Functions of the Digestive System

• The functions of the digestive system are to take in food, break down the food, absorb the digested molecules, and, thus, provide nutrients to the body.

Anatomy and Histology of the Digestive System

• The GI tract is composed of four tunics: mucosa, submucosa, muscularis, and serosa or adventitia.

Oral Cavity

• The lips and cheeks are involved in mastication and
  speech.

• The tongue is involved in speech, taste, mastication, and swallowing.

• There are 32 permanent teeth, including incisors, canines, premolars, and molars. Each tooth consists of a crown, neck, and root.

• The roof of the oral cavity is divided into the hard and soft palates.

• Salivary glands produce serous and mucous secretions. The three pairs of large salivary glands are the parotid, submandibular, and sublingual glands.

Pharynx

• The pharynx consists of the nasopharynx, oropharynx, and laryngopharynx.

Esophagus

• The esophagus connects the pharynx to the stomach. The upper and lower esophageal sphincters regulate movement.

Stomach

• The stomach has a cardiac opening from the esophagus and a pyloric opening into the duodenum.

• The wall of the stomach consists of three muscle layers: longitudinal, circular, and oblique.

• Gastric glands produce mucus, hydrochloric acid, pepsin, gastrin, and intrinsic factor.

Small Intestine

• The small intestine is divided into the duodenum, jejunum, and ileum.

• Circular folds, villi, and microvilli greatly increase the surface area of the intestinal lining.

• Goblet cells and duodenal glands produce mucus.

Liver

• The liver receives blood from the hepatic artery and the hepatic portal vein.

• Bile leaves the liver through the hepatic duct system. The right and left hepatic ducts join to form the common hepatic duct. The cystic duct joins the common hepatic duct to form the common bile duct. The common bile duct joins the pancreatic duct and empties into the duodenum.

• The liver is divided into lobules with portal triads at the corners. Portal triads contain branches of the hepatic portal vein, hepatic artery, and hepatic duct.

• Hepatic cords, formed by hepatocytes, form the substance of each lobule. A bile canaliculus, between the cells of each cord, joins the hepatic duct system.

• Branches of the hepatic artery and hepatic portal vein empty into hepatic sinusoids, which empty into a central vein in the center of each lobe. The central veins empty into hepatic veins, which exit the liver.

Pancreas

• The pancreas is an endocrine and an exocrine gland. Its endocrine function is to control blood nutrient levels. Its exocrine function is to produce bicarbonate ions and digestive enzymes.

Large Intestine

• The cecum forms a blind sac at the junction of the small and large intestines. The appendix is a blind sac off the cecum.

• The colon consists of ascending, transverse, descending, and sigmoid portions.

• The large intestine contains mucus-producing crypts.

• The rectum is a straight tube that ends at the anal canal.

Anal Canal

• The anal canal is surrounded by an internal anal sphincter (smooth muscle) and an external anal sphincter (skeletal muscle).

Peritoneum

• The peritoneum is a serous membrane that lines the abdominal cavity and covers the organs.

• Mesenteries are double layers of peritoneum that extend from the body wall to many of the abdominal organs.

• Retroperitoneal organs are located behind the parietal peritoneum.

Movements and Secretions in the Digestive System

• The digestive system is regulated by neural and hormonal mechanisms. Intramural plexuses are responsible for local reflexes.

Oral Cavity, Pharynx, and Esophagus

• Amylase in saliva starts starch digestion. Mucin provides lubrication.

• Mastication is accomplished by the teeth, which cut, tear, and crush the food.

• During the voluntary phase of deglutition, a bolus of food is moved by the tongue from the oral cavity to the pharynx.

• During the pharyngeal phase of deglutition, the soft palate closes the nasopharynx, and the epiglottis closes the opening into the larynx. Pharyngeal muscles move the bolus to the esophagus.

• During the esophageal phase of deglutition, a wave of constriction (peristalsis) moves the food down the esophagus to the stomach.

Stomach

• Secretions of the stomach.
  • Mucus protects the stomach lining.

  • Hydrochloric acid kills microorganisms and activates pepsin.

  • Pepsin starts protein digestion.

  • Intrinsic factor aids in vitamin B₁₂ absorption.

  • Gastrin helps regulate stomach secretions and movements.

• Regulation of stomach secretions.
  • During the cephalic phase, the stomach secretions are initiated by the sight, smell, taste, or thought of food. Gastrin stimulates stomach secretions.

  • During the gastric phase, partially digested proteins or distention of the stomach also promotes secretion.

  • During the intestinal phase, acidic chyme in the duodenum stimulates neuronal reflexes and the secretion of hormones that induce and then inhibit gastric secretions. Gastrin stimulates stomach secretion. Secretin, gastric inhibitory polypeptide, and cholecystokinin inhibit gastric secretion.

• Movement in the stomach.
  • Mixing waves mix the stomach contents with the stomach secretions to form chyme.

  • Peristaltic waves move the chyme into the duodenum.

Small Intestine

• Secretions of the small intestine.
  • Mucus protects against digestive enzymes and stomach acids.

  • Chemical or tactile irritation, vagal stimulation, and secretion stimulate intestinal secretion.

• Movement in the small intestine.
  • Segmental contractions occur over short distances and mix the intestinal contents.

  • Peristaltic contractions occur the length of the intestine and propel chyme through the intestine.

• Most absorption occurs in the duodenum and jejunum.

Liver

• The liver produces bile, which contains bile salts that emulsify fats.

• The liver stores and processes nutrients, produces new molecules, and detoxifies molecules.

• The liver produces blood proteins.

Pancreas

• The pancreas produces bicarbonate ions and digestive enzymes.

• Acidic chyme stimulates the release of a watery bicarbonate solution that neutralizes acidic chyme. Fatty acids and amino acids in the duodenum stimulate the release of pancreatic enzymes.

Large Intestine

• The function of the large intestine is feces production and water absorption.

• It takes much longer for material to move through the large intestine than the small intestine.

• In the colon, chyme is converted to feces.

• Mass movements occur three to four times a day.

• Defecation is the elimination of feces. Reflex activity moves feces through the internal anal sphincter. Voluntary activity regulates movement through the external anal sphincter.

Digestion, Absorption, and Transport

• Digestion is the chemical breakdown of organic molecules into their component parts. After the molecules are digested, some diffuse through the intestinal wall; others must be transported across the intestinal wall.

Carbohydrates

• Polysaccharides are split into disaccharides by salivary and pancreatic amylases.

• Disaccharides are broken down to monosaccharides by disaccharidases on the surface of the intestinal epithelium.

• Monosaccharides are absorbed by active transport into the blood and carried by the hepatic portal vein to the liver.

• Glucose is carried in the blood and enters most cells by facilitated diffusion. Insulin increases the rate of glucose transport into most cells.

Lipids

• Bile salts emulsify lipids.

• Pancreatic lipase breaks down lipids. The breakdown products aggregate with bile salts to form micelles.

• Micelles come into contact with the intestinal epithelium, and their contents diffuse into the cells, where they are packaged and released into the lacteals.

• Lipids are stored in adipose tissue and in the liver, which release the lipids into the blood when energy sources are needed elsewhere in the body.

Proteins

• Proteins are split into small polypeptides by enzymes secreted by the stomach and pancreas, and on the surface of
  intestinal cells.

• Peptidases on the surface of intestinal epithelial cells complete the digestive process.

• Amino acids are absorbed into intestinal epithelial cells.

• Amino acids are actively transported into cells under the influence of growth hormone and insulin.

• Amino acids are used to build new proteins or as a source of energy.

Water and Minerals

• Water can move either direction across the intestinal wall, depending on osmotic conditions. Approximately 99% of the water entering the intestine is absorbed. Most minerals are actively transported across the intestinal wall.