Week 11 : Digestive System


 The digestive system is divided into regions that specialize in the process of digestion. The tract is primarily composed of a layer of cells, that secrete digestive juices and mucous as well as absorb nutrients, surrounded by muscle. It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.


  • Digestion of specific nutrients
  • Control of metabolism (nutritional requirements/ endocrine regulation
  • Movement of food through the digestive system ( chewing and swallowing / peristalsis)
  • Secretions- exocrine (digestive enzymes and other components)
  • Digestion- catabolism (hydrolysis of macromolecules)
  • Absorption- into blood
  • Elimination –of indigestible substances


  • process by which organisms obtain and utilize their food
  • ingestion- process of taking food into the digestive system so that it may be hydrolized or digested.
  • digestion- the breakdown of food (either chemically or mechanically) in  order to utilize nutrients


  • Micronutrients- vitamins, minerals, & water —  Water, vitamins, and minerals are small molecules that can be absorbed without digestion. (Hydrolysis)
  • Macronutrients- proteins, lipids, carbohydrates, etc…
    • Carbohydrates (sugars and starches) should constitute 50% of the energy for the body. They provide the major source of energy for the body.
    • Roughage is fiber & cellulose found in vegetables that cannot  be digested.
    • Although technically not a nutrient, fiber is essential in the human diet to help exercise the muscles of the digestive tract.
    • An adequate amount of fiber in the diet has been shown to reduce the risks of colon and rectal cancers.


  • In order to be absorbed by the cells most food must be broken down into smaller pieces.
  • The process by which food molecules are broken downs called DIGESTION.


Chemical Digestion

  • Is carried out by the digestive ENZYMES ( Example: Saliva -breaks down starch into disaccharides (contains the enzyme amylase); Stomach acid (HCL or Hydrochloric Acid) – breaks down gastric protease which digests proteins
  • BILE -produced by the liver and stored in the gall bladder breaks down fats

 Mechanical Digestion

  • Where food is crushed, broken or cut into smaller pieces (ingestion into MOUTH)
  •  INCREASES the surface area of the food in order to aid in chemical digestion
  •  Is accomplished by PERISTALSIS and by the churning of the stomach.


Human Digestive Tract

  • Mouth – Ingestion occurs
  • Oral cavity – contains the teeth, tongue & openings for salivary glands
  • Teeth – mechanically breakdown food increasing surface area for chemical digestion by ENZYME action
  • Salivary glands – secrete Saliva (contains the enzyme amylase)

2 Facets of Saliva:

  • Moistens food for easier swallowing
  • Breaks down starches
  • Tongue – pushes food around in your mouth & into the esophagus
  • Esophagus – muscular tube that connects the pharynx with the next specialized section of the digestive tract – the stomach
  • Easier definition: The esophagus is simply a transportation tube from the mouth to the stomach. When we swallow, what we are really doing is closing a trap door in our throat called the epiglottis. This sends food down the esophagus and prevents food from going down the trachea (or windpipe) and into our lungs. Food moves down the esophagus using muscles NOT gravity.

The Stomach

– stretchy sack shaped like the letter J. It has three important jobs:

  • temporarily stores the food you’ve eaten
  • to break down the food into a liquidy mixture (called CHYME)
  • Begins protein digestion by gastric juices

Pyloric sphincter

  • A ring of smooth muscle fibers around the opening of the stomach into the duodenum
  • Controls the movement of food into the small intestines
  • ** Food is usually held 2-4 hours in the stomach.

Small intestine

  • Long, narrow convoluted compartment of the digestive tube all chemical digestion is completed and nutrients are absorbed

Broken into 3 Parts:

  1. Duodenum
  2. Jejunum
  3. Ileum
  • The small intestine is the largest part of the gastrointestinal tract and is composed of the duodenum which is about one foot long, the jejunum (5-8 feet long), and the ileum (16-20 feet long).

Large Intestine

  • Specialized compartment of the digestive tube designed to collect undigested materials and reabsorb water
  • Prepares waste (feces) to be expelled from the body
  • Broken into 3 main parts:
    • Cecum
    • Colon
    • Rectum
  • ** Undigested food and water enter the large intestine where water is absorbed.


  • Sac-like structure in humans found below the junction of the small and large intestine.



  • the act or process by which organisms eliminate solid or semisolid waste material from the digestive tract.
  • Waves of muscular contraction known as peristalsis in the walls of the colon move fecal matter through the digestive tract towards the rectum.


Accessory Structures:

  1. Liver – Organ involved with excretion, the recycling of useable materials, and the production of urea & bile

What does the Liver do?

  • Synthesize, store, and process (metabolize) fats, including fatty acids (used for energy) and cholesterol.
  • Metabolize and store carbohydrates, which are used as the source for the sugar (glucose) in blood that red blood cells and the brain use
  • Eliminate, by metabolizing and/or secreting, the potentially harmful biochemical products produced by the body
  • Detoxify, by metabolizing and/or secreting, drugs, alcohol, and environmental toxins.

2. Gall Bladder – Pouch structure located near the liver which concentrates and stores bile

  • Bile duct – a long tube that carries BILE. The top half of the common bile duct is associated with the liver, while the bottom half of the common bile duct is associated with the pancreas, through which it passes on its way to the intestine.
  • Bile emulsifies lipids (physically breaks apart FATS)
  • Bile is a bitter, greenish-yellow alkaline fluid, stored in the gallbladder between meals and upon eating is discharged into the duodenum where it aids the process of digestion.

3. Pancreas

  • An organ which secretes both digestive enzymes (exocrine) and hormones (endocrine)
  • ** Pancreatic juice digests all major nutrient types.
  • Nearly all digestion occurs in the small intestine & all digestion is completed in the small intestine.


Download the PDF form by clicking the link below:

Digestive System Link



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