The Big Picture
The digestive system breaks food down so the body can absorb and use nutrients. It also removes solid waste from the body.
Food travels through a long tube from the mouth to the anus. Along the way, muscles move and mix the food, digestive juices break it down, and nutrients are absorbed.
The process includes both physical breakdown and chemical breakdown. These two types of breakdown work together during digestion.
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Start Here If You’re Overwhelmed
Focus on these core ideas:
- digestion physically and chemically breaks food down into nutrients the cells can use.
- The digestive system includes the alimentary canal and accessory organs.
- mechanical digestion physically breaks large food particles into smaller pieces.
- chemical digestion breaks food down chemically into small nutrient parts that the body can absorb.
- peristalsis pushes food forward through the digestive tract in one direction.
- Most nutrient absorption takes place in the jejunum and ileum of the small intestine.
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Course Notes: Key Ideas You Need to Know
What the Digestive System Does
The digestive system is also called the gastrointestinal system, or GI system.
Its two major jobs are:
- breaking nutrients down physically and chemically so cells can absorb and use them
- eliminating solid waste from the body
Digestion is the process of physically and chemically breaking food down into nutrients that cells can absorb and use.
The Two Main Parts of the Digestive System
The digestive system has two main groups of structures.
The Alimentary Canal
The alimentary canal, also called the GI tract, is a long tube that extends from the mouth to the anus.
Its major parts are:
- mouth
- pharynx
- esophagus
- stomach
- small intestine
- large intestine
Accessory Organs of Digestion
The accessory organs help with digestion.
They include:
- teeth
- tongue
- salivary glands
- liver
- gallbladder
- pancreas
Layers of the Digestive Tract Wall
The wall of the digestive tract has a similar basic structure throughout its length, although different organs have changes that fit their specific functions.
Mucosa
The mucosa is the innermost layer.
It contains:
- glands that secrete mucus, digestive enzymes, and hormones
- ducts that connect glands that empty into the digestive tract
Submucosa
The submucosa is the middle layer.
It contains loose connective tissue along with:
- blood vessels
- nerves
- glands
- lymphatic vessels
Muscularis
The muscularis, or muscle layer, is the outermost of these three layers.
It contains two layers of smooth muscle. These muscles produce two important types of movement.
One movement repeatedly relaxes and contracts stomach muscles. This back-and-forth movement mixes food with digestive juices and helps with mechanical digestion.
The second movement is peristalsis. Peristalsis is an involuntary smooth-muscle contraction that pushes food forward through the digestive tract in one direction using a wavelike motion.
Digestion Begins in the Mouth
The mouth, or oral cavity, receives food and prepares it for digestion.
Mastication Comes First
Mastication is the first step of digestion.
During mastication:
- Food is bitten.
- The tongue and cheeks position the food.
- The teeth chew, crush, and grind the food.
This process increases the surface area of the food.
Physically breaking large food particles into smaller pieces is called mechanical digestion.
Taste Buds and Saliva
Taste buds are found on:
- the surface of the tongue
- the soft palate
- the upper esophagus
- the epiglottis
They contain nerve endings that allow sweet, sour, bitter, and salty tastes to be sensed.
Taste buds and olfactory nerve stimulation trigger the salivary glands to release saliva.
Saliva is about 99% water. The rest includes digestive enzymes, mucus, electrolytes, glycoproteins, and antibacterial chemicals.
During mastication, saliva:
- moistens food
- helps mix food
- helps with swallowing
Saliva also contains salivary amylase. This enzyme breaks down carbohydrates and starts chemical digestion.
Mechanical Digestion and Chemical Digestion
These two processes are different but both form part of digestion.
| Type | What Happens |
|---|---|
| Mechanical digestion | Large food particles are physically broken into smaller pieces. |
| Chemical digestion | Food is chemically broken down into small nutrient molecules that cells can absorb and use. |
After chewing and mixing with saliva, the food becomes a moist ball called a bolus.
Swallowing and the Esophagus
When swallowing begins, the tongue pushes the bolus into the pharynx.
The pharynx contracts and pushes it into the esophagus.
The esophagus is a muscular tube about 25 cm (10 in.) long. It extends from the pharynx to the stomach.
Peristalsis moves the food down the esophagus.
The food enters the stomach through the cardiac sphincter, a muscular valve that controls the entrance to the stomach.
What Happens in the Stomach
The stomach is a muscular, pouchlike sac in the upper left part of the abdominal cavity.
Strong stomach muscles stir and churn the food into smaller particles.
The stomach has a mucous membrane containing glands that secrete gastric juices.
Gastric juices contain strong acids. These acids kill most microorganisms in the food that has been eaten.
Partially digested food mixes with gastric juices and becomes a semi-liquid substance called chyme.
Other cells produce thicker mucus and bicarbonate. These substances stick closely to the lining of the stomach and small intestine and prevent acidic gastric juices from digesting the body’s own tissue.
Chyme is:
- highly acidic
- foul-smelling
- watery
Peristalsis pushes chyme from the stomach through the pyloric sphincter and into the small intestine.
The Small Intestine
The small intestine is about 6 m (20 ft.) long.
It has three parts:
- duodenum
- jejunum
- ileum
Duodenum
The duodenum is the first part of the small intestine.
More digestive juices are added to chyme here. The pancreas also adds a bicarbonate solution that reduces the acidity of the chyme.
Enzymes in the digestive juices chemically break nutrients down so they can be absorbed.
Jejunum and Ileum
Peristalsis moves chyme through the jejunum and ileum.
Most nutrient absorption takes place in these two parts of the small intestine.
Villi and Nutrient Absorption
Tiny projections called villi line the small intestine. One projection is called a villus.
Villi greatly increase the surface area of the small intestine.
This increased surface area allows digested nutrients to be absorbed efficiently into capillaries.
Bile, the Liver, and the Gallbladder
Bile is a greenish liquid produced by the liver and stored in the gallbladder.
Bile helps with:
- fat digestion
- absorption of fat-soluble vitamins
Bile also gives stool its brownish colour.
Functions of the Liver
The liver has several important functions.
It:
- metabolizes fats, proteins, and carbohydrates
- detoxifies substances from the blood
- stores fat-soluble vitamins
- produces bile, cholesterol, and bilirubin
The Pancreas
The pancreas secretes digestive enzymes.
Key fact: Its enzymes are the most important digestive enzymes because of their role in aiding digestion.
Juices from both the pancreas and the small intestine are added to chyme.
The Large Intestine
The remaining chyme moves from the small intestine into the large intestine, which is also called the large bowel or colon.
It contains undigested nutrients and waste products.
The large intestine has four main functions.
1. Absorbing Water and Electrolytes
The large intestine absorbs much of the water and electrolytes from the chyme.
2. Supporting Beneficial Bacteria
The large intestine contains certain beneficial bacteria.
These bacteria help produce and metabolize substances such as:
- vitamin K
- vitamin B₁₂
- thiamine, or vitamin B₁
3. Storing Feces
The large intestine temporarily stores feces.
Feces are the remaining semi-solid material. They consist of water, solid wastes, some mucus, and microorganisms.
4. Helping Eliminate Feces
The large intestine helps eliminate feces through defecation, or a bowel movement.
Rectum, Anus, and Defecation
Peristalsis moves feces through the colon into the rectum.
The rectum temporarily stores feces.
Defecation is the process of excreting feces from the rectum through the anus.
The term stool refers to feces after they have been excreted.
Exam Clue Words
- First: Mastication is the first step of digestion.
- Most: Most nutrient absorption takes place in the jejunum and ileum.
- One direction: Peristalsis pushes food forward through the digestive tract in one direction.
Common Mix-Ups
| Wrong Idea | Better Idea |
|---|---|
| Mechanical digestion chemically changes food. | Mechanical digestion physically breaks food into smaller pieces. |
| Chemical digestion means chewing and grinding food. | Chemical digestion breaks food into small nutrient molecules using chemical processes. |
| Peristalsis mixes food back and forth. | Peristalsis pushes food forward in one direction. |
| Feces and stool always mean exactly the same stage. | Feces are the remaining semi-solid waste material. Stool means feces after excretion. |
Fast Review
Use these points for a quick last review of the digestive system and the path food follows.
- Digestion physically and chemically breaks food down so nutrients can be absorbed and used by cells.
- The alimentary canal runs from the mouth to the anus. Accessory organs include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
- The digestive tract wall includes the mucosa, submucosa, and muscularis.
- Peristalsis uses involuntary smooth-muscle contractions to push food forward in one direction.
- Mastication is the first step of digestion. It includes biting, chewing, crushing, and grinding food.
- Saliva is about 99% water, and salivary amylase begins the chemical digestion of carbohydrates.
- The esophagus is about 25 cm (10 in.) long and carries food from the pharynx to the stomach.
- The stomach churns food with gastric juices to form chyme.
- The small intestine is about 6 m (20 ft.) long and has three parts: duodenum, jejunum, and ileum.
- Villi increase the small intestine’s surface area, and most nutrient absorption occurs in the jejunum and ileum.
- Bile is produced by the liver, stored in the gallbladder, and helps with fat digestion and absorption of fat-soluble vitamins.
- The large intestine absorbs much of the remaining water and electrolytes, contains beneficial bacteria, temporarily stores feces, and helps eliminate feces.
Final Pep Talk
There are many organs, terms, and steps in this topic, so focus first on the path food follows and what happens at each stop. Once that order feels familiar, the smaller details have a clearer place to fit.