The major function of the respiratory system is gas exchange between the environment and the body tissues.  Every working cell in the body needs a constant supply of life-giving oxygen and nutrients.  Waste products of individual cells must be removed to prevent a toxic overdose of poisons.  Primitive one-celled sea creatures could simply exchange gases with the surrounding water through the cell membrane.  As organisms became more complex, more complex ways of bringing fluid to the cells developed.  The circulatory system was created to provide all the body cells with a constant flow of nutrient-rich fluid.  When living creatures first walked on dry land, life-giving blood brought the fluid along.  However, this fluid also needed a way to replenish and deliver oxygen to the body tissue.  Air-filled lungs became the critical connection to the air environment.  Hemoglobin in red blood cells greatly increased the oxygen carrying capacity of the blood and allowed humans to thrive on land.

What is respiration?

Respiration is the exchange of gases across membranes.  This occurs in two improtant locations in the human body.  First, oxygen-rich air is brought into the lungs.  This air travels through an elaborate system of smaller and smaller airways.  At the end of these airways are tiny air sacs that are surrounded by tiny blood vessels called capillaries.  It is here that oxygen and carbon dioxide diffuse across the cell membranes. 

The other important location of respiration is in the body tissues.  Here too, tiny blood capillaries deliver gases directly to the body's tissue cells.

What is gas exchange?

The composition of dry air is 21% oxygen, 78% nitrogen, 0.04% carbon dioxide and about 1% other gases.  The gases in air have a pressure.  For example, when air is brought next to a liquid with a lower pressure of oxygen, then oxygen will transfer to that fluid.  In the lungs and body tissues, there are membranes that separate body fluids and air.  These membranes are designed to allow oxygen to freely pass across.  The oxygen will transfer across the membrane in either direction until the oxygen pressure becomes equal on both sides.  The same process occurs with other gases as well.

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