The Respiratory System: Structure, Function, and Physiology

The Respiratory System

Overview and Major Functions

The respiratory system is essential for gas exchange, maintaining homeostasis, and supporting cellular metabolism. Its primary functions include:

• Supplying the body with O2: Oxygen is required for cellular respiration and energy production.

• Disposing of CO2: Carbon dioxide is a metabolic waste product that must be removed to prevent toxicity.

• Regulating pH: The respiratory system helps maintain acid-base balance by controlling CO2 levels in the blood.

Processes of Respiration

Respiration involves four key processes that ensure efficient gas exchange and transport:

1. Pulmonary Ventilation (Breathing): Movement of air into and out of the lungs, driven by pressure changes in the thoracic cavity.

2. External Respiration: Exchange of O2 and CO2 between the lungs and the blood across the alveolar-capillary membrane.

3. Transport of Respiratory Gases: Movement of O2 and CO2 in the blood, primarily via hemoglobin and plasma.

4. Internal Respiration: Exchange of gases between systemic blood and tissue cells, supporting cellular metabolism.

Respiratory System Structure

Anatomical Divisions

The respiratory system is divided into upper and lower regions, each with specialized structures:

• Upper Respiratory System: Includes the nose, nasal cavity, and pharynx. Functions in filtering, warming, and humidifying air.

• Lower Respiratory System: Comprises the larynx, trachea, bronchi, bronchioles, and lungs. Responsible for air conduction and gas exchange.

Functional Zones

• Conducting Zone: Transports air to the respiratory zone; includes all respiratory structures except alveoli. Cleanses, warms, and humidifies air.

• Respiratory Zone: Site of gas exchange; includes respiratory bronchioles, alveolar ducts, and alveoli.

Nose and Nasal Cavity

The nose is the only externally visible part of the respiratory system. Its functions include:

• Providing an airway for respiration

• Moistening and warming incoming air

• Filtering and cleaning inspired air

• Serving as a resonating chamber for speech

• Housing olfactory receptors

Pharynx

The pharynx is a muscular tube that connects the nasal cavity to the larynx and esophagus. It is divided into three regions:

• Nasopharynx: Air passageway posterior to the nasal cavity

• Oropharynx: Passageway for food and air

• Laryngopharynx: Connects to the larynx and esophagus

Larynx

The larynx (voice box) is composed of cartilage and functions in:

• Maintaining an open airway

• Routing air and food into proper channels

• Voice production via vocal cords

Trachea

The trachea (windpipe) is a tube supported by C-shaped cartilage rings, lined with ciliated pseudostratified columnar epithelium and goblet cells. Functions include:

• Conducting air to the bronchi

• Filtering and humidifying air

• Maintaining airway patency

Bronchi and Bronchioles

The bronchial tree consists of branching airways:

• Primary bronchi → Secondary bronchi → Tertiary bronchi → Bronchioles → Terminal bronchioles

• Bronchioles have less cartilage and more smooth muscle, allowing for bronchoconstriction and bronchodilation (regulated by the autonomic nervous system).

Lungs and Alveoli

The lungs are divided into lobes and bronchopulmonary segments, each supplied by its own tertiary bronchus. Alveoli are the primary sites of gas exchange:

• Alveoli are lined with simple squamous epithelium and surrounded by capillaries.

• Type II alveolar cells secrete surfactant to reduce surface tension.

• Alveolar macrophages remove debris and pathogens.

Respiratory Membrane and Gas Exchange

Structure of the Respiratory Membrane

The respiratory membrane consists of:

• Alveolar epithelium

• Fused basement membranes

• Capillary endothelium

This thin barrier allows rapid diffusion of gases.

Gas Exchange Principles

• Gas exchange is driven by partial pressure gradients and diffusion distance.

• Surfactant reduces alveolar surface tension, preventing collapse.

Blood Supply to the Lungs

Pulmonary Circulation

• Low-pressure system for gas exchange

• Pulmonary arteries carry deoxygenated blood to the lungs; pulmonary veins return oxygenated blood to the heart

Bronchial Circulation

• High-pressure system supplying nutrients and oxygen to lung tissue (except alveoli)

Pleural Membranes and Pressure

Pleural Structure

• Parietal pleura: Lines the thoracic cavity

• Visceral pleura: Covers the lungs

• Pleural cavity: Contains pleural fluid, maintains negative pressure to keep lungs inflated

Pleural Pressure

• Intrapleural pressure is always lower than atmospheric and alveolar pressure, preventing lung collapse

• Opposing forces of lung recoil and chest wall expansion maintain negative pressure

Mechanics of Breathing

Inspiration and Expiration

• Diaphragm and intercostal muscles contract to increase thoracic volume, decreasing pressure and drawing air in

• Expiration is usually passive, relying on elastic recoil; forced expiration uses abdominal muscles

Boyle's Law

Pressure and volume are inversely related in a closed container:

$P_1V_1 = P_2V_2$

Lung Volumes and Capacities

Definitions and Measurements

• Tidal Volume (TV): Volume of air inhaled or exhaled per breath (~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

Lung Capacities

• Inspiratory Capacity (IC): $IC = TV + IRV$

• Functional Residual Capacity (FRC): $FRC = ERV + RV$

• Vital Capacity (VC): $VC = IRV + TV + ERV$

• Total Lung Capacity (TLC): Sum of all volumes

Gas Exchange and Transport

Partial Pressures

• Each gas in a mixture exerts its own pressure (Dalton's Law)

• Atmospheric pressure at sea level: 760 mm Hg

• O2: 21% (160 mm Hg), CO2: 0.04% (0.3 mm Hg), N2: 79% (600 mm Hg)

Gas Solubility

• CO2 is more soluble in plasma than O2 or N2

O2 Transport

• Most O2 is carried bound to hemoglobin (Hb) in red blood cells

• O2-hemoglobin dissociation curve shows cooperative binding

CO2 Transport

• CO2 is transported in three forms: dissolved in plasma (7%), bound to hemoglobin (23%), and as bicarbonate ions (70%)

• Conversion to bicarbonate is catalyzed by carbonic anhydrase:

$CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow HCO_3^- + H^+$

Control of Respiration

Neural Regulation

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

• Pontine respiratory group fine-tunes breathing during activities

• Respiratory rate is primarily regulated by CO2 levels (chemoreceptors)

Reflexes and Modulation

• Hering-Breuer reflex prevents overinflation of lungs

• Other factors: pain, temperature, voluntary control, emotional states

Respiratory Diseases and Effects of Smoking

Obstructive Pulmonary Diseases

• Includes chronic bronchitis and emphysema

• Characterized by increased airway resistance, decreased expiratory flow, and increased residual volume

Restrictive Pulmonary Diseases

• Difficulty expanding lungs, decreased lung capacity

• Examples: tuberculosis, pulmonary fibrosis

Effects of Smoking

• Constriction of bronchioles, increased mucus production, impaired ciliary function

• Destruction of alveoli, increased risk of COPD and cancer

Summary Table: Lung Volumes and Capacities

Volume/Capacity	Definition	Typical Value (ml)
Tidal Volume (TV)	Air per breath	500
Inspiratory Reserve Volume (IRV)	Max air after normal inspiration	3100
Expiratory Reserve Volume (ERV)	Max air after normal expiration	1200
Residual Volume (RV)	Air remaining after max exhalation	1200
Vital Capacity (VC)	Max air exhaled after max inhalation	4800
Total Lung Capacity (TLC)	Sum of all volumes	6000

Key Equations

• Boyle's Law: $P_1V_1 = P_2V_2$

• Minute Ventilation: $\text{Minute Ventilation} = \text{Tidal Volume} \times \text{Respiratory Rate}$

• Bicarbonate Formation: $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow HCO_3^- + H^+$

Additional info: This guide expands on the original notes by providing definitions, physiological context, and key equations for exam preparation.