Cystic fibrosis

Cystic fibrosis is a disorder effecting plasma membranes. It is caused by a faulty gene.

One of the proteins involved in active transport across membranes is the CFTR protein, which is made by the CFTR gene. It transports chloride ions out of the cell.

Normal

In the respiratory tract, chloride ions are actively transported out of the cells into the mucus which lines the epithelium (remember mucus is produced by the goblet cells). This lowers the water potential of the mucus. This means water moves by osmosis from the cell into the mucus, making the mucus sticky enough to trap dirt and bacteria, but thin enough so it can be moved upwards by the cilia, stopping bacteria settling in the lungs.

Cystic fibrosis

People with CF have a faulty CFTR protein – its tertiary structure is different. Chloride ions cannot be transported out into the mucus. This means the water potential of the mucus increases and water leaves the mucus. This makes it very sticky and thick making it hard for the cilia to move it. This means bacteria settle in the lungs and can cause infections. They develop:

• coughs and wheezing as they try to force the mucus out.
• shortness of breath as mucus blocks bronchioles
• bacteria grow in a layer called a biofilm making it hard for antibiotics and white blood cells to kill them. Lots of phagocytes move to the lungs to destroy them, but end up damaging the lung tissue instead. This lowers the surface area available for gas exchange.
• thick mucus blocks the pancreatic duct
• thick mucus blocks the ducts carrying the gametes in the reproductive tracts – can lead to infertility.

Treating cystic fibrosis

• Chest physiotherapy to help remove mucus from lungs
• Regular exercise and keeping fit is helpful
• Antibiotics to help prevent infections
• Drugs which dilate the airways and thins the mucus
• Take a high fat, protein and carb diet as they don’t absorb their food properly as well as enzyme capsules to help with digestion due to blocked pancreatic duct.
• Gene therapy – putting normal copy of CFTR gene into lung cells
• Lung transplant

 

The lungs

Each bronchus branches into small bronchioles. At the end of each bronchioles are clusters of air sacs – called alveoli.

Underneath the lungs lies the diaphragm.

Breathing in:

• Intercostal muscles contract –> ribs move up and out
• Diaphragm contracts and so flattens

This increases thorax volume and reduces pressure –> air is pulled in.

Breathing out:

• Intercostal muscles relax –> ribs move down and inwards
• Diaphragm relaxes and so becomes dome shaped again.

This reduces the thorax volume and increases the pressure –>air is forced out.

Alveoli – where gas exchange happens

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The inner surface is moist – stops cells drying out. The fluid contains a surfactant which lowers surface tension stopping the alveoli sticking together when breathing in and out. The oxygen also dissolves in this layer before diffusing into the capillaries around the alveoli.

How are alveoli adapted for efficient gas exchange?

• large surface area – millions of alveoli in the lungs and each one is folded increasing the SA even more.
  
• thin permeable surface – epithelium of alveoli is made of very thin flat cells. The wall of the capillary is only one cell thick and the capillary is very close to the alveoli. This all means that oxygen has a very small distance to diffuse across.
• large concentration gradient – breathing in brings in air with high oxygen concentration and low carbon dioxide concentration. The capillaries are bringing deoxygenated blood to the alveoli containing lots of carbon dioxide. This sets up a large concentration gradient for oxygen to diffuse into the blood and carbon dioxide to diffuse into the alveoli and so we breathe out air high in carbon dioxide and low in oxygen.

How are the lungs kept clean?

The trachea and bronchi are lined with an epithelium (outer layer of cells). This has 2 cells in it:

• Ciliated cells – have cilia on the edge (hair like structures that are an extension of the cytoplasm). These bend and beat in time with each other and waft any mucus up the trachea, where it can be swallowed.
  
• Goblet cells – make and secrete mucus. Mucus is slimy and sticky and traps dust and bacteria that is breathed in.

With the mucus sweeping the mucus away this helps to reduce the chance of respiratory infections.