DAE - Introduction to the Digestive System Lesson

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Introduction to the Digestive System

The Functions of the Digestive System


Esophagus: Carries food from the mouth to the stomach. 

Stomach: The organ where digestion of protein begins.

Liver: The largest organ inside the body. Makes bile (fluid that helps break down fats and gets rid of wastes in the body); changes food into energy; and cleans alcohol, some medicines, and poisons from the blood.

Pancreas: A gland that makes enzymes for digestion and the hormone insulin (which helps the body turn food into energy).

Gallbladder: Stores the bile made in the liver, then empties it to help digest fats.

Large intestine: Also called the colon. It absorbs water and sodium from the stool.

Small intestine: The organ where most digestion occurs.

Appendix: A pouch attached to the first part of the large intestine. No one knows its function. 

Rectum: The lower end of the large intestine, leading to the anus.

Anus: The opening at the end of the digestive tract where bowel movements leave the body.

 

When you eat foods—bread, meat, fruits, vegetables, snacks, candy, or desserts—they are not in a form that the body can readily use as nourishment.   The food and drink that is ingested into your body must be changed into smaller molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. Digestion is the process by which food and drink are broken down into their smallest parts so the body can use them to build and nourish cells and to provide energy. In general, the digestive system has three main functions:

 

The three main functions of the digestive system are chemical and mechanical digestion, absorption, and defecation.

Chemical and Mechanical Digestion Absorption Defecation
Digestion includes both mechanical and chemical processes. Mechanical digestion is a physical process that makes food smaller to increase its surface area and make it easier to move.   This type of digestion does not change the chemical nature of the food. Chemical digestion uses digestive secretions such as enzymes and acids to break down complex food molecules into their chemical building blocks (for example, proteins are broken down into amino acids). Nutrients from food that has been broken down enters the bloodstream through a process called absorption, and is transported throughout the body. Defecation is the final step in digestion where undigested materials are removed from the body as feces.

 

The process of digestion

Propulsion:
Swallowing (oropharynx)
Peristalsis (esophagus, stomach, small intestine, large intestine)

Absorption:
Nutrients and water to blood vessels and lymph vessels (small intestine)
Water to blood vessels (large intestine)

Chemical Digestion

Mechanical Digestion:
Chewing (mouth)
Churning (stomach)
Segmentation (small intestine)

 

The Role of Enzymes in the Digestive System

Chemical digestion could not take place without the help of digestive enzymes. An enzyme is a protein that speeds up chemical reactions in the body. Digestive enzymes speed up chemical reactions that break down large food molecules into small molecules.

Did you ever use a wrench to tighten a bolt? You could tighten a bolt with your fingers, but it would be difficult and slow. If you use a wrench, you can tighten a bolt much more easily and quickly. Enzymes are like wrenches. They make it much easier and quicker for chemical reactions to take place. Like a wrench, enzymes can also be used over and over again. But you need the appropriate size and shape of the wrench to efficiently tighten the bolt, just like each enzyme is specific for the reaction it helps.

Digestive enzymes are released, or secreted, by the organs of the digestive system. These enzymes include proteases that digest proteins, lipases that digest lipids, and nucleases that digest nucleic acids. Examples of digestive enzymes are:

*** Information from CK12

Enzyme Produced In Site of Release pH
Carbohydrate Digestion
Salivary amylase Salivary gland Mouth Neutral
Pancreative amylase Pancreas Small intestines Basic
Maltase Small intestines Small intestines Basic
Protein Digestion
Pepsin Gastric glands Stomach Acidic
Trypsin Pancreas Small intestines Basic
Peptidases Small intestines Small intestines Basic
Fat Digestion
Lipase Pancreas Small intestines Basic

 

Our body relies on digestion to provide us with the nutrients needed to maintain homeostasis. Download the "Nutrition" video worksheet here Download here. While watching the video below answer the questions on the worksheet.

Anatomy of the Digestive System

The organs of the digestive system can be divided into two groups: alimentary canal organs and accessory structures. The alimentary canal is comprised of the esophagus, the stomach, the small intestine, and the large intestine.

Alimentary Canal Organs

The alimentary canal is a hollow, muscular tube that runs through the ventral body cavity from the mouth to the anus. Between those two points, the canal is modified as the pharynx, esophagus, stomach, and small and large intestines.

The organs of the digestive system are lined with smooth muscles. These muscles contract in waves to push food through the system in an involuntary process called peristalsis. Without peristalsis, food would not be able to advance through the digestive system. Peristalsis is so powerful that foods and liquids that you swallow would still enter your stomach even if you are standing on your head!

 

These muscles contract in waves to push food through the system in an involuntary process called peristalsis.

The walls of the alimentary canal from the esophagus to the large intestine are made of the same four basic tissue layers:

  1. Mucosa - inner layer that lines the cavity of organs; protects tissues and carries out absorption.
  2. Submucosa - beneath mucosa; contains blood vessels, nerves, and glands.
  3. Muscularis external - a layer of smooth muscle tissue that conducts peristalsis.
  4. Serosa - outermost layer; lubricates the surface by producing serous fluid; lines the abdominopelvic cavity.

 

The layers of the gastrointestinal tract are labeled.

The alimentary canal is also called the gastrointestinal tract.   You may have heard of someone having upper GI tests or lower GI tests. These tests involve examinations of the alimentary canal. The length of the alimentary canal is about five times a person's height. For example, the alimentary canal of a person who is 6 feet tall would be approximately 30 feet long!

Accessory Structures and Organs

Each accessory digestive structure aids in the breakdown of food. Within the mouth, the teeth and tongue begin mechanical digestion, whereas the salivary glands begin chemical digestion.  

The gallbladder anatomy is detailed.

Once food products enter the small intestine, the gallbladder, liver, and pancreas release secretions—such as bile and enzymes—essential for digestion to continue. These structures are called accessory organs because they sprout from the lining cells of the developing gut and assist in its function.   Although food does not pass through these three organs, they play an important part in chemical digestion.

Contribution of Other Body Systems

As is the case with all body systems, the digestive system does not work alone. It requires the cooperative functions of other systems of the body. For example, the digestive system depends upon the cardiovascular system to supply oxygen and nutrients to the digestive organs and to "offload" absorbed nutrients. The digestive and endocrine systems also have a critical relationship. Hormones secreted by several endocrine glands contribute to the control of digestion and metabolism. In turn, the digestive system provides nutrients to "fuel" the endocrine system.

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