasma.

• O2-Hb binding is influenced by local O2 concentration, pH, temperature, CO2, and 2,3-BPG.

• Bohr Effect: Increased CO2 or decreased pH weakens Hb-O2 bond, enhancing O2 unloading in tissues.

Carbon Dioxide Transport

• Transported in three forms:

  1. Dissolved in plasma (~7-10%)

  2. Bound to hemoglobin as carbaminohemoglobin (~20%)

  3. As bicarbonate ion (HCO3-) in plasma (~70%)

• Carbonic anhydrase catalyzes conversion of CO2 and H2O to H2CO3, which dissociates to HCO3- and H+:

• Haldane Effect: Deoxygenated blood can carry more CO2.

Control of Respiration

Neural Regulation

• Medullary respiratory centers set basic rhythm (dorsal and ventral respiratory groups).

• Pontine centers (pneumotaxic and apneustic) modify rhythm for smooth breathing.

• Higher brain centers and chemoreceptors (central and peripheral) adjust rate and depth in response to CO2, O2, and pH changes.

Respiratory Adjustments

Exercise

• Ventilation increases to meet metabolic demands (hyperpnea), but arterial O2 and CO2 remain relatively constant.

Altitude

• Lower atmospheric pressure and O2 at high altitudes can cause acute mountain sickness and chronic adaptations (increased erythropoietin and RBCs).

Respiratory Disorders and Diseases

Obstructive vs. Restrictive Diseases

• Obstructive: Increased airway resistance (e.g., asthma, COPD, bronchitis); difficulty expiring air.

• Restrictive: Reduced lung expansion (e.g., fibrosis, tuberculosis); decreased lung volumes.

Common Disorders

• Asthma: Acute, reversible airway inflammation and bronchospasm.

• Chronic Obstructive Pulmonary Disease (COPD): Irreversible airflow limitation, often due to smoking; includes emphysema and chronic bronchitis.

• Tuberculosis (TB): Infectious disease caused by Mycobacterium tuberculosis.

• Lung Cancer: Leading cause of cancer death; most cases linked to smoking.

• Cystic Fibrosis (CF): Genetic disorder causing thick mucus and frequent infections.

• Sleep Apnea: Temporary cessation of breathing during sleep; can be obstructive or central in origin.

Developmental Aspects

• Respiratory structures develop early in embryogenesis; functional alveoli present by week 28 of gestation.

• At birth, lungs inflate and begin gas exchange; respiratory rate is highest in newborns and decreases with age.

• Respiratory efficiency declines in old age due to decreased compliance and elasticity.