The process of gas exchange in the pulmonary alveolus?

Question

I heard it was through diffusion, but please explain clearly and detailed, in a way I can understand. If possible, find a good image to help clarify.

Thank you =D

Answer 1

Gas molecules can pass through cell membranes freely. Oxygen passes through the red blood cell membrane, the capillary cell and the epithelial cell to the aveolus. Carbon dioxide tranvels the opposite direction.

Answer 2

In order to understand diffusion, you must first understand that molecules, at all temperatures above absolute zero, are constantly vibrating.  That vibration causes the molecules to bounce off of each other and move apart.  However, due to their irregular shape, the molecules move in random directions.  Thus, molecules are constantly moving from their current location and they move in all directions (randomly).

Thus, when you have molecules in different concentrations on different sides of a permeable membrane, molecules will move in all directions including through the membrane.  

Let's say there is a membrane permeable to oxygen (a capillary wall or the wall of the alveoli are permeable to oxygen and to carbon dioxide) .  On one side of the membrane there is a HIGHER concentration of oxygen than on the other side.  Since there is oxygen on both sides of the membrane, molecules of oxygen will bounce off of each other as well as off of other molecules.  This will occur on both sides of the membrane.  However, because there is a greater concentration of oxygen molecules on one side of the membrane than on the other side, a greater number of molecules will move from the side of greater concentration to the side of lesser concentration.  

This will occur until the concentration of molecules is equal on both sides of the membrane.  At that time, the two sides of the membrane are said to be at equilibrium.  Remember, the molecules will still be bouncing and moving, but they will be doing so equally in all regions.

Thus, diffusion is defined as being the movement of molecules from a region of higher concentration to a region of lower concentration.

With regard to an image, perhaps someone putting cologne on will help explain.

The scent of cologne is suspended in an alcohol solution as alcohol molecules start to vibrate more when exposed to a temperature similar to our body's temperature.  So, you put cologne molecules on your skin or clothes.  The cologne molecules are in their greatest concentration on you.  Well, what are the first molecules those cologne molecules will collide with?  The answer is, OTHER cologne molecules.  However, the cologne molecules will bounce off of each other and move in a random fashion.  Some of the cologne molecules will stay close and others will bounce away.  Those cologne molecules that bounce away will then start to collide with and bounce off of and away from molecules of oxygen, nitrogen, hydrogen, etc. that compose the air.  As that occurs, the cologne molecules will ultimately move to other parts of the room where previously there were none.

Thus, the cologne molecules have moved from an area where they were concentrated to an area where they were not concentrated.

Answer 3

The pulmonary alveolus are the small sacs of air in the lungs.  It is here that the exchange of oxygen and carbon dioxide take place between the lungs and the capillary beds of the bloodstream.  The capillary beds are extremely thin, only one cell thick, so the gases easily pass through - oxygen from the lungs into the blood to be circulated throughout the body, and carbon dioxide from the blood into the lungs to be exhaled.