Overview of the Respiratory System

Respiratory System Overview

• A normal human breathes 12-15 times per minute at rest.
• Approximately 6-8 liters of air are inspired and expired per minute.
• Air mixes with gases in the alveoli. Oxygen (O₂) enters the bloodstream in pulmonary capillaries. Carbon dioxide (CO₂) enters the alveoli.

Organization of the Respiratory System

• Airways are tubes that carry air between the external environment and alveoli.
• Upper airways include the nose, pharynx, and larynx.
• Lower airways include the trachea, bronchi, bronchioles, and alveoli.
• Lungs contain multiple branching bronchi.
• The walls of the trachea and bronchi have cartilage, which maintains their shape.
• Bronchioles are the first airways lacking cartilage.
• Alveoli are tiny air sacs where gas exchange occurs.

Zones of the Respiratory System

• The respiratory system is composed of two zones:
  • Conducting zone: No respiration occurs here; includes nose, larynx, pharynx, trachea, bronchi, bronchioles, terminal bronchioles
  • Respiratory zone: Actual gas exchange sites; includes respiratory bronchioles, alveolar ducts, and alveoli

Upper Respiratory Tract

• Nose: Air passageway
• Pharynx: Common passage for air and food.
• Larynx: Branches into the esophagus (food) and trachea (air).

Lower Respiratory Tract

• Trachea: Connects the larynx to the bronchi.
• Bronchi: Branch into smaller bronchioles.
• Bronchioles: Branch into terminal bronchioles.
• Terminal bronchioles: Lead to respiratory bronchioles.
• Alveolar ducts: Lead to alveolar sacs.
• Alveoli: Tiny air sacs for gas exchange.

Lungs

• Two lungs, right and left, divided into lobes.
• Approximately 300 million alveoli in the lungs.
• Huge alveolar surface area (about 70 m²) for efficient gas exchange with blood

Relation of Lungs to Thoracic Wall

• Lungs reside within the thorax.
• Thorax is a closed compartment, separated from the abdomen by the diaphragm.
• Thoracic wall: Spinal column, ribs, sternum, intercostal muscles.
• Pleura: Each lung is enclosed in a double-walled sac. Includes visceral pleura (on lung surface) and parietal pleura (lines thoracic cavity).

Pleura and Pleural Fluid

• Visceral pleura: Attaches directly to the lung.
• Parietal pleura: Lines the thoracic cavity and diaphragm.
• Intrapleural fluid: Small amount of fluid separating the visceral and parietal pleura. Facilitates smooth lung movement during breathing.
• Two pleura layers are close but not attached, allowing for smooth movement during breathing.

Intrapleural Pressure

• Changes in intrapleural fluid pressure cause lungs and thoracic wall to move together.

Definitions

• Inspiration (inhalation): Air movement from the environment into the alveoli.
• Expiration (exhalation): Air movement from the alveoli to the environment.
• Respiratory cycle: One inspiration and one expiration.

Breathing Mechanics

• Inhalation: Rib muscles contract, expanding rib cage. Diaphragm contracts, moving downward, increasing lung volume and decreasing pressure
• Exhalation: Rib muscles relax, decreasing rib cage size. Diaphragm relaxes, moving upwards, decreasing lung volume and increasing pressure

External Respiration

• Alveolo-capillary membranes: Alveoli are surrounded by pulmonary capillaries. Separated by thin membrane (0.5 µm) for efficient gas diffusion.
  • Diffusion of gases (O₂ and CO₂) across the alveolo-capillary membrane.

Internal Respiration

• Gas exchange between blood and cells in the rest of the body.

Regulation of Respiration

• Respiration is a reflex controlled by nervous and chemical mechanisms.
  • Nervous Mechanism: Respiratory centers in the brainstem. Composed of medullary and pontine centers
    • Medullary centers (e.g., dorsal respiratory group, ventral respiratory group) set the basic rhythm of breathing; receive input from chemoreceptors and mechanoreceptors in lungs
    • Pontine centers (e.g., apneustic center, pneumotaxic center) help adjust the depth and rate of breathing.
  • Chemical Mechanism: Chemoreceptors detect blood chemistry changes.
    • Central chemoreceptors: Located in the brainstem; sensitive to changes in CO₂ and H+ (pH) levels in cerebrospinal fluid (CSF)
    • Peripheral chemoreceptors: Located in the carotid and aortic bodies; sensitive to changes in O₂, CO₂, and H+ levels in blood

Types of Chemoreceptors

• Central Chemoreceptors: Located in the medulla oblongata; respond to changes in CO2 and H+ concentration in cerebrospinal fluid (CSF).
• Peripheral Chemoreceptors: Located in the carotid and aortic bodies; respond to changes in O2, CO2, and H+ concentration in blood.