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 offer this series, “The Designed Body.” For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.

Except for molecular oxygen (O₂), which comes in through the lungs, everything else the body needs to survive enters through the gastrointestinal system. This includes things like water, sugars, amino and fatty acids, cholesterol, electrolytes, minerals, and vitamins. But most of the nutrients the body needs are trapped inside more complex molecules, like carbohydrates, proteins, and fats, and are too large to enter the body. The gastrointestinal system must first break down these large molecules into much smaller ones, in a process called digestion, so it can then absorb the nutrients the body needs into the blood. In my last article I showed that digestion is similar to how a pulp and paper mill works. They both use mechanical and chemical means to break down large things into smaller ones and only use their equipment and chemicals when needed.

The process of digestion begins as soon as food enters the mouth. Its presence, along with its taste and smell, are detected by the nervous system, which stimulates the release of saliva from the glands in the mouth. Saliva contains the enzymes amylase and lipase, which begin the chemical breakdown of carbohydrates and fats respectively. As the food mixes with saliva, it is mashed by the teeth and the tongue, formed into a small mushy lump called a bolus, and moved back toward the pharynx.

Sensors in the pharynx detect the bolus and send information to the brain, initiating the swallow reflex. Swallowing involves the coordinated action of about twenty-five different pairs of muscles to protect the airway and propel the bolus into the esophagus, where it is moved by peristalsis down into the stomach. This article will show how the body follows the rules and takes control to continue digestion and absorption within the stomach and beyond.

Seeing, smelling, and tasting food causes the brain to send nerve messages to the stomach, which begins the first or cephalic phase of gastric secretion. This causes the release of mucous, hydrochloric acid, and pepsinogen. The mucous protects the cells that line the stomach from its own chemicals. The acid both kills microbes and converts pepsinogen into a powerful digestive enzyme called pepsin, a protease that begins to chemically breakdown protein. This phase also results in specialized cells in the stomach secreting a hormone called gastrin,which travels in the blood and tells the stomach cells to send out even more mucous, hydrochloric acid, and pepsinogen.

As the stomach fills up and distends with fluid, the stretch-sensitive mechanoreceptors in its walls send out more nerve messages. These stimulate the cells in the stomach to send out even more mucous, hydrochloric acid, and pepsinogen in what is called the second or gastric phase of gastric secretion. The contents of the stomach are then churned and mixed to further help in the digestive process, creating an acidic liquid called chyme.

It is important to note here that besides playing a major role in digestion, the stomach does two other important things: use its acid to help iron be absorbed later on and produce intrinsic factor to protect Vitamin B12 from being broken down by its acid. Both of these nutrients are needed for the production of hemoglobin.

The stomach absorbs very few nutrients (mainly water) and once it has done its part of digestion it passes the chyme into the first part of the intestine called the duodenum. To prevent damage to the duodenum and allow for more efficient digestion and absorption, it is important that the stomach control how fast it releases the chyme. This is accomplished by the pyloric sphincter, a ring-like band of muscle at the end of the stomach that is able to constrict and relax to send out the right amount of chyme for the right situation. Sensors in this region send messages to nerve cells, which help to control gastric emptying. In general, the more fat and protein is present and the more acidic the chyme, the slower the stomach empties its contents into the duodenum. This is why, when you have a heavier meal, your stomach feels full for a longer period of time.

As the stomach works on the acidic chyme and slowly sends it into the duodenum, the stretching of the intestinal walls signals it to start producing its own fluid. Intestinal juice mainly contains saline (NaCl), mucous, bicarbonate (HCO3^–), and digestive enzymes. The alkaline bicarbonate begins to neutralize the acidic chyme that the intestine receives from the stomach. The enzymes produced in the lining of the intestine mainly help to break up the bonds between molecules that contain two sugars. Maltase breaks up the bonds between the two glucose molecules that make up maltose, lactase breaks up the bonds between glucose and galactose which make up lactose, and sucrase breaks up the bonds between glucose and fructose which make up sucrose. The intestine also produces enterokinase, a protease that is important for activating many of the enzymes that come from the pancreas.

As the chyme moves from the stomach into the duodenum, sensors on specialized gland cells detect simple molecules, like fatty and amino acids. The gland cells respond by sending out two hormones, secretin and cholecystokinin, to tell the pancreas to deposit its fluid into the digestive tract. Pancreatic juice contains high amounts of bicarbonate and is very alkaline. The addition of the alkaline pancreatic juice helps to further neutralize the acidic chyme that has entered the intestine from the stomach. The pancreatic juice also contains most of the enzymes needed to finish off the digestion of carbohydrates, fats, and proteins. In addition to amylases and various lipases, the pancreatic juice contains many different proteases that break down proteins. This includes trypsin, chymotrypsins, elastases, and carboxypeptidases. All of these proteases are produced inside the pancreatic cells in the inactive form so they won’t digest the pancreas itself.

Trypsin enters the intestine as trypsinogen and becomes activated by its alkaline environment and enterokinase, which, by snipping a few atoms off, changes its shape so it is ready to go to work. Trypsin then activates the other enzymes and proteases, mentioned above. Finally, since lipids are not very soluble in water, they require the presence of bile from the liver and gall bladder to help in fat digestion. The presence of fatty acids in the duodenum contributes to the release of cholecystokinin, which tells the pancreas to release its juice and the gall bladder to contract and send its concentrated bile into the intestine to help in fat digestion.

The intestine, which consists of the duodenum, jejunum, and ileum, is where most of digestion and absorption take place. In addition to water, glucose, amino acids, cholesterol, and simple fats, the intestine also absorbs other vital chemicals, such as minerals, like calcium and iron, electrolytes, like sodium and potassium ions, and vitamins like A, C, D, E, K, and all of the B vitamins, including Vitamin B12. About 1.5 liters of fluid makes its way from the intestine into the colon daily, where mostly water, sodium, and chloride ions are reabsorbed. The remaining 100-150 gm of feces that daily exits the gastrointestinal tract through the rectum and anus usually consists of about 70 percent water and 30 percent solids from undigested plant fibers, like cellulose, cells shed from the lining of the gastrointestinal tract, and bacteria.

A quick review of gastrointestinal function shows that, to do its job properly, it needs separate control systems to turn on different organs, each using enough fluids and chemicals to adequately digest food and absorb enough nutrients. The cephalic and gastric phases of gastric secretion, along with gastrin, make sure the stomach sends out enough acid to activate pepsin for the digestion of proteins to begin. The amount of fatty and amino acids and the acidity of the chyme determine the rate of gastric emptying to help in proper digestion and absorption.

These chemicals also stimulate certain gland cells to release secretin and cholecystokinin, which together tell the pancreas to release its juices into the intestine and the gall bladder to release concentrated bile. Alkaline intestinal and pancreatic juices neutralize the acid coming from the stomach, which, with the help of enterokinase, activates trypsin. Trypsin then activates many other pancreatic enzymes that do most of the work of digestion. Bile from the liver and the gall bladder are needed to help fat digestion as well. Having completed digestion, the intestine then absorbs the nutrients that have been freed up. Finally, the intestine and colon reabsorb most of the water, sodium, and chloride ions that have been previously secreted so that very little is lost through the feces.

The total absence or significant deficiency of any one nutrient would have made it impossible for our earliest ancestors to survive long enough to reproduce. The gastrointestinal system demonstrates irreducible complexity because every component has to be present for it to be able to do its job. It also demonstrates natural survival capacity because each of its components has to provide enough of the right fluid and chemicals to cause adequate digestion and allow for the absorption of what the body needs for survival. Evolutionary biology usually points to similar systems within simpler organisms to explain how the gastrointestinal system may have come into being. Of course, this does not explain how the simpler systems developed in the first place or how it must work within the laws of nature to allow for survival.

The body must breathe in air every few seconds because its need for oxygen is so acute that without it, it dies in just a few minutes. When it comes to water, because the body is able to move some of it from its cells into the blood to compensate for its loss, it only has to drink fluids every few hours. What about glucose? After all, we know we don’t have to take in glucose as often as oxygen or water to stay alive. So, how does the body go about making sure it has enough for its energy needs and how does evolutionary biology explain the development of the system it uses? That’s what we’ll start to look at next time.

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