Digestive System

Consider for a moment a Big Mac. The purpose in your eating a Big Mac, other than simple hedonism, is to assimilate the nutrients it represents and make them available to build, repair and maintain your own tissues, as well as provide energy for studying and occasional other pursuits.

You may have asked yourself - "Exactly what nutients are present in a Big Mac that I can assimilate?" MacDonald's comes close to full disclosure in this regard, but what they don't tell you is that in order to take advantage of these nutrients, you have to provide the means to carefully break them down into much smaller molecules that can be imported into blood. Luckily, your digestive system takes care of this very complex process so efficiently that most of the time you don't even need to think about it.

At its simplest, the digestive system it is a tube running from mouth to anus. This tube is like an assembly line, or more properly, a dissembly line. Its chief goal is to break down huge macromolecules (proteins, fats and starch), which cannot be absorbed intact, into smaller molecules (amino acids, fatty acids and glucose) that can be absorbed across the wall of the tube, and into the circulatory system for dissemination around your body.

The breakdown of foodstuffs like a Big Mac is accomplished through a combination of mechanical and enzymatic processes. To accomplish this breakdown, the digestive tube requires considerable assistance from accessory digestive organs such as the salivary glands, liver and pancreas, which dump their secretions into the tube. The name "accessory" should not be taken to mean dispensible; indeed, without pancreatic enzymes you would starve to death in short order.

In many ways, the digestive system can be thought of as a well-run factory in which a large number of complex tasks are performed. The three fundamental processes that take place are:

• Secretion: Delivery of enzymes, mucus, ions and the like into the lumen, and hormones into blood.
• Absorption: Transport of water, ions and nutrients from the lumen, across the epithelium and into blood.
• Motility: Contractions of smooth muscle in the wall of the tube that crush, mix and propel its contents.

Each part of the digestive tube performs at least some of these tasks, and different regions of the tube have unique and important specializations.

Like any well-run factory, proper function of the digestive system requires robust control systems. Control systems must facilitate communication among different sections of the digestive tract (i.e. control on the factory floor), and between the digestive tract and the brain (i.e. between workers and managment).

Control of digestive function is achieved through a combination of electrical and hormonal messages which originate either within the digestive system's own nervous and endocrine systems, as well as from the central nervous sytem and from endocrine organs such as the adrenal gland. Different parts of these systems are constantly talking to one another. The basic messages are along the lines of "I just received an extraordinary load of food, so I suggest you get prepared" (stomach to large intestine) or "For goodness sake, please slow down until I can catch up with what you've already given me" (small intestine to stomach).

Finally, a note about differences in digestive anatomy and physiology among animals. The digestive systems of humans, dogs, mice, horses, kangaroos and great white sharks are, to a first approximation, virtually identical. If you look more carefully however, it becomes apparent that each of these species has evolved certain digestive specializations that have allowed it to adapt to a particular diet.

These differences become particularly apparent when you compare a carnivore like a cat with a herbivore like a goat or a horse. Goats and horses evolved from ancestors that subsisted on plants and adapted parts of their digestive tracts into massive fermentation vats which enabled them efficiently utilize cellulose, the major carbohydrate of plants.

In contrast, cats evolved from animals that lived on the carcasses of other animals, and have digestive systems that reflect this history - extremely small fermentation vats and essentially no ability to utilize cellulose. Bridging the gap between carnivores and herbivores are omnivores like humans and pigs, whose digestive tracts attest to a historical diet that included both plants and animals. The image above shows a young omnivore in the company of herbivore and carnivore friends.

Nutrients are mostly absorbed in the small intestine and certain kinds of nutrients are absorbed in the large intestine. Most of the water is absorbed in the large intestine as well. Then the rest of the wastes goes through the rectum.