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Blood Clotting Requires Three Different Processes Working Together


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 some of the lessons that this series has so far provided 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 from the blood 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.

The body must constantly take in oxygen through the respiratory system to provide itself with the energy it needs to live because, unlike glucose, oxygen can’t be stored for future use. About one-quarter to one-half of the energy the body uses while at rest is just for the sodium-potassium pumps within the plasma membrane of its trillions of cells. These pumps push Na+ ions out of the cell and bring K+ ions back in. This action not only maintains the volume and chemical content of the cell, but also the 2/3:1/3 ratio of water inside and outside the cells.

Combined with the control of its water and sodium content and the production of albumin, this process allows the body to maintain enough blood volume. The sodium-potassium pumps also maintain the electrical resting membrane potential of the cell. This is particularly important for proper heart, nerve, and muscle function.

When the cells in the brainstem die (the ones that tell the body to breathe, control its circulation, and make it conscious of its surroundings) the body is considered dead. However, the commonest cardiopulmonary arrest is the most common cause of death. Without respiration the body can’t bring in new supplies of oxygen and get rid of toxic carbon dioxide, and without the heart pumping, there isn’t enough blood flow to the brain. Together, this causes it to malfunction.

Since blood has mass, the heart has to pump it throughout the body against natural forces like inertia, friction and gravity that would prevent it from moving. As blood moves through the circulatory system, it applies a pressure against the vessel walls. This represents the energy that is generating its flow.

In clinical practice, the blood pressure is taken in the upper arm and is a measure of the force of blood against the walls of the brachial artery. The blood pressure is dependent on how well the heart pumps, how much blood is in the systemic arteries, and the resistance to blood flow applied by the downstream arterioles that control how much blood enters the capillaries. Blood flow (Q) is directly related to the blood pressure (P) and inversely related to the vascular resistance (R). This law of nature can be expressed as Q = P/R. The more blood pressure the more blood flow and the less blood pressure the less blood flow. The more vascular resistance the less blood flow and the less vascular resistance the more blood flow.

When we stand up and feel momentarily dizzy, our body must inherently know that if Q = P/R, then P = Q x R. In other words, blood pressure is directly related to the blood flow and vascular resistance. More blood flow and vascular resistance increases the blood pressure and less blood flow and vascular resistance decreases the blood pressure.

Standing up allows gravity to prevent blood from returning to the heart from the veins in the chest, abdomen and legs. It also keeps it from going from the heart to the brain, which reduces the blood pressure and blood flow to the brain. That’s what makes us feel dizzy. The body detects these changes and reacts by sending out nerve messages to make the heart pump harder and faster, pushes blood from the veins back toward the hear,t and increases the vascular resistance applied by the arterioles by increasing the contraction of their surrounding muscles. The first two actions increase blood flow (Q) and the last one increases the vascular resistance (R), so the blood pressure (P) rises and our dizziness usually resolves in a matter of seconds.

The above demonstrates just some of the ways the body takes control and follows the rules in line with the Goldilocks principle: the real numbers must be “just right.” After all, life doesn’t take place within a vacuum or the imaginations of evolutionary biologists. As was the case with our earliest ancestors, to survive we must remain active, exposing our body, and our blood vessels, to the random forces of nature.

Experience tell us that when we run, jump, climb, roll, fall, and generally bang into or scrape up against solid or sharp things, natural forces like friction, momentum, pressure, sheer and gravity all contribute to blood vessel damage, bleeding and blood loss. This takes place because, with damage to the blood vessel wall, the pressure that sustains blood flow within it naturally pushes blood out through the opening. Think of it like when a water pipe in your home bursts. The water flowing through it is under pressure and when the walls of the pipe rupture water naturally flows out. Depending on the location and the severity, if the body can’t stop the bleeding fast enough it runs the risk of serious problems. So to follow the rules that the laws of nature impose on it, the body must have a mechanism in place to prevent excessive bleeding from its blood vessels when they undergo injury.

Experience teaches that when we cut ourselves, a clot forms at the injured site to stop the bleeding and allow healing to take place over the next several days. This process is called hemostasis (Greek: haima = blood + stasis = halt).

Hemostasis generally involves three actions that occur almost simultaneously. Vasoconstriction is contraction of the muscles surrounding the injured blood vessel, which tries to totally close down the opening in the wall. Platelet aggregation is the formation of a soft plug by the platelets coming together and sticking to each other to fill the gap in the blood vessel wall. And activation of the clotting factors is the formation of thousands of sticky strands of fibrin which wrap around the platelet plug to form a molecular meshwork that entraps red blood cells and plasma to form a fibrin clot which closes off the damage and stops the bleeding.

However, when it comes to preventing blood loss from blood vessel injury through hemostasis, the laws of nature present the body with another dilemma. It’s important that the clotting mechanism turn on only when it’s needed, not only to not waste the body’s supply of platelets and clotting factors, but also to prevent the sudden blockage of blood flow to vital organs. A poorly placed blood clot within an artery supplying blood to the heart muscle can cause a heart attack, or to the brain, a stroke, or to the lungs, a sudden drop in oxygen availability. Any one of these situations can result in serious and permanent debility or sudden death.

So, in addition to being able to turn on at the right time, the body must also have a mechanism in place to take control so that hemostasis will turn off and stay off at the right times. How does the body do it and how does evolutionary biology explain how it works in real life? That’s what we’ll begin to look at next time.

Image credit: Paul Goyette, [CC BY-SA 2.0], via Wikimedia Commons.