We live in a world made up of matter. Matter consists 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. Everyone knows that we must breathe in air, drink liquids, and eat food to stay alive. And most people know that it is the oxygen in the air and the water, sugar and other chemicals in what we eat and drink that our body uses to live. Some people even know that all these chemicals must enter the blood and be sent to the tissues so the cells can get what they need to live and work properly. But what most people do not understand, and appreciate, is how the body must contend with the laws of nature to make sure the cells, especially the ones in the brain, are properly supplied with blood. After all, blood consists of chemicals dissolved in water with many different cells floating within it. Therefore, like all matter, blood has mass and is subject to the laws of nature. Blood is subject to inertia, the force that keeps an object at rest if it is already at rest. Blood cannot move unless it is powered to where it is supposed to go. Blood is subject to friction, the force that resists the motion of two moving objects or surfaces that touch. For blood this is called vascular resistance since it resists the flow of blood within the various blood vessels of the body. Finally, blood, especially in the regions above the heart (like the brain), is subject to gravity, the force that makes an object fall to earth. When blood tries to flow to regions above the heart gravity makes it fall down toward the ground.
Blood must overcome the forces of nature, like inertia, friction and gravity, to get to where it is needed. It needs something with enough energy to overcome these natural forces to move it there. Blood pressure, the force which blood applies against the walls of the arteries as it flows through them, is a physical sign of this energy. We have all experienced momentary dizziness when we stand up too quickly. This takes place from having too low of a blood pressure, and consequently, a significant reduction in blood flow to the brain. This happens due to the force of gravity pulling the blood, that is trying to move from our heart to our brain, down to the ground. Having the right blood pressure that results in the right blood flow to the tissues, like the brain, is vital for life. The proof of this is that when our body loses control of its blood pressure, especially when it goes too low, we die. In other words, control is the key to life. But how does the body do it?
As noted in previous articles, one of the most important sets of molecules which work to give the body control are hormones. However, in addition to the hormones released by different glands to control its metabolism, the body also uses nerve cells, which release chemicals called neurohormones, to maintain control as well. After all, the time needed for the glands and their hormones to react and take effect usually requires several minutes. But, when you have just stood up and are about to pass out because your blood pressure has dropped too low, your body has only seconds to react. For our remote ancestors, not being able to stand up and fight, or stand up and run away, would have made life impossible. It would have been the difference between them eating or being eaten. Letís first look at what is needed to control something and then weíll see where the nervous system and some of its neurohormones 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 gland cells into the blood to help regulate specific functions of the body. The last few articles have shown how hormones control the amount of hemoglobin in the blood, the bodyís water, sodium and potassium content, and the blood level of glucose and calcium. These processes take place over several minutes and are continually being adjusted by the body to match its ongoing needs. However, when it comes to managing something like a falling blood pressure on standing up, the body must react much more quickly, usually within seconds, to maintain the blood supply to the brain. The force of gravity waits for no one. It brings everything, and everyone, down to earth. Nerve cells can transmit messages much faster than gland cells, literally in a split second or two. Just like gland cells, nerve cells release different chemicals, called neurohormones, that stimulate or inhibit muscles, glands or other nerve cells. The neurohormones are chemical messengers sent out by nerve cells which are just like the orders sent out by army headquarters. Specialized nerve cells, located in strategic places in the body, have sensors on their surface to detect certain physical signs (like blood pressure). In other words, the nervous system has its own reconnaissance team that can detect a specific physical sign (like blood pressure) which the body must control to survive. The data from these special nerve cells is sent rapidly along nerves to the brain where it is analyzed. The brain then sends out other nerve messages to the target tissues. The nerve cells in the brain act as the integrator, just like army headquarters, to send out orders to direct the activities in the field. These actions, done at a distance from the sensory nerve cells, are designed to achieve a specific goal; the control of a specific physical sign (like blood pressure) so the body can function properly and stay alive. The specific messages from the brain travel along specific nerves which release specific neurohormones in the specific target organs to pass on their orders. The cells in these specific 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 physical sign (like blood pressure) that was sensed by the specialized nerve cells 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 nerve and 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 neurohormone attaches to its specific receptor this signals the target cell to do something specific. And what the target cell does directly affects the physical sign (like blood pressure) that the sensory nerve cells detected in the first place. Now weíll look at how the laws of nature affect blood pressure and how the body takes control to make sure it has the right amount to stay alive and keep standing. Be prepared to exercise your wonder as you never have before!
The cardiovascular system (consisting of the heart and the blood vessels) provides the means for the body to feed its trillions of cells through the blood. The blood travels down the arteries and enters tiny arterioles which in turn empty into the microscopic capillaries inside the tissues. It is within the capillaries that the exchange of oxygen and nutrients takes place between the circulation and the cells. The right side of the heart pumps blood to the lungs through the pulmonary arteries to pick up oxygen and release carbon dioxide and then it returns to the left side of the heart through the pulmonary veins. The left side of the heart pumps blood to all the other organs and tissues through the systemic arteries and picks up its supply of sugar, proteins, fats and other nutrients from the digestive system on its way back to the right side of the heart through the systemic veins. This circuit is repeated over and over again. http://www.youtube.com/watch?v=PgI80Ue-AMo
The blood pressure is the force which the blood applies against the walls of the large systemic arteries (as opposed to the pulmonary arteries) as it flows through them. This is no different than the pressure you feel against the walls of a pipe as water flows through it. And, just as there must be enough water pressure in your home to provide enough water flow to service your needs, so too, there must be enough blood pressure in the large arteries to provide enough blood flow to service the needs of your organs and tissues. In other words, the blood pressure is directly related to, and is the driving force that results in, the blood flow to the cells of the body. Also, the blood pressure is related to mainly three things; (1) how hard and fast the heart pumps, (2) how much blood is in the arteries, and (3) how much resistance the arterioles apply to the blood as it moves to the tissues.
(1) The amount of blood the heart pumps, or cardiac output (CO), is directly related to the heart rate (HR) and how much comes out with each contraction (stroke volume = SV). In fact, the cardiac output can be calculated by multiplying the heart rate by the stroke volume (CO = HR x SV). One can see that if the heart rate and/or the stroke volume increases, the cardiac output increases. And if the heart rate and/or the stroke volume decreases, the cardiac output decreases. In other words, if the heart pumps faster and harder more blood comes out of it. And if it pumps slower and not as hard then less blood comes out of it. An increase in the cardiac output usually causes more blood to enter the arteries and generally results in an increase in the blood pressure. And a decrease in the cardiac output usually causes less blood to enter the arteries and generally results in a decrease in the blood pressure.
(2) The amount of blood in the arterial system is dependent on how much water is in the body and also the relative distribution of blood in the cardiovascular system. Usually, two-thirds of the bodyís water is inside the cells and one-third is outside the cells. Also, of the water outside the cells, usually about 20% is in the circulation. One can then see that if there is more water in the body, there is usually more water in the circulation. And with more water in the circulation, there is usually an increase in the blood pressure. And when there is less water in the body, there is usually less water in the circulation and a decrease in the blood pressure. In addition, at rest, the systemic arteries usually have about 12% of the bodyís total blood supply whereas the systemic veins have about 60%. One can then see that if more blood shifts from the veins into the arteries that the blood pressure is likely to rise. Conversely, if more blood shifts from the arteries into the veins, then the blood pressure is likely to fall.
(3) The blood coming out of the heart flows down the arteries until it meets resistance in the arterioles that feed the tissues. The blood then bounces back towards the heart where it meets resistance at the closed aortic valve and rebounds back to the arterioles again. This ping pong effect of the blood bouncing back and forth between the arterioles and the heart continues while the heart is resting and filling up with blood while waiting to pump again. This vascular resistance to blood flow, applied by the arterioles, tends to make the blood stay within the arteries. One can then see that an increase in the vascular resistance by the arterioles will cause more blood to stay within the arteries resulting in an increase in blood pressure. And a decrease in the vascular resistance by the arterioles will cause less blood to stay within the arteries resulting in a decrease in blood pressure.
When it comes to looking at blood pressure and how it is measured it is important to consider how the heart actually works. At any given moment the heart is either actively contracting and pumping out blood, or relaxing and filling up with blood. This is called the cardiac cycle which consists of systole and diastole. The systolic phase takes up about one-third of the cardiac cycle during which the heart pumps blood into the arteries. The diastolic phase takes up about two-thirds of the cardiac cycle during which the heart fills up with blood. The blood pressure (BP) is generally measured in the main artery that travels through the arm. During systole, as blood is pumped into the arteries, the blood pressure rises to a maximum called the systolic blood pressure (SBP). During diastole, as the blood ping pongs back and forth between the arterioles and the heart, and some of it enters through the arterioles into the tissues, the blood pressure gradually drops to a minimum called the diastolic blood pressure (DBP). The units used to measure the blood pressure are “millimeters of mercury” (mmHg). And the blood pressure is usually read as simply the SBP over the DBP (e.g. 120/80 mmHg). When considering how the blood pressure affects blood flow and tissue perfusion it is better to use the average, or mean arterial pressure (MAP). Since systole takes up one-third, and diastole takes up two-thirds of the cardiac cycle, the MAP can be calculated to be 1/3 SBP + 2/3 DBP. For example the MAP for a BP of 120/60 mmHg would be 80 mmHg (1/3(120) + 2/3(60)). The normal MAP, as measured in the arm, is usually about 70-110 mmHg which translates into a blood pressure range of 90/60 mmHg - 150/90 mmHg
Also, since the force downward caused by gravity and the distance between the heart and the brain are known, the MAP in the brain when the body is in the upright position can be estimated to usually be about 60-75 mmHg. This is because, although the MAP in the arm (at the level of the heart) may be 70-110 mmHg, gravity is preventing the blood from going to the brain and therefore the MAP there is lower (60-75mmHg). Furthermore, on standing up, gravity also forces blood to collect in the veins below the heart, especially in the legs. This pooling reduces the amount of blood that returns to the right side of the heart through the systemic veins. And since the amount of blood the heart pumps is directly related to how much blood is in it, this causes a drop in the cardiac output and, consequently, the blood pressure as well.
So, on standing up quickly gravity packs a one-two punch to try to make us pass out; first, it reduces the arterial blood flow from the heart to the brain by forcing it down towards the ground, and second, it forces more blood to stay in the veins below the heart thereby reducing the cardiac output and the amount of blood being pumped into the systemic arteries in the first place. When the drop in blood pressure is enough to start to affect brain function (usually an MAP < 60 mmHg) we feel dizzy, have blurred vision, and if not quickly corrected, we may pass out and fall down. Experience tells us that the body is usually able to quickly respond and correct this situation. This would have been vital for the survival of our distant ancestors. So, letís see how the body takes control to keep us standing up.
Blood Pressure Control
Recall, the first thing you need to take control is to have a sensor that can detect what needs to be controlled. The body has pressure sensors, called baroreceptors, that are located in the walls of the main arteries that send blood directly to the brain. The baroreceptors detect the amount of stretch in the arterial walls caused by the pressure of blood as it flows past. They respond to the changes in blood pressure by altering the frequency of nerve impulses they send to the brain. The baroreceptors send out the usual frequency of impulses when the MAP is within the normal range (70-110 mmHg). But when the MAP drops below the normal range the frequency of nerve impulses decreases and if it rises above the normal range the frequency of nerve impulses increases.
Recall, the second thing you need to take control is something to integrate the data from the sensors by comparing it with a standard and then deciding what must be done. The nerve impulses from the baroreceptors travel along special nerves to the region in the brain for blood pressure control where they are processed and integrated. The brain sends out the normal amount of nerve impulses to maintain the MAP between 70-110mmHg. When the MAP rises or falls outside the normal range, the brain responds by changing the frequency of its nerve impulses. In particular, if the MAP drops much below 60 mmHg the brain increases its output of nerve impulses along the sympathetic nerves. The more it drops the more it stimulates the sympathetic nervous system. The sympathetic nerves release two neurohormones called norepinephrine and epinephrine.
Recall, the third and final thing you need to take control is an effector that can do something about the situation. Remember the three main factors that affect BP are:
1. the cardiac output: how hard and fast the heart pumps blood into the arteries
2. the amount of fluid in the body and the distribution of blood within the circulation
3. the amount of vascular resistance applied by the arterioles to arterial blood flow
The norepinephrine and epinephrine released by the sympathetic nerves attach to specific receptors in the heart, the blood vessels and other organs as well. These messages signal these organs and tissues to make changes in function which ultimately results in them trying to bring the blood pressure back to where it is supposed to be. In particular, when the blood pressure drops down too low these sympathetic neurohormones affect all three of the main factors that affect blood pressure in order to bring it back to normal.
(1) The heart responds to norepinephrine and epinephrine by pumping harder and faster. This causes an increase in the amount of blood the heart pumps out with each heart beat and also how many times the heart beats per minute. An increase in the stroke volume and the heart rate causes an increase in the cardiac output. And an increase in the cardiac output causes more blood to be pumped into the systemic arteries which usually results in an increase in the blood pressure.
(2) The norepinephrine and epinephrine released by the sympathetic nerves also attaches to specific receptors in organs like the kidneys, the adrenals and the brain which tells the body to hold on to more sodium and water. By holding on to more sodium and water the amount of fluid in the body increases. And when the amount of fluid in the body increases it usually also increases the amount of blood in the circulation and generally causes an increase in the blood pressure. This sympathetic response, affecting the sodium and water content of the body, takes place over several minutes and has a very limited, if any, effect on preventing the body from passing out when standing up. However, another effect of these neurohormones is to make the systemic veins push more blood back up toward the heart. This effect partially reverses the tendency for the blood to pool in the leg veins and increases the venous return to the right side of the heart. An increase in the venous return to the heart causes an increase in the cardiac output. And an increase in the cardiac output causes an increase in the blood pressure.
(3) The norepinephrine and epinephrine released by the sympathetic nerves also attaches to specific receptors on the smooth muscle that surrounds the arterioles. They signal the smooth muscle to contract more which causes the diameter of the arteriole to diminish. A smaller opening in the arteriole makes it harder for the blood to flow from the arteries through to the capillaries which causes an increase in the vascular resistance. An increase in the vascular resistance makes more blood stay in the arteries which causes an increase in the blood pressure. In addition, hormones generated from the kidneys and the brain also tell the smooth muscle around the arterioles to contract more to increase the vascular resistance and the blood pressure as well. However, it takes several minutes for these hormones to take effect so they have a very limited, if any, effect on preventing the body from passing out on standing up.
When our prehistoric hominid ancestors were trying to survive in a world full of predators, they had to have had a properly functioning sympathetic nervous system to prevent them from passing out every time they stood up. Experience teaches that on standing up, gravity makes our blood move toward the ground making us feel dizzy and at risk of passing out. Medical science knows that gravity reduces the blood pressure in the brain by preventing it from flowing up through the systemic arteries and pooling it in the leg veins causing a reduction in the cardiac output. The baroreceptors, located in the arteries that supply blood to the brain, respond to the changes in blood pressure by changing the frequency of their messages to the brain. The higher the blood pressure the higher the frequency of nerve impulses and the lower the blood pressure the lower the frequency of nerve impulses. The brain processes and integrates the information from the baroreceptors and, in particular, responds to decreases in blood pressure by stimulating the sympathetic nerves more. More stimulation of the sympathetic nerves makes them send out more norepinephrine and epinephrine. These neurohormones lock onto specific receptors in the heart, the blood vessels and other organs and tissues as well. They make the heart pump harder and faster which makes the cardiac output rise. A rise in the cardiac output causes the blood pressure to rise. They make the veins send more blood back to the heart and also tell the kidneys to hold on to more sodium and water, which causes more fluid to be present in the arterial system. More blood in the arteries causes the blood pressure to rise. And finally, they tell the muscles around the arterioles to contract more and raise the vascular resistance which keeps more blood in the arteries. An increase in the vascular resistance causes the blood pressure to rise. Every time you stand up quickly and feel a bit dizzy, take a moment to consider that this is what your body is doing to keep you from passing out.
Points to Ponder
The way your body controls its blood pressure, allowing you to stay conscious when in the upright or standing position, requires it to have several different parts, located in the right positions, each doing the right thing. The control of blood flow to the brain is of critical importance for survival. There are many different places in the body that have sensors which can detect blood pressure. Many of these are connected to organ systems that secrete hormones which take several minutes to take effect. When youíve just stood up quickly to defend yourself, or flee from danger, and the blood flow to your brain is dropping because of gravity, your body doesnít have the luxury of having several minutes to correct the situation. It must know instantaneously what is going on with the blood flow to the brain and respond within a few seconds. The (1) baroreceptors are located precisely where they need to be; within the main arteries that channel blood to the brain. They must not only be able to detect the changes in blood pressure but also adjust the frequency of their nerve messages according to these changes. The (2) area of the brain that controls the blood pressure must be properly calibrated (like the gas gauge in a car) so that when it receives the data from the baroreceptors its analysis reflects the truth about the blood flow to the brain. It must then adjust accordingly the nerve impulse frequency it sends along the sympathetic nervous system. The (3) specific receptors for norepinephrine and epinephrine must be located on (4) the cells of the heart which on stimulation tell it to pump harder and faster, (5) the kidneys, the adrenals, and the veins, which on stimulation ultimately makes the body hold onto more sodium and water, and pushes blood from the veins into the arteries, and (6) the smooth muscle surrounding the arterioles which when stimulated contracts more and increases the vascular resistance. If any one of these six parts were to be missing, or not working properly, the whole system would fail and the body would not be able to stand up and defend itself without always immediately passing out. Each part that contributes to the sensor, the integrator, and the effector is needed to perform this vital function for body survival. Dr. Michael Behe has called a system where the absence of any one part renders it useless as being irreducibly complex. The system our body uses to control its blood pressure on standing up demonstrates irreducible complexity.
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 previously to control oxygen transport, blood glucose, water content, sodium, potassium, and calcium, or this one for blood pressure? Remember, without any one of these abovementioned systems working properly we die. In addition to these there are other irreducibly complex systems each of which is absolutely vital for life. For example there is a control system in the body for temperature as well. 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 make it automatically capable of supporting life? If you think about it youíll realize that thereís one more piece of the puzzle thatís needed, a piece that goes beyond irreducible complexity, to enable these systems to keep us alive within the laws of nature.
Firefighters battling a three story blaze must spray enough water over the building as soon as possible to prevent it from being completely destroyed. They do this by using a pump to propel water upwards against its inertia, the friction within the pump and hose and the surrounding air, and the force of gravity involved in climbing three stories. Their success depends on, not only how much water they apply, but also how fast they can get it there. For if they have enough water but it takes too long to get there, the fire will destroy the building. And if they can get the water there quickly, but they don't have enough, then the building will still go down in flames. The right amount of water in the right amount of time is what is needed to put out the fire. Similarly, our brain cells need the right amount of oxygen and glucose to function properly. When we stand up the body must send the right amount of blood in the right amount of time to the brain to prevent us from passing out. Real numbers have real consequences when it comes to dealing with the laws of nature. The same applies to the body and how it functions. Not just any blood pressure will do. Based on what we know about how the body actually works our ancestorsí ability to survive and reproduce depended on them being able to maintain the right blood pressure on standing up (MAP > 60 mmHg) to ensure that they had the right blood flow to the brain to keep them aware and functioning. But what if the system that uses the sympathetic nerves to control the blood pressure had been set differently? What if it could only raise the MAP to 40 mmHg? Then, all clinical experience tells us that our prehistoric ancestors could never have survived.
Real numbers have real consequences when it comes to dealing with the laws of nature. For, not just any blood pressure and blood flow is needed for survival. It has to be the right blood pressure and it has to result in the right amount of blood flow to keep the brain and all the other organs and tissues in the body working properly. 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 “natural survival capacity”. 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 a knowledge of 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 the sympathetic nerves to keep the blood pressure and blood flow to the brain at the right level, even after standing up, seems to inherently know what is needed to get the job done and it does it naturally. The same can be said for each of the other control systems that manage oxygen transport, glucose, water, sodium, potassium, calcium 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 pathways to death. 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 solely by random chemicals coming together to form cells, then simple organisms, and then complex ones like us? In other words, without “a mind at work” to make it happen? Engineers have designed many different pressure sensitive devices that are used in the production of all sorts of things. Likewise, our body uses pressure sensitive devices to maintain the blood pressure and blood flow to the brain when we stand up. No, when it comes to the origin of life it seems to me that 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.
Comments and questions about this article or any of the previous ones are welcome.
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