In Zazen we sit on a cushion on the floor with both legs crossed, and with our lower spine, upper spine, and head held straight vertically. Keeping the spine straight has a direct and immediate effect on the autonomic nervous system that controls many of our body’s functions. Its effects include control of heart rate and force of contraction, constriction and dilatation of blood vessels, contraction and relaxation of smooth muscle in various organs, the ability to focus the eyes and the size of the pupils, and the secretion of hormones from various glands directly into the blood stream.
The autonomic nervous system is composed of two subsystems: the sympathetic and the parasympathetic nervous systems. When the sympathetic nervous system is stimulated, our heart rate increases, arteries and veins constrict, the lungs relax, and our pupils dilate; in short, we become tense and alert. When the parasympathetic nervous system is stimulated, the opposite happens; our heart rate decreases, arteries and veins dilate, the lungs contract, and the pupils constrict. You can see that the two systems prepare the body for an active or passive response sometimes known as the “fight or flight” syndrome. When the effect of the two systems on the organs is in balance, we are neither ready to fight, nor ready to run away; we are in a normal state.
The parasympathetic nerves emerge from the spinal chord at the base of the spine (the second, third and fourth sacral vertebrae) and through the cranial vertebrae in the neck, whereas the sympathetic nerves emerge from the spinal chord through the middle vertebrae in the back (the T1 to L2 vertebrae). Keeping the spine normally upright, with the head sitting squarely on the top of the vertebral column minimizes the compression of the nerves of these two systems at the points where the nerves emerge through the vertebrae, and ensures an uninterrupted supply of blood, allowing them to function normally. When the parasympathetic and sympathetic systems are both working normally, they function in opposition to give us a state of balance of body-and-mind; not too tense, and not too relaxed, not overly optimistic or pessimistic; not too aggressive and not too passive. It is this physical state of balance in the autonomic nervous system that give rise to what we call a balanced body-and-mind.
In addition to this, sitting in the upright posture, where the force of gravity acts down through the spine onto the pelvis, is a position in which our body’s reflexes can work efficiently to integrate the functioning of the whole body.
(p 11-12 here)