7 The Process of Digestion and Absorption


  • The Body’s Organ System
  • How Your Digestive System Works
  • Probiotics and Prebiotics
  • Common Digestive Diseases and Disorders


When you smell coffee or fresh baked cookies, what happens? Do those aromas stimulate your desire to drink or eat? The sight and smell of foods are enough to prime your digestive tract and stimulate saliva production. This chapter will look at the steps of digestion and absorption and how your body breaks down the food into usable components. Additionally, we will investigate digestive diseases and disorders.

Human Anatomy & Systems

Human bodies are made of a system of cells.  Cells are the basic structure and functional unit of all life; groups of cells form tissues, and tissues form organs.  The human body depends on organs working together, as organ systems, to do specific jobs in the body.

The Eleven Organ Systems in Your Body

 The Eleven Organ Systems in the Human Body and Their Major Functions

Organ System Organ Components Major Function
Circulatory heart, blood/lymph vessels, blood, lymph Transport nutrients and waste products
Digestive mouth, esophagus, stomach, intestines Digestion and absorption
Endocrine all glands (thyroid, ovaries, pancreas) Produce and release hormones
Immune white blood cells, lymphatic tissue, marrow Defend against foreign invaders
Integumentary skin, nails, hair, sweat glands Protective, body temperature regulation
Muscular skeletal, smooth, and cardiac muscle Body movement
Nervous brain, spinal cord, nerves Interprets and responds to stimuli
Reproductive gonads, genitals Reproduction and sexual characteristics
Respiratory lungs, nose, mouth, throat, trachea Gas exchange
Skeletal bones, tendons, ligaments, joints Structure and support
Urinary kidneys, bladder, ureters Waste excretion, water balance


We obtain nutrition by eating food.  Unlike gasoline, which is perfectly designed to power a car, food is not ready for the body to use in its current form.  The tacos you ate last night doesn’t go right into your body and power your body; A LOT has to happen to it for the body to be able to use it.  The process of transforming food into usable nutrition for the body is called digestion.  There are 4 steps to digestion:

  1. Eat food.
  2. Break down the food into tiny pieces.
  3. Absorb nutrition into the body:  move the small particles out of the digestive system and the rest of the body.
  4. Get rid of the waste, which is anything your body can’t use.

The digestive tract is a tube through the body, starting at the mouth and ending with the anus.  The digestive system includes the mouth, esophagus, stomach, small intestine, pancreas, liver, gallbladder, and large intestine.


Watch the Video on Digestion and Absorption




As food moves through the body, it is broken down by mechanical and chemical breakdown.

Mechanical breakdown is when you physically break food into pieces.  Mechanical breakdown starts in the mouth, with the teeth tearing, ripping, and grinding food into smaller pieces.  The mechanical breakdown also occurs with the muscular action of the esophagus, stomach, and small intestines.  Have you ever bought a bag of grated cheese, and it’s all clumped up, so you whack the bag on the counter or massage it with your fingers to break up all the cheese?  That’s essentially what the body does; the digestive system’s muscles massage and separate and move food along through your body.  This process is strong and happens even if working against gravity; if you drink a glass of water and then quickly do a headstand, your body will still be able to move that water through the digestive system even if you are upside down.

The chemical breakdown is when the body makes enzymes (chemicals) that break food down into small molecules.  Enzymes are added to the mechanically broken down food from the salivary glands, stomach, pancreas, and small intestine.  Additionally, the liver makes a chemical called bile, stored in the gallbladder, which helps the body digest fat.  These chemicals are necessary to break the food down into molecules.

On the first half of the journey of a piece of food through the body, mechanical and chemical breakdown occurs.  However, in the small intestine, absorption is also happening:  the body absorbs the molecules from the food, taking them through the intestine wall and into the blood where the energy and building blocks can be delivered throughout the body.  But hold on, the process of digestion is not done yet!  No breakdown occurs in the large intestine, but some molecules and reabsorption of water are absorbed.  Lastly, the large intestine packages and pushes the remaining undigested and indigestible food out of the body through the anus; it eliminates the waste.  Digestion is complete!


Can You Identify The Digestive Organs?


What should your indigestible food look like as it exits your body?

Bristol Stool Chart

Use the link below to access the Bristol Stool Chart and then compare the categories to your stools.


Your Gut Microbiome

Another feature of the large intestine is that it is where an enormous quantity of bacteria live.  That may seem kind of gross to discover that your gut is home to more bacteria than there are cells in your body, but most of these bacteria are harmless, and some are even beneficial.  You may have heard about probiotics and prebiotics in the news or seen them listed on food labels.  Probiotics are defined as live bacteria that provide health benefits to the host (us), such as warding off diarrhea and reducing the effects of lactose intolerance.  On the other hand, prebiotics is a type of fiber in some foods, which serves as a food source for probiotics.  An analogy is to think of your gut as a farm.  The probiotics are farm animals; the more farm animals on the farm, the better the farm is doing.  But to keep the farm animals in good condition, you have to feed them.  The hay you feed them is the prebiotics.  We call this ‘farm’ your gut biome; the more and the healthier the probiotics, the healthier your gut biome.  Foods such as yogurt contain probiotics, which can add to your gut biome.  Some foods are prebiotics, like onions, bananas, and oats, which can feed your gut biome.  A strong gut biome is thought to provide a host of health benefits.

Watch the Video on Pro/PreBiotics

Digestive Diseases and Disorders

Many people suffer from digestive irregularity from time to time.

  • LACTOSE INTOLERANCE:  Lactose intolerance happens when the body can’t break down the sugar found in milk, which causes gas, bloating, pain, and diarrhea.  People can avoid dairy products or take supplements such as Lactaid to digest the sugar (lactose) found in milk.
  • DIARRHEA:  Diarrhea is a term used to describe very watery stools.  It typically occurs with stomach pain and frequent trips to the bathroom. Diarrhea can be the symptom of stress, foodborne illness, or other infection.  If a person has diarrhea, they should pay special attention to hydration as they lose more water than normal through diarrhea.  Acute diarrhea often resolves on its own and needs no other treatment than rest, hydration, and over the counter medication.  On the other hand, it can be a symptom of a more severe disorder and disease.
  • CONSTIPATION:  Constipation occurs when a person has difficulty passing stool and generally has three or fewer bowel movements a week.  There can be numerous causes of constipation, including underlying digestive disorders and diseases, hormonal imbalances such as thyroid issues, or the slowing of food transit that happens in pregnancy.  The first line of defense against constipation is increasing water, dietary fiber, and activity levels.

Many diseases and disorders related to the digestive system.  Unbalanced diets can cause/effect disease; for example, high-fat diets can exacerbate GERD, IBS, and IBD symptoms.

  • PEPTIC ULCERS: Peptic ulcers are sores in the stomach lining brought on by microbiome disturbances, stress, and infection.  GERD medications are often provided to decrease stomach acid and allow healing.
  • GERD:  Gastroesophageal Reflux Disease.  This disease is a type of persistent acid reflux that tends to occur in people with an unbalanced high-fat diet.  Additionally, some food and drink such as spicy foods and chocolate make the symptoms worse.  Treatment for GERD is making lifestyle changes, including dietary modifications to reduce weight and pressure on the stomach.
  • IBS:  Irritable Bowel Syndrome.  This syndrome is characterized by muscle spasms in the colon, resulting in abdominal pain, bloating, constipation, and/or diarrhea.  An unbalanced diet and stress primarily cause IBS.  Treatment includes diet and lifestyle modifications.
  • IBD:  Irritable Bowel Disease.  This disease is inflammation and damage to the GI tract.  If the damage is mostly in the small intestines, it is called Crohn’s Disease; if it is only in the large intestine, it is called Ulcerative Colitis (UC).  The cause is unknown.  The proper course of treatment is unclear but may include drugs and surgery.
  • Celiac Disease:  This disease is an autoimmune disorder that damages the villi in the small intestines, resulting in IBS and IBD symptoms.  Damage is caused by the body overreacting to gluten, a common component in grains and found in most bread.  Once diagnosed properly, a person who avoids gluten will heal their gut and stop the negative symptoms within six months.

What is Gluten?


Learning Objectives

  • Identify the parts of the Digestive System:  mouth, esophagus, stomach, small intestine, pancreas, liver, gallbladder, large intestine. (MCCCD Competency 10)
  • Define what probiotics and prebiotics and their contribution to health. (MCCCD Competency 6)
  • Define common digestive diseases and disorders, including GERD/Gastritis, Crohn’s Disease, Ulcerative Colitis, Constipation, Diarrhea, Celiac disease, and lactose intolerance. (MCCCD Competency 10)


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Nutrition Essentials Copyright © 2020 by Stephanie Green and Kelli Shallal is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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