2.14 Cystic Fibrosis

Question 1

What is cystic fibrosis? What causes CF?

Answer 1

• a genetic disorder where mucus produced by goblet cells is abnormally thick and sticky
• caused by a mutation in the CFTR (cystic fibrosis transmembrane regulator) gene on chromosome 7

Question 2

What is CFTR? What is its function?

Answer 2

• cystic fibrosis transmembrane regulator
• it is a channel protein responsible for transporting chloride ions out of cells and into mucus
• this causes water to move out of the cell and into the mucus by osmosis

Question 3

Explain CFTR’s role in regulating the amount of water in mucus (what happens when there is too little water in mucus):

Answer 3

• chloride ions are actively pumped into the cell from the tissue fluid
• this increases the concentration of chloride ions inside the cell
• therefore, the chloride ions diffuse through the open CFTR channel into the mucus down the concentration gradient (the concentration of Cl- in the cell is higher than the concentration of Cl- in the mucus)
• because of the Cl- ions in the mucus, the mucus is now more negatively charged than the tissue fluid - there is an electrical gradient between the mucus and the tissue fluid)
• this means that positive sodium ions will diffuse down the electrical gradient (from the tissue fluid to the mucus) to even out the charge
• this causes a large increase of Na+ and Cl- ions in the mucus
• therefore water is drawn out of the cell and into the mucus by osmosis, preventing the mucus from becoming too thick

Question 4

Why do people with CF have abnormally thick mucus?

Answer 4

• the CFTR protein is either missing or dysfunctional
• this means that Cl- ions are unable to diffuse through the CFTR channel into the mucus
• this means that the concentration of Cl- ions inside the cell will always be higher than the concentration of Cl- ions in the mucus
• so water will be continuously drawn out of the mucus and into the cell by osmosis, and the mucus will be too thick
• if the CFTR protein is partially functional, it is still much less efficient at transporting chloride ions out of the cell, meaning less water moves out of the cell into the mucus by osmosis, and so the mucus will still be abnormally thick

Question 5

What are goblet cells?

Answer 5

epithelial cells that produce mucus

Question 6

What is the role of mucus in the respiratory system? How is it transported through the system?

Answer 6

• mucus coats the tubes in the respiratory system
• it traps dust, debris, and microorganisms that enter the airways
• this helps prevent lung infections as microorganisms are trapped
• the mucus is continuously removed and transported to the lungs by the beating of cilia that cover epithelial cells
• (cilia = small hair-like structures, epithelial cells = cells that line the internal organs, body cavities, skin, etc)

Question 7

How does CF affect the respiratory system (infection)?

Answer 7

• the cilia cannot move and transport the mucus as the mucus is too thick
• because mucus production continues, there is a build up of mucus in the airways
• therefore, the pathogens that are trapped in the mucus are not transported away and killed
• this will increase the chances of lung infection
• repeated lung infections will damage the lungs

Question 8

How does CF affect the respiratory system (gas exchange)?

Answer 8

• the cilia cannot move and transport the mucus as the mucus is too thick
• because mucus production continues, there is a build up of mucus in the airways
• the mucus can block the narrow airways
• this means that air can no longer reach the areas below the blockage
• therefore the surface area available for gas exchange is reduced (as the number of alveoli that can access air is reduced due to blockage)

Question 9

How are the problems in the respiratory system caused by CF treated?

Answer 9

• antibiotics are given to kill bacteria trapped in mucus, which prevents lung infections
• physiotherapy can help dislodge mucus (to prevent blockages in the airways) and improve gas exchange

Question 10

How does CF affect the digestive system (pancreatic duct)?

Answer 10

• the pancreatic duct (the tube connecting the pancreas to the small intestine) becomes blocked by the thick mucus
• this means that the digestive enzymes produced by the pancreas are not able to reach the small intestine
• the lower concentration of enzymes in the small intestine reduces the rate of digestion, so less nutrients are absorbed
• additionally, the mucus lining the intestine is thick, so this inhibits the absorption of nutrients
• because energy and nutrients are lost from the food, those with CF need to eat much more than those without

Question 11

How does CF affect the digestive system (pancreatic cysts)?

Answer 11

• because the pancreatic duct is blocked, digestive enzymes are trapped in the pancreas
• this causes pancreatic cysts - the cysts of hard fibrous tissue give the cystic fibrosis its name
• this inhibits the production of digestive enzymes which further reduces the absorption of nutrients
• the production of insulin is also inhibited, so there is a higher risk of diabetes

Question 12

How does CF affect the reproductive system?

Answer 12

• in females, the cervical mucus is thickened, which stops the sperm from reaching the egg (thick mucus reduces the motility of the sperm), so fertility is reduced
• in males, the sperm ducts (tubes connecting the testes to the penis) are absent or blocked by thick mucus, so the sperm cannot reach the penis, and fertility is reduced