Study Guide: The Respiratory System (Chapter 22, Human Anatomy & Physiology)

The Respiratory System: Overview

Major Functions and Processes

The respiratory system is essential for gas exchange, supplying oxygen to cells for cellular respiration and removing carbon dioxide. It works closely with the cardiovascular system to accomplish four key processes collectively known as respiration:

• Pulmonary ventilation: Movement of air into and out of the lungs (breathing).

• Pulmonary gas exchange: Exchange of gases (O2 and CO2) between lungs and blood.

• Transport of respiratory gases: Movement of gases in the blood between lungs and tissues.

• Tissue gas exchange: Exchange of gases between blood and body tissues.

Functional Anatomy of the Respiratory System

Major Organs and Divisions

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

• Upper respiratory system: Nose, paranasal sinuses, pharynx

• Lower respiratory system: Larynx, trachea, bronchi (and branches), lungs, alveoli

• Respiratory muscles: Classified as part of the muscular system (e.g., diaphragm, intercostals)

The Nose and Paranasal Sinuses

The nose is the only externally visible part of the respiratory system and serves several functions:

• Provides an airway for respiration

• Moistens, warms, filters, and cleans inspired air

• Serves as a resonating chamber for speech

• Houses olfactory receptors

The nose is divided into the external nose and the nasal cavity. The nasal cavity is lined with mucous membranes, including olfactory mucosa (for smell) and respiratory mucosa (for air cleaning and warming).

The Pharynx

The pharynx (throat) connects the nasal cavity to the larynx and mouth to the esophagus. It is divided into three regions:

• Nasopharynx: Posterior to nasal cavity, serves only as an airway

• Oropharynx: Posterior to oral cavity, passageway for food and air

• Laryngopharynx: Posterior to larynx, passageway for food and air

The Larynx

The larynx (voice box) provides an open airway, routes air and food, and houses the vocal folds for voice production. Its framework consists of nine cartilages, including the thyroid (Adam's apple), cricoid, and epiglottis.

• Epiglottis: Covers the laryngeal inlet during swallowing

• Vocal folds: Vibrate to produce sound

• Vestibular folds: Help close the glottis during swallowing

The Trachea

The trachea (windpipe) is a flexible tube supported by C-shaped cartilage rings. Its wall consists of three layers: mucosa, submucosa, and adventitia. The trachealis muscle contracts during coughing to expel mucus.

The Bronchi and Subdivisions

The bronchial tree undergoes about 23 generations of branching, ending in the respiratory zone structures (respiratory bronchioles, alveolar ducts, alveoli). Conducting zone structures transport air, while respiratory zone structures are sites of gas exchange.

Alveoli and the Respiratory Membrane

Alveoli are the primary sites of gas exchange. The respiratory membrane is extremely thin, allowing efficient diffusion. It consists of alveolar and capillary walls with a shared basement membrane. Type II alveolar cells secrete surfactant to reduce surface tension.

Gross Anatomy of the Lungs

Lung Structure and Lobes

Each lung is surrounded by pleurae and connected to the mediastinum. The right lung has three lobes (superior, middle, inferior), while the left lung has two lobes (superior, inferior) and a cardiac notch to accommodate the heart.

Blood Supply and Innervation

Lungs are perfused by pulmonary and bronchial circulations. Pulmonary arteries carry deoxygenated blood to the lungs, while pulmonary veins return oxygenated blood to the heart. Bronchial arteries supply lung tissue except alveoli. Innervation is by parasympathetic (bronchoconstriction) and sympathetic (bronchodilation) fibers.

The Pleurae

Pleurae are double-layered serous membranes (parietal and visceral) that surround the lungs. Pleural fluid lubricates and creates surface tension, helping keep lungs attached to the thoracic wall.

Pulmonary Ventilation: Mechanics of Breathing

Pressure Relationships and Boyle’s Law

Pulmonary ventilation consists of inspiration and expiration. Volume changes cause pressure changes, which drive air movement. Boyle’s law describes the inverse relationship between pressure and volume:

• Boyle’s Law: $P \propto \frac{1}{V}$

Pressure Relationships in the Thoracic Cavity

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

• Intrapulmonary pressure: Pressure in alveoli, fluctuates during breathing

• Intrapleural pressure: Pressure in pleural cavity, always negative relative to intrapulmonary pressure

• Transpulmonary pressure: Difference between intrapulmonary and intrapleural pressures; keeps lungs inflated

Inspiration and Expiration

Inspiration is an active process involving contraction of the diaphragm and external intercostals, increasing thoracic volume and decreasing pressure. Expiration is usually passive, relying on relaxation and lung recoil. Forced expiration uses abdominal and internal intercostal muscles.

Physical Factors Influencing Ventilation

• Airway resistance: Friction in airways; greatest in medium-sized bronchi

• Alveolar surface tension: Surfactant reduces surface tension, preventing alveolar collapse

• Lung compliance: Measure of lung distensibility; high compliance means easier ventilation

Pulmonary Volumes and Capacities

Measuring Ventilation

Pulmonary volumes and capacities are measured to assess respiratory status. Key volumes include:

• Tidal volume (TV): Air moved in/out with each breath (~500 mL)

• Inspiratory reserve volume (IRV): Air forcibly inspired beyond TV

• Expiratory reserve volume (ERV): Air forcibly expired beyond TV

• Residual volume (RV): Air remaining in lungs after forced expiration

Capacities are combinations of volumes:

• Inspiratory capacity (IC): TV + IRV

• Functional residual capacity (FRC): RV + ERV

• Vital capacity (VC): TV + IRV + ERV

• Total lung capacity (TLC): TV + IRV + ERV + RV

Gas Exchange: Diffusion Between Blood, Lungs, and Tissues

Basic Properties of Gases

• Dalton’s Law: Total pressure of a gas mixture equals the sum of partial pressures of individual gases.

• Henry’s Law: The amount of gas dissolved in a liquid is proportional to its partial pressure and solubility.

Pulmonary Gas Exchange

Gas exchange across the respiratory membrane is driven by partial pressure gradients and gas solubilities. Oxygen diffuses from alveoli to blood; carbon dioxide diffuses from blood to alveoli.

Tissue Gas Exchange

In systemic tissues, oxygen diffuses from blood to tissues, and carbon dioxide diffuses from tissues to blood. Venous blood returning to the heart has lower oxygen and higher carbon dioxide.

Oxygen and Carbon Dioxide Transport

Oxygen Transport

• 98.5% of oxygen is bound to hemoglobin (Hb) in red blood cells; 1.5% is dissolved in plasma.

• Hemoglobin can carry four O2 molecules; loading and unloading are influenced by partial pressure, temperature, pH, and BPG concentration.

Carbon Dioxide Transport

• CO2 is transported in three forms: dissolved in plasma (7–10%), bound to hemoglobin (20%), and as bicarbonate ions (70%).

• Formation of bicarbonate involves CO2 combining with water to form carbonic acid, which dissociates into H+ and HCO3-.

Regulation of Respiration

Neural Mechanisms

Respiratory centers in the medulla and pons control breathing rate and rhythm. The ventral respiratory group (VRG) generates the basic rhythm, while the dorsal respiratory group (DRG) integrates sensory input. Pontine centers smooth transitions between inspiration and expiration.

Chemical Factors

• Arterial CO2 and pH are the most important factors affecting ventilation.

• Central chemoreceptors (in brain stem) and peripheral chemoreceptors (in aortic arch and carotid arteries) detect changes.

• Rising CO2 stimulates increased ventilation; low O2 becomes a major stimulus only when levels drop below 60 mm Hg.

Clinical Applications and Disorders

Common Disorders

• Rhinitis: Inflammation of nasal mucosa

• Laryngitis: Inflammation of vocal folds

• Pleurisy: Inflammation of pleurae

• Atelectasis: Lung collapse

• Infant Respiratory Distress Syndrome (IRDS): Lack of surfactant in premature infants

• Chronic Obstructive Pulmonary Disease (COPD): Includes emphysema and chronic bronchitis

• Asthma: Acute, reversible airway obstruction

• Tuberculosis: Infectious disease caused by Mycobacterium tuberculosis

• Lung Cancer: Leading cause of cancer deaths, often due to smoking

• Sleep Apnea: Temporary cessation of breathing during sleep

• Cystic Fibrosis: Genetic disorder causing thick mucus and respiratory infections

Developmental Aspects

Embryonic Development

Upper respiratory structures develop first, followed by the larynx, trachea, bronchi, and alveoli. By week 28, most premature babies can breathe on their own. Lungs continue to mature until young adulthood, and respiratory efficiency decreases with age.

Summary Table: Upper and Lower Respiratory System

Region	Main Structures	Functions
Upper Respiratory	Nose, paranasal sinuses, pharynx	Airway, filtration, humidification, olfaction, speech resonance
Lower Respiratory	Larynx, trachea, bronchi, lungs, alveoli	Airway, voice production, gas exchange

Summary Table: Pulmonary Volumes and Capacities

Volume/Capacity	Definition	Average Value (Adult)
Tidal Volume (TV)	Air moved per breath	500 mL
Inspiratory Reserve Volume (IRV)	Extra air inspired	2100–3200 mL
Expiratory Reserve Volume (ERV)	Extra air expired	1000–1200 mL
Residual Volume (RV)	Air remaining after forced expiration	1200 mL
Vital Capacity (VC)	TV + IRV + ERV	~4800 mL
Total Lung Capacity (TLC)	TV + IRV + ERV + RV	~6000 mL

Key Equations

• Boyle’s Law: $P \propto \frac{1}{V}$

• Inspiratory Capacity: $IC = TV + IRV$

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

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

• Total Lung Capacity: $TLC = TV + IRV + ERV + RV$

Additional info:

• Some clinical and developmental details were expanded for completeness.

• Tables were recreated based on textbook context.