According to Henry's law, the amount of a given gas that dissolves in a liquid at a constant temperature is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

 What happens to this pressure as one descends below sea level and why?
Water Depth and Pressure
The amount of force acting on a given region is referred to as pressure. The pressure produced by the water grows progressively as you go deeper into the ocean. This is due to an increasing amount of water pressing down from above on you.

What is the composition of atmospheric air (think of the 4 major gases only)?
Earth's atmosphere is composed of about 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, and 0.1 percent other gases. Trace amounts of carbon dioxide, methane, water vapor, and neon are some of the other gases that make up the remaining 0.1

Which of these gases move into the blood stream and which move out of the blood stream in the lungs?
Gases pass through the membranes at the respiratory membrane, where the alveolar and capillary walls meet, with oxygen entering the bloodstream and carbon dioxide exiting.

Why do we not get the ‘bends’ at normal atmospheric pressure?
Unfortunately, our bodies aren't adapted to breathing under pressure (because we normally breathe air under normal atmospheric conditions). Henry's Law states that as the air pressure in the lungs rises, gases such as nitrogen, helium (when used in diving gas mixtures), and oxygen become more soluble in the blood.

 How do the gases move from one compartment to another in the body?
Gas exchange occurs in the lungs' millions of alveoli and the capillaries that surround them. Inhaled oxygen flows from the alveoli to the blood in the capillaries, and carbon dioxide moves from the blood in the capillaries to the air in the alveoli, as illustrated below.

 What is the partial pressure of oxygen in the alveolar air and in the pulmonary capillary blood? In which direction will oxygen flow and why?
However, the partial pressure difference is only 5 mm Hg, which is smaller than that of oxygen. Carbon dioxide partial pressure in capillary blood is approximately 45 mm Hg, but it is approximately 40 mm Hg in alveoli.