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.

Ingestion  The first step in digestion is getting food into the system.  Ingestion is the process of putting food into your mouth—the opening to the digestive tract.

Digestion  Food is broken down in two ways—by mechanical and chemical digestion.

Mechanical vs Chemical Digestion

Absorption  Once food has been broken into small molecules, it can be absorbed by the cells of the small intestine.  From there, the molecules enter the circulatory system, which transports them throughout the body.

Elimination  The digestive system cannot digest and absorb all food products that enter the body.  Some materials travel through the large intestine and are eliminated from the body as feces.

The Process of DigestionThe Process of Digestion  The human digestive system is built around an alimentary canal—a one-way tube that passes through the body.

The MouthThe 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.

Saliva  The salivary glands secrete saliva, which helps to moisten the food 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  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.  Saliva also has lysozyme, an enzyme that fights infection by digesting the cell walls of many bacteria that enter the mouth with food.

The MouthThe Mouth  Once food is chewed, the combined actions of the tongue and throat muscles push the clump of food, called a bolus, down the throat.

The Process of DigestionThe Process of Digestion  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.  The action of the epiglottis prevents food from moving into the air passageways to the lungs as it passes through the pharynx and into the esophagus.

The EsophagusThe 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.

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.  A backflow of stomach acid into the esophagus results in a burning sensation in the center of the chest known as heartburn. 

The StomachThe Stomach  The stomach is a large muscular sac that continues chemical and mechanical digestion of food.

Chemical Digestion in the StomachChemical 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 polypeptide fragments.

Mechanical Digestion in the StomachMechanical 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.  A mixture with an oatmeal-like consistency called chyme is produced.

Mechanical Digestion in the StomachMechanical Digestion in the Stomach  After an hour or two, the pyloric valve—located between the stomach and small intestine—opens, and chyme begins to spurt into the small intestine.

Digestion in the Small IntestineDigestion in the Small Intestine  As chyme is pushed through the pyloric valve, it enters the duodenum.  The duodenum, where almost all of the digestive enzymes enter the intestine, is the first of three parts of the small intestine.

Digestion in the Small IntestineDigestion in the Small Intestine  As chyme enters the duodenum from the stomach, it mixes with enzymes and digestive fluids from the pancreas, the liver, and the lining of the duodenum.

Pancreas  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.

Pancreas  2. It also produces enzymes that break down carbohydrates, proteins, lipids, and nucleic acids.  3. The pancreas also produces sodium bicarbonate, a base that quickly neutralizes stomach acid as chyme enters the duodenum.

The Liver and GallbladderThe 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, pouch-like organ called the gallbladder.

The Liver and GallbladderThe Liver and 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.

Effects of Digestive EnzymesEffects of Digestive Enzymes  Digestive enzymes hasten the breakdown of foods and make nutrients available to the body.

Absorption and Elimination  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.

Absorption in the Small IntestineAbsorption in 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 projections known as microvilli, which provide an enormous surface area for absorption of nutrient molecules.

Absorption in the Small IntestineAbsorption in 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.  Most fats and fatty acids are absorbed by lymph vessels.

Small and Large IntestineSmall and Large Intestine  When chyme leaves the small intestine, it enters the large intestine, or colon. This is usually just water, cellulose, and other undigestible substances.  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.

Large IntestineLarge 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.

Elimination  The concentrated waste material—the feces—that remains after most of the water has been removed passes into the rectum and is eliminated from the body through the anus.

Elimination  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.