Respiratory System

General structure and function

  • gas exchange, thermoregulation
    • In lungs: oxygen diffuses into blood. Carbon dioxide diffuses out of blood.
    • The mechanism of this gas exchange follows Henry's law, which basically says that there is an equilibrium concentration of oxygen that should be dissolved in blood.
    • When blood reaches the lungs, it has less than the equilibrium concentration of oxygen because the body used the oxygen up. Therefore, oxygen diffuses into blood.
    • The CO2 in blood that reaches the lungs is higher than the equilibrium concentration because of the body releases them. Therefore, CO2 diffuses out of blood.
    • Thermoregulation: breathing causes you to lose heat (you breath out warm, moist air).
  • protection against disease, particulate matter
    • Nostril hair filters out particles.
    • Mucus lining of respiratory tract traps pathogens and particles.
    • Cilia on mucus lining of respiratory tract sweeps pathogen and particles out, where you either spit it out or swallow it into stomach acid.
    • Macrophages reside in alveoli.

Breathing mechanisms

diagram of the lungs showing negative pressure breathing mechanism
  • diaphragm, rib cage, differential pressure
    • Diaphragm = muscle that pulls downward when contracting, which increases chest volume, decreases pressure, and sucks air into lungs.
    • Rib cage = expands outward during breath intake. Intercostal muscles help this expansion. At rest, the rib cage maintains lung volume, prevent lung from collapsing, forms a cage around lungs for protection.
    • Differential pressure = difference between intrapulmonary (inside lung) pressure and intrapleural (outside lung) pressure.
      • Intrapulmonary pressure = atmospheric pressure (lung is open to the outside, so has same pressure as outside).
      • Intrapleural pressure = less than atmospheric pressure = sucks on the lungs, prevent lung from collapsing. During breath intake, intrapleural pressure decreases even further, causing the lung to expand.
    • Negative pressure mechanism in breathing is just a fancy term for sucking. You breathe in by establishing negative pressure in the lung (sucking). However, when someone gives you mouth-to-mouth, that's positive pressure.
  • resiliency and surface tension effects
    • Lung is elastic, it recoils as soon as you relax after breath intake. If not for the rib cage, the lung would collapse even further.
    • Surface tension causes the lung to collapse. Surfactants produced in the alveoli decreases surface tension, and helps alveoli to stay open.