Science Courses/College Biology: Help and ReviewCourse
- AuthorBrittany W.
Brittany has a bachelor's degree in biological sciences and biochemistry.
View bio - InstructorAmanda Robb
Amanda has taught high school science for over 10 years. She has a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. She is also certified in secondary special education, biology, and physics in Massachusetts.
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Learn about the complete digestive system, and its advantages, over an incomplete digestive system. Discover how the complete digestive tract is structured.Updated: 11/21/2023
Table of Contents
- Digestive System Definition
- Complete Digestive System
- Advantages of a Complete Digestive System
- Lesson Summary
Frequently Asked Questions
What is the difference between complete and incomplete gut?
A complete gut is one that has two openings to the outside world. Usually, the two openings are the mouth and the anus. An incomplete gut has one opening to the outside world. This means that food is consumed, and waste is released from the same opening.
What organisms has a complete digestive system?
Many organisms possess a complete digestive system such as mammals, reptiles, amphibians, fish, birds, and insects. A complete digestive system is one with two openings to the outside world (i.e., mouth and anus).
Table of Contents
- Digestive System Definition
- Complete Digestive System
- Advantages of a Complete Digestive System
- Lesson Summary
The digestive system is a collection of organs and glands designed to:
- Digest and absorb food
- Eliminate waste
The old French word digestif is the precursor for the term digestive system meaning "of or pertaining to physiological digestion." Digestion is the breakdown of food into biological molecules that can be used by the body. Absorption is the intake of those molecules so that they can be used by certain tissues and organs. The breakdown of food can occur:
- Mechanically: a purely physical process to break down food without changing its chemical composition
- Enzymatically: using enzymes (proteins that help biochemical reactions occur) to chemically change consumed food into various other active chemical substances
There are two main types of digestive systems:
- Incomplete digestive system: a digestive system with a single opening to the outside world
- Complete digestive system: a digestive system with two openings to the outside world
The single opening in an incomplete digestive system serves as both the mouth and the anus. In a complete digestive system, the mouth and anus are independent of one another. Humans have a complete digestive system where food enters through the mouth, is digested, and is then expelled through the anus, as waste. The series of organs that make up the digestive system, and the path food takes as it is being digested, is called the digestive tract. A more formal digestive tract definition is the series of organs that food travels in order to be swallowed, digested, absorbed, and expelled.
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The complete digestive system, also known as the complete digestive tract, begins with the mouth and ends with the anus. Another term for the complete digestive tract is the alimentary canal; therefore, the alimentary canal definition or the gastrointestinal tract definition is the same as that of the digestive system.
The alimentary canal includes:
- Mouth
- Pharynx (throat)
- Esophagus
- Stomach
- Small intestine
- Large intestine
- Rectum
- Anus
The alimentary canal begins with the mouth because it is the organ where food is consumed: bitten, chewed, and swallowed. Here, food is both mechanically and chemically altered:
- Mechanical alterations occur through the grinding of food by teeth.
- Chemical alterations occur through the enzyme amylase. Amylase breaks down the bonds of carbohydrates into simple sugar molecules.
The tongue pushes food back to the throat where it continues on to the next stages of digestion. Once food enters the pharynx, a biological covering, called the epiglottis, protects the trachea so that food does not enter the windpipe. Aspiration is the medical term for unintentionally sucking food into the airway, and it can cause significant damage to the respiratory tract. Once blocked from the trachea, food enters the esophagus.
The esophagus is a long muscular tube that mechanically pushes food from the throat to the stomach. The stomach contains a highly acidic environment of stomach acid and enzymes, that help break down the food even further. The stomach is also lined with muscles that help push the food into the small intestine.
The small intestine is where most of the absorption of food takes place. Bile, a substance released from the gallbladder, helps break down fat. Additional digestive enzymes from the pancreas are also released into the small intestine. The large intestine is where the majority of water, any remaining nutrients, and electrolytes are also removed. By the time food leaves the large intestine, it is now considered feces. It passes from the large intestine to the rectum. The rectum stores stool until it is ready to be expelled through the anus.
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The advantages of a complete digestive system include:
- The ability to use multiple digestive organs at one time
- The ability to maintain continuous food intake
- Efficient absorption of nutrients
A complete digestive system takes advantage of multiple organs to perform various jobs concerning digestion. Those jobs include:
- Ingestion: consuming or eating food
- Digestion: breaking down food
- Absorption: extracting nutrients from food
- Defecation: expelling food as waste
Each of these jobs occurs in a different portion of the digestive tract, allowing the body to maximize efficiency and output. Another component of maximizing efficiency, is the fact that complete digestive systems have two openings. This means that an organism does not have to wait for food waste to be expelled before it can ingest more food. This is particularly helpful in times of famine where the digestive system slows down, and waste is excreted less often. Humans are able to ingest food whenever it becomes available and accessible, rather than succumb to any biological constraints imposed by incomplete digestive systems. In this way, scientists believe that the evolution of a complete digestive system allowed for more complex organisms to exist.
Additionally, many of the organs of the digestive tract are folded and compacted to fit into the confined space of the thoracic cavity and abdomen. For example, the small intestine contains folds in its inner lining to increase its surface area and capacity for absorption. For many of these organs, form matches function, and their shapes optimize maximum productivity.
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A complete digestive system is a digestive system with a separate start and endpoint. In humans, the start is the mouth, and the endpoint is the anus. A digestive system is a collection of organs and glands designed to digest and absorb food as well as eliminate waste. Digestion is the process of breaking down food into useful substances for the cells in the body, and absorption is the intake of those substances. The organs of the digestive system include:
- Mouth
- Pharynx (throat)
- Esophagus
- Stomach
- Small intestine
- Large intestine
- Rectum
- Anus
A complete digestive system allows for the evolution of more complex organisms by allowing for the continuous intake of food and, thus, more nutrients. This is because each organ can specialize, which leads to a more efficient absorption of nutrients. Advantages of a complete digestive system include:
- The organism can have a continuous intake of food
- The organism can have multiple digestive organs
- The organism can absorb nutrients more efficiently
Most complex organisms have a digestive tract that consists of organs whose form matches their particular function. For example, the small intestine contains folds in its inner lining to increase its major role in absorbing nutrients. The entire digestive tract is designed to maximize productivity and the extraction of nutrients to sustain life.
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Video Transcript
What Is a Digestive Tract?
As we snack on chips, veggies, or fruit while we work, we're getting some good energy to fuel our brains. However, where does this energy come from, and how do our bodies extract it from food? The answer is the digestive tract, which is a group of tissues or organs designed to break down food. Some animals, including humans, have a complete digestive tract, meaning it has a clear beginning (the mouth) and a separate end (the anus). Other, less complex animals, like sea sponges, or jellyfish, have only one opening.
These animals take in food from the same opening they release waste. Sound gross? Good thing humans have a complete digestive system! Let's look more closely at the human digestive tract.
The Human Digestive Tract
Humans, like other more complex animals, have a complete digestive tract. It starts at the mouth, where food is ground up and the digestion of carbohydrates, like those found in bread or sweets, begins. Next, the food is transported by the esophagus down to the stomach, which is a specialist in breaking down carbohydrates and some protein. It does this using tiny proteins called enzymes that speed up the chemical reaction of breaking down food. It also has an abundance of acid, which dissolves food into liquid.
Next, the food goes into the small intestine, a very different environment. The small intestine is basic, or lacks acid, unlike the stomach. This environment is suited for absorbing nutrients and breaking down mainly protein and fat. Here, we can see the advantage of efficient digestion. The stomach has a very harsh environment, perfect for liquefying food. But, the small intestine has a totally different environment for absorbing nutrients.
Since each organ is separate, they can do their jobs to the best of their abilities. After the small intestine is the large intestine, which absorbs water. Again, the structure and environment of the large intestine is different, so it can do a different job. After the large intestine is the rectum for storing waste and the anus for expelling waste. While the food moves through the digestive tract, a person is free to continue eating to gain more energy and nutrients.
Complete Digestive Tract Advantages
More recently evolved and more complex organisms usually have a complete digestive tract. Scientists think it evolved to provide two main advantages: to allow for continuous intake of food and to more efficiently absorb nutrients.
The first advantage of a complete digestive tract is continuous food intake. Organisms with an incomplete digestive tract have a limited amount of space. Everything has to come out the same way it came in. Think of it like a line at the cafeteria. If there was only one entrance and exit, the cafeteria would be a mess. People would get their trays and then have to wade back through a sea of people that are already in line to get a seat. The line would probably back up, and the cafeteria wouldn't be able to serve as many people.
Now, think of what normally happens when there is a separate entrance and exit. People get their food and keep moving in the same direction. They can easily leave the line to find a seat, so the line moves quickly. This is analogous to a complete digestive system. The food comes in one end and moves in that direction until it exits. Our digestive system can hold way more food this way, allowing us to extract more nutrients.
The second advantage of a complete digestive tract is its efficient absorption of nutrients. Since the digestive tract is a long tube, organisms can house lots of organs. Different organs in our digestive system perform different functions, so each organ can specialize in a particular task. Think about group work for a large project. The project will run a lot smoother if each group member is an expert in their part. For example, one student might have excellent notes, and another might be a PowerPoint whiz. The project would, ideally, be done faster and better than if there was only one person doing each job.
Lesson Summary
A complete digestive system is a digestive system that has different start and end points. Organs in the digestive system break down food into usable parts for the body and help us get energy. The other type of digestive system is an incomplete digestive system, which has the same opening for food and exit for waste.
Complete digestive systems are better able to absorb nutrients, since each organ is separate and can specialize. It also allows for continuous intake of food, which lets animals grow bigger and develop more complex structures. Humans have a complete digestive system with specialized organs, like the stomach and small intestine, that do different jobs, allowing for more efficient absorption of nutrients.
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