I think I mentioned when I started writing these columns that I am a Physiologist by training and I did research in Neurophysiology in a previous life. Despite that, I haven’t written a single column on physiology, yet. So, let’s fix that. It’ll be fun to play in my own sandbox. So much to pick from…I could go on for days about how your eyes work or how nerves work or how learning and memory function or how breathing is controlled or why you get hungry. Where to start? Kidneys. Kidneys are good.
I think I mentioned way back when that one of the things of biology that inspires such awe in me is how elegant and controlled it is. Almost all biological processes are as simple as they can be, while still being extremely effective and exquisitely controlled. The work of the kidney is a marvelous case in point. Pretty much everyone is familiar with the role your kidneys play in producing urine, but that’s usually as deep as most people’s understanding goes. Unless you’ve had kidney stones. If you’ve had kidney stones, you are probably WAY more aware of your kidneys than most other people. But kidney stones are bad. Bad kidneys! Let’s look at good kidneys, instead.
Mostly what your kidneys do is to control water balance in your body, which also has a significant effect on blood pressure. Your kidneys are also responsible for helping maintain pH levels (acidity) of your blood and body fluids. In addition, you kidneys play a primary role in keeping the levels of electrolytes, like sodium, in your blood and body fluids within a narrow range. Your adrenal glands, which produce adrenaline and a number of other really important hormones are located on top of your kidneys like little hats, but that’s sort of a separate thing, so we will leave that aside for now. Your kidneys do all that, all day, every day, and they only weigh around 4 or 5 ounces apiece. That’s about as much as a hamburger.
Source: Blausen.com
Figure legend: This a a cross-section of a kidney. Blood enters through the renal artery. It passes through the the kidney tissue where toxins are removed, creating urine. How much water is released during the process depends on how well-hydrated you are. The urine is collected in the calyx, which flow into the renal pelvis, which is connected to your ureter, which carries the urine to your bladder for storage until you urinate. The filtered blood passes into the renal veins and back out to the rest of the body.
You might have heard that the human body is about 70% water. It actually depends on whose body we’re talking about, but the average water content in an adult human is about 60% by weight. Babies are more like 75%. Men tend to be a little more watery than women because muscle tissue contains quite a lot of water while fat contains very little and women usually carry a bit more adipose tissue than men, who tend to be a little more muscular. That is a general statement, so I’d rather not hear about my sexist stereotypes from any of you female body-builder types. People in good shape, with lower body fat, have higher percentages of water than people who are a little better padded. A 175-pound (80 kilogram) man, then, would generally contain about 12 and a half gallons (48 liters) of water. About 8 of those gallons (32 liters) are in what is called the intracellular space, or inside the cells of the body. Each of your 35 trillion or so cells that make up your body is basically a little fluid-filled balloon. That leaves 4 and half gallons or so (16 liters) in the extracellular space, or the space outside your cells. This is the fluid surrounding all the cells, filling various spaces (like the spaces around your joints) and, importantly, your blood. That 80 kg guy has somewhere in the neighborhood of 6-7 liters (about 7 or 8 quarts) of blood. Women, who are (again, generally) smaller, also have less blood—more like 5-6 liters.
It is really important to understand that everything is fine if this guy keeps his 48L of water, but if he loses just 4 liters, he will be badly dehydrated. Losing 8 liters is life-threatening. You can lose 4 L of water thorough sweat in about 3 hours of moderate activity on a hot day, assuming you were well-hydrated to start with. It’s much worse for babies, which is why diarrhea is the number one cause of death in small children, worldwide. Babies are little and don’t have a lot of fluid reserve. They can become dangerously dehydrated very easily, and diarrhea, particularly chronic diarrhea, takes lots and lots of fluid from the body. That is why is it so important to make sure kids with diarrhea remain well-hydrated. Anyway, the problem with dehydration is inside your cells. Remember when I was going on and on about water and how important it was in biology? We went through how the chemical reactions inside your cells were the key to everything. Those biochemical reactions, like most chemical reactions have to occur in solution, meaning the chemical compounds that are participating in the reactions are dissolved in the water inside your cells. When the fluid level in your cells drops, your biochemistry gets messed up. Your nerve cells are particularly sensitive to dehydration and start to malfunction pretty rapidly when they are dehydrated. Water can move freely between the inside of your cells (intracellular space) and the outside (extracellular space), so as your total body water goes down, it goes down everywhere at once, so when blood levels are low, it means the level of hydration inside your cells is also low. This is why you can look at the veins on the back of your hand to get an idea on how well hydrated you are (or how dehydrated) you may be. Assuming you can normally see the veins on your hands, if they are puffed up, you’re hydrated. If they are flatter and harder to see than normal, you are most likely dehydrated.
Anyway, your body realizes how important it is to maintain proper fluid levels in your body and it has mechanisms to help you to take in more water when you need it and get rid of excess water when there is too much. Your kidneys are the primary organs responsible for this. All the blood in your body passes through your kidneys about every 6 minutes or so, meaning your total blood volume goes through your kidneys about 10 times an hour. The primary reason why your blood passes through your kidneys is that your kidneys are responsible for removing toxins from your blood. Lots of pharmaceuticals are processed by the kidneys, which is why you sometimes have to get a kidney function test before you can take certain drugs, and also why certain drugs can damage your kidneys (the rest of the toxins you take in are removed by your liver). If your kidneys aren’t working well, they may not be able to remove the drugs, or their breakdown products, from your blood and the levels could build up and become toxic. One of the main “natural” toxins your kidneys get rid of is called urea, which is something your body produces when it breaks down proteins. Urea is pretty toxic and we make a lot of it, so your kidneys have to get rid of it. Urea is why urine is called urine. It’s also partially responsible for much of both the color and smell of urine. In addition to removing toxins, however, the kidneys also determine if we have the proper amount of water in our bodies. If the amount of blood passing through the kidneys is low, the kidneys will sense this and release chemical signals (hormones) that do several things. One thing that happens is that, when the chemical signals get to your brain, your brain tells you that you are thirsty and you’ll try to find some water to drink. By the way, by the time you feel thirst, you are already slightly dehydrated, so you should drink before you get thirsty, particularly if you are sweating. The second thing the chemical signals do is to tell your kidneys to stop making so much urine.
When you are well-hydrated, your kidneys may produce 2 liters a day of dilute urine, or even more. When you start to become dehydrated, your kidneys will adjust and start producing a much more concentrated urine to preserve water in the blood. One indicator of the state of your hydration is the color and odor of your urine (and how much you produce). When you are well-hydrated, your urine will be clear, almost colorless to very pale yellow and fairly odorless. When you are moving toward dehydration, your urine is darker and has a stronger smell. So if your pee is dark, or deep yellow and has a significant odor, it is likely a sign you are dehydrated and need to take in fluids. Your kidneys can only retain so much water, though. In order to remove the toxins from your blood, your kidneys MUST produce somewhere around 400-500 milliliters (about a pint) of urine a day in order to flush the toxins out. If you’ve ever heard the thing about the guy trapped in a car or whatever who survived by drinking his urine, you should know that doesn’t work very well. It would probably be helpful if he drank the first urine he produced after getting trapped (before he started getting dehydrated), because it is mostly water with relatively small amount of toxins. Once he starts getting dehydrated though, and the toxin levels rise, it’s actually worse to drink the urine, because there is less water recovered and the toxins go back into the blood and cause all sorts of problems.
I already mentioned that babies are prone to dehydration, but another group that experiences a lot of dehydration is old folks. Older people often have aches and pains, so they tend to do a bit of a cost-benefit analysis when they think about getting up and moving around. When a healthy person gets thirsty, they get up and get a drink. When a creaky old person gets thirsty, they might think that if they wait a while, they will be hungry, too, or maybe they’ll be thirsty, hungry AND have to go to the bathroom, so they wait before they get up and get it all done at once. Less cost, more benefit. The problem is that older folks tend to have lost some muscle mass, so they, like the children, have less excess fluid reserve than younger adults, and they get dehydrated more easily. Emergency rooms see a lot of older people come in, seemingly in late-stage dementia, who walk out in a couple hours, right as rain, after getting some IV fluids. Brain cells are some of the most sensitive to dehydration, so cognitive issues are a common symptom.
The lesson to be learned here is to stay hydrated. You won’t wear out your kidneys and the consequences of dehydration can be very serious, indeed. Staying well-hydrated also helps prevent kidney stones, and I promise you do not want kidney stones. Drink before you get thirsty. Pay attention to your pee. It should be pretty colorless and odorless. Look out for the old folks. Make sure they are getting enough to drink. A final note for you younger females—some of you have a tendency to under-hydrate, as well, for whatever reason. If you restrict your water intake because you think it might lead to weight loss—don’t. It won’t. Drink plenty of water and your kidneys will remove any excess water you don’t need. Remember, the consequences of dehydration can be serious.