Cystic fibrosis - just one small change in a protein

Lung structure

• Nose
  • Air is filtered in nostrils with small hairs
  • Air is moistened and warmed by nasal cavities
  • Mucus traps foreign particles while cilia propels particles towards the throat
• Air passes into the pharynx → larynx → trachea
  • The epiglottis is found within the larynx
  • Breathing: epiglottis projects upwards → larynx is open
  • Swallowing: larynx pulled up / epiglottis flaps back and blocks larynx / prevents food from entering airway
• Trachea
  • Contains C-shaped cartilage rings / prevents collapse of tube
  • Divides into 2 tubes with smaller diameter called bronchi
  • Bronchus is supported with ciliated epithelia to prevent microorganisms
  • Right bronchus is bigger than the left one → common site for inhaled foreign bodies
• Bronchi further divide into bronchioles
  • Their diameter can be controlled by smooth muscles
  • Form alveoli (100µm in diameter)

Fick’s law

• Rate of diffusion is proportional to (surface area x conc. difference) / distance
• Applies to exchange of food, waste, gases, and heat with surroundings
• Large organisms
  • Have a small surface area : volume ratio
  • Decreases the rate of diffusion
  • Large animals loose less heat than small animals
  • Don't require a high metabolism to maintain body temperature
  • Feed only once
• Small organisms
  • Lose heat very readily
  • Need a high metabolism to maintain body temp
  • Must feed continuously
• More detail in Unit 2 Section 3.2.4
• [exam] Efficient gas exchange requires:
  • Large surface area
  • Large concentration gradient (low O2 on one side, high O2 on the other site of the membrane)
  • Short diffusion pathway (thickness of membrane molecules must travel to diffuse across)

Alveolar Gas Exchange

• Greater partial pressure of O2 in alveolar air / more O2 dissolves in blood (Henry's Law)
• Two types of alveoli cells
• Type I cells
  • Composed of endothelium - layer of two thin cells
  • This allows diffusion of gases (short diffusion pathway) down their conc. gradients
  • O2diffuses from air to blood; CO2diffuses from blood to air
• Type II cells
  • Secrete surfactant that keep alveoli constantly moist
  • Allows oxygen to dissolveand to diffuse through the cells into the blood
  • In the blood, it is taken up by haemoglobin
• Alveoli contain phagocytes to kill bacteria that have not been trapped by mucus
• Ventilation
  • Flow of air in and out of alveoli
  • Maintains large concentration gradient

Ventilation

• Tidal volume, VT, volume of air inhaled and exhaled in a normal single breath (≈0.5 L)
• Functional residual capacity, FRC, volume remaining in lungs after exhalation of tidal volume (≈2.5 L)
• Expiratory reserve volume, ER, volume of a maximal exhalation (≈1.5 L)
• Residual volume, RV, volume remaining in lung after maximal exhalation (≈1L)
• Inspiratory reserve volume, IR, additional volume that can be inhaled after inhalation of tidal volume
• Vital capacity, VC, maximum volume of exhalation after lungs are maximally filled
  • Best clinical indicator of breathing
• Minute ventilation is the overall flow of air into lungs (analogous to cardiac output)
  • Minute Ventilation = Tidal Volume x Respiratory Rate
  • (0.5 litre/breath * 10 breaths/min = 5 litres per minute)
• "Dead space" - not all O2 available in air is available to alveoli
  • Fresh air mixes with exhaled air during inspiration
  • Alveolar ventilation takes dead space into account
  • Alveolar ventilation = (Tidal Volume - Dead Space) x Respiratory Rate
  • (350 ml x 10 breaths per minute = 3500 ml or 3.5 litres)

Measurements of Ventilation

• A spirometer is used to measure expired breath
• Restrictive disorders, such as pulmonary fibrosis, reduce compliance and vital capacity
• Four measures are called respiratory volumes
  • Tidal volume
  • Inspiratory reserve volume
  • Expiratory reserve volume
  • Residual volume
• Others, called respiratory capacities, are calculated by adding 2 or more of the respiratory volumes