We live in a world made from matter. Matter is made up of atoms and molecules that follow the laws of nature. All life is made up of atoms and molecules that are organized into cells. Our body has trillions of them. Water is vital for life. Most people know that by drinking water it is brought into the body by the digestive system. And many people know that the digestive system puts water into the blood where it is sent to the organs and tissues by the cardiovascular system. There are even some who know that it is the kidneys that are involved in regulating the bodyís water content. However, what most people do not know, and appreciate, is where water is located in the body and how the body actually controls its water content. Having the right amount of water in the body is not as simple as just drinking it. Nor is it as simple as just being able to swallow water and having properly working digestive and cardiovascular systems and kidneys. Without the body being able to control its water content life as we know it would be impossible. The proof of this is that when our body is no longer able to control how much water it has, we die. In other words, control is the key to life. But how does the body do it? One of the most important sets of molecules which work to give the body control are hormones. Letís first look at what is needed to control something and then weíll see where hormones fit into the scheme to keep us alive.
To be able to control something requires having at least three different parts all working together in harmony. The first thing you need is a sensor to detect what needs to be controlled. If you have no way of being aware of what needs to be controlled how can you control it? The sensor is like the reconnaissance team that an army sends out to check on the whereabouts and activities of its enemy. Without this information the army would be working in the dark. The second thing you need to control something is an integrator which interprets the information from the sensors, makes decisions about what needs to be done, and then sends out orders. If you donít understand the information from the sensors and canít make decisions about what should be done then what use are the sensors in the first place and how can you control something? The integrator is like army headquarters, where the information from the reconnaissance team is analyzed, decisions are made about what needs to be done and orders are sent out. Without army headquarters there would be no coordinated action in the field. The third thing you need to control something is an effector which receives the orders from the integrator and does something. If you have a sensor to detect what needs to be controlled and an integrator to know what needs to be done, but not an effector to do it, then whatís the use of having the first two and how can anything be controlled? The effector is like the soldiers, who at the orders received from headquarters go and do what needs to be done. Without soldiers there is no army and the battle is already lost.
Hormones are protein molecules that are sent out by special gland cells into the blood to help regulate specific functions of the body. The hormones are chemical messengers sent out by gland cells just like the orders sent out by army headquarters. The gland cells have sensors on their surface that can detect how much of a specific chemical (like water) is present in the blood. So, the gland cells have their own reconnaissance team that can detect a specific chemical which the body must control to survive. The gland cells take the information from their sensors, analyze it, and then send out the right amount of a specific hormone into the blood. The gland cells act as the integrator, just like army headquarters, to send out orders to direct activities in the field. These actions, done at a distance from the gland cells, are designed to achieve a specific goal; the control of a specific chemical (like water) so the body can stay alive. The hormones from the different gland cells travel in the blood to specific target organs to pass on their orders. The cells in these organs act as the effector, which, like the soldiers in the field, receive the orders and perform a specific action. This effect, done at the direction of the integrator, helps to control the specific chemical (like water) that was sensed by the gland cells which sent out the hormone in the first place.
However, army headquarters must send out different orders to different soldiers telling them to do different things. So too, the bodyís different gland cells must send out different messages to different target cells to get different things done. And just as the soldiers canít take just any message or do whatever they want, the target cells must respond to the right message and do the right thing, otherwise the body wouldnít be able to control anything. The way the body ensures that the right target cells receive the right orders so they can do the right thing is for them to have specific receptors. The receptors in the target cells are proteins with a special shape that allow them to attach to specific molecules when they come in contact with them. Think of it like a key fitting into a lock, or tuning your radio or television to a specific station. When the hormone attaches to its specific receptor this signals the target cell to do something. And what the target cell does directly affects the specific chemical (like water) that the gland cell which sent out the hormone detected in the first place. Now weíll look at why the body must control its water content and how it actually does it. Be prepared to exercise your wonder as you never have before!
Water is our bodyís most abundant molecule. It makes up 60% of the body by weight. This means that a man weighing 70 kg (154 lbs) has about 42 L (liters) of water in his body. The water in our body is located in one of two places; it is either inside our cells or outside our cells. Two-thirds of our bodyís water is called the intracellular fluid because it is located inside our trillions of cells. This means that a man weighing 70kg has a total of about 28 L of water in the cells of his body. Each human cell is surrounded by a very thin wall called the plasma membrane. The plasma membrane defines the limits of the cell and separates it from other cells and the outside world. The main substance of the cell is water. All the different parts of the cell are suspended within its water. The water in the cell also acts as a solvent for dissolved chemicals that are important for life. These chemicals, in solution, include sodium and potassium ions and many different proteins. The amount of water that takes up space in the cell is called its volume. The total number of chemical particles dissolved in a given volume of the cellís water is called its total chemical concentration. If water leaves the cell, its volume falls and its total chemical concentration rises. And if water enters the cell, its volume rises and its total chemical concentration falls. To live and work properly the laws of nature demand that each cell keep its volume of water and total chemical concentration relatively constant.
The remaining one-third of our bodyís water is called the extracellular fluid because it is located outside our cells. This means that a man weighing 70 kg has about 14 L of water outside the cells of his body. About 20% of the water outside the cells is in the blood. This is called the plasma. Water provides a way for the body to feed its cells with what they need to live. Oxygen enters the blood from the lungs and attaches to red blood cells to be transported to the tissues. But the red blood cells need the water thatís in blood to be able to float inside the blood vessels. The digestive system takes in vital atoms and molecules (like salt and sugar) and places them in the blood. But these chemicals must dissolve in the water thatís in blood so they can be transported to the tissues. If there isnít enough water in the bloodstream then the body may not be able to properly feed the tissues due to low blood pressure, and consequently, low blood flow. The remaining 80% of the water outside the cells is located around and between the cells and is called the interstitial fluid. By the laws of nature water is able to pass freely through biological membranes. This means that water can pass from the blood into the interstitial fluid and from there into the cells and back again. In other words, the interstitial fluid acts as a bridge between the circulation and the cells, allowing water to pass through it in either direction. If there is too much interstitial fluid in some organs, like the brain and the lungs, we can die. Cerebral edema is a condition where there is too much water between the cells in the brain. This causes pressure on the nerve cells which can lead to significant brain malfunction and even death. Pulmonary edema is a condition where there is too much water between the cells in the lungs. This makes it very difficult for proper gas exchange which can lead to significant weakness, shortness of breath and even death.
Just as our body requires a certain amount of oxygen to live, grow and work properly so too, it needs to have the right amount of water as well. As noted above, each of our cells must regulate its volume and total chemical concentration within a narrow range to survive and function properly. Having too much water, or too little water, in the cell can be fatal to it. In addition, the amount of water in the circulation must be regulated to be sure thereís the right amount of blood flow to the tissues. And having too much water in the interstitial fluid of certain vital organs, like the brain and the lungs, can result in death as well. So, having the right amount of water, inside and outside your cells is very important for life.
The body is always losing water from the circulation mainly due to three processes, each of which is absolutely necessary for life. The first process is respiration. When glucose releases energy in the presence of oxygen it forms carbon dioxide and water. Having too much carbon dioxide in the body can be deadly so it must be breathed out regularly. But every time we breathe air out of our body we also release some water vapor as well. So, the amount of water we lose by respiration depends on how hard and fast we are breathing, which depends on what we are doing. The second process is perspiration. The body must maintain its core temperature within a narrow range so its organs and tissues can work properly. The chemical reactions the body uses to live and work properly always give off heat which affects the bodyís core temperature. The more active the body is, the more heat it gives off. One way for the body to control its core temperature is to release heat through perspiration. When we perspire our body places water on the surface of the skin. Heat is then released from our body as it is used to evaporate the water on our skin into the surrounding air. The more active we are and the hotter and more humid it is, the more water we must release through perspiration to help keep our core body temperature under control. The third process is the formation of urine in the kidneys. Protein metabolism in the body produces a nitrogen containing chemical called ammonia which is converted in the liver into a more soluble chemical called urea. Just like carbon dioxide, the build-up of ammonia and urea in the body can be toxic and cause death. The kidneys constantly filter some of the water in the blood and place it into millions of tiny tubules. This fluid moves through the tubules and becomes more concentrated with urea (and other chemicals) as it becomes urine. The body produces a daily amount of urea (and other nitrogen containing chemicals) which it must get rid of through the urine. Since the kidneys are limited in how much they can concentrate chemicals there is a minimum amount of urine the body must excrete to rid itself of the toxins it makes on a daily basis. Clinical experience teaches that the average male must produce at least 500 mL of urine daily, no matter how much water he has in his body, to be sure he gets rid of these toxic chemicals. On top of that, at rest, he usually loses at least another 500 mL of water through respiration and perspiration as well.
So, at rest, our body loses, at a minimum, about one liter of water from its extracellular compartment through respiration, perspiration and urine formation. If we have a fever and diarrhea we lose even more water. And if we are vomiting as well, not only do we lose more water, but we canít replace it by drinking. Despite the body always losing water from the extracellular space it is able to make adjustments so that it doesnít have to take in a new supply right away as it does for oxygen. When water is lost from the circulation and the interstitial fluid the total chemical concentration in the extracellular fluid rises above what is inside the cells. This higher concentration of total chemicals makes water naturally move out of the cells into the extracellular fluid by osmosis. This movement of water from the intracellular to the extracellular fluid causes the total chemical concentration on both sides to become the same (at a level somewhere between the original two). It also returns the ratio between the intracellular and the extracellular fluid back to 2/3:1/3. In doing so it helps to partially replace the water losses from the circulation. So, water coming from the cells to the circulation allows the body to maintain an adequate blood pressure and blood flow so it can feed the tissues with what they need.
When the cells give up water to the circulation it causes their volume to drop and their total chemical concentration to rise. But, as noted above, to live, grow and work properly our cells must maintain their volume and total chemical concentration within a narrow range. Therefore there is a definite limit to how much water the cells can give up to the extracellular fluid before they begin to malfunction and die. Recall, the average male has about 42 L of water in his body. If he loses 10% (4 L) he will usually become sluggish and tired. A loss of 15% (6 L) will probably make him weak, dizzy and confused. Let it progress to 20% (8 L) and heís likely to become lethargic and poorly responsive. Further progression to 25% (10 L) and heís most likely to die from dehydration. So how does our body make sure we have enough water so we can survive within the laws of nature?
One way for our body to take care of its water content is to make us drink liquids. The thirst center in the hypothalamus receives information from various sources and tells us when to drink. Water is readily taken up (absorbed) by the digestive system and is put into the bloodstream where it can easily pass into the interstitial fluid. With the intake of water, its movement into the extracellular fluid makes the total chemical concentration within it lower than what is within the cells. The lower concentration of total chemicals makes water naturally move out of the circulation and interstitial space into the cells by osmosis. The movement of water from the extracellular to the intracellular fluid causes the total chemical concentration on both sides to become the same (at a level somewhere between the original two). It also returns the ratio between the intracellular to the extracellular fluid back to 2/3:1/3. This natural movement of water into the cells replenishes them with what they had previously lost to the circulation through respiration, perspiration and the formation of urine.
Another way to help control the water content of the body is through the kidneys, which are always making urine. Experience teaches us that if we have been working or playing outside in the heat and havenít been drinking liquids, we form small amounts of very concentrated urine. In contrast, if we are relaxing in an air-conditioned room and drink several ounces of our favorite beverage, we soon pass large amounts of very dilute urine. So, how do the kidneys know when to form small amounts of very concentrated urine when we are at risk of becoming dehydrated, and large amounts of very dilute urine to prevent us from having too much water in our body? Letís see how the body does it.
Recall, the first thing you need to take control is to have a sensor that can detect what needs to be controlled. As noted above, when the total chemical concentration in the blood is higher than in the cells water naturally moves, by osmosis, from the cells into the blood to make both sides equal. This causes the cells to shrink a little. In contrast, if the total chemical concentration in the blood is lower than in the cells, then water naturally moves, by osmosis, from the blood into the cells to make both sides equal. This causes the cells to expand a bit. One can therefore see that the osmotic pull of water into, or out of, the cells is directly related to the relationship between the total chemical concentration in the blood and the cells. The movement of water in either direction results in both sides ultimately having the same total chemical concentration which is somewhere between the original two. But it also results in both sides having a different amount of water as well, returning the ratio between the intracellular to the extracellular fluid back to 2/3:1/3.
There are sensory nerve cells in the hypothalamus that can detect these changes in cell volume brought about by osmotic forces. They are called osmoreceptors and what they detect in the way of water movement into, or out of, the cell affects the frequency of their nerve impulses. The more water loss the osmoreceptors detect as the cell shrinks, the stronger the nerve messages they send out. And the less water loss they detect as the cell shrinks less or even expands, the weaker the nerve messages they send out. By being able to monitor cell volume and the movement of water into, or out of, the cell the osmoreceptors are able to indirectly sense the total chemical concentration of the blood and with it the amount of water that is present in the body.
Recall, the second thing you need to take control is something to integrate the data by comparing it with a standard and then deciding what must be done. The osmoreceptors in the hypothalamus send nerve impulses to the posterior pituitary gland. The stronger the signal the more hormone it releases and the weaker the signal the less hormone it releases. The hormone the posterior pituitary gland sends out is called Anti-Diuretic Hormone (ADH). Diuretics are chemicals that cause more urine formation. Therefore, an anti-diuretic is a chemical that causes less urine formation. When there is more water loss and cell shrinkage, the posterior pituitary gland responds to the stronger messages from the osmoreceptors by sending out more ADH. In contrast, when there is less water loss as noted by less cell shrinkage or even expansion, the posterior pituitary gland responds to the weaker messages from the osmoreceptors by sending out less ADH.
Recall, the third and final thing you need to take control is an effector that can do something about the situation. One thing that ADH does is stimulate the thirst center to tell you to drink liquids to increase your water content. There are many other chemicals that affect thirst as well. A high level of ADH will make you very thirsty whereas a low level of ADH will tend not to stimulate your thirst at all. ADH also travels in the blood to the kidneys and attaches to specific receptors on some of its tubules and tells them to bring more water back into the body from the urine that is presently in production. The more ADH sent out by the posterior pituitary gland the more water is taken back into the body by the kidneys and the less urine is produced. And the less ADH sent out by the posterior pituitary gland, the less water is taken back into the body and the more urine is produced.
So, as the body loses more water, through respiration, perspiration and urine formation, more water goes from the cells into the blood. This makes the osmoreceptors shrink more and send out stronger nerve messages to the posterior pituitary gland. The stronger nerve messages from the osmoreceptors make the posterior pituitary gland send out more ADH. More ADH in our blood makes us more thirsty and tells our kidneys to hold onto more water and make smaller amounts of very concentrated urine. And when the body brings in water from the digestive system, more water goes from the blood into the cells. This makes the osmoreceptors expand more and send out weaker nerve messages to the posterior pituitary gland. The weaker nerve messages from the osmoreceptors make the posterior pituitary gland send out less ADH. Less ADH in our blood reduces our thirst and tells our kidneys to let go of more water and make large amounts of very dilute urine. This is why we produce small amounts of very concentrated urine when we work or play hard outside in the heat without drinking liquids and large amounts of very dilute urine when we relax and drink lots of fluids.
Finally, it is important to keep in mind that our kidneys filter about 180 liters of water from the blood every day (7.5 L/hr) and normally release about 1 to 1.5 liters of urine. Recall, the average male dies if he loses just 25% of his total water (10 L). In other words, if none of the fluid filtered by the kidneys were brought back into the body we would die in just under 90 minutes. Normally, about 90% of the water filtered by the kidneys is brought back into the body regardless of the its overall water content. But that still leaves the remaining 10% to manage. Clinical experience shows that a person with a total absence of ADH puts out about 20 L of very dilute urine per day. This amount represents about one-half of their total body water content (42 L) and twice the loss needed to die from dehydration (10 L). If a person were unable to replace any of this water loss they would die in about 12 hours. One can therefore see that the mechanism behind the production and regulation of ADH secretion plays a vital role in the bodyís ability to control its water content so it can survive within the laws of nature.
Points to Ponder
The way our body makes sure it has enough water for its cells and circulation is not just as simple as drinking water. Neither is it just as simple as having properly working digestive and cardiovascular systems and kidneys. To control its water content the body must also have (1) special cells in the hypothalamus that can (2) sense the osmotic pull of water into and out of the cell and can (3) send appropriate nerve messages to (4) special cells in the posterior pituitary gland that can produce (5) ADH and send it into the blood to (6) the thirst center in the hypothalamus and (7) special cells in the kidneys which have (8) ADH receptors. If any one of these eight parts is missing the whole system fails and the body dies because it canít control its water. Each part that contributes to the sensor, the integrator, and the effector is needed to perform its vital function for body survival.
Dr. Michael Behe calls such a system, where the absence of any one part renders it useless, as being irreducibly complex. One must then wonder how an irreducibly complex system with so many vital parts could have come into existence? Does it make sense that this system could have come about one step at a time? First the sensor, with no integrator or effector, or the integrator with no sensor or effector, or the effector with no sensor or integrator? The idea is totally absurd. They must have all come together as a system to perform a function to keep the body alive. And which system came first? The ones mentioned last time for oxygen transport and blood glucose control, or this one for water? Remember, if any one of these systems is absent or not working properly, we die. In addition to these there are many other irreducibly complex systems each of which is absolutely vital for life. There are control systems in the body for sodium, potassium, calcium, blood pressure and temperature just to name a few more. Each of these systems has its own sensor(s), integrator(s) and effector(s). And if just one of these parts is missing the whole system fails and the body dies. But if a system is irreducibly complex does that automatically make it capable of supporting life? If you think about it youíll realize that thereís one more piece of the puzzle thatís needed; a piece thatís beyond irreducible complexity, to enable these systems to keep us alive within the laws of nature.
Imagine if the flow of water in your home were only a trickle. Do you think you and your family would have enough for drinking and all your other household needs besides? The average adult uses about 200-300 L of water per day for drinking, washing herself, her dishes and clothes, toileting, and even brushing her teeth. To have enough water requires that the flow of water into her residence be fast enough to meet her needs. Real numbers have real consequences when it comes to dealing with the laws of nature. Based on what we know about how the body actually works our ancestorsí ability to survive and reproduce depended on them producing at least 500 mL of urine per day to get rid of enough toxic chemicals. And to prevent death from dehydration meant that their kidneys couldnít let go of a lot more than the normal amount of 1 to 1.5 L per day of urine as well. In other words, not just any amount and quality of urine production would have been adequate. For our ancestors to survive and reproduce they would have needed to produce the right amount of urine with right amount of chemicals for the right set of circumstances. But what if the system that uses ADH to control the bodyís water content were set differently? What if the amount of ADH produced, or its effects, didnít let the kidneys produce enough urine to get rid of enough toxins? Alternatively, what if the amount of ADH, or its effects, allowed too much water be released from the body resulting in dehydration? Clinical experience teaches that our ancestors could never have been able to survive and reproduce.
Real numbers have real consequences when it comes to dealing with the laws of nature. For, not just any amount of water is enough to keep the body alive. It has to be the right amount. Just because a system is irreducibly complex does not automatically mean that it will be able to function well enough to allow for life. Besides being irreducibly complex, systems that allow for life must also have a ďnatural survival capacityĒ. By this I mean that each system must give the organism the capacity to survive by taking into account the laws of nature. This usually involves having knowledge about what is needed to keep the organism alive within the laws of nature and then being able to do what needs to be done. The system that uses ADH to control the bodyís water content seems to inherently know that the kidneys must produce at least 500 mL of urine per day to get rid of enough toxins and it does it naturally. It also seems to know that, in normal circumstances, it must not release much more than 1-1.5 L of urine per day to prevent dehydration, and it does it naturally. The same can be said for each of the other control systems that manage oxygen, glucose, sodium, potassium, calcium, blood pressure and temperature as well. Not only are each of these systems irreducibly complex with a natural survival capacity, but without any one of them the body dies.
The laws of nature have put up many obstacles to prevent life from existing. Just as a car can die from not having enough gas for energy, or oil for seizing parts, or anti-freeze for engine overheating, so too, all physicians know that there are many different ways for us to die. If you really want to begin to understand how life came into existence, you first have to understand how easily it can become non-existent. Did life really come about by random chemicals coming together to form cells, then simple organisms, and then complex ones like us? If you were walking along a beach and saw your name clearly marked out in the sand would you think that it had come about only by the forces of the wind and the waves? Science still has a lot of explaining to do!
Be sure to catch all of the articles in Dr. Glicksman's series, "Beyond Irreducible Complexity."
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.
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