The respiratory system is a complex network of organs and structures that work together to bring oxygen into the body and remove carbon dioxide. From the nose to the lungs, each part plays a crucial role in breathing and gas exchange.

The system is divided into upper and lower tracts, with the lungs being the main site of gas exchange. Alveoli, tiny air sacs in the lungs, are where oxygen enters the bloodstream and carbon dioxide is removed, powering our cells and keeping us alive.

Respiratory System Anatomy

Structures of respiratory tracts

• Upper respiratory tract
  • Nose and nasal cavity
    • Nasal hairs and mucus trap particles (dust, pollen)
    • Warms and humidifies incoming air prepares air for entry into lungs
  • Pharynx (throat)
    • Nasopharynx, oropharynx, and laryngopharynx regions of the pharynx
    • Passageway for both air and food/drink shared pathway
  • Larynx (voice box)
    • Contains vocal cords produce sound for speech
    • Epiglottis covers larynx during swallowing prevents food/drink from entering airways (aspiration)
• Lower respiratory tract
  • Trachea (windpipe)
    • Lined with ciliated pseudostratified columnar epithelium and mucus-producing goblet cells trap and remove particles, humidify air
    • C-shaped cartilage rings prevent collapse maintains open airway
  • Bronchi
    • Right and left primary bronchi branch from trachea carry air to respective lungs
    • Secondary (lobar) bronchi branch to each lobe of the lungs (3 right, 2 left)
    • Tertiary (segmental) bronchi further divide into bronchioles progressively smaller airways
  • Lungs
    • Right lung has three lobes; left lung has two lobes accommodates heart on left side
    • Pleural membranes cover lungs and line thoracic cavity reduces friction during breathing
    • Contain alveoli, the site of gas exchange tiny sacs surrounded by capillaries
    • Alveoli are lined with surfactant, which reduces surface tension and prevents collapse

Gas exchange in lungs

• Ventilation: movement of air in and out of lungs
  • Inhalation: diaphragm and external intercostal muscles contract, increasing volume of thoracic cavity and decreasing pressure, causing air to flow into lungs (Boyle's Law)
  • Exhalation: diaphragm and external intercostal muscles relax, decreasing volume of thoracic cavity and increasing pressure, causing air to flow out of lungs passive process
• Gas exchange occurs in alveoli
  • Alveoli are tiny air sacs surrounded by capillaries (300 million per lung)
  • Oxygen diffuses from alveoli into blood; carbon dioxide diffuses from blood into alveoli (concentration gradients)
  • Diffusion occurs due to concentration gradients gases move from high to low concentration
  • Partial pressure of gases drives the diffusion process
• Oxygen is transported by hemoglobin in red blood cells to tissues (oxyhemoglobin)
• Carbon dioxide is transported in blood plasma and red blood cells to lungs for exhalation (carbaminohemoglobin, bicarbonate ion)
• Gas exchange is essential for cellular respiration (ATP production) and removal of metabolic waste product (carbon dioxide)

Conducting vs respiratory zones

• Conducting zone
  • Includes nose, pharynx, larynx, trachea, bronchi, and bronchioles
  • Functions:
    1. Provides passageway for air to reach respiratory zone (conduit)
    2. Warms, humidifies, and filters incoming air (air conditioning)
    3. No gas exchange occurs in this zone (anatomical dead space)
• Respiratory zone
  • Includes respiratory bronchioles, alveolar ducts, and alveoli
  • Functions:
    1. Site of gas exchange between air and blood (primary function)
    2. Thin, moist alveolar walls allow for efficient diffusion of gases (0.2-0.6 μm thick)
    3. Dense capillary network surrounding alveoli facilitates gas exchange (short diffusion distance)
    4. Alveolar macrophages remove debris and pathogens (dust cells)

Respiratory Mechanics

• Thoracic cavity houses the lungs and other vital organs
• Compliance refers to the lungs' ability to expand and contract during breathing
• Dead space is the volume of air that does not participate in gas exchange, including anatomical and physiological dead space