Chapter 22: The Respiratory System – Structure, Function, and Clinical Relevance

Chapter 22: The Respiratory System

Functional Anatomy of the Respiratory System

The respiratory system is responsible for gas exchange, supplying oxygen to the body and removing carbon dioxide. It is divided into the upper and lower respiratory tracts, each with distinct anatomical structures and functions.

• Upper respiratory system: Includes the nose, nasal cavity, pharynx, and larynx. Functions to warm, humidify, and filter air.

• Lower respiratory system: Includes the trachea, bronchi, bronchioles, and alveoli. Responsible for conducting air and facilitating gas exchange.

• Conducting zone vs. respiratory zone: The conducting zone transports air (nose to terminal bronchioles), while the respiratory zone (respiratory bronchioles, alveolar ducts, alveoli) is where gas exchange occurs.

Key Terms: Pharynx, Larynx, Trachea, Bronchi, Alveoli

Upper Respiratory System: Structure and Function

The upper respiratory tract prepares inhaled air for entry into the lungs by warming, humidifying, and filtering it.

• Nose and Nasal Cavity: Contains nasal conchae, olfactory epithelium, and respiratory mucosa. Functions include filtering particles, humidifying air, and housing olfactory receptors.

• Pharynx: Divided into nasopharynx, oropharynx, and laryngopharynx. Involved in both respiratory and digestive pathways.

• Larynx: Contains vocal cords (true and false), glottis, and epiglottis. Responsible for sound production and protecting the airway during swallowing.

• Clinical Note: Rhinitis and sinusitis are common inflammatory conditions affecting the upper respiratory tract.

Lower Respiratory System: Conducting and Respiratory Zones

The lower respiratory tract conducts air to the lungs and is the site of gas exchange.

• Trachea: Supported by tracheal cartilage; lined with mucosa to trap particles.

• Bronchi and Bronchioles: Branch into primary, secondary, and tertiary bronchi, ending in terminal and respiratory bronchioles.

• Alveoli: Microscopic air sacs where gas exchange occurs. Composed of type I (gas exchange) and type II (surfactant production) cells, and alveolar macrophages.

• Surfactant: Reduces surface tension in alveoli, preventing collapse. Deficiency leads to Infant Respiratory Distress Syndrome (IRDS).

Pleural Cavities and Pleura

Each lung is housed in its own pleural cavity, lined by pleural membranes.

• Parietal pleura: Lines the thoracic cavity.

• Visceral pleura: Covers the lungs.

• Pleural cavity: Space between pleurae, contains lubricating fluid.

Mechanics of Breathing: Pressure and Volume Changes

Breathing is driven by changes in thoracic volume and pressure, allowing air to move in and out of the lungs.

• Boyle's Law: Pressure and volume are inversely related: $P_1V_1 = P_2V_2$

• Pressures in respiration: Atmospheric pressure ($P_{atm}$), intrapulmonary pressure ($P_{pul}$), intrapleural pressure ($P_{ip}$), transpulmonary pressure ($P_{tp}$).

• Pneumothorax: Air in the pleural cavity disrupts pressure balance, causing lung collapse.

• Muscle contractions: Diaphragm and intercostal muscles change thoracic volume during inspiration and expiration.

• Bronchodilation and Bronchoconstriction: Sympathetic stimulation dilates bronchioles; parasympathetic constricts.

Gas Exchange: Diffusion Across the Respiratory Membrane

Oxygen and carbon dioxide diffuse across the thin respiratory membrane between alveoli and pulmonary capillaries.

• Partial pressure gradients: Drive diffusion of gases ($O_2$ and $CO_2$).

• External respiration: Gas exchange between alveoli and blood.

• Internal respiration: Gas exchange between blood and tissues.

• Factors affecting gas exchange: Membrane thickness, surface area, and partial pressure differences.

• Diseases: Edema, emphysema, and COVID-19 impair gas exchange.

Transport of Oxygen and Carbon Dioxide

Oxygen and carbon dioxide are transported in the blood by different mechanisms.

• Oxygen transport: Mostly bound to hemoglobin; a small amount dissolved in plasma.

• Hemoglobin affinity: Influenced by $pH$, $CO_2$, temperature, and 2,3-BPG (Bohr and Haldane effects).

• Carbon dioxide transport: Dissolved in plasma, bound to hemoglobin, or as bicarbonate ions ($HCO_3^-$).

• Carbonic anhydrase: Enzyme that catalyzes conversion of $CO_2$ and $H_2O$ to $HCO_3^-$ and $H^+$.

• Chloride shift: Exchange of $Cl^-$ and $HCO_3^-$ across red blood cell membranes during $CO_2$ transport.

Control of Respiration: Brainstem Centers

Breathing is regulated by centers in the brainstem, responding to chemical and neural signals.

• Medullary centers: Ventral respiratory group (VRG) and dorsal respiratory group (DRG) set basic rhythm.

• Pontine centers: Modify rhythm for smooth transitions between inspiration and expiration.

• Chemoreceptors: Monitor $CO_2$, $O_2$, and $pH$ levels in blood and cerebrospinal fluid.

• Higher brain centers: Hypothalamus and cerebral cortex can influence breathing rate and depth.

• Breathing patterns: Apnea (no breathing), eupnea (normal), and dyspnea (labored).

Respiratory Diseases and Clinical Considerations

Respiratory diseases are major causes of morbidity and mortality, affecting gas exchange and ventilation.

• Hypoxia: Inadequate oxygen delivery; types include anemic, ischemic, histotoxic, and hypoxemic hypoxia.

• COPD (Chronic Obstructive Pulmonary Disease): Includes emphysema and chronic bronchitis; causes airflow limitation.

• Asthma: Reversible airway obstruction due to inflammation and bronchoconstriction.

• Treatments: Bronchodilators, corticosteroids, oxygen therapy.

• Comparison of COPD and Asthma: COPD is progressive and largely irreversible; asthma is episodic and reversible.

Table: Comparison of Oxygen and Carbon Dioxide Transport Mechanisms

Transport Mechanism	Oxygen	Carbon Dioxide
Dissolved in Plasma	~1.5%	~7%
Bound to Hemoglobin	~98.5% (as oxyhemoglobin)	~23% (as carbaminohemoglobin)
As Bicarbonate Ion	0%	~70%

Table: Types of Hypoxia

Type	Cause	Example
Anemic Hypoxia	Low hemoglobin	Anemia
Ischemic Hypoxia	Impaired blood flow	Heart failure
Histotoxic Hypoxia	Cells unable to use oxygen	Cyanide poisoning
Hypoxemic Hypoxia	Low arterial $O_2$	Pneumonia

Additional info: Academic context and definitions have been expanded for clarity and completeness. Tables have been inferred and constructed based on standard textbook knowledge.