Lesson Overview
Lesson Overview
The Digestive System
THINK ABOUT IT
Lesson Overview
The Digestive System
Functions of the Digestive System
Lesson Overview
The Digestive System
Functions of the Digestive System
What are the functions of the digestive system?
The digestive system converts food into small molecules that can be used by the cells of the body. Food is processed by the digestive system in four
Lesson Overview
The Digestive System
Ingestion
The first step in digestion is getting food into the system.
Lesson Overview
The Digestive System
Digestion
Food is broken down in two
ways—by mechanical and chemical digestion.
Mechanical digestion is the
physical breakdown of large pieces of food into smaller pieces.
During chemical digestion,
Lesson Overview
The Digestive System
Absorption
Once food has been broken into small molecules, it can be absorbed by the cells of the small intestine.
Lesson Overview
The Digestive System
Elimination
The digestive system cannot digest and absorb all food products that enter the body.
Lesson Overview
The Digestive System
The Process of Digestion
Lesson Overview
The Digestive System
The Process of Digestion
What occurs during digestion?
Lesson Overview
The Digestive System
The Process of Digestion
Lesson Overview
The Digestive System
The Process of Digestion
Lesson Overview
The Digestive System
The Mouth
Chewing begins the process of mechanical digestion.
Lesson Overview
The Digestive System
Teeth
The teeth are anchored in the bones of the jaw.
The incisors, cuspids, and bicuspids cut into and tear at food.
The molars grind and crush food into a fine paste that can be swallowed.
Lesson Overview
The Digestive System
Saliva
The salivary glands secrete saliva, which helps to moisten the food and make it easier to chew.
Lesson Overview
The Digestive System
Saliva
Saliva also begins the process of chemical digestion.
Saliva contains an enzyme called amylase that begins to break the chemical bonds in starches, forming sugars.
Lesson Overview
The Digestive System
The Mouth
Lesson Overview
The Digestive System
The Process of Digestion
The bolus enters the area at the back of the throat called the pharynx.
Lesson Overview
The Digestive System
The Process of Digestion
Lesson Overview
The Digestive System
The Esophagus
The bolus passes through a tube called the esophagus into the stomach. Contractions of smooth muscles, known as peristalsis, provide the force that moves food through the esophagus toward the stomach.
Lesson Overview
The Digestive System
Esophagus
After food passes into the stomach, a thick ring of muscle called the cardiac sphincter closes the esophagus. This prevents the contents of the stomach from flowing back.
Lesson Overview
The Digestive System
Chemical Digestion in the Stomach
Lesson Overview
The Digestive System
Chemical Digestion in the Stomach
The lining of the stomach contains millions of microscopic gastric glands that release hydrochloric acid and an enzyme called pepsin that functions best in acidic conditions.
Pepsin begins the digestion of proteins by breaking them into smaller
Lesson Overview
The Digestive System
Chemical Digestion in the Stomach
Another stomach gland produces mucus, a fluid that lubricates and protects the stomach wall.
If this mucous layer fails, acids may erode the stomach lining and cause a sore called a peptic ulcer.
Most peptic ulcers are the result of infection with the bacterium
Helicobacter pylori (shown in the micrograph) and can be cured with
Lesson Overview
The Digestive System
Mechanical Digestion in the Stomach
Alternating contractions of the stomach’s three smooth muscle layers
thoroughly churn and mix the swallowed food, breaking it down and allowing enzymes greater access to the food.
Lesson Overview
The Digestive System
Mechanical Digestion in the Stomach
Lesson Overview
The Digestive System
Digestion in the Small Intestine
As chyme is pushed through the pyloric valve, it enters the duodenum.
Lesson Overview
The Digestive System
Digestion in the Small Intestine
Lesson Overview
The Digestive System
Just behind the stomach is the pancreas, a gland that serves three important functions.
1. One function is to produce hormones that regulate blood sugar levels.
Lesson Overview
The Digestive System
Just behind the stomach is the pancreas, a gland that serves three important functions.
2. It also produces enzymes that break down carbohydrates, proteins, lipids, and nucleic acids.
Lesson Overview
The Digestive System
Just behind the stomach is the pancreas, a gland that serves three important functions.
3. The pancreas also produces sodium bicarbonate, a base that quickly neutralizes stomach acid as chyme enters the duodenum.
Lesson Overview
The Digestive System
The Liver and Gallbladder
Assisting the pancreas in fat digestion is the liver.
The liver produces bile, a fluid loaded with lipids and salts.
Lesson Overview
The Digestive System
The Liver and Gallbladder
When fat is present in the duodenum, the gallbladder releases bile through a duct into the small intestine.
Lesson Overview
The Digestive System
Effects of Digestive Enzymes
Lesson Overview
The Digestive System
Absorption and Elimination
Lesson Overview
The Digestive System
Absorption and Elimination
How are nutrients absorbed and wastes eliminated?
Most nutrients from food are absorbed through the walls of the small
Lesson Overview
The Digestive System
Absorption From the Small Intestine
After leaving the duodenum, chyme moves along the rest of the small intestine.
Lesson Overview
The Digestive System
Absorption From the Small Intestine
The folded surfaces of the small intestine are covered with fingerlike projections called villi.
The surfaces of the cells of the villi are covered with thousands of tiny
Lesson Overview
The Digestive System
Absorption From the Small Intestine
Nutrient molecules are rapidly absorbed into the cells lining the small intestine.
Most of the products of carbohydrate and protein digestion are absorbed into the capillaries in the villi.
Lesson Overview
The Digestive System
Absorption From the Small Intestine
By the time chyme is ready to leave the small intestine, complex organic molecules have been digested and absorbed, leaving only water, cellulose, and other undigestible substances behind.
As materials move from the small intestine to the large intestine, they pass the appendix.
In some mammals, the appendix processes cellulose and other materials.
Lesson Overview
The Digestive System
Absorption From the Small Intestine
When chyme leaves the small intestine, it enters the large intestine, or colon.
Lesson Overview
The Digestive System
Absorption From the Small Intestine
The primary function of the large intestine is to remove water from the undigested material that is left.
Water is absorbed across the wall of the large intestine, leaving behind the undigested materials.
Lesson Overview
The Digestive System
Absorption From the Small Intestine
Colonies of bacteria present in the large intestine produce compounds that the body is able to use, including vitamin K.
Lesson Overview
The Digestive System
Elimination
Lesson Overview
The Digestive System
Elimination
If not enough water is absorbed by the large intestine, a condition known as diarrhea occurs.
