Why is the movement of each gas essentially in one direction only (including appropriate numerical values)? Is the oxygen pressure inside cells...

The gas exchange process happening between the alveoli and the surrounding capillary bed is driven by diffusion.  Diffusion is a passive transport mechanism that causes gasses to move from areas of high concentration to areas of lower concentration.  

The blood being pumped to the lungs is low in oxygen content and high in carbon dioxide content because it's oxygen and glucose has been converted to carbon dioxide through the process of cellular respiration at each cell.  That carbon dioxide needs to be removed from the body, so it is carried to the lungs in order to be exhaled.  

Upon inhalation, the alveoli become filled with outside air.  It's mostly nitrogen, but about 20% of the inhaled breath contains oxygen and almost zero carbon dioxide.  The surrounding capillaries contain a higher concentration of carbon dioxide than the alveoli, so the gas diffuses out of the capillaries and into the alveoli.  The oxygen does the opposite. While the capillary blood does contain some oxygen, it is around 10-14 percent.  That's lower than what's in the alveoli, so oxygen diffuses out of the alveoli and into the capillary blood.  

The oxygen pressure in most cells will be lower than the oxygen pressure in the blood.  That's why the oxygen moves out of the blood and into the cells.  They need it to perform cellular respiration in order to produce ATP.