When we stand up quickly why do we sometimes feel light-headed for a moment or two?
The laws of nature demand that for your organs to work properly they must have enough energy. Your cells get this energy from the glucose and oxygen carried to them from within your blood. Normally, your heart pumps blood with enough pressure to push it throughout your circulatory system so that all of your cells can get enough of what they need to live and work properly.
Gravity is a force of nature that pulls all matter towards the ground. When you’re in the upright position (your head is above your heart) gravity works against the blood as it tries to flow up from your heart, through the arteries, and into your brain. Without enough blood flow to your brain it begins to malfunction and you feel light-headed. In normal circumstances, even though you’re in the upright position, your heart generates enough force (blood pressure) to overcome gravity so that there’s enough blood flow to your brain and you can continue your activities.
But, when you stand up quickly, gravity also prevents the blood in your legs from returning to your heart. With less blood coming back to the heart from the tissues this can significantly reduce how much blood your heart can pump out with each beat.
So, on standing up quickly, the effects of gravity reducing the blood return from your legs to your heart, and with it, how much blood it can pump out, and resisting blood flow from your heart to your brain, can sometimes combine to drop your blood pressure and blood flow to your brain low enough to cause you to feel light-headed. Common experience teaches us that since this light-headedness usually lasts only a second or two, we know that our body must know what to do to correct it. So how does it do it?
Your body has sensors in the main arteries leading to your brain which detect your blood pressure and sends this information to your brain for analysis. When the blood pressure drops low enough so that it may compromise the blood flow to your brain (and result in malfunction) your brain responds by sending out specific nerve messages throughout your body at one-third the speed of sound. These nerves release a neurohormone called norepinephrine which, like a key in a lock, attaches to specific adrenergic receptors in the heart, the veins and the arterioles and signals them to do something to correct the situation. What do you think that would be?
Remember, from the last article, that the three main factors that affect the blood pressure are:
1) how much blood the heart pumps out,
2) the amount of blood within the arteries (as opposed to the veins), and
3) the vascular resistance applied by the downstream arterioles.
The effects of norepinephrine are:
1) to make the heart beat harder and faster to pump out more blood,
2) to make the veins transfer blood to the arteries, and
3) to make the muscles surrounding the arterioles tighten up more to increase the vascular resistance.
In real life, these three quickly performed actions are usually enough to raise the blood pressure and preserve the blood flow to your brain so that your light-headedness only lasts a moment or two.
Three Questions for Mr. Darwin
- Where did the information come from to tell my body how to make the sensors that detect my blood pressure and place them exactly where they’re needed to be so my brain can control its blood flow?
- How does my brain know how low its blood pressure can go before its blood flow (and function) may be compromised and how does it know what to do about it?
- How did intermediate organisms develop this blood pressure control system one step at a time if all of the parts (the sensors in the arteries, the brain’s integration and ability to produce and release norepinephrine, and the adrenergic receptors in the heart, the veins and the arterioles) had to be present in exactly the right locations and do exactly the right things fast enough for survival?
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Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.
Copyright 2017 Dr. Howard Glicksman. All rights reserved. International copyright secured.
