6.4 - Gas Exchange

• Ventilation maintains concentration gradient of carbon dioxide and oxygen between air in alveoli and blood flowing in capillaries
• In humans gas exchange occurs through small air sacs called alveoli in the lungs
• Gasses diffuse because there is a higher concentration of oxygen and a lower concentration of carbon dioxide in the alveolus than in the capillary
• Ventilation is the maintenance of pumping fresh air into the alveoli and removing stale air 
• Type 1 pneumocytes are extremely thin alveolar cells adapted to carry out gas exchange
• Lungs have many alveoli to increase surface area for diffusion
• Wall of each alveolus is a single layer of cells called the epithelium
• The walls of the adjacent capillaries to the alveolus are also a single layer of thin cells
• The short diffusion path of the alveoli and capillary walls is an adaptation that allows a greater rate of gas exchange
• Type 2 pneumocytes secrete a solution containing surfactant that creates a moist surface in alveoli to prevent sides of alveoli adhering to each other by reducing surface tension
• The film of moisture that type 2 pneumocytes secrete allows oxygen to dissolve and diffuse to blood in the capillaries
• The film of moisture that type 2 pneumocytes secrete also provides an area from which carbon dioxide can evaporate into the air and be exhaled
• Fluid secreted by type 2 pneumocytes contains pulmonary surfactant
• Pulmonary surfactant has molecules similar to phospholipids
• Surfactant molecules form a monolayer with hydrophilic heads facing water and hydrophobic tails facing the air in alveoli
• Surfactant molecules reduce surface tension and prevent water from causing the sides to adhere to each other when air is exhaled.

  Prevents collapse of lung
• Premature babies are often born with insufficient pulmonary surfactant
• Treatment for babies with less surfactant is giving them oxygen or surfactant extracted from animal lungs
• Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles 
• Air enters ventilation system through mouth or nose then passes down trachea
• Rings of cartilage in trachea wall keep it open even when pressure inside is low or surrounding tissue pressure is high
• Trachea divides to two bronchi which both have rings of cartilage
• Inside lungs, bronchi divide repeatedly to form narrow, tree-like airways called bronchioles
• Bronchiole walls have smooth muscle fibres allowing variation in airway width
• Muscle contractions cause pressure changes inside the thorax that force air in and out of the lungs to ventilate them
• 1. During ventilation, muscle contractions cause pressure inside the thorax to drop below atmospheric pressure
• 2. When pressure inside thorax is below atmospheric pressure, air is drawn into the lungs via inspiration until lungs have reached atmospheric pressure
• 3. Muscle contractions cause pressure inside thorax to rise above atmospheric pressure, causing air to be forced out of lungs via expiration
• Different muscles are required for inspiration and expiration because muscles only do work when they contract
• Muscles do work when contracting by exerting a pulling force that causes a particular movement, making them shorter
• Muscles passively lengthen when relaxing and are pulled into an elongated state by the contraction of another muscle
• An antagonistic pair of muscles is when one muscle contracts and causes a movement and the second muscle is elongated by the first (and vice versa)
• Inspiration and expiration involve opposite movements so different muscles work as antagonistic pairs 
• Inspiration

  • Diaphragm contracts and moves downwards, pushing abdominal wall out
  • Abdominal wall muscles relax allowing pressure from diaphragm
• Expiration

  • The diaphragm relaxes and moves upwards
  • Abdominal wall muscles contract pushing abdominal organs and diaphragm upwards
• Inspiration

  • External intercostal muscles contract pulling the ribcage upwards and outwards
  • Internal intercostal muscles relax and are pulled into elongated state
• Expiration

  • External intercostal muscles relax and are pulled into elongated state
  • Internal intercostal muscles contract, pulling ribcage inwards and downwards
• Emphysema patients have smaller number of alveoli with thicker walls
• For emphysema patients surface area for gas exchange is reduced and distance for diffusion increases, causing insufficient ventilation 
• Emphysema patients also lose elasticity in their lungs