The Respiratory System: Structure, Function, and Physiology

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Functional Anatomy of the Respiratory System

Overview of Respiratory Organs

The respiratory system is responsible for gas exchange, supplying oxygen to the body and removing carbon dioxide. It consists of a series of organs and structures that facilitate the movement and conditioning of air.

Nasal cavity: Warms, moistens, and filters incoming air; contains paranasal sinuses that lighten the skull and produce mucus.
Pharynx: Connects nasal cavity and mouth to larynx and esophagus; serves as a passageway for air and food.
Larynx: Routes air and food into proper channels; contains vocal cords for sound production.
Trachea: Windpipe; conducts air to the bronchi; lined with ciliated pseudostratified epithelium.
Bronchi and subdivisions: Conducting zone consists of right and left primary bronchi, which branch into secondary and tertiary bronchi, ultimately leading to bronchioles.
Lungs: Main organs of respiration; contain alveoli for gas exchange; divided into lobes and served by pulmonary arteries and veins.

Additional info: The respiratory membrane consists of alveolar and capillary walls, facilitating efficient gas exchange.

Protective Mechanisms and Respiratory Zone Structures

Mucus and cilia: Trap and remove particles and pathogens from inhaled air.
Alveolar macrophages: Engulf and digest foreign particles in the alveoli.
Respiratory membrane: Thin barrier for gas exchange between alveolar air and blood.

Structure of the Lungs and Pleura

Visceral pleura: Covers the external lung surface.
Parietal pleura: Lines the thoracic cavity.
Pleural cavity: Space between pleurae containing lubricating fluid to reduce friction during breathing.

Mechanics of Breathing

Atmospheric and Intrapulmonary Pressures

Breathing involves changes in pressure within the thoracic cavity, allowing air to flow into and out of the lungs.

Intrapulmonary pressure: Pressure within the alveoli; rises and falls during respiration.
Intrapleural pressure: Pressure within the pleural cavity; always lower than intrapulmonary pressure.
Boyle's Law: The pressure of a gas is inversely proportional to its volume.

Role of Respiratory Muscles

Diaphragm and intercostal muscles: Contract during inspiration, increasing thoracic volume and decreasing pressure.
Expiration: Usually passive, resulting from relaxation of respiratory muscles.

Physical Factors Influencing Pulmonary Ventilation

Airway resistance: Friction encountered by air in the airways; reduced as airways widen.
Alveolar surface tension: Surfactant reduces surface tension, preventing alveolar collapse.
Lung compliance: Determined by lung tissue and surrounding thoracic cage.

Respiratory Volumes and Pulmonary Function Tests

Respiratory volumes and capacities are measured to assess lung function.

Tidal volume (TV): Amount of air inhaled or exhaled with each breath (~500 mL).
Inspiratory reserve volume (IRV): Amount of air that can be forcibly inhaled after normal inspiration (~2100-3200 mL).
Expiratory reserve volume (ERV): Amount of air that can be forcibly exhaled after normal expiration (~1000-1200 mL).
Residual volume (RV): Air remaining in lungs after forced expiration (~1200 mL).

Volume/Capacity	Description
TV	Tidal Volume
IRV	Inspiratory Reserve Volume
ERV	Expiratory Reserve Volume
RV	Residual Volume

Additional info: Pulmonary function tests help diagnose obstructive and restrictive lung diseases.

Gas Exchanges Between the Blood, Lungs, and Tissues

Gas Laws and Properties

Gas exchange is governed by Dalton's law and Henry's law.

Dalton's Law: Total pressure exerted by a mixture of gases is the sum of the pressures exerted by each gas.
Henry's Law: The amount of gas that dissolves in a liquid is proportional to its partial pressure.

External and Internal Respiration

External respiration: Exchange of O2 and CO2 between alveoli and blood.
Internal respiration: Exchange of gases between blood and tissue cells.

Factors Affecting Gas Exchange

Thickness and surface area of respiratory membrane: Thinner and larger surface area increases efficiency.
Partial pressure gradients: Drive diffusion of gases.
Ventilation-perfusion coupling: Matches air flow and blood flow for optimal gas exchange.

Transport of Respiratory Gases by Blood

Oxygen Transport

Hemoglobin: Most oxygen is transported bound to hemoglobin in red blood cells.
Oxygen loading and unloading: Affected by partial pressure, pH, temperature, and BPG (2,3-bisphosphoglycerate).

Carbon Dioxide Transport

Dissolved in plasma: Small percentage of CO2 is transported this way.
Bound to hemoglobin: CO2 binds to globin portion of hemoglobin.
Bicarbonate ions: Majority of CO2 is converted to bicarbonate ions in plasma.

Control and Adjustments of Respiration

Neural and Chemical Regulation

Medullary respiratory centers: Control basic rhythm of breathing.
Pontine centers: Modify and fine-tune breathing patterns.
Chemoreceptors: Detect changes in CO2, O2, and pH levels.

Respiratory Adjustments

Hyperventilation: Increased rate and depth of breathing; can lead to decreased CO2 levels.
Acclimatization: Physiological adjustments to high altitude, including increased red blood cell production.

Homeostatic Imbalances and Developmental Aspects

Common Respiratory Disorders

Chronic bronchitis, emphysema, asthma, tuberculosis, lung cancer: Affect structure and function of the respiratory system.

Developmental Changes

Embryonic development: Formation of respiratory structures begins early in gestation.
Infancy to old age: Respiratory efficiency and structure change throughout life.

Summary Table: Key Respiratory Volumes and Capacities

Term	Definition	Typical Value (mL)
Tidal Volume (TV)	Air inhaled/exhaled per breath	500
Inspiratory Reserve Volume (IRV)	Max air inhaled after normal inspiration	2100-3200
Expiratory Reserve Volume (ERV)	Max air exhaled after normal expiration	1000-1200
Residual Volume (RV)	Air remaining after forced expiration	1200

Example: Application of Boyle's Law in Breathing

During inspiration, the diaphragm contracts, increasing thoracic volume and decreasing pressure, causing air to flow into the lungs.
During expiration, the diaphragm relaxes, decreasing thoracic volume and increasing pressure, causing air to flow out of the lungs.