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How Animals Digest Their Food

3D rendering of the human digestive system

3D rendering of the human digestive system (myboxpra, iStockphoto)

3D rendering of the human digestive system

3D rendering of the human digestive system (myboxpra, iStockphoto)

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Learn about how different animals digest their foods and the organs of their digestive systems.

All animals need nutrients from their environment. Nutrients provide energy and materials to build new cells and tissues. Nutrients include proteins, and sugars, vitamins, minerals and fats. 

Different types of animals have different ways of getting their nutrients. We are going to explore some of those ways by comparing different animal groups.

Invertebrates with Primitive Digestion Systems

Let’s first consider flatworms. One of the best known types of flatworms are tapeworms. As their name suggests, they look like long pieces of tape! All tapeworms are parasites that live in the digestive systems of other animals. Instead of finding their own nutrients, they absorb the pre-digested nutrients from their hosts directly through their bodies. Living this way means that they do not need to have a true digestive system.

Microscope view of a tapeworm. On its head are hooks that let it latch on to the interior of an animal’s digestive system
Microscope view of a tapeworm. On its head are hooks that let it latch on to the interior of an animal’s digestive system (Source: tonaquatic via iStockphoto).
Image - Text Version

Shown is the front end of a pale pink tapeworm as seen through a microscope. It has many flat body segments that make it look like a piece of tape. On its head are round suckers and barb-like hooks that allow it to hold on to the interior of its host animal.

Other types of free-ranging flatworms have developed a very basic digestive system. They have a simple gastrovascular cavity. This means they have only one opening that serves as both the mouth and the anusPlanarians are a type of flatworm. They take food in by extending their pharynx out of their body. This is like sticking your throat out of your mouth and using it like a straw to suck in food!

Planaria moving and feeding under the microscope (2016) by Sci- Inspi (3:20 min.).

Other animals also do not have true digestive systems. These include sponges, jellyfish, and anemones.

Complete Digestive Systems 

Roundworms and annelids, like earthworms, are more advanced. They have a complete digestive system. Other, such as insects, spiders and crabs also have a complete digestive system.

A complete digestive system has a gastrointestinal tract (GI tract). This tract is a one-way system that starts at the mouth and ends at the anus. A GI tract is a much more efficient system than a gastrovascular cavity because it prevents the mixing of waste and food.

Basic structure and function of a GI tract
Basic structure and function of a GI tract (Let’s Talk Science using an image by Zuperia via iStockphoto).
Image - Text Version

Shown is an cartoon illustration of a dog eating out of a bowl of food.

When the food is taken in, we call this ingestion. Next the food moves along the GI tract to where it is broken down. We call this stage digestion. The food continues to move along the GI tract where nutrients from the broken down food is absorbed. We call this stage absorption. Finally, any material not used by the animal leaves the body as waste. We call this phase elimination.

Some invertebrates have developed a way of using their internal digestive system on the outside. This is called external digestion. For instance, sea stars put a part of their stomachs outside their bodies when they digest their prey. This part of the stomach produces digestive enzymes. These are molecules that help break down the prey’s tissues. Nutrients are then directly absorbed by the cells in the stomach.

Sunflower star has weird way of eating (2012) by Vancouver Aquarium (1:18 min)

Did you know?

The common housefly secretes enzymes from its mouth that it puts on its food. When the food is broken down, it can suck it into its digestive system. 

Digestive Systems of Vertebrate Animals 

 are a group of animals that includes fish, amphibians, reptiles, mammals and birds. All vertebrates have a complete digestive system with a similar design. The system starts at the mouth. The mouth connects via a tube, called the esophagus, to the stomach. The stomach is usually an acidic environment where food is broken down into smaller molecules. These molecules then travel to the small intestine where most of the nutrients are absorbed. What remains then travels to the large intestine where water and salts are absorbed and wastes are readied for removal. The length of the digestive tract is related to the diet of the animal.  and animals that feed only on fruit, have the shortest GI tracts.  that consume a lot of fibrous plant matter tend to have the longest tracts. This is because the cellulose in plants takes a lot of time to be digested inside the GI tract. Omnivores, like humans, are in between those two extremes.

Did you know?

Plant cell walls are made from cellulose. It is the most common type of fibre we eat. But no animals can digest it. It is microbes in the digestive system that break it down. The process of breaking it down results in the production of smelly gases, we call farts!

Mammals 

Let’s start by looking at our own digestive system. Knowing how ours works will help you to understand the differences with other groups of mammals.

Parts of the human digestive system include:

The human digestive system and its organs
The human digestive system and its organs (Let’s Talk Science using an image by Monalisa Dakshi via Vecteezy).
Image - Text Version

Shown is a colour illustration of the human digestive system, beginning at the esophagus and ending at the anus. The esophagus is a pale pink tube that leads from the mouth to the pink, j-shaped stomach. Sitting above the stomach is the liver, which is a reddish-brown, wedge-shaped organ. Just below the liver is the gallbladder, which is a green bean-shaped organ. Below the stomach is the pancreas, which is a long, flattened yellow gland. Leading away from the stomach is another pink tube. This tube, which is the small intestine, winds its way up down and sideway in the abdomen. Eventually it connects to the large intestine. The large intestine is a pink, inverted u-shaped tube. The large intestine has a wider diameter than the small intestine and has a repeating pattern of bulges. The start of the large intestine is identified as the cecum. Connected to the cecum is a narrow worm-like structure identified as the appendix. The appendix is on the right side of the body. The colon makes up the majority of the rest of the large intestine. At the far end of the large intestine is the rectum. This tube narrows to an opening identified as the anus.

Human Digestive System

Part Description Location Function
Mouth Contains teeth and tongue Head Breaks down food through chewing in a process called mastication. During mastication, saliva containing enzymes is mixed with the food to start the process of digestion.
Esophagus Transport tube Connects mouth and stomach Moves food to the stomach. Muscular contractions called peristalsis push the food through the esophagus.
Stomach Muscular organ where food is broken down Between esophagus and small intestine Physical digestion of food is done in a process called churning. Chemical digestion is done by the stomach’s acidic gastric juices and enzymes. The gastric juices are also useful in destroying the majority of microorganisms that come into our bodies with food.
Small Intestine Narrow muscular tube Between stomach and large intestine Absorption of the majority of nutrients. Absorption is increased by the many folds in the tissues and by the millions of intestinal villi. Villi are finger-like extensions that line the intestine. The villi make the surface area for nutrient absorption hundreds of square meters!
Appendix Finger-shaped pouch on the right side of the body Near where the small intestine and large intestine meet Scientists once thought this was a useless organ inherited from our ancestors. The current theory is that it may act as storage for our bacterial flora.
Large Intestine Wider muscular tube; made up of the cecumcolon  and rectum Between small intestine and anus The pouch-like cecum is involved in the absorption of salts. It also adds a coating to the solid waste, making it easier to move. The colon absorbs water, salt and some vitamins. The rectum is the final part of the large intestine. The rectum acts as a temporary storage area for feces.
Anus Opening the outside of the body End of the digestive tract Elimination of food wastes

Did you know?

Food can take up to 3 days to get completely digested.

Several other important organs participate in digestion but are not within the GI tract. One of these is your liver. Your liver produces bile. Bile is a chemical that helps in digesting fat. Some of the bile is transported to the stomach and some to the gallbladder. The gallbladder stores bile and releases it in the small intestine during meals. The pancreas is another organ. It produces enzymes for the stomach. The pancreas and liver have other important functions, in addition to those in the digestive system.

Ruminants 

Ruminants are a group of hooved herbivorous animals. This group includes cows, sheep, goats, deer, llamas, camels and giraffes. Ruminants have a very large stomach. It takes up ¾ of their. Unlike ours, it has four compartments.

Diagram showing the four chambers and the direction that food travels in the ruminant digestive system
Diagram showing the four chambers and the direction that food travels in the ruminant digestive system (Let’s Talk Science using an image by ttsz via iStockphoto).
Image - Text Version

Shown is a colour illustration of the ruminant stomach.

The organ as a whole is roughly oval in shape and pink in colour. Projecting off of the top right side is a tube identified as the esophagus. This tube connects to a large chamber that takes up about half of the stomach. This chamber is the rumen. Blue arrows indicate that food travels in through the esophagus, around the rumen and back out through the esophagus. Another chamber that takes up about a quarter of the stomach also connects to the esophagus. This chamber is the reticulum. Green arrows indicate that food from the rumen reenters the stomach through the esophagus and into the rumen. Food travels through the rumen and exits through a hole into a small, oval chamber identified as the omasum. Purple arrows indicate that the food travels around the omasum then exits through a hole into the final chamber identified as the abomasum. Dark blue arrows indicate the food passes through the very spongy abomasum and exits through a tube identified as the beginning of the small intestine.

Food travels through them in this order:

  1. Rumen
    The rumen is the largest of the four chambers.This is where lighter food, like grass, first goes. It is also used to store food for later. The rumen contains microbes that can digest plant fibres, which make up the majority of the diet of ruminants. The rumen has the perfect temperature and pH for the microbes to thrive. Ruminant animals do not chew their food very much. The partly-chewed food that goes into the rumen can be taken back to the mouth where it can be chewed some more. The term for this is regurgitation, which is also known as chewing cud. Regurgitation does not just happen in ruminants. Mother birds also regurgitate food for their offspring.
  2. Reticulum
    Heavier items, like twigs or even nails, go directly into the reticulum. Regurgitated food also goes to the reticulum. The reticulum keeps the larger objects for further digestion, and sends the smaller food particles onwards.
  3. Omasum
    This chamber is where water is absorbed into the body from the partially digested food. The interior of the omasum has many folds, which resemble the pages in a book. These folds increase the surface area, much like the pili in the human digestive system.
  4. Abomasum
    This is the “true stomach” of ruminants. Like non-ruminants, this chamber is very acidic and contains digestive enzymes. After leaving the abomasum, food enters the small intestine, much like in other mammals.

Did you know?

Ruminants typically spend one-third of their time grazing, one-third of their time chewing their cud and one-third of their time doing neither!

Birds 

Because stomachs are heavy muscular organs, birds have small ones to make flying easier. The term for the stomach of birds is proventriculus. Before entering the proventriculus, food is first stored in the crop. Unlike mammals, birds do not have teeth and cannot chew their food. The physical breakdown of their food mostly happens in an organ called the gizzard.

The gizzard is a muscular organ located after the stomach. It contains small stones that the bird swallows on purpose. The movement of the stones by the muscles of the gizzards breaks down the bird’s food. Food is passed back and forth between the proventriculus and the gizzard. This results in a repeated cycle of physical and chemical digestion.

The small and large intestines of birds are similar to those of mammals. At the end of the digestive tract is the cloaca. Like in reptiles, this is a tubular structure that serves as a shared opening for the digestive, reproductive, and urinary systems.

Digestive system of a chicken
Digestive system of a chicken (Let’s Talk Science using an image by VectorMine via iStockphoto).
Image - Text Version

Shown is a colour illustration of a cutaway side view of an adult chicken with emphasis on the digestive system.
At the head end of the chicken is an orange triangular shape identified as the beak. From there, a white tube extends down the chicken's throat. This tube is identified as the esophagus. The esophagus is connected to a roughly oval white organ identified as the crop. The crop is connected via a tube to a bulging section identified as the proventriculus. Connected via a tube to the proventriculus is another bulging white organ identified as the gizzard. The gizzard connects to the small intestine. This is a long, folded pale pink tube. Connected to the small intestine is another pink tube with a slightly larger diameter identified as the large intestine. The large intestine empties waste to the outside world via a funnel-shaped opening identified as the cloaca.

 

Learn More

The Digestive System (2012)
This video by Crash Course (11:52 min.) presents the human digestive system.

Your Digestive System
This page gives information on the human digestive system.

Structure Of The Small Intestine - Functions Of The Small Intestine - What Are Villi (2018)
This video (3:10 min.) by What's Up Dude presents the anatomy and function of the small intestine.

Introduction to Worms
This page by the University of Hawaii presents the characteristics of different groups of worms and how they are related.

Ruminant digestion
This page by the Science learning Hub presents the science of ruminant digestive systems.

Why is bird poop white : avian digestive system (2019)
This animated video (4:01 min.) by Ample Wise explains how bird digestive systems work and why their poop is white.

Bird Digestion - Adaptations for Flight
This video (3:58 min.) by BioBush explains how the digestive system of birds is adapted for flight.

References

Bowen, R. (n.d.). Rumen Physiology and Rumination. Pathophysiology of the Digestive System, Colorado State University. Retrieved from http://www.vivo.colostate.edu/hbooks/pathphys/digestion/herbivores/rumination.html.

Karasov, W. H., & Douglas, A. E. (2013). Comparative digestive physiology. Comprehensive Physiology3(2), 741–783. 

Parish, J (Feb. 2011). Ruminant Digestive Anatomy and Function. Cattle Business in Mississippi. Retrieved from https://extension.msstate.edu/sites/default/files/topic-files/cattle-business-mississippi-articles/cattle-business-mississippi-articles-landing-page/mca_feb2011.pdf

Stevens, C. E. (2019). Digestive System. AccessScience, McGraw-Hill Education, Mar. 2019. 

Vertebrate Digestive Systems. (2021, March 6). Retrieved from https://bio.libretexts.org/@go/page/13843

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