The Digestive System
The Digestive System: An Overview, p. 863
All living organisms must obtain nutrients from their environment to sustain life.
These substances are
used as raw materials for synthesizing essential compounds (anabolism
) or are decomposed to provide
energy that cells need to continue functioning (catabolism
The catabolic reactions require two
essential ingredients: (1) oxygen and (2) organic molecules (such as carbohydrates, fats, or proteins) that
can be broken down by intracellular enzymes.
The digestive system consists of a muscular tube, the digestive tract
, also called the gastrointestinal (GI)
or alimentary canal
, and various accessory organs.
The digestive tract begins at the oral cavity and continues through the pharynx, esophagus, stomach,
small intestine, and large intestine, which opens to the exterior at the anus.
Functions of the Digestive System
occurs when materials enter the digestive tract via the mouth. Ingestion is an active process
involving conscious choice and decision making.
is crushing and shearing that makes materials easier to propel along the digestive
tract. It also increases their surface area, making them more susceptible to enzymatic attack.
refers to the chemical breakdown of food into small organic fragments suitable for absorption
by the digestive epithelium.
Simple molecules in food, such as glucose, can be absorbed intact, but
epithelial cells have no way to absorb molecules the size and complexity of proteins, polysaccharides, or
triglycerides. These molecules must be disassembled by digestive enzymes prior to absorption.
is the release of water, acids, enzymes, buffers, and salts by the epithelium of the digestive
tract and by glandular organs.
is the movement of organic substrates, electrolytes (inorganic ions), vitamins, and water
across the digestive epithelium and into the interstitial fluid of the digestive tract.
is the removal of waste products from body fluids.
The digestive tract and glandular organs
discharge waste products in secretions that enter the lumen of the tract.
Most of these waste products,
after mixing with the indigestible residue of the digestive process, will leave the body.
The lining of the digestive tract also plays a protective role by safeguarding surrounding tissues
against (1) the corrosive effects of digestive acids and enzymes; (2) mechanical stresses, such as
abrasion; and (3) bacteria that either are swallowed with food or reside in the digestive tract.
The digestive epithelium and its secretions provide a nonspecific defense against these bacteria.
When bacteria reach the underlying layer of areolar tissue, the lamina propria
, they are attacked
by macrophages and other cells of the immune system.
The Digestive Organs and the Peritoneum