Gas exchange

• Gas exchange is most efficient when the exchange surface has a large area, a large diffusion gradient is maintained and the barrier to diffusion is thin.

• Gas exchange in plants is involved in both respiration and photosynthesis
• In plants, during the night (when only respiration can take place) oxygen is absorbed and carbon dioxide is released. During the day, when the rate of photosynthesis exceeds the rate of respiration, carbon dioxide is absorbed and oxygen is released.
• In plant leaves, the mesophyll is the main gas exchange surface. It has a large surface area, is thin, has an extensive air-space system connected to the atmosphere by stomata that allow the entry of air to maintain a concentration difference.

• In mammals, the alveoli in the lungs are the specialised gas exchange surface through which oxygen diffuses into the blood and carbon dioxide diffuses out.

• The millions of sac-like alveoli provide a large surface area.

• The extremely thin walls of the alveoli and the capillaries (consisting only of squamous epithelium) provide a short diffusion distance.

• The ventilation of the lungs and the circulation of blood across the alveolar surface both maintain high concentration gradients between the alveolar air and the blood.

• Ventilation occurs because breathing movements create pressure differences between the atmosphere and the air in the lungs.

• Inhalation - contraction of external intercostal muscles and diaphragm muscle (flattens), while ribs move up and out, so increasing the volume of the thorax and decreasing the pressure around the lungs and air enters lungs

• Exhalation - relaxation of external intercostal muscles and diaphragm (returns to dome), while ribs move down and in, so volume of thorax decreases and pressure around the lungs increases while the elastic alveoli recoils and air exits lungs.

A number of diseases associated with smoking:

• chronic bronchitis - inflammatory response, airways narrow, excessive mucus produced. Tar paralyses cilia (remove mucus and microbes from lungs) increasing risk of infection. People suffer with breathlessness, coughing and increased susceptibility to infection.
• emphysema - inflammatory response leads to breakdown of alveolar walls. This reduces area available for gas exchange, becomes difficult to get enough oxygen. Loss of elastic fibres in alveolar walls, exhalation becomes more difficult due to reduced ability of alveoli to recoil
• lung cancer - tar contains carcinogens which cause damage to DNA in epithelial cells lining the lungs. Cells with damaged DNA may divide in a modified and uncontrolled way producing a mass of unspecialised cells (tumour).