Study Notes: The Respiratory System

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Respiratory System

Overview and Functions

The respiratory system is essential for gas exchange, supplying oxygen (O2) to cells and removing carbon dioxide (CO2), a metabolic waste product. It also plays roles in olfaction (smell) and speech.

Supply O2 to cells: Required for cellular respiration and energy production.
Dispose of CO2: Prevents toxic accumulation of metabolic waste.
Olfaction and speech: Air movement through the nasal cavity and larynx enables smell and vocalization.

Processes of Respiration

Respiration involves four main processes:

Ventilation: Movement of air into and out of the lungs.
External respiration: Exchange of gases between alveoli and blood.
Transport (gases): Movement of O2 and CO2 in the blood.
Internal respiration: Exchange of gases between blood and tissues.

Organizational Schema of the Respiratory System

Conducting Zone

The conducting zone consists of structures that transport air, clean, humidify, and warm it, but do not participate in gas exchange. This zone is also called "anatomic dead space."

Includes: Nasal cavity, pharynx, larynx, trachea, bronchi, and terminal bronchioles.
Function: Pulmonary ventilation (air movement).
Epithelium: Humidifies air and traps particles/microbes via mucus and cilia.
Cartilage: Larynx, trachea, and bronchi contain cartilage to maintain an open airway.

Respiratory Zone

The respiratory zone is where gas exchange occurs between air and blood.

Begins at respiratory bronchioles (lined by simple cuboidal epithelium).
Ends with alveoli (lined by simple squamous epithelium).
Site of external respiration with pulmonary capillaries.

Air Conducting Zone Details

Bronchioles

Bronchioles are the smallest tubes of the bronchial tree (diameter < 1 mm).

Lined by simple cuboidal epithelium with cilia.
No cartilage; walls contain smooth muscle.
Bronchoconstriction: Increases resistance, limits entry of unwanted particles.
Bronchodilation: Decreases resistance (mediated by norepinephrine).
Analogous to "resistance" vessels (arterioles) in the vascular system.

Airway Resistance

In healthy individuals, airway resistance is physiologically insignificant due to large lumen and extensive branching.

Asthma: Inflammation and bronchospasms (often due to histamine) increase resistance and can obstruct airflow.

Respiratory Zone Details

Alveoli (Air Sacs)

Alveoli are small out-pockets of simple squamous epithelium, surrounded by pulmonary capillaries and elastin fibers.

Site of gas exchange.
Respiratory membrane is essential for gas exchange; if nonfunctional, it creates "physiologic dead space."

Respiratory Membrane (Air-Blood Barrier)

The respiratory membrane consists of alveolar and capillary walls with fused basement membranes, forming a thin barrier for gas diffusion.

Type I cells: Simple squamous epithelium for gas exchange.
Type II cells: Secrete surfactant to reduce surface tension.
Resident macrophages: Remove debris and keep alveolar surfaces sterile.
Energy requirement: Gas diffusion across the membrane is passive and does not require energy.

Pulmonary Surfactant

Pulmonary surfactant is an oily fluid produced by type II alveolar cells. It reduces surface tension at the water-air interface, preventing alveolar collapse.

Composed of phospholipids.
Decreases "attraction" forces between water molecules.
Prevents collapse when alveoli are packed closely together.

Pressures in the Thoracic Cavity

Intrapulmonary and Intrapleural Pressures

Intrapulmonary pressure (Ppul): Pressure in alveoli; normalized to 0 mmHg (1 atm = 760 mmHg).
Intrapleural pressure (Pip): Pressure between parietal and visceral pleurae; always negative, typically -4 mmHg.

Causes of Negative Intrapleural Pressure

Lung collapse: Due to lung recoil and surface tension.
Compliance (expansion): Ability of lungs and thoracic cage to expand.
Compliance factors:
- Degree of lung distention/elasticy.
- Flexibility of thoracic cage.
The greater the lung compliance, the easier it is to expand the lungs at any given transpulmonary pressure.

Healthy vs. Decreased Lung Compliance

Healthy Lung	Decreased Compliance (e.g., Asbestosis)
Normal, open alveolar spaces	Fibrotic, thickened tissue; reduced expansion

Pulmonary Ventilation

Boyle's Law and Ventilation

Pulmonary ventilation is governed by changes in volume and pressure, described by Boyle's Law:

Boyle's Law: (Pressure × Volume = constant)
Increasing lung volume decreases pressure, allowing air to flow in.
Excitation of inspiratory muscles (e.g., diaphragm) increases lung volume and decreases intrapulmonary pressure.

Sequence of Inspiration

Diaphragm: Primary inspiratory muscle; contracts and flattens, increasing thoracic volume.
Chest wall is elastic and expands.
Air rushes into lungs as pressure drops.

End of Inspiration and Expiration

At the end of inspiration, equilibrium is reached.
During expiration, chest volume recoils as diaphragm relaxes, increasing pressure and expelling air.

Regulation of Ventilation

Chemoreceptor Control

Central chemoreceptors: Located in the medulla; respond to rising arterial PCO2 (most potent respiratory stimulus).
Peripheral chemoreceptors: Located in carotid arteries and aortic arch; also respond to changes in blood gases.

Central and Peripheral Control

Central control: Medullary respiratory center regulates basic rhythm.
Peripheral control: Carotid and aortic bodies monitor blood chemistry.
CO2 "blow off" refers to increased ventilation to remove excess CO2.
Relevant equation:

Neural Regulation

VRG (Ventral Respiratory Group): Inspiratory/expiratory center; signals intercostal and phrenic nerves.
Pontine center: Modifies and fine-tunes breathing rhythms during vocalization, sleep, and exercise.
"C3, 4, 5 keeps the diaphragm alive" refers to the cervical spinal nerves that innervate the diaphragm.

Summary Table: Zones and Functions

Zone	Main Structures	Function
Conducting Zone	Nasal cavity, pharynx, larynx, trachea, bronchi, terminal bronchioles	Air transport, cleaning, humidifying, warming
Respiratory Zone	Respiratory bronchioles, alveolar ducts, alveoli	Gas exchange (O2, CO2)

Key Terms and Definitions

Alveoli: Tiny air sacs for gas exchange.
Surfactant: Substance reducing surface tension in alveoli.
Compliance: Measure of lung expandability.
Boyle's Law: Relationship between pressure and volume in gases.
Chemoreceptors: Sensory receptors detecting chemical changes in blood.

Example: Asthma

Asthma is characterized by inflammation and bronchospasms, leading to increased airway resistance and difficulty in breathing. Airways may become thickened with exudate, further magnifying the effect of bronchospasms.

Additional info:

Gas exchange efficiency depends on the integrity of the respiratory membrane and surfactant production.
Diseases such as asbestosis decrease lung compliance, making breathing more laborious.