Low Blood Pressure and Evolutionary Biology
Editor's note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that's because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News & Views is delighted to present this series, "The Designed Body." Dr. Glicksman practices palliative medicine for a hospice organization.
The body is made up of trillions of cells which must each receive what they need to live. The cardiovascular system circulates blood throughout the body to accomplish this task. However, being made of matter, the body must follow the laws of nature. These laws demand that there be enough power to move the blood against natural forces such as inertia, vascular resistance, and gravity.
The blood pressure is the force the blood applies against the arterial walls as it flows by, and represents the energy that moves the blood where it needs to go. It's like how a city must supply its residents with enough water to meet their hygienic needs. If the pressure in the city's water system isn't high enough, then customers won't be serviced properly. The same thing applies to the blood pressure and the body. If there isn't enough blood pressure in the systemic arteries, the organs of the body won't be serviced properly and they'll malfunction or die because their metabolic needs aren't being met.
The three main things that affect arterial blood pressure are how well the heart pumps blood, how much blood is in the arteries, and the vascular resistance applied to blood flow by the downstream arterioles. The body controls its blood pressure mainly through systems which involve the brain, the heart, the kidneys and the blood vessels. Evolutionary biologists only show how these systems look and then imagine how they may have come about without considering how they actually work within the laws of nature to keep the body functioning properly. The last article in this series showed what it takes for the body to be able to stand up to gravity, something that would have been absolutely necessary for the survival of our earliest ancestors. But sometimes circumstances can still overwhelm the body's control mechanisms to allow the blood pressure to go too low.
It is important to understand that the lower limit of blood pressure resulting in blood flow is directly related to the metabolic needs of the organs. It's the same as having enough gas and air entering a car's engine so it has enough energy to perform properly. Without enough blood pressure to provide enough blood flow to the organs, they are as good as dead.
My previous article showed that vital organs like the brain use autoregulation to maintain a constant blood flow. They do this by changing the vascular resistance in their arterioles to adjust to constant changes in blood pressure. The normal range of the mean arterial pressure (MAP) that results in proper blood flow in the brain, without the need for autoregulation, is 70-100 mmHg. This translates into a BP of 90/60-140/80.
However, when it comes to significantly low blood pressure, there are limits to how low the vascular resistance in the arterioles can go in an attempt to maintain adequate blood flow. For critical organs like the brain, this lower limit of MAP is about 50 mmHg, which is equal to a blood pressure of only 70/40.
People who have a blood pressure less than 90/60 mmHg are said to have hypotension. Many of them have to live relatively sedentary lives because they suffer from fatigue, weakness and dizziness on standing up. Since cardiac performance is a major determinant of blood pressure, it is not surprising that many people with heart failure, valvular disorders and arrhythmias commonly suffer from hypotension. People with disorders of the sympathetic nervous system, as seen in diabetics and people with neurological conditions like Parkinson's Disease and Multiple Sclerosis, are particularly prone to severe drops in blood pressure when standing up, sometimes to the point of passing out (syncope). This condition is called orthostatic hypotension and takes place because the body cannot compensate fast enough through the sympathetic nervous system to the sudden drop in blood pressure brought on by the force of gravity.
Recall, from the last article, that when the MAP drops below 70 mmHg, the baroreceptors in the main arteries supplying blood to the brain trigger the sympathetic nervous system to increase the cardiac output and peripheral vascular resistance to keep the MAP where it is supposed to be. However, sometimes circumstances arise that overwhelm the body's ability to maintain an MAP above 50 mmHg. When this happens, the body is unable to adequately perfuse its organs and tissues and it is said to be in circulatory shock (as opposed to emotional shock).
Without aggressive resuscitation, irreversible cellular dysfunction and multi-organ failure is likely, often leading to death. Since blood pressure is largely dependent on three factors, blood volume, cardiac performance, and vascular resistance, shock is generally divided into three categories that reflect this reality.
A significant and quick drop in total blood volume can lead to what is called hypovolemic shock. Severe and rapid blood loss from traumatic rupture of a large blood vessel or a major organ or severe bleeding from the gastrointestinal tract are some of the more common causes. Non-hemorrhagic fluid loss, such as severe and persistent vomiting and diarrhea, or extensive third degree burns, can cause hypovolemic shock as well.
Conditions of the heart resulting in a marked reduction in cardiac output can quickly lead to what is called cardiogenic shock. A sudden heart attack causing severe damage to the left ventricle and a marked reduction in contractility and stroke volume or the sudden rupture of the mitral valve, are common causes of cardiogenic shock. Additionally, an already damaged and failing heart that suddenly starts beating too slowly or too quickly can lead to a quick drop in cardiac output and cardiogenic shock as well.
In my last article I showed that the relationship between blood pressure, blood flow and vascular resistance can be expressed as P = Q x R. Blood pressure (P) is directly related to total blood flow (Q) and the vascular resistance (R) in the tissues. As noted above, hypovolemic and cardiogenic shock are caused by a significant reduction in cardiac output (Q) due to reduced total blood volume and inefficient cardiac function, respectively. The body's natural response to both hypovolemic and cardiogenic shock is to increase the vascular resistance through the sympathetic nervous system and other hormones in an attempt to bring the blood pressure back towards normal.
In contrast to both hypovolemic and cardiogenic shock, the third type of shock involves a significant reduction in the peripheral vascular resistance. Since P = Q x R, a major drop in R can also lead to severe hypotension in what is called distributive shock. In this setting, the muscles surrounding the arterioles throughout the body relax and allow too much blood to enter the capillaries which results in excessive loss of fluid into the tissues.
One of the commonest causes of distributive shock is severe infection, often in the bloodstream. Another name for it is septic shock, or SIRS (systemic inflammatory response syndrome). Not only do the bacteria release toxins that cause the muscles surrounding the arterioles to relax too much causing a catastrophic drop in the vascular resistance, but the body's immune system in trying to fight the infection does the same thing as well.
The body has many different mechanisms in place to overcome the laws of nature and thus survive in the world. The fact that we die indicates that eventually these mechanisms become overwhelmed, resulting in cardiopulmonary arrest and brain death.
As is common with many of the chemical and physiological parameters the body must control to stay alive, blood pressure must follow theGoldilocks principle. Everything has to be just right for the body to be able to function well enough to survive. The current theory of evolution does not even begin to explain how any of this works in real life. Real numbers have real consequences and clinical experience bears this out by showing that not only is low blood pressure hazardous to your health, but so is high blood pressure. That's what we'll consider next time.
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