1)   One theory is that it’s actually not so much about relaxation and has more to do with helping the body get salt out.  When people are under stress, they tend to take shallow breaths.  This “inhibitory breathing” in turn makes the blood more acidic and makes the kidneys less efficient at removing sodium from the blood.

In research conducted by Dr. David Anderson of the National Institute of Health, inhibitory breathing in animals was linked to elevated salt and higher blood pressure.  A similar process is suspected to be at play in people.  

“If you sit there under-breathing all day, as most people do, and you have high salt intake, your kidneys may be less effective at getting rid of salt,” said Anderson.

When people do slow, deep breathing, “They may be changing their blood gases and the way their kidneys are regulating salt,” Anderson hypothesizes.

2)   Another theory is that deep breathing increases nitric oxide transmission in our blood.  Nitric oxide (NO) is a key “signaling gas”.  The endothelium cells (the inner lining of our blood vessels) use nitric oxide to tell the muscles that surround them to relax.  This process is known as “vasodilation” and results in increased blood flow.

In research conducted by Dr. Nick Vaziri, of the UC Irvine College of Medicine, high blood pressure is often associated with impaired nitric oxide pathways.

“Nitric oxide is a substance that helps keep our blood vessels open,” says Dr. Elijah Saunders, head of hypertension research at the University of Maryland School of Medicine’s Cardiology Department.

3)   A third theory is that slow, deep breathing helps us oxygenate.  Our brains are only 2% of our body weight yet they consume 20% of the oxygen we inhale.

“Slight changes in oxygen content in the brain can alter the way a person feels and behaves,” says Dr. Daniel G. Amen, author of Change Your Brain, Change Your Life.

The shallow breaths associated with stress and negative emotions don’t allow us to get enough oxygen.  “The oxygen content in the angry person’s blood is lowered,” says Dr. Amen, leading to more negative behavior in a downward spiral.

The answer is to break the cycle by recognizing when our physiology isn’t serving our best interests.  Slow, deep breathing gives our body the oxygen it needs and signals the body to get things back in balance.

4)   The fourth theory centers around a concept known as "entrainment."  This is the tendency of the brain to mimic a stimulus.  For example, when you hear slow, mellow Jazz, the electrical currents in your brain will get calm too.  It's thought that through slow, deep breathing, your brain takes a cue to break the stress cycle.

The fascinating thing is that the break in the stress cycle isn't just temporary. 

In research conducted at the University of Wisconsin-Madison, Dr. Richard Davidson compared brain activity and immune response in two groups: "relaxation" vs. control group.  The "relaxation" group showed increased immune response and increased activity in the left prefrontal cortex (associated with positive temperament) of the brain.  It appears that regular relaxation isn't temporary - it actually helps the brain rewire.

"What we've found is that the trained mind, or brain, is physically different from the untrained one," says Davidson.