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 phases—ingestion, digestion, absorption, and elimination.
Lesson Overview
The Digestive System
Lesson Overview
The Digestive System
The Process of Digestion
What occurs during digestion?
During digestion, food travels through the mouth, esophagus, stomach, and small intestine. Mechanical digestion and chemical digestion are the two processes by which food is reduced to molecules that can be absorbed.
Mechanical digestion is the physical breakdown of large pieces of food into smaller pieces.
During chemical digestion, enzymes break down food into the small molecules the body can use.
Lesson Overview
The Digestive System
The Mouth
Chewing begins the process of mechanical digestion.
Chemical digestion begins as digestive enzymes in saliva start the breakdown of complex carbohydrates into smaller molecules.
Lesson Overview
The Digestive System
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.
Meanwhile, your tongue moves food around so that it comes in contact with your teeth.
Lesson Overview
The Digestive System
The salivary glands secrete 1 liter of saliva per day, which helps to moisten the food with mucous and make it easier to chew.
The release of saliva is under the control of the nervous system and can be triggered by the scent of food.
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 (maltose).
Saliva also has lysozyme, an enzyme that fights infection by digesting the cell walls of many bacteria that enter the mouth with food.
Lesson Overview
The Digestive System
The bolus enters the area at the back of the throat called the pharynx.
As this occurs, a flap of connective tissue called the epiglottis closes over the opening to the trachea.
Once food is chewed, the combined actions of the tongue and throat muscles push the clump of food, called a bolus, down the throat.
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
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.
A backflow of stomach acid into the esophagus results in a burning sensation in the center of the chest known as heartburn.
Lesson Overview
The Digestive System
Stomach
Alternating contractions of the stomach’s three smooth muscle layers thoroughly mechanically churn and mix the swallowed food, breaking it down and allowing enzymes greater access to the food.
A mixture with an oatmeal-like consistency called chyme is produced.
Lesson Overview
The Digestive System
The lining of the stomach contains millions of microscopic gastric glands that release hydrochloric acid, mucus and an enzyme called pepsin that functions best in acidic conditions.
Pepsin begins the chemical digestion of proteins by breaking them into smaller polypeptide fragments.
Lesson Overview
The Digestive System
Small Intestine
After an hour or two, the pyloric valve—located between the stomach and small intestine—opens, and chyme begins to spurt into the duodenum of the small intestine to continue chemical digestion.
Chyme mixes with enzymes and
digestive fluids from the pancreas,
the liver, and the lining of the
duodenum.
Lesson Overview
The Digestive System
Just behind the stomach is the pancreas, a gland that serves three important functions.
Pancreas
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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.
Bile is stored in a small, pouchlike organ called the gallbladder.
When fat is present in the duodenum, the gallbladder releases bile through a duct into the small intestine.
Bile breaks up the globs of fat into smaller droplets, making it possible for enzymes to reach the smaller fat droplets and break them down.
Lesson Overview
The Digestive System
Effects of Digestive Enzymes
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 intestine. The large intestine absorbs water and several vitamins and prepares waste for elimination from the body.
After leaving the duodenum, chyme moves along the rest of the small intestine. The chyme is now a rich mixture of small- and medium-sized nutrient molecules that are ready to be absorbed.
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The Digestive System
Small Intestine is Designed for Nutrient Absorption
Small intestine wall is folded into many finger-like projections called villi
Epithelial cells of each villus have microscopic projections called microvilli
Total surface area of all villi in small intestine is ~surface area of a tennis court!
Nutrient Absorption Occurs in Epithelial Cells of Small Intestine
Each villus absorbs nutrients into circulatory and lymphatic vessels
Fats: absorbed into lymph
Sugars & amino acids: move into blood
Four Tiers of Surface Area?
The Large Intestine
When chyme leaves the small intestine, it passes by the appendix and enters the large intestine, or colon. Complex organic molecules have been digested and absorbed, leaving only water, cellulose, and other undigestible substances behind.
The large intestine, which is actually much shorter than the small intestine, gets its name due to its diameter, which is much greater than the small intestine’s diameter.
Lesson Overview
The Digestive System
The Large Intestine
The primary function of the large intestine is to remove water from the undigested material that is left. Water is reabsorbed across the wall of the large intestine, leaving behind the undigested materials.
Also, colonies of bacteria present in the large intestine produce compounds that the body is able to use, including vitamin K. When large doses of antibiotics are given to fight an infection, they can destroy these bacteria, and vitamin K deficiency can occur.
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The Digestive System
Elimination
The concentrated waste material— the feces —that remains after most of the water has been removed passes into the rectum (last 2.5 cm) and is eliminated from the body through the sphincter controlled anus.
If not enough water is absorbed by the large intestine, a condition known as diarrhea occurs.
If too much water is absorbed from the undigested materials, a condition known as constipation occurs.
Four stages of the “Digestive” System?
Lesson Overview
The Digestive System