Cystic fibrosis problems

Microorganisms become trapped in the mucus in the lungs. Some of these can cause illness - they are pathogens.The mucus is normally moved by cilia into the back of the mouth cavity where it is either coughed out or swallowed, thus reducing the risk of infection. 

With CF the mucus layer is so sticky that the cilia cannot move the mucus. Mucus production still continues, as it would in a normal lung, and the airways build up layered of thickened mucus. There are low levels of oxygen in the mucus, partly because oxygen diffuse slowly through it, and partly because the epithelial cells use up more oxygen in CF patients. Harmful bacteria can thrive in these anaerobic conditions.

White blood cells fight the infections within the mucus but as they die they break down, releasing DNA which makes the mucus even stickier. Repeated infections can eventually weaken the body's ability to fight the pathogens, and cause damage to the structures of the gas exchange system.

How sticky mucus reduces gas exchange

Gases such as oxygen cross the walls of the alveoli into the blood system by diffusion. To supply enough oxygen to all the body's respiring cells, gas exchange must be rapid. The fine structure of the lungs helps to maximise this.

The effect of increase in size on surface area

In unicellular organisms the whole cell surface membrane is the exchange surface. Substances that diffuse into or out of the cell move down a concentration gradient (from high to a low concentration). The gradients are maintained by the cell continuously using the substances absorbed and producing waste. For example, oxygen diffusing into a cell is used for respiration which produces carbon dioxide. 

The larger an organism, the more exchange has to take place to meet the organism's needs. Larger multicellular organisms have more problems absorbing substances because of the organism's surface area compared with its volume. This is known as the surface area to volume ratio, calculated by dividing an organism's total surface area by its volume.