The Respiratory System: Functions and Anatomy

• The respiratory system facilitates gas exchange, taking in oxygen and expelling carbon dioxide
• Key structures included: the nose, nasal cavity, pharynx, larynx, trachea, bronchi, and lungs

Functions of the Respiratory System

• Gas exchange occurs as oxygen from inhaled air enters the blood, and carbon dioxide from the blood enters the lungs to be exhaled
• Airflow is essential for moving air to and from the exchange surfaces of the lungs
• Protection is provided to respiratory surfaces against dehydration, temperature changes, and pathogens
• Sound production enables speech and other vocalizations
• Regulation of blood volume, blood pressure, and body fluid pH happens by influencing arterial concentrations of carbon dioxide
• Olfactory sensation occurs by way of providing olfactory sensations to the olfactory bulb in the brain

The Nose and Nasal Cavity

• Air enters the respiratory system via the nose and nasal cavity
• The nasal cavity is divided into left and right portions by the nasal septum
• The nasal mucosa warms and humidifies inhaled air
• Nasal conchae (turbinates) create turbulence in the nasal cavity, increasing the contact time between the air and the mucosa
• The olfactory region, located in the superior nasal cavity, contains receptors for the sense of smell

The Pharynx

• The pharynx (throat) is a shared passageway for both the respiratory and digestive systems
• It is divided into three regions:
  • Nasopharynx: The superior portion, connected to the nasal cavity via the internal nares, contains the pharyngeal tonsil (adenoids)
  • Oropharynx: The middle portion, located posterior to the oral cavity, contains the palatine tonsils and lingual tonsils
  • Laryngopharynx: The inferior portion, extends from the hyoid bone to the entrance of the larynx and esophagus

The Larynx

• The larynx (voice box) surrounds and protects the glottis
• The glottis is a narrow opening between the vocal cords
• The larynx is made of nine cartilages including:
  • Thyroid cartilage forms the anterior and lateral walls of the larynx, commonly known as the Adam's apple
  • Cricoid cartilage presents as a ring-shaped cartilage inferior to the thyroid cartilage
  • Epiglottis acts as a flap-like structure that covers the glottis during swallowing to prevent food and liquids from entering the trachea
• The vocal cords vibrate as air passes over them, producing sound

The Trachea

• The trachea (windpipe) is a tube that extends from the larynx to the main bronchi
• C-shaped tracheal cartilages support it, preventing collapse
• The posterior tracheal wall contains the trachealis muscle, which can contract to reduce the diameter of the trachea
• The trachea is lined with pseudostratified ciliated columnar epithelium, which traps debris and sweeps it toward the pharynx

The Bronchi

• The trachea branches into the right and left main bronchi
• The primary (main) bronchi enter the lungs
• The right main bronchus is wider, shorter, and more vertical compared to the left main bronchus
• Each main bronchus divides into lobar (secondary) bronchi, which enter the lobes of the lungs
• Lobar bronchi branch into segmental (tertiary) bronchi, supplying specific lung segments
• The branching pattern from the trachea to the terminal bronchioles is called the bronchial tree

The Lungs

• The lungs are paired organs located in the thoracic cavity
• The right lung has three lobes: superior, middle, and inferior
• The left lung has two lobes: superior and inferior
• The lobes are separated by fissures
• The outer surfaces of the lungs are covered by the pleura, a serous membrane
  • The visceral pleura covers the outer surface of the lungs
  • The parietal pleura lines the inner surface of the thoracic cavity
  • The pleural cavity exists as the space between the visceral and parietal pleura, containing a small amount of pleural fluid for lubrication

Bronchioles and Alveoli

• The bronchioles are small airways that branch from the segmental bronchi
• They lack cartilage and are primarily composed of smooth muscle
• Terminal bronchioles branch into respiratory bronchioles
• Respiratory bronchioles lead into alveolar ducts, which end in alveolar sacs
• Alveoli are small, sac-like structures where gas exchange occurs
• The alveolar walls contain capillaries, creating a short diffusion distance for oxygen and carbon dioxide
• Type I alveolar cells are the primary cells forming the alveolar walls
• Type II alveolar cells secrete surfactant, which reduces surface tension in the alveoli and prevents their collapse
• Alveolar macrophages (dust cells) patrol the alveoli and remove debris and pathogens
• The respiratory membrane serves as the thin barrier across which gas exchange occurs
  • It consists of the alveolar epithelium, the capillary endothelium, and their fused basement membranes

Mechanisms of Breathing

• Breathing, or pulmonary ventilation, involves two phases: inspiration (inhalation) and expiration (exhalation)
  • Inspiration (inhalation) involves air moving into the lungs
  • Expiration (exhalation) involves air moving out of the lungs
  • Air moves from an area of high pressure to an area of low pressure
• During inspiration, the diaphragm and external intercostal muscles contract
  • The diaphragm moves downward, increasing the volume of the thoracic cavity
  • The external intercostals elevate the ribs, further increasing thoracic volume
  • The increase in volume decreases the pressure within the thoracic cavity (intrapleural pressure), causing air to flow into the lungs
• During expiration, the diaphragm and external intercostal muscles relax
  • The diaphragm moves upward, decreasing the volume of the thoracic cavity
  • The ribs depress, further decreasing thoracic volume
  • The decrease in volume increases the pressure within the thoracic cavity, causing air to flow out of the lungs
• Accessory muscles such as the sternocleidomastoid, scalenes, pectoralis minor, and abdominal muscles are involved in forced breathing

Lung Volumes and Capacities

• Tidal volume (TV) is the volume of air inhaled or exhaled during a normal breath, approximately 500 mL
• Inspiratory reserve volume (IRV) is the amount of air that can be forcefully inhaled after a normal tidal volume inhalation, approximately 3100 mL
• Expiratory reserve volume (ERV) is the amount of air that can be forcefully exhaled after a normal tidal volume exhalation, approximately 1200 mL
• Residual volume (RV) is the volume of air remaining in the lungs after a maximal exhalation, approximately 1200 mL
• Inspiratory capacity (IC) is the sum of tidal volume and inspiratory reserve volume (TV + IRV)
• Functional residual capacity (FRC) is the sum of residual volume and expiratory reserve volume (RV + ERV)
• Vital capacity (VC) is the sum of tidal volume, inspiratory reserve volume, and expiratory reserve volume (TV + IRV + ERV)
• Total lung capacity (TLC) is the sum of all lung volumes (TV + IRV + ERV + RV)

Control of Respiration

• The respiratory rhythmicity centers in the medulla oblongata and pons control breathing
  • The medulla oblongata contains the ventral respiratory group (VRG) and the dorsal respiratory group (DRG)
  • The VRG is responsible for forceful breathing
  • The DRG modifies the rate and depth of breathing
  • The pons contains the pneumotaxic center and the apneustic center, which influence the activity of the medullary centers
• Chemoreceptors monitor the levels of carbon dioxide, oxygen, and pH in the blood and cerebrospinal fluid
  • Central chemoreceptors are located in the medulla oblongata and respond to changes in pH and carbon dioxide levels in the cerebrospinal fluid
  • Peripheral chemoreceptors are located in the carotid bodies and aortic bodies and respond to changes in oxygen, carbon dioxide, and pH levels in the blood
• The Hering-Breuer reflex involves stretch receptors in the lungs that prevent over-inflation during forced breathing