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Defending the City: The Body’s Two-Pronged Defense Strategy


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 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 a few things that this series has shown are needed for human survival. 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 it is made up of matter, the body is subject to the laws of nature, which demand that it constantly take in oxygen to provide itself with the energy it needs to live. Unlike with glucose, the body can’t store oxygen for future use.

These laws also demand that the body have the right amounts and distributions of water, sodium, and potassium for blood volume, and the proper resting membrane potential for adequate nerve, muscle, and heart function. Additionally, since blood has mass, it needs the heart to pump it through the circulatory system to the tissues with enough pressure against natural forces like inertia, vascular resistance, and gravity..

If the body doesn’t have the right levels of oxygen, water, sodium, potassium, blood pressure, or blood flow, then cell death takes place. When the cells in the brainstem die — the ones that tell the body to breathe in air, control its cardiovascular system, and make it conscious of its surroundings — the body is considered dead. The most common pathway to death is by cardiopulmonary arrest. Without respiration, the body can’t bring in new supplies of oxygen and get rid of toxic carbon dioxide. Without the heart pumping, there is no blood flow to the brain. So, together, cardiopulmonary arrest causes death very soon after.

Life does not exist in a vacuum or merely in the imagination of evolutionary biologists. As we saw in the most recent articles in this series, small blood vessels of the body constantly undergo injury from the everyday activities of life. For our earliest ancestors to survive long enough to reproduce, they needed a well-controlled clotting mechanism (hemostasis) in place that would turn on only when it was needed and turn off and stay off when it wasn’t.

Bleeding disorders — where the clotting mechanism won’t turn on — can cause a brain hemorrhage from even minimal trauma, or hypovolemic shock from spontaneous gastrointestinal bleeding. Hypercoagulable states — where the clotting mechanism turns on at the wrong time — can easily cause death from a heart attack, stroke, or pulmonary embolism. Either way, unless hemostasis is properly controlled, the body is as good as dead. But well-controlled hemostasis is dependent on having a finely tuned system of pro- and anti-clotting factors that must be produced in adequate quantities by the liver and the endothelium that lines the blood vessels.

Hemostasis is a type of defense system the body uses to prevent itself from bleeding to death from injuries and accidents. But that’s not the only one it has. The bones, muscles, and nerves work together to allow the body to detect danger and avoid or defend against it. However, survival also requires us to defend ourselves from enemies that we can’t detect with the senses. We are perpetually exposed to germs: microorganisms that are too small to be seen with the naked eye. These consist mostly of bacteria, viruses, and fungi. If such microbes invade the body and become widespread, then serious disease, debility, and even death can result.

Against microbial attack, the body has a two-pronged defense strategy. The first line of defense is the epithelium.This tissue separates and protects the interior cells of the body from the effects of the outside world. The skin is an epithelial tissue consisting of many different types of cells that provides passive resistance to invasion by microbes. Skin also protects the body from mechanical and chemical injury, ultraviolet radiation, extreme heat and cold, excessive fluid loss, and helps to control body temperature. The respiratory, gastrointestinal, and genitourinary systems also have an epithelial lining that separates their underlying tissue from the effects of the environment. Microbes that are inhaled, or swallowed, or are able to enter the urinary tract, come up against these barriers.

If the invading microorganisms breach the first line and enter into the tissues, then the second line of defense, the immune system, swings into action. The immune system consists of many different cells and proteins. In ancient times, when invaders breached the walls of a town, they usually met armed resistance. By using their weapons and shields for protection, the intruders would kill and loot their way through the town, thus conquering it. Similarly, after breaching a passive barrier like the skin, usually through a cut or scrape, invading microbes attempt to loot the body by using the nutrients in its fluids to live, grow, and multiply.

As with a town stormed by a finite number of attackers, a microbial infection usually involves a relatively small invading force. But once inside the body, the infection is able to multiply rapidly by using the resources of its host. It’s the job of the immune system to limit this activity as much as possible to preserve organ function.

Although there are many different types of bacteria, viruses, and fungi, the few that have developed the ability to breach the first line of defense and do battle with the immune system are called pathogens (Gk. pathos = disease + gennan = to produce). Some of these pathogenic organisms enter the cells, take over their metabolism, rapidly reproduce and then send out the next generation of microbes into the body after the cell dies. Many others can live within the tissue fluid between the cells and multiply and spread locally.

Infections are possible in almost every organ of the body. Progression of infection within a given organ system can lead to severe body malfunction. If the lungs develop pneumonia, this can significantly diminish their ability to bring in oxygen and release carbon dioxide and, particularly in people with emphysema, can lead to respiratory failure and cardiopulmonary arrest. If the gastrointestinal tract develops gastroenteritis, the associated vomiting and diarrhea, particularly in the very young and old, can lead to dehydration, chemical imbalance, hypovolemic shock, and cardiopulmonary arrest. If the brain develops encephalitis or meningitis, the nerve malfunction aggravated by the increased pressure can lead to brain death.

If the pathogens are not stopped within the tissues they initially infect, they can migrate into the lymphatics. The lymphatic system consists of very thin walled tiny channels that carry lymph (L. lympha = water), a liquid that comes from the fluid not reabsorbed at the venous end of the capillary. Every tissue and organ in the body is drained by lymphatic vessels, which eventually come together to drain into the venous system.

It is through the lymphatic system that microbes gain access to the bloodstream and all of the tissues and organs of the body. By working their way through the lymphatics and into the bloodstream, these organisms can cause septicemia and irreversible shock, resulting in death for about 250,000 people in this country every year.

Without the epithelial tissue of the body protecting it from microbial invasion, life would have been impossible for our earliest ancestors. But the experience of death-dealing infections throughout the world also tells us that without a properly working immune system, the same applies. How the immune system works and what it takes to control it so we can live within the world of microbes will be the subject of my next few articles.

Image: Siege of Peking, Boxer Rebellion, via Wikicommons.