Anatomy and Physiology: The Digestive System

The Digestive System

We are biological machines who need a constant supply of materials and fuel in order for us to function. In addition to being a fuel source (as in the case of carbohydrates), food contains vital raw materials, in the form of small monomers and chains of polymers that we use for repair, growth, defense, and so on. Simply put, if we stop eating we will ultimately die. The need for fuel and materials is so strong that our bodies will start to break down what we have already built (such as our fat reserves and even our muscles) in order to survive.

Despite the intimate relationship we all have with food, far too many people are ill informed on the basics of the digestive system. This is a shame, because this system of tubes, transport, breakdown, and ultimately distribution runs the gamut from the wonderful to the disgusting. The organs, our protagonists, run the risk of destroying themselves because they are made of the same basic materials as the food we eat, and they have thus evolved many clever ways of protecting themselves. Given that we are what we eat, we will also follow the many twisted pathways from the moment our food touches our lips, to the point where it reaches our hips (whether it stays for a life-time or not!).

Function Junction

The glory of the digestive system exists, in part, because of the many different functions it carries out. Eating alone is not enough. As you have seen before, the structure of the different organs will vary according to their functions. The functions of the digestive system are …

  • Ingestion. Ingestion is the act of eating. Food is fuel, and we have to get it inside somehow!
  • Mechanical digestion. Much of what we eat is rather large by cellular standards, and it needs to be broken down into smaller pieces (without actually changing the molecules) to increase the surface area available for the enzymes. Mechanical digestion consists of tearing, chewing, and the churning effects of peristalsis.
  • Peristalsis. Once we swallow, the ride begins! Food needs to travel about 30 feet (9 meters) before all is said and done; luckily muscle tone in our organs shortens that length by about half. Peristalsis is the rhythmic contractions of our digestive organs that propel the food.
  • Chemical digestion. As the food travels along the digestive tract, various enzymes are released, which chemically change the complex polymers into the basic monomers that our bodies will ultimately use.
  • Secretion. It is important to note that digestive enzymes can digest your own body! The organs thus need to protect themselves by secreting various fluids.
  • Absorption. The monomers and water in the gastrointestinal tract need to be transported to the body via the bloodstream, but they first need to be absorbed into the blood.
  • Storage and toxin processing. All of the blood from the abdominal digestive organs passes through the liver, which stores some sugar (as glycogen) for emergencies, converts other sugars into fat, and breaks down toxic chemicals before too much gets to the body.
  • Excretion and egestion. Egestion is the release of wastes (feces) we do not use, via the anus; this release is also called defecation (to “de-feces” oneself).
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Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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