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The Human Body Continues to Give Evolutionary Biologists High Blood Pressure


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." For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.

the-designed-body4.jpgLet's review what this series of articles has shown so far about human life. The body is made up of trillions of cells, each of which must control its volume and chemical content while receiving what it needs to live, grow and work properly. Since the body is made up of matter, it is subject to the laws of nature, which demand that it have enough energy to do what it needs to do to survive.

To get enough oxygen to meet its metabolic needs, the lungs must have fast enough airflow, large enough volume and efficient enough gas exchange in addition to having enough hemoglobin in the blood. Since it is the blood in circulation that provides the cells with what they need, the body must also make sure it has enough water, sodium and albumin to maintain enough blood volume. Moreover, since blood has mass, it needs the heart to pump it throughout the body against forces like inertia, friction and gravity that naturally prevent it from moving anywhere. To do this adequately, the heart must have a properly working electrical system, which is dependent on having the right amount of potassium in the blood, and enough coronary blood flow, proper valve function and sufficient ventricular relaxation and contractility.

Having enough of whatever is needed to follow the rules set down by nature also requires being able to take control and each of the systems the body uses has a sensor, an integrator and an effector to get the job done right. And since life doesn't take place inside a vacuum or within the imaginations of evolutionary biologists, when it comes to living and dying, real numbers have real consequences. This means that to survive within the laws of nature, life must make sure that each of its numerous chemical and physiological parameters must stay within a certain range.

We've already looked at how the blood volume and cardiac output are properly maintained. But it is possible for both of these parameters to be normal and death can still take place due to an abnormal blood pressure. How does that happen?

Blood pressure is the force which blood applies against the walls of a chamber or blood vessel through which it is moving. Think of it like the pressure that can be felt as water rushes through a garden hose. Since blood flows from the heart through the arteries to the capillaries in the tissues, and back again through the veins, this means that there are many different blood pressures throughout the cardiovascular system. However,the blood pressure used in clinical practice is understood to be the force of blood exerted against the walls of the brachial artery that is located in the upper arm. In other words, the blood pressure is synonymous with the force the blood applies against the walls of the large systemic arteries.

When it comes to measuring blood pressure, it is important to understand the cardiac cycle and how blood flows within the arterial system. Systole makes up one-third of the cardiac cycle during which the left ventricle pumps blood through the aortic valve into the aorta and from there into the large arteries. As this blood is added to what is already present within the brachial artery, this causes the blood pressure to rise to a maximum. This is called the systolic pressure (SP). Diastole makes up two-thirds of the cardiac cycle, during which the left ventricle relaxes and fills with blood.

Once pumped out of the left ventricle, the blood travels down narrowing vessels until it meets resistance in the arterioles. Some of the blood flows through to the capillaries while some rebounds back towards the heart where it again meets resistance, this time from the closed aortic valve. The blood then continues to ping-pong back and forth between the arterioles and the closed aortic valve during diastole, allowing some of it to pass into the capillaries. By the time diastole ends and systole is about to begin, the blood pressure within the brachial artery has dropped to a minimum and is called the diastolic pressure (DP). In other words, at the end of diastole there is still a lot of blood within the large arteries.

The blood pressure is measured by pumping up a pressure cuff, placed around the upper arm, high enough to block the blood flow in the brachial artery. While slowly releasing the pressure, the doctor uses a stethoscope to listen for the first sound of turbulent blood flow (SP) and then its total disappearance (DP). The blood pressure is measured in units called millimeters of Mercury (mmHg)and is usually expressed as the SP/DP, such as "120/60." Since systole lasts for one-third of the cardiac cycle and diastole, two-thirds, it is possible to calculate the mean arterial pressure (MAP) as 1/3 SP + 2/3 DP. So the MAP for a BP of 120/60 would be 80 mmHg (1/3 (120) + 2/3 (60)).

It is important to remember that the blood pressure represents the driving force that moves blood throughout the circulatory system to feed the cells what they need to live, grow and work properly. This means that, in general, blood flow is directly related to blood pressure. It is the MAP that is often used when considering blood flow because it represents the average blood pressure. Like all matter, blood is affected by natural forces such as inertia and gravity, which must be overcome by the pumping heart to move it where it needs to go. Just as friction slows the movement of an object on the ground, blood flow meets its counterpart, called vascular resistance in the blood vessels. In particular, the vascular resistance that is generated by the peripheral arterioles is called the peripheral vascular resistance (PVR).

Inertia, vascular resistance and gravity are the three main forces that prevent blood from going where it needs to go in the body. The body must generate enough blood pressure to maintain enough blood flow throughout the circulation. The three main factors that affect the blood pressure are the cardiac output, the total blood volume and its distribution within the circulation, and the peripheral vascular resistance. The harder and faster the heart pumps, the more blood enters the arterial system and the higher the blood pressure, and vice versa.

The total blood volume is dependent on how much water and sodium is in the body. The systemic arteries usually contain about 12 percent of the body's blood while the veins have about 60 percent. So, the more water and sodium in the body, the more blood volume, and when the body shifts more of it from the systemic veins into the arteries, the higher the blood pressure and vice versa. The vascular resistance applied to blood flow by the peripheral arterioles causes less blood to go into the capillaries and more to stay inside the arteries. So more peripheral vascular resistance results in higher blood pressure and less peripheral vascular resistance, lower blood pressure.

It is the laws of nature that demand that each cell have enough energy and nutrients to live and do what it is supposed to do inside the body. However, it is these same laws that the body must overcome by having enough arterial blood pressure to get enough blood flow to its cells and provide them with what they need. In other words, as clinical experience has shown, too low of a blood pressure is a big problem. Since blood vessels are made of tissue that can withstand only so much pressure, too high of a blood pressure is just as bad. Next time we'll look at how the body takes control of its blood pressure to be able to survive within the laws of nature.

Image credit: FASTILY (Own work) [GFDL or CC BY-SA 3.0], via Wikimedia Commons.