Respiratory System Overview

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. It involves a series of organs and structures that facilitate breathing, protect the airways, and regulate blood pH.

• Primary Functions: Gas exchange, regulation of blood pH, protection from inhaled pathogens, and vocalization.

• Comparison with Cardiovascular System: Both systems involve bulk flow, but the respiratory system moves air, while the cardiovascular system moves blood.

Cellular vs. External Respiration

Definitions and Processes

• Cellular Respiration: The metabolic process by which cells use oxygen to produce ATP, generating carbon dioxide as a waste product.

• External Respiration: The exchange of gases (O2 and CO2) between the atmosphere and the blood, and between the blood and the body tissues.

• Integrated Processes: Ventilation, gas exchange at the lung, gas transport in the blood, and gas exchange at the tissues.

Structure of the Respiratory System

Major Components

• Upper Respiratory Tract: Nose, nasal cavity, pharynx, larynx.

• Lower Respiratory Tract: Trachea, bronchi, bronchioles, alveoli.

• Thoracic Cage: Ribs, sternum, thoracic vertebrae, and associated muscles (e.g., diaphragm, intercostal muscles).

• Pleural Sacs: Each lung is enclosed in a pleural sac containing pleural fluid, which reduces friction and helps keep the lungs inflated.

Bones and Muscles of the Thorax

• Major Bones: Ribs, sternum, thoracic vertebrae.

• Major Muscles: Diaphragm, external and internal intercostal muscles.

• Three Thoracic Sacs: Right pleural sac (right lung), left pleural sac (left lung), pericardial sac (heart).

Pleural Sacs and Pleural Fluid

• Pleura: Double-layered membrane surrounding each lung.

• Pleural Fluid: Lubricates the surfaces, allowing smooth movement during breathing and creating surface tension that keeps the lungs expanded.

• Relationship: The lungs are not directly attached to the thoracic wall but are held against it by the pleural fluid.

Airways and Gas Exchange

Pathway of Air

• Air passes from the nose/mouth → pharynx → larynx → trachea → bronchi → bronchioles → alveoli.

• As airways branch, their diameter decreases, but the total cross-sectional area increases.

• Velocity of Air Flow: Highest in the trachea, lowest in the alveoli.

Airway Functions

• Warming, Humidifying, Filtering: Airways warm and humidify air to protect delicate alveolar tissue and filter out particles using mucus and cilia.

Alveoli

• Structure: Tiny air sacs at the end of bronchioles, surrounded by capillaries.

• Cell Types: Type I alveolar cells (gas exchange), Type II alveolar cells (produce surfactant).

• Function: Site of gas exchange between air and blood.

Pulmonary Circulation

Characteristics

• High-Flow, Low-Pressure: Pulmonary circulation receives the entire cardiac output at a lower pressure than systemic circulation.

• Blood Volume: Only a portion of pulmonary blood is involved in gas exchange at any moment.

• Average Pulmonary Arterial Pressure: Approximately 25/8 mmHg (systolic/diastolic).

• Effect on Capillary Exchange: Lower pressure reduces fluid filtration into alveoli, preventing pulmonary edema.

Gas Laws in Respiration

Air as a Mixture of Gases

• Partial Pressure: The pressure exerted by a single gas in a mixture. Total pressure is the sum of partial pressures.

• Humidity: Water vapor reduces the partial pressures of other gases in inspired air.

Pressure Gradients

• Gases move from areas of higher partial pressure to lower partial pressure.

Boyle's Law

• Definition: The pressure of a gas is inversely proportional to its volume at constant temperature.

• Application: Changes in thoracic volume during breathing create pressure gradients that drive air flow.

Ventilation

Definitions

• Ventilation: The movement of air into and out of the lungs.

• Respiratory Cycle: One inspiration plus one expiration.

• Spirometer: Device used to measure lung volumes and capacities.

Lung Volumes and Capacities

• Tidal Volume (TV): Volume of air inhaled or exhaled in a normal breath.

• Inspiratory Reserve Volume (IRV): Additional air that can be inhaled after a normal inspiration.

• Expiratory Reserve Volume (ERV): Additional air that can be exhaled after a normal expiration.

• Residual Volume (RV): Air remaining in the lungs after maximal exhalation.

• Capacity: The sum of two or more lung volumes (e.g., vital capacity = TV + IRV + ERV).

Mechanics of Breathing

Muscles Involved

• Quiet Breathing: Diaphragm and external intercostal muscles.

• Forced Breathing: Involves accessory muscles such as internal intercostals and abdominal muscles.

Pressure-Flow-Resistance Relationship

• Equation:

• Air flows from high to low pressure; resistance is mainly determined by airway diameter.

Inspiration and Expiration

• Inspiration: Thoracic volume increases, alveolar pressure decreases, air flows in.

• Expiration: Thoracic volume decreases, alveolar pressure increases, air flows out.

• Passive Expiration: Due to elastic recoil of lungs and thoracic cage.

• Active Expiration: Uses internal intercostals and abdominal muscles.

Intrapleural Pressure

• Definition: Pressure within the pleural cavity, normally subatmospheric to keep lungs inflated.

• Pneumothorax: Puncture of the pleural membrane equalizes pressure, causing lung collapse.

Lung Compliance and Elastance

• Compliance: The ability of the lung to stretch (change in volume per change in pressure).

• Elastance: The ability of the lung to return to its original shape after stretching.

• High Compliance: Lungs stretch easily (e.g., emphysema).

• Low Compliance: Lungs are stiff (e.g., fibrosis).

• High Elastance: Lungs recoil easily; low elastance means poor recoil.

• Diseases: Conditions with reduced compliance are called restrictive lung diseases.

Surfactant and Work of Breathing

• Surface Tension: Created by the thin fluid layer lining the alveoli; opposes lung expansion.

• Surfactant: A substance produced by Type II alveolar cells that reduces surface tension, making it easier to inflate the lungs.

• Law of LaPlace: , where P = pressure, T = surface tension, r = radius.

• Smaller alveoli would have higher pressure without surfactant; surfactant equalizes pressure among alveoli.

• Premature Infants: May lack surfactant, leading to respiratory distress syndrome.

Airway Resistance

• Determinants: Airway diameter (most significant), length, viscosity of air.

• Poisuelle's Law: , where R = resistance, η = viscosity, L = length, r = radius.

• Highest resistance is usually in medium-sized bronchi.

• Bronchioles are the site of variable resistance, affected by smooth muscle tone.

• Factors Affecting Bronchial Tone: CO2 (dilates), histamine (constricts), parasympathetic (constricts), sympathetic (dilates), epinephrine (dilates).

Ventilation Efficiency

• Total Pulmonary Ventilation: Total volume of air entering or leaving the lungs per minute.

• Alveolar Ventilation: Volume of fresh air reaching the alveoli per minute; a more accurate measure of gas exchange efficiency.

• Calculation:

Alveolar Gas Composition

• Partial pressures of O2 and CO2 in alveoli remain relatively constant during normal breathing.

• Hyperventilation increases alveolar O2 and decreases CO2; hypoventilation has the opposite effect.

Ventilation-Perfusion Matching

• Ventilation (air flow) is matched to perfusion (blood flow) in the lungs for optimal gas exchange.

• Pulmonary capillaries can constrict or dilate in response to local O2 and CO2 levels to direct blood flow to well-ventilated areas.

• Low alveolar O2 causes vasoconstriction; high O2 causes vasodilation.

Assessment of Pulmonary Function

• Auscultation: Listening to breath sounds with a stethoscope to detect abnormalities.

• Spirometry: Measures lung volumes and air flow to diagnose obstructive and restrictive lung diseases.

• Obstructive Diseases: Conditions where air flow is diminished (e.g., asthma, chronic bronchitis, emphysema).