May 10, 2012
Every day, millions of people have their blood pressure (BP) checked. Whether it is while sitting in a doctor’s office, being prepped for surgery, or lying critically ill in the I.C.U., the blood pressure, like temperature, pulse, and respiration, is a measure of a person’s health and even their capacity to live and function in the world. But what exactly is the blood pressure and why is it so important for life? Moreover, how does the body control its blood pressure, making adjustments to it so it can stand up to gravity to survive within nature? Finally, what problems can arise if blood pressure control is lost?
After all, the body does not exist within a vacuum. Like all living creatures, it too must contend with the laws of nature to survive. The phenomenon of pressure, like friction, wind resistance, and gravity, is a random force of nature that directly impacts body function and its capacity to survive in the world. It is only in the last century that medical science has been able to more accurately measure the blood pressure, and in the last fifty years, has come to better understand the connection between blood pressure and health and longevity. Meanwhile, from time immemorial, the body, by necessity, has not only been measuring its own blood pressure, but also has been making constant adjustments to allow for survival. This article will explain what the blood pressure is, how it is measured, and why it is so important for life. In addition, it will show how the body controls its blood pressure so it can stand up to gravity so it can survive and how when these control mechanisms are not working properly they can lead to significant debility and even death.
What is important to note here is that the mere presence of the various organ systems that are affected by, and have an effect on, the blood pressure does not in itself explain the body’s ability to control it to stay alive. To think so would be like saying that one understands how an airplane can fly merely by looking at its individual parts without considering how the laws of nature can make it crash. The same can be said of the human body. It is made up of hundreds of trillions of cells, organized into numerous different organ systems and various other structures, which together must contend with the laws of nature to survive. Presently, many people claim that the idea that life came about solely by the random forces of nature is logical and makes perfect sense. To think that one has explained how life came about by only developing a theory of how each component may have come into being, but not how all of these parts came together to perform numerous different vital functions that give life the capacity to survive, as determined by the laws of nature, is irrational and unscientific. A truly rational theory of life must address, not only how living organisms developed the ability to form the parts that make up their bodies, but also how these parts came together to perform different functions that control their internal environment allowing them to survive within nature. For, it has been said that the body achieves perfect equilibrium with its environment only when it is dead. Therefore, it is impossible to understand and appreciate how life came into being without first knowing how easily the forces of nature can make it malfunction and die.
Blood Pressure: What Is It?
Pressure is defined as “the force per unit area applied in a direction perpendicular to the surface of an object”. In other words, pressure is the force, the weight, or the stress applied at a right angle to the surface of an object. The body is able to detect pressure. You can experience what pressure feels like by pressing your thumb as hard as you can against the bone just below your eye. The high pressure applied by your thumb to your face gives way to pain so that you will not cause damage to the tissues below the surface.
The 17th century Italian scientist, Evangelista Torricelli, determined the weight of air on the surface of the earth (atmospheric pressure) by inverting a long glass tube filled with mercury (Hg) into a cup which also contained mercury. The level of mercury in the tube then fell depending on how much air pressure was being exerted on the mercury in the cup. Thus, Torricelli invented the first barometer and determined that, at sea level, the atmospheric pressure is about 760 mmHg (millimeters of mercury), since the height of the column of mercury in the glass tube was 760 mm above the surface of the cup.
Another way to experience pressure is to think about what happens when you inflate a bicycle tire. The air being pumped into the tire applies pressure against the inner wall of the tube so that it inflates, like a balloon, making it firm. As the tire inflates, it becomes increasingly difficult to indent it by pressing your thumb against it. The higher the inflation, and the firmer the tire, the higher the air pressure within it. Rather than being measured in mmHg, tire pressure is often measured in “pounds per square inch” (psi).
Finally, place your hand below a water tap and slowly open it more and more. By doing this you will see, hear, and feel, the physical effects of water pressure as the water splashes off your hand. Water pressure is the force that water exerts against a confined space, like the walls of the pipe in which it is flowing. Like tire pressure, water pressure is often measured in psi as well.
Similarly, blood, which is the fluid that flows within the cardiovascular system to feed the tissues, exerts pressure against the walls of the vessels in which it is contained. However, as the blood circulates throughout the body, it enters and applies pressure against the walls of many different types of blood vessels, and the four chambers of the heart as well. The circuit for the blood begins in the left ventricle where it is pumped into the aorta and the large arteries of the systemic circulation. The blood then travels through smaller arteries into the tiny arterioles and then the microscopic capillaries, where the exchange of oxygen and other chemicals needed for life, takes place within the tissues. From the capillaries the blood enters the venules which come together to form smaller and then larger veins eventually entering the right side of the heart. The blood enters the right atrium and then moves to the right ventricle. From the right ventricle the blood is pumped to the lungs and enters the pulmonary circulation, ultimately returning through the pulmonary veins to the left atrium and then into the left ventricle, completing the circuit.
So the question is: which of these “blood pressures” is the one that medical science is talking about when it refers to the BP? The pressure applied by the blood against the walls of the left or right ventricles, the left or right atria, the aorta, the large or small arteries, the large or small veins, the arterioles, the venules, or the capillaries? The answer is that the “blood pressure” generally refers to the force that blood exerts against the walls of the larger arteries of the systemic circulation (as opposed to the pulmonary circulation).
In summary, fluid within a confined space, like water in a pipe, or blood within a chamber of the heart or blood vessel, exerts pressure against its walls. The common use of the term “blood pressure” generally refers to the force of blood exerted against the walls of the large arteries within the systemic circulation of the cardiovascular system.
Blood Pressure: How Is It Usually Measured?
The blood pressure is usually measured from the blood flowing within the brachial artery (L. brachium = arm) at the inner aspect of the elbow. The instrument used to measure the BP is called a sphygmomanometer (Gk sphygmos = pulse + manos = thin + metron = pressure). It was invented and improved upon in the late 19th and early 20th centuries. It initially consisted of an inflatable cuff connected to a column of mercury. When the cuff was wrapped around the middle of the upper arm and inflated, the pressure being applied would begin to close off the brachial artery resulting in turbulent blood flow. This turbulent blood flow could be heard by using a stethoscope, a device to listen to the sounds of the heart and lungs, which was invented in the 19th century. The amount of pressure that was being applied to the brachial artery could be determined by looking at the amount of mercury in the column (recall: air pressure at sea level is 760 mmHg). See: http://en.wikipedia.org/wiki/File:Clinical_Mercury_Manometer.jpg
Modern sphygmomanometers now use aneroid gauges, which contain no liquids. See: http://en.wikipedia.org/wiki/File:Sphygmomanometer%26Cuff.JPG
Many of them also have automatic sensors embedded within the cuff that can detect turbulent blood flow and give an automatic digital read out. See: http://en.wikipedia.org/wiki/File:BloodPressure.jpg
Before considering how the blood pressure is measured, it is important to first understand what happens to the blood when it is pumped from the heart into the systemic arteries. There are two phases of heart function and therefore two different BP readings. When the left ventricle contracts and pumps blood into the arterial system this is called systole (Gr. systole = contraction). When the heart relaxes and is filling with blood it is called diastole (Gk. diastole = expansion). With systolic contraction, as blood exits the left ventricle, it is pumped into the aorta and the arteries of the systemic circulation. However, blood flow in the arteries is not like water flowing down a river on its way to the ocean. In nature, smaller brooks and streams feed into larger creeks and rivers that deposit their water contents into an even larger reservoir like a lake or the sea. This is actually similar to how the veins of the body work. However, for the arteries, the opposite is true. The large arteries branch out into progressively smaller arteries which then feed the even smaller arterioles. The arterioles feed into the microscopic capillaries in the tissues. It is within these thin-walled capillaries where the exchange of oxygen, glucose, water, and other nutrients, takes place between the circulation and the tissues. In effect, this process involves not only a redistribution of blood throughout the body, but also a slowing down as well, as it approaches the capillaries.
To get an idea of what happens in the arterial system, imagine rush hour traffic on a superhighway as each vehicle slows to enter an exit ramp to their destination. However, in the case of the blood in the arterial system, all of the vehicles are trying to enter the exit ramps at the same time. Obviously, not all of them are going to make it. What happens in the arterial system is that the blood flowing down the larger arteries (superhighway) encounters the resistance of the arterioles (exit ramps), and while some of the blood passes through to the capillaries, the rest rebounds back towards the heart. In effect, a reversal of blood flow takes place and now the blood travels back along the larger arteries eventually encountering the resistance of the closed aortic valve in the heart. The blood then springs back off the closed aortic valve towards the tissues until it, once again, encounters the resistance of the arterioles, where some blood passes through to the capillaries and the rest pulses back again. This "ping-pong effect" of the blood rebounding between the arterioles and the closed aortic valve, as the blood travels back and forth in the arteries, continues throughout diastole as the heart rests and fills with blood in preparation for the next systolic contraction.
The pressure of the blood against the walls of the brachial artery as the heart is resting and filling with blood is called the diastolic pressure. With systolic contraction, the heart pumps more blood into an arterial system that already contains blood with a diastolic pressure, which causes a rise in the blood pressure to what is called the systolic pressure. In other words, the blood pressure within the large arteries hits its maximum during systole and is called the systolic blood pressure (SBP) and drops to its minimum during diastole and is called the diastolic blood pressure (DBP).
In order to determine the SBP and the DBP the cuff is inflated high enough to stop the flow of blood in the brachial artery. This is determined by hearing the stoppage of turbulent blood flow. The cuff pressure is then released slowly. When the first sound of turbulent blood flow escaping through the brachial artery is heard, the reading on the manometer is observed, and the SBP is determined (e.g.120 mmHg). As the cuff pressure is released further, the sounds heard through the stethoscope progressively lessen and become more muffled. This takes place due to there being less turbulent blood flow. When the sound of blood flow has disappeared, the reading on the manometer is noted and the DBP is determined (e,g. 80 mmHg). The blood pressure is usually expressed by simply saying the SBP "over" the DBP such as "120/80 mmHg". See: http://homepage.smc.edu/wissmann_paul/anatomy1/1bloodpressure.html
Another important blood pressure reading is the mean arterial pressure (MAP). The MAP represents the average blood pressure within the large arteries of the systemic circulation. Since the systolic blood pressure generally occurs during one-third of the cardiac cycle and the diastolic for about two-thirds, the MAP can be roughly calculated as 1/3 SBP + 2/3 DBP, or alternatively as (SBP + 2DBP)/3. The normal range for the MAP is 70 - 110 mmHg. This translates into a range in BP of 90/60 mmHg (90 + 2(60))/3 = 70) to about 150/90 mmHg (150 + 2(90)/3 = 110).
To summarize, the blood pressure is usually measured in the brachial artery by using a sphygmomanometer applied to the middle of the upper arm and listening for turbulent blood flow at the inner aspect of the elbow. The systolic blood pressure is determined by observing the reading at the first sound of turbulent blood flow. The diastolic blood pressure is determined by observing the reading when the sound of turbulent blood flow stops. The mean arterial pressure is the average pressure applied against the walls of the large arteries by the blood and can be estimated by the formula (SBP + 2DBP)/3.
Blood Pressure: Why It Is So Important And The Body’s Dilemma
Every cell in the body requires the right amount of oxygen and glucose for the energy needed to survive and function properly. These vital chemicals are brought into the body through the respiratory and gastrointestinal systems respectively, where they enter the bloodstream. The cardiovascular system has the task of sending enough blood to the tissues so that the cells of the body can receive what they need to live. The blood enters the thin-walled capillaries, under pressure, and like squeezing a sponge, fluid, containing various chemicals, is pushed out of the circulation from the capillaries into the tissues, where the chemical exchange takes place. However, when it comes to providing enough blood to the tissues to adequately feed its cells the body faces a dilemma. A dilemma that takes place because of the laws of nature.
The first problem relates to the fact that just as friction slows the movement of an object on the ground, blood flow meets its counterpart, vascular resistance, in the blood vessels. In other words, nature tends to slow down the blood as it moves through the circulation, just as it does a ball rolling across a field. Therefore, the blood pressure must be high enough to meet the resistance to blood flow that is built into the arterial system. Without enough blood pressure to overcome the vascular resistance within the arterial system, the tissues would not be fed properly and death would take place.
The second problem relates to how pressure affects the filtering of fluid out of the blood when it is in the capillary. Experience teaches that the harder you squeeze a sponge, the more water comes out of it. In other words, the higher the pressure of the blood entering the thin-walled capillaries, the more fluid will be squeezed out of the circulation into the tissues. The problem is that, not only would too much fluid in the tissues cause them to malfunction, but also, this could seriously alter the distribution of water within the body. This means that if the blood enters the capillaries with too much pressure, too much fluid can be removed from the circulation resulting in debility and even death.
So, the dilemma the body faces is that the cardiovascular system must have enough blood pressure to feed the tissues while not overwhelming the capillaries to maintain proper organ function and fluid balance. The first problem can be explained by looking at how a community provides its residents with enough water and compare that with how the body generates enough blood pressure to feed its tissues properly. The second problem can be explained by looking at how electricity is supplied for safe use in the home and compare that with what the body does to control its blood pressure within the tissues to make it safe for use in the capillaries.
Problem One: Resistance To Blood Flow
Hold onto a water tap while gradually opening it wide up and you will not only see, but also hear, and feel, what flow and pressure are all about. The maximum water velocity will determine how long it will take for you to fill up the bathtub or sink, or even whether you can take a proper shower or not. The vibration felt as the water flows out of the tap represents the force of water against the walls of the pipe. This water pressure is the force that is responsible for how much and how fast the water is able to flow out of the tap. You might even say that the water pressure determines the maximum flow rate which represents the functional capacity of your water delivery system. To have an adequate flow of water in the home often requires a pump to provide the energy needed to move it under pressure to where it is needed. Moreover, as the water travels along the pipes, branches into others to go to specific homes, and then branches again within the home to service its water needs, energy is naturally used up and the water pressure subsides. In other words, the resistance to water flow is directly related to the length of the pipe, how often it branches, and the number of households on-line. This means that the more homes that are being provided water, the more energy is required to provide enough water pressure to service the community. Therefore, the problem the laws of nature impose on every community, regarding “resistance to flow”, is to be able to generate enough pressure to ensure that every household has an adequate flow of water to service its needs.
Similarly, the cardiovascular system provides the means for the body to feed its tissues through the blood in the circulation. Therefore, there must be enough blood pressure within the system to provide enough energy to accomplish this task. It might be useful to think of the blood pressure as being the driving force that results in blood flow from the heart, through the arteries to the capillaries, and back again through the veins to the heart. In other words, the blood pressure in some way is a measure of the blood flow throughout the body. However, as noted above, just as friction slows the movement of an object on the ground, blood flow meets its counterpart, vascular resistance, in the blood vessels. Moreover, the lesson on water pressure above tells us that, in principle, the longer the vessels, the more branching within the system, and the more tissues receiving blood, the higher the resistance to blood flow. Therefore, to provide enough blood to the tissues the circulation must have enough blood pressure to overcome the natural resistance to blood flow that is built into the arterial system.
The cardiovascular system uses the power generated by the heart to pump enough blood with enough pressure to adequately feed the tissues through the capillaries. The power of the heart is determined by looking at how much blood it pumps out per minute. This is called the cardiac output (CO), which at rest is usually about 5 liters per minute (L/min). The cardiac output is dependent on the heart rate (HR) (beats per minute) and the stroke volume (SV), the amount of blood pumped out per heart beat. In fact the cardiac output can be calculated by multiplying the heart rate by the stroke volume and can be expressed as CO = HR x SV. For example, at rest the usual HR is about 70 beats per minute (bpm) and the SV is about 70 mL (milliliters). If CO= HR x SV then 70 bpm x 70 mL/beat = about 5,000 mL/min or 5 L/min. Medical experience teaches that, at rest, a cardiac output of 5L/min is usually more than sufficient for feeding the tissues.
So, the laws of nature, regarding the resistance to flow, impose the same problem on the body, and its need for enough blood flow to feed its tissues, as it does for a community trying to provide enough water to its residents. Moreover, the body’s solution to the problem is almost identical to the one for the community. The use of a pump (the heart) to generate enough pressure to service the tissues with blood.
Problem Two: The Filtering Of Blood
Being able to use various electrical devices without them being destroyed is dependent on having the right amount of electricity running through the home. Electrical generating stations send out electricity with hundreds of thousands of volts along transmission lines which is later scaled down to tens of thousands of volts by distribution substations. Eventually the electricity passes through transformers which lowers the voltage to much lower levels making it usable for the consumer in the home. In order to generate so much electrical power for transport and then be able to scale it down for consumer use, each component must be properly constructed and positioned within the electrical grid.
So too, as noted above, the heart must generate enough power to transmit blood through the arteries so it can reach the capillaries in the tissues and return back to it through the veins. Moreover, the aorta, and other larger vessels, that handle the blood coming directly out of the heart, must have walls that are strong and elastic enough to withstand the high pressure. The diameter of the channel in which blood flows within the aorta is about 25 mm. This quickly drops down to about 4 mm for the larger arteries (like the brachial artery in the arm). The arteries that branch out from the larger ones to supply the different tissues with blood are surrounded by muscle and have smaller diameters, in the order of only 0.1- 1.0 mm. Since the downstream arteries have smaller diameters this makes it harder for the blood coming from the heart to enter them and resistance to blood flow takes place. In fact, by the time the blood has flowed from the heart through the larger and then smaller arteries, and has reached the arterioles, the mean arterial pressure (MAP) has usually dropped from 100 mmHg in the aorta to about 85 mmHg. The arterioles are also surrounded by muscle, which is often contracted, meaning that the diameter of the arterioles is extremely small, usually about 0.02 - 0.03 mm. This causes further resistance to blood flow. So by the time the blood has passed through the arterioles to enter the capillaries, the pressure within them has dropped to only 30 mmHg. This means that the arterioles are the main resistance blood vessels of the peripheral circulation. In this way, the blood enters the capillaries at a much reduced pressure (30 mmHg) which is sufficient to allow the body to filter the right amount of fluid and chemicals out of the circulation without losing too much fluid to the tissues.
So, just as the laws of nature regarding the safe use of electricity within the home requires it to be scaled down, so too, the blood pressure must be scaled down so that the body can feed the tissues properly without at the same time losing too much fluid to them. The solution for both problems is similar. The distribution substations and transformers apply resistance to lower the electrical voltage just as the smaller arteries and arterioles apply vascular resistance to lower the pressure of the blood as it enters the capillaries.
Summary: The Dilemma Solved
When it comes to supplying the tissues with enough blood without causing tissue damage and compromising fluid balance, the laws of nature present the body with a dilemma. Not only must it have a sufficient blood pressure to generate enough blood flow to the tissues, it must also be able to scale down the pressure of blood as it enters the capillaries to prevent excessive fluid loss, tissues damage, and organ malfunction. In addition to having enough blood within the circulation, the pumping of the heart is one of the main forces responsible for generating enough blood pressure within the circulation. Also, the resistance to peripheral blood flow, by the smaller arteries and the arterioles downstream, makes sure that the blood does not enter the capillaries at too high of a pressure. So, when it comes to how the laws of nature affect blood flow and its filtration within the capillaries, this is how the body deals with them to survive. But before considering how the body controls its blood pressure and is able to stand up to gravity, it is important to first understand the relationship between blood pressure, blood flow, and peripheral vascular resistance.
Blood Pressure: Its Relationship With Blood Flow and Vascular Resistance
As noted above, blood pressure is the driving force that results in blood flow throughout the cardiovascular system. However, as noted above, just as friction slows the movement of an object on the ground, blood flow meets its counterpart, vascular resistance, in the blood vessels. It is important to realize that the peripheral resistance to blood flow from the smaller arteries and arterioles downstream, causes the blood upstream to stay within the larger arteries during the cardiac cycle. Recall, this is why the blood bounces back and forth between the arterioles and the closed aortic valve during diastole (see above). Therefore, if more peripheral resistance to blood flow causes more blood to stay within the larger arteries (like the brachial artery), this will make the BP rise. So, the amount of blood pressure is directly related to the amount of peripheral vascular resistance within the arterial system. This means that a rise in the peripheral vascular resistance causes a rise in the BP and a drop in the peripheral vascular resistance causes a drop in the BP.
In addition, we know that the force of blood against the walls of the arteries is also dependent on the amount of blood being pumped into them by the heart. The stronger and/or the faster the heart pumps, the higher the cardiac output, which represents the systemic blood flow. So we can see that the BP is directly related to, not only the downstream vascular resistance, but the cardiac output as well. It might be simpler to remember this relationship between blood pressure (BP), blood flow (BF), and vascular resistance (VR) with the equation BP = BF x VR. Therefore, you can see that a rise in either the blood flow or the peripheral vascular resistance will elevate the BP and a drop in either of them will drop the BP.
Another very important thing to notice is that if BP = BF x VR, then BF = BP/VR. In other words, blood flow is directly related to blood pressure, but is inversely related to the vascular resistance. This means that if the BP rises, or falls, blood flow will rise, or fall as well, since the blood pressure is the driving force behind blood flow in the first place. However, if the vascular resistance rises, or falls, blood flow will be affected conversely. It will fall, or rise instead, since the vascular resistance by definition means a resistance to blood flow. Therefore a rise in vascular resistance means a drop in blood flow, and a fall in vascular resistance means a rise in blood flow.
Blood Pressure: The Dilemma Provided By Gravity
Besides its relationship to the cardiac output and the peripheral vascular resistance, one other very important random force of nature that impacts blood pressure, and blood flow (especially to the brain), is gravity. Gravity is the force that keeps us on the ground. In other words, gravity pulls everything down towards the earth. When your body is lying horizontal, and everything is at approximately the same level, the MAP at the level of the heart is about 100 mmHg. When you sit or stand up, placing your head above your heart, gravity pulls the blood back down towards your heart and your feet. Therefore, when the body is in the upright position the blood traveling from the heart into the cerebral arteries to feed the brain is up against, not only peripheral vascular resistance, but also gravity as well. Since the gravitational pull towards the earth is known, and so is the distance between the brain and the heart, it can be estimated that the MAP within the cerebral circulation in the upright position is only about 60 - 70 mmHg.
A drop in the MAP within the brain (going from 100 mmHg to only 60- 70 mmHg) will tend to greatly diminish cerebral blood flow as well. However, the equation BF = BP/VR tells us that blood flow is inversely related to vascular resistance. So, if the vascular resistance within the arterioles of the brain can be lessened this might compensate for the drop in the MAP brought about by gravity and restore normal cerebral blood flow. In fact, the brain, and the other organs of the body, are actually able to adjust the vascular resistance within their local circulation to compensate for fluctuations in blood pressure to keep their blood flow relatively constant. This process is called autoregulation. The problem for the brain is that an MAP of 60 - 70 mmHg, is the lower limit at which autoregulation can compensate enough to maintain an adequate blood flow to meet its metabolic needs.
When it comes to how gravity affects the blood pressure and blood flow to the brain, the body faces another dilemma. Experience tells us that, on standing up quickly, especially after lying down or being bent over for a while, it is not uncommon to feel dizzy for a few seconds. If this light-headedness continues, you may actually pass out (syncope) and fall to the ground. The reason why this happens is that gravity has caused a significant drop in blood flow to your brain which has been enough to make it malfunction. When you stand up quickly, gravity not only reduces the blood flow coming to the brain from the heart, but it also reduces the blood flow coming back to the heart from the veins in the legs as well. This drop in the venous return of blood to the heart can be enough to cause a significant reduction in the cardiac output because the heart has less blood within it. Recall, blood pressure is directly related to blood flow. Therefore, a drop in the blood flow out of the heart into the arteries will drop the blood pressure as well. The problem the body faces in this setting is that even with a normal MAP at the level of the heart (100 mmHg), the upright position causes the MAP in the cerebral circulation to drop to 60- 70 mmHg. Now, on standing up quickly, the additional drop in the MAP at the level of the heart, due to the pooling of blood within the veins of the legs, will make the MAP within the brain drop even further. As noted above, an MAP of 60 - 70 mmHg is the lower limit at which the brain is able to sufficiently compensate (by autoregulation) the vascular resistance of its arterioles to preserve adequate blood flow. Therefore, in this setting, there is a real danger of the brain not receiving enough blood flow for its metabolic needs. This brain malfunction, due to a reduction in blood flow on standing up, we experience as dizziness, blurred vision, and if not corrected, can result in syncope.
Regarding gravity, and how it affects blood pressure and blood flow, this is another dilemma that faces the body. For, the brain is the organ in the body that is the most sensitive to reductions in blood flow since it is always working so hard and has a high metabolic rate. When the blood pressure in the brain starts to drop below 60 - 70 mmHg it is no longer able to make enough local adjustments to provide itself with enough blood flow. A drop in the blood pressure within the brain below the 50 - 60 mmHg range causes it to malfunction, which, as noted above, leads to dizziness and sometimes syncope. Passing out and falling to the ground usually will resolve the problem quickly because once the body is in the horizontal position the venous return from the legs to the heart will immediately correct itself and the effect of gravity on the blood coming from the heart to the brain will also be eliminated. However, we know that human survival depended on our ancestors being able to stand up to gravity so they could prey upon others rather than being preyed upon instead. The dilemma that gravity imposed on the bodies of our human ancestors was a matter of life and death. If, on standing up, the body hadn’t been able to immediately keep the MAP within the brain above 60 - 70 mmHg, then it wouldn’t have had enough blood flow to function properly in the upright position and human life, as we know it, would have been impossible. Since, we know that on standing up quickly the dizziness we feel usually disappears within a few seconds, we have evidence that the body must have figured out what to do to combat the effects of gravity, otherwise we wouldn’t be here. Let’s take a look at the main way in which the body is able to quickly control its blood pressure so that is can stand up to gravity to survive in the world. It is important to note here that the body must have known about the equation BP = BF x VR millions of years before any scientist ever figured it out.
Blood Pressure: How Does The Body Control It So It Can Stand Up To Gravity?
How do you control anything? Think about it. First, you need to be able to detect what you want to control. For example, when driving your car, you want to be sure that you don’t run out of fuel, so the manufacturer puts a sensor in the gas tank to detect the level of gas within it. Next, you need to be able to take the information provided by the sensor and compare it with a standard to know if the amount of what you are trying to control is acceptable. For example, the car maker provides a fuel gauge, located on the dashboard, that integrates the information from the sensor in the gas tank to let you know how much fuel there is in it and whether it is sufficient. Finally, if after integrating the data from the sensor, you determine that the result is unacceptable, then you have to be able to do something to correct the situation. For example, if the fuel gauge shows that the level of fuel in the gas tank is running too low, then you must effect a change to correct the situation by pulling into a gas station and putting some gas into the gas tank. Once this has been done the fuel gauge on the dashboard will verify how much gas is in the gas tank and you can feel confident to drive to your destination.
In summary, to control something you need a “sensor” to detect it, an “integrator” to process the data and compare it with a standard to know if something needs to be done, and an “effector” that can make the necessary change(s) to return what you are controlling to an acceptable level, a level that can then be verified by the sensor.
Blood flow to the brain, and the other organs of the body, is vital for life. Since BF = BP/VR, and the force behind blood flow is due to blood pressure, this means that blood flow is directly related to blood pressure. If the blood pressure rises, the blood flow will increase, and if the blood pressure drops, the blood flow will decrease. Therefore, one way for the body to ensure that it has enough blood flow to the brain (and its other organs) is to make sure that the blood pressure is at an adequate level to accomplish this task. For, since the brain, and the other organs of the body, require a certain amount of oxygen and glucose to meet their metabolic needs, if the blood pressure and blood flow drop below certain numerical limits, then significant organ malfunction and even death can take place. In other words, when it comes to the blood pressure, real numbers can be a matter of life and death. So, it is vital for the body to have a control mechanism in place for its blood pressure so that there can be adequate blood flow to the brain and its other organs. This becomes very important when the body tries to stand up to gravity.
One of the main set of sensors that detect the arterial blood pressure are the baroreceptors (Gk baros = weight). These sensors are located in the walls of the aortic arch and the carotid arteries. See: http://education.yahoo.com/reference/gray/illustrations/figure?id=505
The baroreceptors are mechanoreceptors (Gk. mechane = machine) that respond to the degree of stretch within the arterial wall produced by the pressure of blood being exerted against them. Generally, when the MAP in the aorta is within the normal range (about 70-110 mmHg), the frequency of nerve impulses from the baroreceptors remains stable. However, as the MAP deviates up or down from this normal range, the baroreceptors, on detecting these changes, alter their impulse frequency. The degree of stretch detected by the baroreceptors directly affects the frequency of nerve impulses they send to the brain. A rise in the MAP, above the normal range, causes more stretch, and therefore, more nerve messages to be sent. A drop in the MAP, below the normal range, causes less stretch, and therefore, fewer nerve messages to be sent. The messages from the baroreceptors travel along special nerves to the region in the brainstem for blood pressure control where they are processed and integrated. The brainstem usually sends out a stable amount of nerve impulses to maintain a normal MAP. If the MAP rises or falls outside the normal range (70-110 mmHg), the brainstem will respond to the data it has received from the baroreceptors by changing the frequency of its nerve impulses. In particular, when the MAP drops below 60 -70 mmHg, the brainstem responds to the drop in nerve messages from the baroreceptors by sending out more nerve impulses that cause an increase in the release of certain neurotransmitters (a messenger of nerve information from one cell to another). These neurotransmitters are called epinephrine and norepinephrine which are released by a division of autonomic control called the sympathetic nervous system. Epinephrine and norepinephrine attach to specific receptors in the target organs, namely, the heart and the blood vessels, to cause changes in their function to raise the blood pressure. These sympathetic neurotransmitters stimulate the heart to beat harder and faster which results in an increase in cardiac output and systemic blood flow. They also stimulate the muscles surrounding the arterioles in non-muscular tissue to contract, which causes a rise in the peripheral resistance to blood flow. Since BP = BF x VR, an increase in cardiac output and blood flow (BF) into the large arteries, with an increase in the vascular resistance (VR)from the peripheral blood vessels, will cause a rise in blood pressure (BP).
So, when the baroreceptors detect a serious drop in blood pressure, the brainstem activates the sympathetic nervous system which sends out neurotransmitters, called epinephrine and norepinephrine. These chemicals combine to stimulate the heart and the blood vessels in a way to bring the blood pressure back up towards normal. It is important to note here that the body does have other ways of controlling its blood pressure but they do not work as quickly as the autonomic nervous system. Also, like most other proteins in the body, a given quantity of epinephrine and norepinephrine is only effective for a few minutes since they are quickly used up and broken down within the body.
Once the MAP rises back into the normal range, the baroreceptors respond to this increased stretching of the vessel wall by increasing the frequency of nerve impulse transmissions to the brainstem. The brainstem responds to this rise in baroreceptor impulse frequency by reducing its stimulation of the sympathetic nervous system toward normal levels. In addition, after having stood up and started to walk, the leg muscles help push more blood back to the heart. This action improves the venous return, thereby correcting the cardiac output and stabilizing the blood pressure and cerebral blood flow as well. In this way, after standing up, the body is able to respond to the sudden, but usually temporary, drop in blood pressure and cerebral blood flow, by increasing the cardiac output and peripheral vascular resistance until the venous return improves toward normal by other means. This is why, if we feel a bit dizzy on standing up, it usually lasts only a few seconds. The baroreceptors, the brainstem, the neurotransmitters of the sympathetic nervous system, and the specific receptors in the heart and the peripheral blood vessels, all work together so that we can stand up to gravity and fight for survival.
In summary, most of the time the dizziness we experience on standing up quickly, especially from a lying or bent over position, usually resolves in a matter of seconds. This means that the detection of these sudden drops in blood pressure, brought on by gravity, and the ability of the brainstem to correct them quickly, through the sympathetic nervous system, allows the body to maintain moment to moment control of the blood pressure. Being able to closely monitor the blood pressure, and have moment to moment control, allows the body to maintain adequate blood flow to the brain, which is vital for survival. In other words, not just any blood pressure is sufficient to feed the brain with enough blood flow for it to function properly. When it comes to survival, real numbers have real consequences. Remember, once the MAP drops below 50 - 60 mmHg, blood flow to the brain is often seriously compromised which can result in brain malfunction manifesting as dizziness, weakness, and syncope. Therefore, vascular resistance and gravity are two very important forces of nature that affect blood pressure and blood flow which must be taken into account by the cardiovascular system for human life to continue.
In particular, medical science knows that there are many conditions which can result in sympathetic nerve malfunction (e.g. Diabetes, Parkinson‘s Disease, Multiple Sclerosis). Malfunction of the sympathetic nervous system can often cause the body to have difficulty in maintaining a proper blood pressure on standing up. As noted above, being unable to compensate quickly and well enough to a significant drop in blood pressure when standing up can result in an inadequate blood flow to the brain, and syncope. People who suffer from a severe malfunction of the sympathetic nervous system are often so prone to dizziness and passing out when standing up that they must live their lives in either the lying or sitting position and must be very careful when getting up to transfer from one place to another. Clinical experience, regarding people with these sympathetic disorders, demonstrates in practical terms how gravity can affect life and how important this autonomic function is for survival. It is difficult to imagine how our human ancestors could have survived if, on quickly standing up to run after prey, or to avoid being preyed upon, they would have promptly fallen to the ground unconscious due to a critical drop in blood pressure and blood flow to the brain.
Blood Pressure Control: How Did All Of This Come About?
When thinking about how the body developed the ability to control its blood pressure to maintain the blood flow to its brain so that it could stand up to gravity, it is important to take into account much more than just the mere presence of the components involved. At a bare minimum, the parts considered should include; the sensors (baroreceptors), the integrators (the brainstem cells for blood pressure control and the sympathetic nervous system), and the effectors (epinephrine and norepinephrine, their specific receptors, and the heart and muscle surrounding the arterioles). Evolutionary biologists, the people who teach the public that life has come about solely by the random forces of nature, should be expected to explain, not only how each of these components came into being (which is hard enough in itself), but also several other associated features as well. These additional features should include; the proper placement of the sensors so they can detect the right thing to do their job correctly, the proper connection between the sensors and the right types of cells to integrate the information that has been detected, and the ability for the integrating cells to effect the right sorts of changes in target organ function to correct the situation.
Comparing the body’s ability to control its blood pressure, to maintain an adequate blood flow to the brain on standing up, with how you make sure you have enough fuel in your gas tank, so your car doesn’t die on you, will be used to illustrate the above points. As human beings, we have the ability to detect whether something has been designed or not. Without this ability we would not be able to determine if a person’s death was a murder, a suicide, an accident, or due to natural causes. Without our ability to detect design we would not be able to have justice, through the discipline of forensics, nor a knowledge of our ancient past, through archaeology. More to the point, without our ability to detect design, rather than assigning merit or blame to an unwitnessed event, we would only be able to conclude that it had occurred by chance. Please review what is presented below and then consider what society should be demanding from evolutionary biologists when it comes to teaching us about the origin of life.
The first component required to control anything is a sensor that is located in the right position and is able to detect what needs to be controlled. The car requires gas to provide the energy to power its motion. Without gas the car cannot run. Therefore, the car manufacturer provides a sensor placed within the gas tank to detect the amount of fuel present. The car consists of many different parts. So, there are many other places within the car where this sensor could have been placed. For example, it could have randomly been placed within the lubrication system, the radiator, or the transmission. If the sensor for fuel had been located in any of these places it would be impossible to know how much gas is in the car. The gas sensor has been constructed in such as way, and has been put exactly where it needs to be (in the gas tank), to be able to do its job.
Similarly, the brain (and the other organs and tissues of the body) requires adequate blood flow to have enough energy to survive and function properly. As noted above, the blood pressure is the driving force that powers blood flow throughout the body. The baroreceptors, which detect the arterial blood pressure, are sensors that are located within the main arteries that channel blood into the brain. However, as noted above, the blood that circulates in the different regions of the body passes through different types of vessels while still applying pressure against their walls. So, there are many other places within the cardiovascular system where the baroreceptors could have been positioned at random to detect blood pressure. For example, they could have been put in the arteries that carry blood to the arms, or the legs, or in the veins that drain the liver or the kidneys, or in the capillaries that bring in oxygen and release carbon dioxide within the alveoli of the lungs. It is safe to say that, if the baroreceptors had been placed in any of these other positions instead of the main arteries that direct blood to the brain, the body would not be able to properly monitor its blood pressure and be able to preserve adequate blood flow to the brain on standing up. The baroreceptors detect stretching of the arterial wall and are located exactly where they need to be (within the aortic arch and the carotid arteries) to be able to do their job.
If life has come about solely by the random forces of nature, then how do scientists explain the presence of the baroreceptors precisely where they need to be to maintain blood flow to the brain? No other location within the human body would allow for survival, so a pre-human life form would not have been able to reproduce itself. The body would have had to get it right the first time. There would not have been any second chances. Is it reasonable to believe that the sensor needed to provide information on how much fuel you have in your car was placed in the gas tank by random chance?
Having a sensor to detect something that needs to be controlled is useless without having an integrator to take the information it detects and compare it with a standard to know if a change in function is needed. To know if your car has enough fuel the manufacturer has provided a fuel gauge that is located on the dashboard. Besides the fuel gauge, there are many other gauges on the dashboard that help you keep track of other important functions within the car. So, there are other gauges on the dashboard to which the fuel sensor in the gas tank could have been randomly connected. For example, it could have been connected to the gauge that tracks oil pressure, or the one that monitors the temperature within the engine. Clearly, if the information from the fuel sensor in the gas tank was sent to the oil or temperature gauges, then it would be impossible to know how much fuel is in the gas tank. In addition, just like zeroing a weigh scale, the fuel gauge must be properly calibrated to be sure that what it says reflects the actual level of fuel in the gas tank. For if the tank is almost empty, but the fuel gauge indicates that there is still plenty of gas in the car then the ability for it to adequately warn the driver of problems ahead has been lost. So, to be able to really know how much fuel is in a car, the data from the sensor in the gas tank must be connected to a properly calibrated fuel gauge which can readily be seen by the driver.
Similarly, the information about the blood pressure within the arteries leading into the brain must be transmitted from the baroreceptors to the region within the brainstem that manages the blood pressure. However, the brainstem performs many other important functions as well. So, there are many other regions in the brainstem to which the information from the baroreceptors could go instead. For example, the data from the baroreceptors could be connected to the areas within the brainstem that control respiration, or thirst, or hunger, or salt intake, or consciousness. If the data from the baroreceptors is not sent to the proper region in the brainstem then the body will not be able to control the blood flow to the brain on standing up. Moreover, just like zeroing a weigh scale, the brainstem must be calibrated so that it properly interprets the information it receives from the baroreceptors. For, if the blood pressure were to actually be lower than what the brainstem perceives it to be, then, although we would experience a warning about low cerebral blood flow (by being dizzy), the brainstem might not respond well enough to remedy the situation. In that case we would faint (unless we were to lie down immediately). So, to control the blood pressure and the blood flow within the brain, the information detected by the baroreceptors must connect up to the proper region in the brainstem where it must be properly integrated so that it reflects the truth about the blood pressure within the main arteries leading into the brain.
If life has come about solely by the random forces of nature, then how do scientists explain the fact that the data from the baroreceptors goes to the correct region in the brainstem where it is properly integrated to control blood pressure and cerebral blood flow to prevent dizziness or syncope on standing up? Without this proper connection human life would be physically impossible, so a pre-human life form would not have been able to reproduce itself. The body would have had to get it right the first time. There would not have been any second chances. Is it reasonable to believe that the sensor in the gas tank of your car was randomly connected to a randomly calibrated fuel gauge so you can know if you have to fill up with gas?
It is no use having a sensor and integrator to know that there is a problem with something if you cannot do anything about it. When a properly calibrated fuel gauge tells you that your car is low on gas, you are the agent that can effect a change to prevent your car from running out of gas and dying on the road. However, you cannot go just anywhere or do just anything to solve the problem. You have to pull into a gas station and use a gas pump to fill your gas tank with fuel. Besides a gas station there are many other places that you could go at random. For example, you could go to a zoo, or an amusement park, or a nightclub, instead of a gas station. It is clear that if you went to any of these places rather than a gas station you would not be able to solve your problem. However, just going to a gas station would not automatically correct the situation either. For, a gas station sells more than just gas. So, at random you could decide to buy a soda, or engine oil, or anti-freeze and put one of them into the gas tank instead. In addition, there are reservoirs for fluids other than just for gas in your car. So, you could randomly choose to pump gas into the windshield fluid reservoir, the radiator, or the transmission, instead of the gas tank. Obviously, if you put the wrong fluid in the gas tank or pump gas into the wrong reservoir you will not be able to correct the problem. In other words, when something is not working right (car running out of gas) to effect a change that allows proper function to continue (keep the car running) you cannot just randomly do anything, you must do the right thing (put enough fuel in the gas tank).
Similarly, if you stand up quickly and feel dizzy, this is a sure sign that the MAP at the level of your heart has probably dipped below 70 mmHg, and below 50 mmHg at the level of your brain. If they are working correctly, the baroreceptors in the main arteries leading to your brain have immediately informed your brainstem of the situation. If you are going to be able to remain standing, with all your wits about you, then your brainstem is going to have to do the right thing very quickly. It will have to immediately send out enough of the right nerve messages to the right target organs and stimulate them enough to do the right things to correct the situation as soon as possible. The body could randomly produce any type of chemicals but the ones that are specifically needed to effect the right changes in the target organs are epinephrine and norepinephrine. The cells in the heart, and the muscles surrounding the arterioles, could randomly have many different receptors on their surfaces, but if they do not have the ones that are specific for epinephrine and norepinephrine, then the right things needed to correct the situation will not be done. However, even if the brainstem were to respond correctly, and epinephrine and norepinephrine were to lock onto their specific receptors in the heart and the arterioles, causing an increase in cardiac output and peripheral vascular resistance, although the right things may have been done, if it does not have enough of an effect and is not done quickly enough, then syncope will still take place. In other words, to be able to remain standing, with all your wits about you, not just any increase in cardiac output and/or peripheral vascular resistance will do. Real numbers have real consequences and in this case, time is of the essence. For, if you remain standing, as the brainstem tries to correct the situation (unless you lie down or put your head below your heart) the MAP will continue to drop. Therefore, unless the MAP at the level of the heart can be quickly brought back up to above 70 mmHg, and 50 mmHg at the level of the brain, you are going to pass out and fall to the ground.
If life has come about solely by the random forces of nature, then how do scientists explain that the body is able to do, quickly and efficiently enough, exactly what needs to be done to maintain an adequate blood flow to the brain on standing up? Clinical experience (with people who have sympathetic nerve malfunction) shows that, without all of these parts working to perform this function properly, human survival would have been impossible. The body would have had to get it right the first time. There would not have been any second chances. Is it reasonable to believe that when you realize your car is running out of gas, begin to look for a gas station, drive in, park your car, get out, place the nozzle in the gas tank, and pump enough gas in the gas tank to get you to your destination, that you have done all of these things at random?
To summarize, it is clear that the body needs to have several different parts, located in exactly the right positions, reacting in the right ways, all doing the right things, to be able to stand up to gravity and remain conscious. The same can be said of every other system in the body that performs a specific function for survival.
In conclusion, experience teaches us that the human body has physical limitations. Jump to the ground from a ledge thirty feet in the air and a broken leg is the likely result. This happens because of the physical law of gravitational acceleration and the fact that the leg is made of bone, not rubber. Thrust a hand into a hot flame and burnt fingers are the inevitable consequence. This takes place because of the nature of heat energy and the fact that what covers the body is flesh, not asbestos. The human body exists within the physical universe and therefore is subject to the laws of nature. This article has shown that vascular resistance and gravity are two important forces of nature that together can seriously reduce the blood flow to the brain when a person stands up. This reduction in blood flow to the brain can be enough to compromise brain function and lead to dizziness, and, if not correctly quickly enough, unconsciousness. The inability for our human ancestors to stand up to gravity without always passing out, would have rendered them unfit for survival. All medical experience teaches us that the purposeless laws of nature cause malfunction, debility, and death, not functional ability, survival capacity, and life. In fact, an understanding of how the body controls its blood pressure to stand up to gravity, shows that life has had to come up with ways to combat the effects of the random forces of nature to survive. Remember, when the body is in total equilibrium with its environment, when the laws of nature are unencumbered by its physiological functions, the body is said to be "dead”. When it comes to the development of life, it would appear that the random forces of nature are much less forgiving than the imaginations of evolutionary biologists. Next time we’ll look at how the body controls the blood flow to its different tissues and organs during exercise, making sure that it goes where it needs to go in order to survive.
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 column or any of the previous ones are welcome at firstname.lastname@example.org
Copyright 2012 Dr. Howard Glicksman. All rights reserved. International copyright secured.