The Respiratory System is involved in getting gases to and from individual cells. The body will use the circulatory system which already has indirect contact, through the capillaries to every living cell, to present O² to the cells and remove excess CO² from the cells. We need a way to load & remove O² & CO² in blood, bearing in mind the physical properties of gasses and liquids.

1- Living cells can only exist in a liquid environment, i.e., a salt water environment similar to ancient sea water.

2- Gasses have a very limited ability to dissolve in water... For O² it's about 2%; for CO² about 7% The O² value (2%) is too low to keep us (a mega-celled organism) alive.

3- Keep the body's energy cost at a minimum by using DIFFUSION (high concentration      to low concentration) and REVERSABLE REACTIONS (sensitive to diffusion concentrations) for the exchanges.

4- Diffusion times increase as the square of the increases in distance to be traversed... If the distance becomes 3 times as large, the diffusion time will be 3² or 9 times longer.

5- If we have a large surface area with short diffusion distances and access to a molecule (hemoglobin) that, when bonded with gases REVERSIBLY, forms an insoluable compound, we can move very large quantities of O² through the 2% O²  carrying water layers into 'on the shelf' (out of solution) storage within the red blood cells.

In response, lung tissue has evolved that unites a 2nd large capillary bed with millions of tiny ALVEOLAR SACS of the same order of magnitude; where the surface areas are again very large, and the distances to traverse for exchange are, once again, very short. With this configuration, the O² loading process will be complete before the blood gets half way across the capillary surrounding the alveolar sac providing a large safety margin. Unfortunately, smoking tars, scar tissue, excess mucus, etc., add thickness causing quickly lengthening diffusion times (T=X²) which will quickly eliminate the safety margin.

1. Describe the changes in partial pressures of O² as one goes from outside air, to alveolar sacs, to capillaries, to cells, and the reverse for CO² (note CO² is carried in the blood in three ways).

2. Describe the detection method for determining changes in O² concentration leading to the desire to breath more rapidly and the negative feedback system that helps regulate the detection system.