When one stands up, until the body’s normal mechanisms kick into gear, blood begins to pool downward. The first place that
looses blood perfusion is the highest place in the body. That’s the head for a person sitting or standing. This can best be understood
thought the use of a simple analogy.
The Body as an Inflatable Jumper:
Indeed, the cardiovascular system can be thought of a little like a kid’s birthday party jumper castle.
The castle only stays “plumply inflated” if:
- the sum of the parts that hold the air inside (the tank) doesn’t expand,
- there is no more than a small leak from the tank that is relatively constant and
- there is a pump that is constantly pumping more air into the tank at a sufficient rate to replace the loss.
Think of what happens when the circuit breaks and the pump stops pumping. The jumper tank immediately begins to deflate. Likewise, if the jumper workmen come along and decide the castle needed to be twice as big and opened a valve suddenly to inflate a new part of the tank or if one of the kids plays a prank and opens a valve such that air escapes more rapidly than can be pumped in, then the castle begins to deflate. Indeed, the parts that crumple first are the castle turrets which are at the top of the jumper.
The sum total of all the arteries in the body, large and small, are like the tank of the jumper. These parts need to be pumped “plump-full” in order for the part at the top, the brain, to function. If the pump, the heart, is pumping strongly and at the right rate, it keeps the exact amount of blood pumped from the venous pools into the arteries to keep the tank plump-full. But if the heart rate suddenly becomes too low, then the tank, which is always returning blood back to the veins after the nutrients are used, loses it plumpness and deflates – particularly in the turrets (the brain). Likewise, if parts of the tank are suddenly repositioned higher above the pump, such a when a person stands suddenly, the turrets are always the first to pay the price with some deflation – if only briefly. So perfusion of blood to the brain is briefly interrupted in this way.
This is where the vagus nerve comes into the story. It is responsible for fine-tuning the heart rate and the diameter of the arteries (vascular tone) which is another way of saying the size of the tank. Activation of the vagus nerve (increased vagal tone) slows the heart rate and dilates vessels. Removal of vagal tone increases heart rate and constricts vessels. This fine tuning keeps the blood pressure exactly where it needs to be in order to perfuse the brain. When a person stands up quickly, the vagus nerve must act instantaneously to cause the heart rate to increase and blood vessels to constrict in order to ‘keep the turret plumply pumped’. If it doesn’t do this just right, perfusion of the brain is poor and the series of symptoms sometime leading to fainting begins.