I enjoyed this NPR article Just Breathe: The Body Has a Built in Stress Reliever.
Here are some highlights:

• “As it turns out, deep breathing is not only relaxing, it’s been scientifically proven to affect the heart, the brain, digestion, the immune system — and maybe even the expression of genes.”
• “…breathing can have a profound impact on our physiology and our health.”
• ‘Our breaths will either wake us up or energize us. It will relax us, or it will just balance us’…
• “Research has shown that breathing exercises like these (see article) can have immediate effects by altering the pH of the blood, or changing blood pressure.”
• “But more importantly, they can be used as a method to train the body’s reaction to stressful situations and dampen the production of harmful stress hormones”
• “She says rapid breathing is controlled by the sympathetic nervous system. It’s part of the “fight or flight” response — the part activated by stress. In contrast, slow, deep breathing actually stimulates the opposing parasympathetic reaction — the one that calms us down.”
• “Research shows that breathing can even change the expression of genes. He says that by using your breath, you can alter the basic activity of your cells with your mind.”
• “What’s happening when you’re stressed is that your own body is giving itself multiple shots of that anti-inflammatory hormone, and so that tunes down your immune system’s ability to do its job to fight infection,” Sternberg says.