I’ve mentioned it before, but I am, by training, a Physiologist. That means I’ve spent a lot of time learning about the structure and function of body systems. You might have noticed that I like science—all the sciences. I don’t understand nearly as much about most of them as I would like to, but I’ve dabbled enough to know that nothing compares with Physiology for sheer “gee-whiz” factor. If people had any idea how amazing the act of touching their thumb to their forefinger is, they would never do anything else but look at their hands in amazement, tapping their fingers together.
I would never in a million years try to take anything away from how amazing humans are, as a species of organism. We sometimes (often) fail to live up to our potential as sentient creatures, but as living organisms, we are pretty great. Before you get to feeling too impressed about that, though, you should realize that there are a number of things a petunia, for instance, can do that you can’t. Having established our general wonderfulness, I would like to go back to something I alluded to quite a while ago, when we were going through some of that chemistry stuff. Remember the “Biology is really Chemistry, Chemistry is really Physics, and Physics is really hard” joke? It’s true. Pretty much everything we do as organisms is the product of biochemical reactions, from our digestion to brain function to muscle contraction and everything else. When you start looking into things like nervous system function or immunity, it’s easy to get lost in the sheer intricacy of the architecture or the complexity of cellular function, but there is a whole other layer of Physiology on top of that that is even more incredible. It is the level of control. EVERYTHING that goes on in an organism happens exactly when it needs to and only when it needs to. Many of our more dreadful diseases, like cancer, stem from a failure in the normal control mechanisms.
A person is composed of somewhere around 37 trillion human cells, give or take a few, and a similar number of microbes, most of which inhabit the large intestine. Yes, humans contain about the same number of non-human cells, like bacteria, as they do human cells, and that’s usually a good thing. There are skin cells and muscle cells and nerve cells and fat cells and blood cells and all kinds of other cells, each one of which is specialized to perform a particular function. Skin cells protect your insides from the outside, muscle cells contract, nerve cells conduct information, fat cells store fats for energy and so on. In order to perform those functions, these cells have to be able to do many other tasks. They have to be able to use energy, so there are thousands of chemical reactions going on in the cell to allow it to use energy (remember where that energy originally came from?). They have to be able to repair themselves and grow, so there are all sorts of reactions going on with reading information from the cell’s DNA and producing new proteins. There are other reactions that cause muscles to contract or liver cells to produce digestive enzymes or kidney cells to allow more water to leave the body. There is SO much more to all of that, but those are stories for another day. The point I’m trying to make here is that each of those 37 trillion cells is doing something, or it is waiting to do something at the proper time, and all of that activity has to be coordinated so only the right things happen at the right times. We have two systems that work together to control everything else—the nervous system and the endocrine system.
Source: Alan Sved, Wikimedia Commons
Everyone probably has a basic concept of the nervous system—the brain is connected to the spinal cord and the nerves lead in and out to convey sensory information into the system or instructions out to the various parts of the body. In a nutshell, that is what the nervous system does. As you may imagine, there are some details involved that make the process a wee tad more complicated than that, but we can save most of that for, perhaps, another day if anyone is interested. There are a couple of things that are germane to today’s topic, though. One is that your nervous system monitors pretty much everything going on inside and outside your body. It tells you when it’s cold outside. It tells you when you step on a nail. It tells you what you’re looking at, what you’re hearing and what you’re smelling. The sheer volume of physical sensory information coming into your body to inform you of your physical environment is astounding. There is so much coming in that your brain filters out the vast majority of it before you even become aware of it, because most of it isn’t important and you couldn’t process it all if you had to pay attention to every input, all the time. Imagine if you were actually aware of the pressure on your backside all the time while you’re sitting down. In addition to all that external sensory information, your nervous system is also monitoring your internal environment. It is keeping track of your blood pressure, your body temperature, the positions of your arms and legs, the orientation of your body and whether or not it is moving, whether or not you need to eat and a million other parameters. Collecting all that information is only part of the process, though. Once you, or actually your brain (since almost all of this monitoring is subconscious), gets the information, your body needs to respond to it.
Some of the responses will be mediated by the nervous system. Just as sensory information comes into the nervous system, information also goes out to the various parts of the body to make things happen. Your brain may, for instance, cause your muscles to twitch in response to being cold. This twitching (shivering) causes your muscles to generate heat, thereby helping to keep you warm. There are a bunch of other responses to being cold, but this is an example of a nervous system-mediated response to a particular sensory input. Another example is that, when you step on that nail (let’s say you step on it with your right foot), your nervous system will cause certain muscles in your left leg to contract to support your weight while it also causes other muscles in your right leg to contract, pulling your foot off the nail. All that happens by reflex—you have no conscious need to think or do anything. Speed is the primary virtue of a nervous system-mediated response. Since the nervous system sends information by electrical impulses, that information moves very fast, so you can almost instantaneously pull your foot off the nail. The downside is that nervous system-mediated responses tend to be short-lived. One nervous system input causes one thing to happen, then it stops, unless the nervous system sends another signal. The withdrawal reflex that got your foot off the nail, for instance is a single response. Shivering, on the other hand, requires constant input to keep the twitching going.
So, if you need a response that lasts longer, you need another control system, and here is where the endocrine system comes into play. The endocrine system is a collection of specialized structures, generally referred to as glands. Your pituitary gland, thyroid gland, adrenal glands, part of your pancreas and quite a few others are examples. These glands produce chemical signals (generally proteins, but sometimes lipid-based) called hormones that are released into the bloodstream to turn responses on or off. Any cell that is able to “receive” the chemical signal will respond to it as long as it is present in the blood. Depending on what kind of cell it is, it will be responsive to certain hormones and will not respond to others. There are certain hormones produced by the kidneys, for instance, that regulate sodium levels in the blood and blood pressure. We’ve talked about kidneys a little before, so I thought we’d use a renal system as an example. When the amount of blood flow through the kidneys is too low, the kidneys release an enzyme called renin. In the blood, renin (through a multiple step process) converts another molecule that is found in the blood into a hormone called angiotensin Angiotensin can be recognized by the muscle cells lining your arteries, causing the muscle cells to constrict, making your arteries smaller in diameter, which causes your blood pressure to go up, just like running water through a smaller hose (or a nozzle) increases the pressure. By increasing your blood pressure, the flow rate through your kidneys increases, which shuts off the release of more renin. Angiotensin also causes the adrenal glands to release another hormone called aldosterone. Aldosterone causes the kidneys to retain more water, which causes the amount of water in the blood to increase, which also aids in increasing blood flow. All of those reactions, as you might imagine, take a while to exert an effect, so hormones are relatively slow-acting, but as long as the chemical signals are present in the blood, their effect will continue. That’s why renin production stops when blood flow through the kidneys increases. It makes the signaling mechanism self-limiting. No more renin means no more angiotensin production, so as soon as the angiotensin that was produced originally, before the blood flow increased again, is used up and broken down, the effect will stop, the arteries will relax and your blood pressure will return to normal. This pathway is actually the target of many drugs used to treat hypertension. By blocking the renin-angiotensin system, the drugs prevent the increase in blood pressure caused by abnormal activation of the system.
So, in summary, there are two main control mechanisms that direct the activity of all the physiological systems of the body. The nervous system can act very fast, but it only exerts its effect when it is sending signals, so its effects generally don’t last very long–seconds to minutes. The endocrine system, because it has to produce chemicals that then are secreted into the blood and have to find their way to their targets, activates more slowly, but can cause changes that last a long time–minutes, sometimes, but sometimes hours or even days, or longer—as long as the chemical signals are present. Often, the nervous system and endocrine system work together to provide an integrated response that can be very precisely controlled. It is better than magic!