Functional Anatomy of the Respiratory System

Primary Functions

• Gas Exchange: Facilitates the movement of oxygen (O2) into the body and carbon dioxide (CO2) out of the body.

• Regulation of Blood pH: By controlling CO2 levels, the respiratory system helps maintain acid-base balance.

• Vocalization: Air movement through the larynx enables sound production.

• Olfaction: The sense of smell is enabled by airflow over olfactory epithelium.

• Conditioning of Inspired Air: Air is warmed, humidified, and filtered before reaching the lungs.

Respiratory Tract Divisions

Upper Respiratory Tract

• Nose

• Nasal Cavity

• Paranasal Sinuses

• Pharynx

Lower Respiratory Tract

• Larynx

• Bronchial Tree

• Alveoli

Anatomy of the Upper Respiratory Tract

The Nose & Nasal Cavity

• External Nares: Nostrils; entry points for air.

• Nasal Septum: Divides the nasal cavity into left and right portions.

• Conchae: Bony projections that increase surface area and create turbulence, enhancing air conditioning.

• Respiratory Epithelium: Pseudostratified ciliated columnar epithelium with goblet cells that produce mucus to trap particles.

• Olfactory Epithelium: Located in the superior region, responsible for smell.

Palates

• Hard Palate: Formed by maxillae and palatine bones; separates nasal and oral cavities.

• Soft Palate: Extends posteriorly from the hard palate; closes off the nasopharynx during swallowing.

Pharynx

• Nasopharynx: Air passage only; lined with pseudostratified ciliated columnar epithelium; contains auditory tube openings.

• Oropharynx: Passage for air and food; lined with stratified squamous epithelium; located behind the oral cavity.

• Laryngopharynx: Passage for air and food; lined with stratified squamous epithelium; leads to larynx and esophagus.

Anatomy of the Lower Respiratory Tract

Larynx

• Connects pharynx to trachea; maintains an open airway and produces sound.

• Composed of three main cartilages:

  • Thyroid Cartilage: Forms the "Adam's Apple".

  • Cricoid Cartilage: Provides support below the thyroid cartilage.

  • Epiglottis: Covers the glottis during swallowing to prevent aspiration.

Vocal Cords

• Elastic ligaments within vestibular folds; muscles adjust tension to produce sound as air passes through.

Trachea

• Supported by C-shaped cartilage rings to maintain airway patency.

• Trachealis muscle and annular ligament adjust tracheal diameter.

• Branches into left and right primary bronchi.

Bronchial Tree

• Primary bronchi branch at the mediastinum into right and left bronchi.

• Further divides into lobar (secondary) bronchi and then into smaller passageways.

• Right Bronchus: Larger diameter, steeper angle—more likely to receive foreign objects.

Bronchioles

• Lack cartilage; composed of smooth muscle to regulate airflow.

• Bronchodilation: Sympathetic nervous system increases diameter.

• Bronchoconstriction: Parasympathetic nervous system decreases diameter; can be triggered by anaphylaxis.

• Respiratory bronchioles are the thinnest branches where gas exchange begins.

Lungs

• Right Lung: Three lobes (superior, middle, inferior); larger than left.

• Left Lung: Two lobes (superior, inferior); smaller due to cardiac notch.

• Hilum: Entry/exit point for bronchi, blood vessels, and nerves.

• Pleura: Each lung is surrounded by visceral and parietal pleura; pleural fluid reduces friction.

Gas Exchange

Alveoli

• Composed of simple squamous epithelium (type I pneumocytes) for gas exchange.

• Type II pneumocytes secrete surfactant to reduce surface tension and prevent alveolar collapse.

• Alveolar macrophages remove debris and pathogens.

• Gas exchange occurs across the blood-air barrier.

• Pneumonia can disrupt this barrier, impairing gas exchange.

Blood Supply

• Respiratory Portion: Supplied by the pulmonary circuit (right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium).

• Conducting Portion: Removes CO2 from blood via pressure gradients.

Gas Laws and Partial Pressures

• Gases move from areas of high to low partial pressure.

• Dalton’s Law: The total pressure of a gas mixture is the sum of the partial pressures of each individual gas.

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

Respiration Types

External Respiration

• Gas exchange between alveoli and pulmonary capillaries (external environment and interstitial fluids).

• Occurs in three steps: pulmonary ventilation, gas diffusion, and transport of gases.

Internal Respiration

• Gas exchange between systemic capillaries and tissue cells (within body tissues and organs).

Pulmonary Ventilation (Breathing)

• Physical movement of air into and out of the lungs.

• Air flows from high to low pressure.

• Intrapulmonary Pressure: Pressure within alveoli (measured in mm Hg).

• Intrapleural Pressure: Pressure within the pleural cavity.

• Surfactant: Reduces surface tension on alveolar surfaces, preventing lung collapse.

Respiratory Muscles

• Primary Muscles: Diaphragm and external intercostals.

• Accessory Muscles: Internal intercostals and some abdominal muscles (active during forced breathing).

Gas Transport in Blood

Oxygen Transport

• 98.5% of O2 is bound to hemoglobin (Hb) in red blood cells.

• 1.5% is dissolved in plasma.

• Each hemoglobin molecule contains four heme units, each with an iron ion that binds O2.

• When all four heme units are bound to O2, the molecule is called oxyhemoglobin.

• O2 can be released from Hb as needed by tissues.

Factors Affecting Hemoglobin-Oxygen Binding

• Increased temperature decreases Hb affinity for O2 (more O2 released).

• Decreased pH (more acidic) decreases O2 saturation of Hb.

• Increased biphosphoglycerate (BPG) promotes O2 release from Hb.

Carbon Dioxide Transport

• CO2 is produced in peripheral tissues.

• Transported in three forms:

  • Dissolved in plasma

  • As carbaminohemoglobin (bound to Hb)

  • As bicarbonate ion (after conversion to carbonic acid in RBCs)

Mechanics of Breathing

Boyle’s Law

• Pressure and volume are inversely related:

• Increasing the volume of the thoracic cavity decreases pressure, causing air to flow in.

• Decreasing the volume increases pressure, causing air to flow out.

Pulmonary Volumes

• Tidal Volume (TV): Amount of air moved in or out during a normal breath.

• Anatomic Dead Space: Volume of air in the respiratory tract not involved in gas exchange.

• Residual Volume (RV): Air remaining in lungs after maximal exhalation.

Control of Respiration

Respiratory Centers (Medulla)

• Dorsal Respiratory Group (DRG): Controls inspiration.

• Ventral Respiratory Group (VRG): Controls expiration.

Receptors

• Chemoreceptors: Detect changes in O2, CO2, and pH levels.

• Baroreceptors: Detect changes in blood pressure via stretch receptors in the aorta and carotid sinuses.

• Hering-Breuer Reflexes:

  • Inflation Reflex: Prevents overexpansion of the lungs.

  • Deflation Reflex: Stimulates inspiration to prevent lung collapse.

Disturbances in Homeostasis

• Hypoxia: Low oxygen levels in tissues.

• Anoxia: Complete lack of oxygen in tissues.

• Pulmonary Embolism: Blockage of a pulmonary artery, impeding blood flow to lung tissue.

Example Table: Comparison of Upper and Lower Respiratory Tract

Additional info: The study guide covers all major anatomical and physiological aspects of the respiratory system as outlined in standard college-level Anatomy & Physiology courses. For exam preparation, students should also be familiar with the clinical implications of respiratory disturbances and the integration of respiratory function with other body systems.