Gas exchange in humans

• Mammals have a low surface area to volume ratio and they maintain their body temperature which requires an increased rate of aerobic respiration. Because of this, mammals have a very high oxygen demand.
• Humans have two lungs found in the thorax cavity
• The nasal cavity contains hairs that trap dust and some pathogens.
• The nasal cavity also has a humid environment that warms and moistens the air before entering the lungs to maintain homeostasis of the airway and to provide conditions for gas exchange.
• The walls of the trachea contain cartilage 'c' rings that help stop the trachea from collapsing.
• The trachea cartilage has a c shape as it allows the absence of cartilage near the oesophagus, allowing food to easily pass through.
• Trachea walls are also lined with ciliated epithelia. The beating of the cilia moves the mucus up the trachea and to the throat, where it can be swallowed and stomach acid can digest it.
• The trachea also contains goblet cells which secrete mucus that can trap dust and pathogens, stopping them from reaching the lungs.
• The trachea divides into two bronchi, leading to either lung. The bronchi then further split into many bronchioles that lead to the alveoli.
• Bronchi also contains cartilage, ciliated epithelia and goblet cells.
• The wider bronchioles have cartilage, but as they get deeper into the lungs, bronchioles lose their cartilage.
• All bronchioles contain smooth muscle that can relax, widening the bronchioles - allowing more air to reach deeper in the lungs.
• Features of a specialised exchange surface are:

  • Large surface area
  • Moistened surface
  • Thin and freely permeable
  • Located close to a transport system
• Alveoli are the sites of gas exchange.
• The walls of alveoli are moistened and the alveoli are lined with squamous epithelial cells which are only one cell thick - providing a short distance for diffusion.
• The alveoli have an extensive network of blood capillaries, providing a constant supply of deoxygenated blood thus maintaining a steep concentration gradient.
• Between alveoli are elastic fibres that stretch and recoil during breathing to increase the surface area for diffusion.
• There are hundreds of millions of alveoli, providing a large surface area for diffusion.
• Capillaries have a very narrow lumen, meaning red blood cells are very close to their outer walls - shortening even more the diffusion distance.
• Some alveolar cells secrete surfactant which reduces surface tension between alveoli and increases rate of diffusion.
• Breathing ensures a high concentration of oxygen in the alveoli and a low concentration of carbon dioxide, causing a very steep concentration gradient.
• Oxygen in the air dissolves in the moisture of the inside alveolar wall. This oxygen then diffuses into red blood cells where it binds with haemoglobin.