Gas Exchange (Core)Modified from: Stephen Taylor

i-Biology.net

Why do we need a ventilation system? We are large organisms. Oxygen cannot diffuse into all our cells directly from the air, nor can waste products be directly ejected from the body. We have specialised organ systems, which are efficient, but need delivery of nutrients and removal of waste. The ventilation system ensures the blood can be the medium for this.

We are land-borne. Gases need moist surfaces (membranes) in order to diffuse. Our lungs are moist membranes, allowingoxygen to diffuse into the blood and carbon dioxide to diffuse out.

The ventilation system maintains a large concentration gradient between the alveoli and the blood. The constant flow of past the alveoli brings blood with a high CO2 concentration and low O2 concentration. Breathing out keeps the CO2 concentration in the alveoli low, so it diffuses out of the blood. Breathing in keeps O2 concentration in the alveoli high, so it diffuses into the blood.

Diagram from: http://www.sciencequiz.net/jcscience/jcbiology/gapfilling/breathingsystem.htm

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter25/animation__gas_exchange_during_respiration.html

Which process(es) of membrane transport are being used in gas exchange at the membranes of the alveoli?

http://www.footprints-science.co.uk/alveoli.htm

http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP15104

Characteristics of the alveoli walls• The walls of the alveoli consist of a

single layer of cells, called epithelium. There are two types of cells in the epithelium:

– Type I cells are very flattened, about 0,15 µm thick.

– Type II cells are rounded and secrete surfactant, which coats the inner surface of the alveoli with a film of moisture, preventing collapse of alveoli during expiration and help oxygen to dissolve.

http://diatronic.co.uk/nds/webpub/resp_memb.jpg

• There is a dense network of blood capillaries between adjacent alveoli. The walls of these capillaries is also composed of flattened cells, about 0,15 µm thick, and it is called endothelium.

• An extracellular layer of protein gel, called basement membrane, lies between the epithelium of the alveolus and the endothelium of blood capillaries. It is about 0,1µm thick. This is the gas exchange surface.

• About 0,4 µm in allhttp://diatronic.co.uk/nds/webpub/resp_memb.jpg

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