The photo energy releases nitric oxide from the hemoglobin and possibly surrounding tissue. Nitric oxide is a signaling molecule that is known to relax smooth muscle cells in the arteries, veins, capillaries and lymph vessels. When these muscles relax, they dilate the blood vessel, thus allowing increased circulation. Nitric oxide has been shown to be essential for wound healing even if chronic or traumatic. Scientific literature shows that nitric oxide promotes collagen synthesis, collagen fibril alignment, and angiogenesis. Nitric oxide is also a neurotransmitter.