Biology, The Science of Life

The Digestive System: Introduction

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Overview: This lecture will consist of the instructor lecturing for 50 minutes in an authoritative fashion and a take-home review crossword will be given out at the end of class. A short video clip on the digestive system will be presented at the end of the lecture to summarize the topic visually. In this section, we turn from nutritional requirements (previous lecture not presented on this website) to the mechanisms by which humans process food. There will be a brief introduction to how other animals ingest food as well but this will be covered in a later lecture in the course.

Materials: The teacher will need the class computer to project the power point of the lecture onto the screen in the front on the class (file is available for download above). The Power Point presentation will be used as a media teaching tool for the whole duration of the class. Another digital media teaching tool in this lecture is the video which will be presented at the end of class to briefly summarize the topic (video is found at the bottom of the page). The instructor may also use the white board to further emphasize the material when they feel the need to use it. Students will need paper, pen or pencil, or their laptops to take down notes. Also, the Power Point presentation is made available to the students, therefore they are required to take down additional notes that are not presented on the Power Point.

Assessment: A take-home crossword will be distributed at the end of class for a total worth of 2% of the final grade (available above for download). The crossword provides questions similar to the exam presented at the end of the 10 lectures therefore it is a great practice and review tool. Crosswords are chosen as the method of assessment for these authoritarian lectures because they turn a 50 min lecture into an interactive game. A lot of material is covered in lessons set-up like this one and a crossword is more amusing to complete than a simple questionnaire; it provides clues and to a certain level, a confirmation of your answer. It also covers the material in as much detail as any questionnaire.

Extra Notes: The students are responsible for knowing the material found on the Power Point presentation and what is covered by the instructor in class for the exam. Students have prior knowledge of the building blocks which compose food (i.e. amino acids, glucose, glycerol and fatty acids, etc).


  • Ingestion (7 minutes)
  • Digestion (12 minutes)
  • Absorption (12 minutes)
  • Elimination (10 minutes) 
  • Other Digestive Systems (5 minutes)
  • Video Summary (2 minutes)
Before beginning today's lecture material, briefly provide an overview of where various digestive compartments are localized in the human body using the image above which is also found in the Power Point presentation.

Introduce the following definitions:
  • Ingestion is the act of eating. Food can be ingested in many liquid and solid forms.
  • Digestion is the process by which food is broken down into smaller molecules which the body can absorb.
  • Absorption is when the animal’s cell takes up (absorb) small molecules such as amino acids and simple sugars.
  • Elimination completes the process as undigested material passes out of the digestive system.

Open the Power Point presentation and go through the material.

Organs specialized for sequential stages of food processing form the mammalian digestive system
Because most animals, including mammals, have an alimentary canal, we can use the human digestive system as a representative example of the general principles of food processing.
In mammals, the digestive system consists of the alimentary canal and various accessory glands that secrete digestive juices (enzymes that break down the food) through ducts into the canal. The accessory glands of the mammalian digestive system are three pairs of salivary glands, the pancreas, the liver, and the gallbladder.
Food is pushed along the alimentary canal by peristalsis, which is alternating waves of contraction and relaxation in the muscles lining the canal. It is peristalsis that enables us to process and digest food even while lying down.
At some of the junctions between specialized compartments, the muscular layer forms ringlike valves called sphincters. Sphincters act like drawstrings to close off the alimentary canal and they regulate the passage of material between compartments. Sphincters are found at the junction between the esophagus and the stomach and between the stomach and the small intestine.

The Oral Cavity, Pharynx, and Esophagus
Ingestion and the initial steps of digestion occur in the mouth, or oral cavity. Mechanical digestion begins as teeth of various shapes cut, smash, and grind food, making the food easier to swallow and it also increases surface area for digestion.
The presence of food stimulates a nervous reflex that causes the salivary glands to deliver saliva through ducts to the oral cavity. Saliva initiates chemical digestion by secreting amylase which hydrolyzes starch.
When food arrives at the oral cavity, the tongue plays a critical role in distinguishing which foods should be processed further. After chewing, the tongue manipulates the food, shaping it into a ball called the bolus. The tongue pushes the bolus to the back of the oral cavity and into the pharynx.
The Pharynx, or throat region, opens to two passageways: the esophagus and the trachea (windpipe). The esophagus connects to the stomach, whereas the trachea leads to the lungs. When you swallow, a flap of cartilage called the epiglottis prevents food from entering the trachea. If swallowing reflex fails, food or liquids can reach the windpipe and cause choking, a blockage of the trachea.
The esophagus contains both striated and smooth muscle. The striated muscle is situated at the top of the esophagus and is active during swallowing. Throughout the rest of the esophagus, smooth muscle functions in peristalsis. The rhythmic cycles of contraction move each bolus to the stomach.

Ask students if they have any questions up to this point. Question period should be no more than 2-3 minutes.

Continue going through the material.

Digestion in the Stomach
The stomach is located just below the diaphragm in the upper abdominal cavity (as seen in a previous lecture). The stomach primarily stores food and continues digestion. The stomach secretes digestive fluid called gastric juice and mixes this secretion with the food through a churning action. This mixture of ingested food and digestive juice is called chyme.
Two components of gastric juice carry out chemical digestion: hydrochloric acid (HCl), which disrupts the extracellular matrix that binds cells together. The concentration of HCl is so high that the pH of gastric juice is about 2, acidic enough the dissolve iron nails.
The exposed bonds are attacked by the second component of gastric juice – a protease, or protein-digesting enzyme called pepsin. Pepsin works best in an acidic environment and cleaves proteins.
The stomach lining protects against self-digestion by secreting mucus, a viscous and slippery mixture of glycoproteins, cells, salts, and water.
The sphincter located where the stomach opens to the small intestine helps regulate the passage of chyme into the small intestine.

Digestion in the Small Intestine
Most enzymatic hydrolysis of macromolecules from food occurs in the small intestine.
Fun Fact (not covered on exam): It is over 6m long in humans, the small intestine is the alimentary canal’s longest compartment. Its name refers to its small diameter, compared with that of the large intestine.
The first 25cm or so of the small intestine form the duodenum, a major crossroad in digestion. It is here that chyme from the stomach mixes with digestive juices from the pancreas, liver, and gallbladder.

Bile Production by the Liver
Digestion of fats and other lipids begins in the small intestine and relies on the production of bile, a mixture of substances that is made in the liver. Bile contains bile salts, which act as detergents that aid in digestion and absorption of lipids. Bile is stored and concentrated in the gallbladder.

Absorption in the Small Intestine
To reach body tissues, nutrients in the lumen must first cross the lining of the alimentary  canal. Most of this absorption occurs in the small intestine. Large folds in the lining have finger-like projections called villi.

Absorption in the Large Intestine
The alimentary canal ends with the large intestine, which includes the colon, cecum, and rectum. The small intestine connects to the large intestine at a T-shaped junction, where a sphincter controls the movement of material.
One arm of the T is the colon, which leads to the rectum and the anus. The other arm forms a pouch called the cecum. The cecum is important for fermenting ingested material.
A major function of the colon is to recover water that has entered the alimentary canal as a solvent of digestive juices.
The feces, the wastes of the digestive system, become increasingly solid as they are moved along the colon by peristalsis.
A rich flora , E. Coli, is common in human digestive systems. Within the intestine, E. Coli live on the unabsorbed organic material. As by-products of their metabolism, many colon bacteria generate gases, including methane, which has an offensive odor, hence why you fart.
The terminal portion of the large intestine is the rectum, where feces are stored until they can be eliminated. Between the rectum and the anus are two sphincters, the inner one being involuntary and the outer one being voluntary.

We have followed a meal from one opening (the mouth) of the alimentary canal to the other (anus).
Briefly overlook four main feeding mechanisms of animals:

Suspension Feeders
Many aquatic animals are suspension feeders, which sift small food particles from the water. For example, attached to the upper jaw of the humpback whale are comb-like plates called baleen, which strain small invertebrates and fish from enormous volumes of water. Clams and oysters are also suspension feeders. They use their gills to trap tiny morsels; cilia then sweep the food particles to the mouth in a film of mucus.

Substrate Feeders
Substrate feeders are animals that live in or on their food source. For example, maggots, which burrow into animal carcasses.

Fluid Feeders
Fluid Feeders suck nutrient-rich fluid from living host. For example, a mosquito pierces the skin of its human host with hollow, needle-like mouthparts and is consuming a blood meal.

Bulk Feeders
Most animals, including humans, are bulk feeders, which eat relatively large pieces of food. Their adaptations include tentacles, pincers, claws, poisonous fangs, jaws, and teeth that kill their prey or tear off pieces of meat. For example, a rock python ingests a gazelle whole (they have a loosely hinged jaw).

Ask students if they have any questions. Question period man stay open for the time remaining of class but make sure to save 2 minutes for the video.
Close the Power Point presentation.
Show video below as a summary.
If there is time remaining after the video, allow the students to start work on the crossword in groups.