The Gut and Digestion

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This chapter describes the various organs that make up the gut of mammals and birds and how they contribute to the digestion of food.


After completing this section, you should know:

  • what is meant by the terms: ingestion, digestion, absorption, assimilation, egestion, peristalsis and chyme
  • the characteristics, advantages and disadvantages of a herbivorous, carnivorous and omnivorous diet
  • the 4 main functions of the gut
  • the parts of the gut in the order in which the food passes down it


  • The gut breaks down plant and animal materials into nutrients that can be used by animals’ bodies.
  • Plant material is more difficult to break down than animal tissue. The gut of herbivores is therefore longer and more complex than that of carnivores. Herbivores usually have a compartment (the rumen or functional caecum) housing micro-organisms to break down the cellulose wall of plants.
  • Chewing by the teeth begins the food processing. There are 4 main types of teeth: incisors, canines, premolars and molars. In dogs and cats the premolars and molars are adapted to slice against each other and are called carnassial teeth.
  • Saliva is secreted in the mouth. It lubricates the food for swallowing and contains an enzyme to break down starch.
  • Chewed food is swallowed and passes down the oesophagus by waves of contraction of the wall called peristalsis. The food passes to the stomach where it is churned and mixed with acidic gastric juice that begins the digestion of protein.
  • The resulting chyme passes down the small intestine where enzymes that digest fats, proteins and carbohydrates are secreted. Bile produced by the liver is also secreted here. It helps in the breakdown of fats. Villi provide the large surface area necessary for the absorption of the products of digestion.
  • In the colon and caecum water is absorbed and micro organisms produce some vitamin B and K. In rabbits, horses and rodents the caecum is enlarged as a functional caecum and micro-organisms break down cellulose cell walls to simpler carbohydrates. Waste products exit the body via the rectum and anus.
  • The pancreas produces pancreatic juice that contains many of the enzymes secreted into the small intestine.
  • In addition to producing bile the liver regulates blood sugar levels by converting glucose absorbed by the villi into glycogen and storing it. The liver also removes toxic substances from the blood, stores iron, makes vitamin A and produces heat.

Learning Activities

Library of Resources

Presentations and Blackboard quizzes can be accessed by students of Otago Polytechnic.