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Chapter 21: 1-13

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The Digestive System

Web Lecture

Components of Digestive Systems

You may want to review the development of tissue types in the gastrula of animals that lack a coelom, animals with a pseudocoelom, and animals with a true coelom. The development of the coelom will determine the complexity of the digestive tract. In fact, as we go through the different systems with emphasis on human anatomy and physiology, you may find it useful to compare these "complex" systems with their counterparts in other animals by refer to the chart in the weblecture on invertebrates.

Simple Digestive Systems

Simple digestive systems have a single opening, through which food enters and wastes leave. Primary digestion occurs when cells lining this cavity release enzymes into the cavity to break down food; digestion continues in food vacuoles within cells, and the resultant nutrients diffuse into other cells. Undigested food is forced out through the mouth opening.

Complex Digestive Systems

As we have already seen, during empbryonic development in more complex animals, the developing gastrula structure folds up and forms three rings:

These complex digestive systems have two openings; food enters through the mouth, travels through the digestive tract, which may have several specialized regions for breaking down different substances, and undegested products are eliminated through the second opening (called the anus.

In most vertebrates, food undergoes different digestive processes; starches, proteins and fats are broken down by specialized enzymes secreted by different organs at different points along the way. In the human, food is digested as follows:

Mouth → Ingestion (plus some digestion)

Teeth grind food into smaller particles

Saliva glands secrete amylases to break down starches

Swallowing passes food to

Enzymes include:
sucrase (breaks bonds between dissachyride monomers)
Pharynx → epiglottis → esophagus → Ingestion

Bolus traveling; no digestive activity

No enzyme activity
 Stomach → Digestion

Glands in stomach lining secrete hydrochloric acid to produce pepsin to break down proteins. Note that these also break down salivary amylase so that starch digestion slows to a halt in the stomach. The stomach pushes food out the pylorus.

Enzymes include:
pepsin (breaks peptide bonds in amino acids of proteins)
(Small intestine)

Duodenum → jejunum →

ileum →


Food in the duodenum is broken down chemically by bile from the liver and enzymes from the pancreas, as well as enzymes from the walls of the intestine. The base nutrients are absorbed through the cells lining the villi, projections which increase the surface area of the small intestine to the size of a tennis court.

Enzymes include:
bile salts produced by the liver and gall bladder to break down fatty acids and nucleic acids
 (Large intestine)

cecum → colon → rectum

Absorption and Elimination

The walls of the large intestine absorb water and sodium from any undigested food, and bacteria in the intestine continue the breakdown (and incidentally produce vitamins for their host organism). These wastes are then eliminated from the body.

Limited enzyme activity.
Liver Filtration

Blood from the intestinal lining is filtered to eliminate contaminants.

Limited enzyme activity.

The Human Digestive Systems

The main components of the human digestive system are listed in the table above. Trace the passage of food through the diagram below, and try to identify each numbered organ. When you hover over the diagram, you'll be able to see the actual names. Use the diagram to drill yourself, and be sure to identify the function of each organ as well.

Test yourself

Where are the three salivary glands?

What are the main subdivisions of the small intestine?

The appendix is attached to which part of the intestine?

Check The Innerbody for animations and more details on aspects of the digestive system that interest you.