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General Education
Lesson 7: Digestive System
(Human Systems)
What is the Digestive System?
The digestive system is a series of organs and structures that work together to break down food into smaller particles that your body can use for energy and growth. The process of breaking down food and absorbing nutrients is called digestion. Your digestive system is like a long factory that takes in large pieces of food and transforms them into tiny nutrients that your body can absorb and use.
- The digestive system breaks down food so your body can use it for energy and to build and repair tissues.
Two Types of Digestion
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| Image from Superprof |
Digestion happens in two ways: physically (breaking food into smaller pieces) and chemically (breaking down food molecules using enzymes).
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| Image from Adobe Stock |
Physical Digestion
The mechanical breakdown of food into smaller pieces without changing the chemical structure of the food.
Where it happens:
- Mouth - Chewing breaks food into smaller pieces
- Stomach - Churning action mixes and breaks down food
- Small intestine - Muscular contractions continue to mix food
Physical digestion is when your teeth and stomach physically break food into smaller pieces.
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| Image from BBC |
Chemical Digestion
The breakdown of large food molecules into smaller molecules using enzymes. Enzymes are special proteins that speed up chemical reactions.
Where it happens:
- Mouth - Saliva contains enzymes that start breaking down carbohydrates
- Stomach - Gastric juices break down proteins
- Small intestine - Multiple enzymes break down carbohydrates, proteins, and fats
- Pancreas - Produces enzymes that help in the small intestine
Chemical digestion is when enzymes cut apart food molecules, the molecules change at the chemical level.
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| Image from Gastroenterology |
The Digestive Tract
Food travels through a long tube called the digestive tract (or gastrointestinal tract). Let's follow food on its journey through your digestive system.
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| Image from Vecteezy |
Mouth
Food enters your body and begins to be broken down.
Processes:
Physical digestion: Your teeth chew and break food into smaller pieces
Chemical digestion: Salivary glands release saliva containing enzymes that begin breaking down carbohydrates
Saliva's job:
- Moistens food so it is easier to chew and swallow
- Contains amylase enzyme that breaks down starch into simpler sugars
- Lubricates food for easier swallowing
Your mouth chews food into smaller pieces and saliva starts breaking down carbohydrates chemically.
Pharynx (Throat)
Food is swallowed and moves from your mouth to your esophagus.
Process:
- Swallowing is a reflex action (automatic)
- The epiglottis (a flap) covers your windpipe so food goes down the esophagus, not into your lungs
- Muscles contract to push food downward
The pharynx is where swallowing happens. A flap prevents food from going into your lungs. Time in pharynx: Less than a second
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| Image from Cleveland Clinic |
Esophagus
Food travels from your throat to your stomach.
Characteristics:
- A tube about 10 inches long
- Muscular walls contract in waves called peristalsis
- Peristalsis pushes food down toward the stomach
- No digestion happens here; it is just a passageway
- Works even when you are upside down because of peristalsis
- Peristalsis: Wavelike muscular contractions that move food through the entire digestive tract from esophagus to rectum.
The esophagus is a tube that carries food from your throat to your stomach using muscular waves called peristalsis. Time in esophagus: About 5-10 seconds
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| Image from GeeksforGeeks |
Stomach
Food is mixed with gastric juices and begins to break down, especially proteins.
Characteristics:
- A muscular, bag-like organ
- Can expand to hold up to 4 liters of food
- Produces gastric juices containing:
- Hydrochloric acid (HCl) - kills bacteria and helps break down proteins
- Pepsin - enzyme that breaks down proteins
- Mucus - protects stomach lining from acid
Physical processes:
- Stomach muscles churn and mix food with gastric juices
- Creates a soupy mixture called chyme
- Chyme: The semi-liquid mixture of food and gastric juices in the stomach. This is what leaves the stomach and enters the small intestine.
The stomach is like a mixer that churns food and mixes it with acids and enzymes. The result is a soupy mixture called chyme. Time in stomach: 2-4 hours (depending on the type and amount of food)
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| Image from Cleveland Clinic |
Small Intestine
Final digestion occurs and most nutrient absorption happens.
Characteristics:
- About 20 feet long (but called "small" because of its narrow diameter)
- Divided into three parts: duodenum, jejunum, and ileum
- Very important for digestion and absorption
Villi:
- finger-like structures that line the inside of the small intestine
- increase surface area for absorption
- each villus has many capillaries (tiny blood vessels) for absorbing nutrients
- nutrients pass through villi into the bloodstream
What digestion happens here:
- Enzymes from the pancreas and liver continue breaking down food
- Carbohydrates are broken down into simple sugars (glucose)
- Proteins are broken down into amino acids
- Fats are broken down into fatty acids and glycerol
- Most digestion and absorption happens in the small intestine, taking 3-5 hours
The small intestine is where most nutrients are absorbed. Its finger-like villi absorb nutrients into your blood so your body can use them.
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| Image from DNA Labs |
Large Intestine (Colon)
Water is absorbed from the remaining food, and waste is prepared for elimination.
Characteristics:
- About 5 feet long (wider than small intestine, so called "large")
- Removes water from the remaining food material
- Contains beneficial bacteria that help break down remaining food
What happens in the colon:
- Water absorption: Most of the water from the remaining food is absorbed back into the blood
- Vitamin K production: Beneficial bacteria in the colon produce vitamin K, which is needed for blood clotting
- Waste formation: The remaining material becomes feces (solid waste)
The large intestine absorbs water from waste and helps your body get vitamin K. It prepares waste for elimination. Time in large intestine: 12-48 hours (food can stay here much longer than in other parts)
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| Image from Kaizen Hospital |
Rectum
Feces (solid waste) is stored here until it is ready to be eliminated.
Characteristics:
- Last part of the digestive tract before the anus
- Stores waste
- Stretches as feces accumulates
- Signals your brain when it is full (the urge to have a bowel movement)
- Easy explanation: The rectum is like a storage area for waste before it leaves your body.
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| Image from Cleveland Clinic |
Anus
Waste (feces) is eliminated from your body.
Characteristics:
- Opening at the end of the digestive tract
- Controlled by two sphincters (rings of muscle)
- One sphincter is involuntary (automatic), one is voluntary (you control it)
- This is where defecation (elimination of waste) occurs
- Defecation: The process of eliminating feces from the body through the anus.
The anus is the opening where solid waste leaves your body during defecation.
Accessory Organs: Helping Digestion
While the main digestive tract does much of the work, several accessory organs help by producing important substances needed for digestion.
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| Image from Cleveland Clinic |
Salivary Glands - glands in and around your mouth that produce saliva.
- Produce saliva (about 1.5 liters per day)
- Saliva contains enzymes (like amylase) that start breaking down carbohydrates
- Saliva contains mucus that lubricates food
- Saliva is triggered by smell, taste, or even thinking about food
located at:
- Under the tongue
- Below the jaw
- Near the ears
Salivary glands make saliva that starts digestion and makes food easier to swallow.
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| Image from Mayo Clinic |
Liver - the largest internal organ in the human body.
- Produces bile, a greenish liquid that helps digest fats
- Bile breaks apart large fat molecules into smaller ones (this is called emulsification of fats)
- Stores excess glucose as glycogen for energy later
- Breaks down toxins and poisons so they can be eliminated
- Produces blood proteins
- Stores vitamins and minerals
located at:
- Upper right part of your abdomen, protected by the rib cage
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| Image from Biology Online |
Bile:
- Emulsification of fats: Bile breaks large fat droplets into smaller ones, making them easier for enzymes to break down
- Makes fats dissolve so they can be absorbed in the small intestine
- Gives stool its brown color
The liver is the biggest organ inside you. It makes bile that breaks apart fats so they can be digested and absorbed. It also cleans your blood and stores energy.
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| Image from National Cancer Institute |
Gallbladder - a small, pear-shaped organ attached to the liver.
- Stores bile produced by the liver
- Concentrates bile (removes water to make it stronger)
- Releases bile into the small intestine when fatty food enters
How it works:
- Bile is constantly produced by the liver
- The gallbladder stores extra bile
- When you eat fatty food, the gallbladder contracts and squirts bile into the small intestine
- This gives you enough bile to digest the fat
- Located at underneath the liver
The gallbladder is like a storage tank for bile. It holds bile until your body needs it to digest fat.
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| Image from Cleveland Clinic |
Pancreas - a gland that has two main jobs: producing enzymes for digestion and producing hormones for blood sugar control.
Digestive:
- Produces powerful digestive enzymes that break down food
- Produces amylase - breaks down carbohydrates
- Produces protease - breaks down proteins
- Produces lipase - breaks down fats
- Releases these enzymes into the small intestine to help final digestion
- Produces insulin and glucagon - hormones that control blood sugar levels
- Located behind the stomach, below the liver
The pancreas makes enzymes that break down carbohydrates, proteins, and fats. These enzymes are sent to the small intestine to finish digestion.
Digestive System Quiz: click here
