Human Digestive System

1 Anatomy of the Digestive System

Digestive System

1. The digestive system consists of a digestive tube and its associated accessory digestive organs.
2. The digestive system includes the oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, and anus.
3. Accessory digestive organs, such as the salivary glands, liver, gallbladder, and pancreas, are connected to the digestive tract.

2 Functions of the Digestive System

The functions of the digestive system are ingestion, motility, secretion, digestion, absorption, and elimination.

3 Histology of the Digestive Tract

Digestive Tract Histology

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

Mucosa

1. The mucosa consists of a mucous epithelium, a lamina propria, and a muscularis mucosae.
2. The epithelium extends into the lamina propria to form intestinal glands.

Submucosa

The submucosa is a connective tissue layer containing the submucosal plexus, blood vessels, and small lands.

Muscularis

1. The muscularis consists of an inner layer of circular smooth muscle and an outer layer of longitudinal smooth muscle.
2. The myenteric plexus is between the two muscle layers.
3. Interstitial pacemaker cells are located throughout the myenteric plexus.

Serosa or Adventitia

The serosa or adventitia forms the outermost layer of the digestive tract.

4 Regulation of the Digestive System

Nervous, hormonal, and local chemical mechanisms regulate digestion.

Nervous Regulation of the Digestive System

Nervous regulation involves the ENS and CNS reflexes.

Chemical Regulation of the Digestive System

1. Over 30 neurotransmitters are associated with ENS.
2. The digestive tract produces hormones that regulate digestion.
3. Other chemicals produced by the digestive tract exercise local control of digestion.

5 Peritoneum

1. The peritoneum is a serous membrane that lines the abdominal cavity and organs.
2. Mesenteries are peritoneum that extends from the body wall to many of the abdominal organs.
3. Retroperitoneal organs are located behind the peritoneum.

6 Oral Cavity

Oral Cavity

The oral cavity includes the vestibule and the oral cavity proper.

Lips, Cheeks, and Palate

1. The lips and cheeks are involved in facial expression, mastication, and speech.
2. The roof of the oral cavity is divided into hard and soft palates.
3. The palatine tonsils are located in the lateral wall of the fauces.

Tongue

1. The tongue is involved in speech, taste, mastication, and swallowing.
2. The intrinsic tongue muscles change the shape of the tongue, and the extrinsic tongue muscles move the tongue.
3. The anterior two-thirds of the tongue is covered with papillae; the posterior one-third is devoid of papillae.

Teeth

Molar

1. Teeth are responsible for mastication. They are collectively known as dentition.
2. Twenty deciduous teeth are replaced by 32 permanent teeth.
3. The types of teeth are incisors, canines, premolars, and molars.
4. A tooth consists of a crown, a neck, and a root.
5. The root is composed of dentin. Within the dentin of the root, tooth is the pulp cavity, which is the center of the tooth. The pulp cavity is filled with pulp, blood vessels, and nerves. The crown is dentin covered by enamel.
6. Periodontal ligaments hold the teeth in the alveoli.

Mastication

The muscles of mastication are the temporalis, masseter, medial pterygoid, and lateral pterygoid.

Salivary Glands

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

7 Swallowing

Swallowing involves the pharynx and esophagus. It is divided into three phases.

Pharynx

The pharynx consists of three parts: the nasopharynx, oropharynx, and laryngopharynx.

Esophagus

1. The esophagus connects the pharynx to the stomach. The upper and lower esophageal sphincters regulate movement.
2. The esophagus consists of an outer adventitia, a muscular layer (longitudinal and circular), a submucosal layer (with mucous glands), and a stratified squamous epithelium.

Swallowing Phases

1. During the voluntary phase of swallowing, a bolus of food is moved by the tongue from the oral cavity to the pharynx.
2. The pharyngeal phase is a reflex caused by the stimulation of stretch receptors in the pharynx.
• The soft palate closes the nasopharynx, and the epiglottis and vestibular folds close the opening into the larynx.
• Pharyngeal muscles move the bolus to the esophagus.
3. The esophageal phase is a reflex initiated by the stimulation of stretch receptors in the esophagus. Peristalsis(or peristaltic wave is a wave of contraction) moves the food to the stomach.

8 Stomach

Anatomy of the Stomach

The openings of the stomach are gastroesophageal (to the esophagus) and pyloric (to the duodenum).

Anatomy and Histology of Stomach

Histology of the Stomach

1. The wall of the stomach consists of an external serosa, a muscle layer (longitudinal, circular, and oblique), a submucosa, and simple columnar epithelium (surface mucous cells).
2. Rugae are the folds in the stomach when it is empty.
3. Gastric pits are openings to the gastric glands, which contain mucous neck cells, parietal cells, chief cells, and endocrine cells.

Secretions of the Stomach

1. The mucus protects the stomach lining.
2. Pepsinogen is converted to pepsin, which digests proteins.
3. Hydrochloric acid promotes pepsin activity and kills microorganisms.
4. Intrinsic factor is necessary for vitamin B12 absorption in the small intestine.
5. The sight, smell, taste, or thought of food initiates the cephalic phase. Nerve impulses from the medulla oblongata stimulate gastric secretion (hydrochloric acid, pepsinogen, gastrin, and histamine secretion.)
6. Distension of the stomach, which stimulates gastrin secretion and activates CNS and local reflexes that promote secretion, initiates the gastric phase.
7. Acidic chyme, which enters the duodenum and stimulates neuronal reflexes and the secretion of hormones that inhibit gastric secretions, initiates the intestinal phase.

Movements of the Stomach

1. The stomach stretches and relaxes to increase volume.
2. Mixing waves mix the stomach contents with stomach secretions to form chyme.
3. Peristaltic waves move the chyme into the duodenum.
4. Gastrin and stretching of the stomach stimulate stomach emptying.
5. Chyme entering the duodenum inhibits movement through neuronal reflexes and the release of hormones.

9 Small Intestine

Small Intestine

The small intestine is divided into three parts: the duodenum, jejunum, and ileum.

Anatomy and Histology of the Small Intestine

1. Circular folds, villi, and microvilli greatly increase the surface area of the intestinal lining.
2. Absorptive, goblet and endocrine cells are in intestinal glands. Duodenal glands produce mucus.

Secretions of the Small Intestine

1. The mucus protects against digestive enzymes and stomach acids.
2. Digestive enzymes (disaccharidases and peptidases) are bound to the intestinal wall.
3. The vagus nerve, secretin, and chemical or tactile irritation stimulate intestinal secretion.

Movement in the Small Intestine

Anatomy and Histology of Duodenum

1. Segmental contractions mix intestinal contents. Peristaltic contractions move materials distally.
2. Stretch of smooth muscles, local reflexes, and the parasympathetic nervous system stimulate contractions. Distension of the cecum initiates a reflex that inhibits peristalsis.

10 Liver

Anatomy of the Liver

1. The liver has four lobes: right, left, caudate, and quadrate.
2. The liver is divided into lobules.
• The hepatic cords are composed of columns of hepatocytes separated by the bile canaliculi.
• The sinusoids are enlarged spaces filled with blood and lined with endothelium and hepatic phagocytic cells.

Histology of the Liver

1. The portal triads supply the lobules.

• The hepatic arteries and the hepatic portal veins take blood to the lobules and empty into the sinusoids.
• The sinusoids empty into central veins, which join to form the hepatic veins, which leave the liver.
• Bile canaliculi converge to form hepatic ducts, which leave the liver.

1. Bile leaves the liver through the hepatic duct system.

• The hepatic ducts receive bile from the lobules.
• The cystic duct from the gallbladder joins the hepatic duct to form the common bile duct.
• The common bile duct joins the pancreatic duct at the point at which it empties into the duodenum.

Functions of the Liver

1. The liver produces bile, which contains bile salts that emulsify lipids.
2. The liver stores and processes nutrients detoxify harmful chemicals and synthesize new molecules.
3. Hepatic phagocytic cells phagocytize red blood cells, bacteria, and other debris.
4. The liver produces blood proteins.

11 Gallbladder

1. The gallbladder is a small sac on the inferior surface of the liver.
2. The gallbladder stores and concentrates bile.
3. Cholecystokinin stimulates gallbladder contraction.

Pancreas and Gallbladder

12 Pancreas

Anatomy of the Pancreas

1. The pancreas is both an endocrine and an exocrine gland. Its exocrine function is the production of digestive enzymes.
2. The pancreas is divided into lobules that contain acini. The acini connect to a duct system that eventually forms the pancreatic duct, which empties into the duodenum.

Pancreatic Secretions

1. Digestive enzymes, including inactive proteolytic enzymes that are activated in the small intestine
2. A watery bicarbonate solution that neutralizes acidic chyme

Regulation of Pancreatic Secretion

Cholecystokinin and the vagus nerve stimulate the release of digestive enzymes. Secretin stimulates the release of bicarbonate ions and water.

13 Large Intestine

Anatomy of the Large Intestine

1. The cecum forms a blind sac at the junction of the small and large intestines. The vermiform appendix is a blind tube off the cecum. The vermiform appendix is a blind tube off the cecum.
2. The ascending colon extends from the cecum superiorly to the right colic flexure. The transverse colon extends from the right to the left colic flexure. The descending colon extends inferiorly to join the sigmoid colon.
3. The sigmoid colon is an S-shaped tube that ends at the rectum.
4. Longitudinal smooth muscles of the large intestine wall are arranged into bands, called teniae coli, that contract to produce pouches called haustra.
5. The mucosal lining of the large intestine is simple columnar epithelium with mucus-producing crypts.
6. The rectum is a straight tube that ends at the anus.
7. An internal anal sphincter (smooth muscle) and an external anal sphincter (skeletal muscle) surround the anal canal

Large Intestine

Secretions of the Large Intestine

1. The mucus protects the intestinal lining.
2. Epithelial cells secrete HCO₃
− Sodium is absorbed by active transport, and water is absorbed by osmosis.
3. Microorganisms are responsible for vitamin K production, gas production, and much of the bulk of feces.

Movement in the Large Intestine

1. Segmental movements mix the colon’s contents.
2. Mass movements are strong peristaltic contractions that occur three or four times a day.

Defecation is the elimination of feces. Reflex activity moves feces to get through the internal anal sphincter. Voluntary activity regulates the movement of feces or indigestible food through the external anal sphincter.

14 Digestion and Absorption

1. Digestion is the breakdown of organic molecules into their components.
2. Absorption is the means by which molecules are moved out of the digestive tract and distributed throughout the body.
3. Transport from the intestinal epithelium occurs by two routes.
• Water, ions, and water-soluble products of digestion are transported to the liver through the hepatic portal system.
• The products of lipid digestion are transported through the lymphatic system to the circulatory system.

Carbohydrates

1. Carbohydrates consist of starches, glycogen, sucrose, lactose, glucose, and fructose.
2. Polysaccharides are broken down into monosaccharides by a number of different enzymes.
3. Monosaccharides are taken up by intestinal epithelial cells by symport that is powered by a Na+ gradient by facilitated diffusion.
4. The monosaccharides are carried to the liver, where the nonglucose sugars are converted to glucose.
5. Glucose is transported to the cells that require energy.
6. Glucose enters the cells through facilitated diffusion.
7. Insulin influences the rate of glucose transport.

Lipids

1. Lipids include triglycerides, phospholipids, steroids, and fat-soluble vitamins.
2. Lipase digests lipid molecules to form free fatty acids and monoglycerides.
3. Emulsification, the transformation of large lipid droplets into smaller droplets, is accomplished by bile salts.
4. Micelles form around lipid digestion products and move to epithelial cells of the small intestine, where the products pass into the cells by simple diffusion.
5. Within the epithelial cells, free fatty acids are combined with a monoglyceride to form triglycerides.
6. Proteins coat triglycerides, phospholipids, and cholesterol to form chylomicrons.
7. Chylomicrons enter lacteals within intestinal villi and are carried through the lymphatic system to the bloodstream.
8. Triglycerides are stored in adipose tissue, converted into other molecules, or used as energy.
9. Lipoproteins include chylomicrons, VLDL, LDL, and HDL.
10. LDL transports cholesterol to cells, and HDL transports it from cells to the liver.
11. LDLs are taken into cells by receptor-mediated endocytosis, which is controlled by a negative-feedback mechanism.

Proteins

1. Pepsin in the stomach breaks proteins into polypeptide chains.
2. Trypsin and other proteolytic enzymes from the pancreas produce smaller peptides.
3. Peptidases, bound to the microvilli of the small intestine, break down peptides
4. Amino acids, dipeptides, and tripeptides are absorbed by symport that is powered by Na+ or H+ gradients or by facilitated diffusion.
5. Amino acids are transported to the liver, where the amino acids can be modified or released into the bloodstream.
6. Amino acids are actively transported into cells under the stimulation of growth hormones and insulin.
7. Amino acids are used as building blocks or for energy.

Water

Water moves in either direction across the wall of the small intestine, depending on the osmotic gradients across the epithelium.

Ions

1. Sodium, potassium, calcium, magnesium, and phosphate are actively transported.
2. Chloride ions move passively through the wall of the duodenum and jejunum but are actively transported from the ileum.
3. Calcium ions are actively transported, but vitamin D is required for transport, and the transport is under hormonal control.

15 Effects of Aging on the Digestive System

The mucous layer, the connective tissue, the muscles, and the secretions of the digestive tract all tend to decrease as a person ages. These changes make an older person more open to infections and toxic agents.