Digestive System Anatomy and Physiology

You manage this system constantly: NPO status, tube feeds, bowel sounds, ostomy output, swallowing precautions, B12 deficiency. Knowing how food moves and breaks down from mouth to anus tells you what a symptom or a missing function actually means at the bedside.

Functions of the Digestive System

1. Ingestion. Placing food in the mouth. An active, voluntary process.
2. Propulsion. Moving food from one organ to the next. Swallowing starts it; peristalsis (involuntary alternating waves of contraction and relaxation in the organ wall) keeps it going.
3. Mechanical digestion. Physically fragmenting food into smaller pieces: the tongue mixing food, the stomach churning, segmentation in the small intestine.
4. Chemical digestion. Enzymes breaking large food molecules down into their building blocks.
5. Absorption. Moving digested end products from the GI lumen into blood or lymph. Foods must first cross the mucosal cells by active or passive transport.
6. Defecation. Eliminating indigestible residue through the anus as feces.

Anatomy of the Digestive System

The digestive organs split into two groups: those forming the alimentary canal, and the accessory organs.

Organs of the Alimentary Canal

The alimentary canal (gastrointestinal tract) is a continuous, hollow muscular tube that winds through the ventral body cavity, open at both ends.

Mouth

Food enters through the mouth (oral cavity), a mucous-membrane-lined cavity.

• Lips (labia). Protect the anterior opening.
• Cheeks. Form the lateral walls.
• Palate. The hard palate forms the anterior roof, the soft palate the posterior roof.
• Uvula. A fleshy fingerlike projection extending inferiorly from the soft palate.
• Vestibule. The space between the lips/cheeks and the teeth/gums.
• Oral cavity proper. The area enclosed by the teeth.
• Tongue. Fills the floor of the mouth; anchored to the hyoid bone and the styloid processes of the skull.
• Lingual frenulum. A mucous-membrane fold securing the tongue to the floor and limiting its posterior movement.
• Palatine tonsils. Paired masses of lymphatic tissue at the posterior oral cavity.
• Lingual tonsil. Covers the base of the tongue.

Pharynx

From the mouth, food passes posteriorly into the oropharynx (behind the oral cavity) and the laryngopharynx (continuous with the esophagus below). Both are common passageways for food, fluids, and air.

Esophagus

The esophagus (gullet) runs from the pharynx through the diaphragm to the stomach.

• Size and function. About 25 cm (10 inches) long. A passageway that conducts food to the stomach by peristalsis.
• Wall structure. From the esophagus to the large intestine, the canal walls share the same four tissue layers (tunics).
• Mucosa. Innermost moist membrane lining the lumen: surface epithelium, a little connective tissue (lamina propria), and a scant smooth muscle layer.
• Submucosa. Soft connective tissue beneath the mucosa, holding blood vessels, nerve endings, lymph nodules, and lymphatic vessels.
• Muscularis externa. Typically an inner circular and an outer longitudinal layer of smooth muscle.
• Serosa. Outermost layer, a single layer of serous-fluid-producing cells, the visceral peritoneum.
• Intrinsic nerve plexuses. The submucosal and myenteric nerve plexuses, networks that are part of the autonomic nervous system and regulate GI motility and secretion.

Stomach

• Location. The C-shaped stomach sits on the left side of the abdominal cavity, mostly hidden by the liver and diaphragm.
• Function. A temporary storage tank and a site for food breakdown.
• Cardiac region. Surrounds the cardioesophageal sphincter, where food enters from the esophagus.
• Fundus. The expanded part lateral to the cardiac region.
• Body. The midportion; narrows inferiorly into the pyloric antrum, then the funnel-shaped pylorus.
• Pylorus. The terminal part, continuous with the small intestine through the pyloric sphincter (valve).
• Size. Varies from 15 to 25 cm in length; diameter and volume depend on contents. Full, it holds about 4 liters (1 gallon); empty, it collapses on itself.
• Rugae. Large folds the empty stomach mucosa throws into.
• Greater curvature. The convex lateral surface.
• Lesser curvature. The concave medial surface.
• Lesser omentum. A double layer of peritoneum from the liver to the lesser curvature.
• Greater omentum. A peritoneal extension that drapes over the abdominal organs like a lacy apron, riddled with fat that insulates, cushions, and protects.
• Stomach mucosa. Simple columnar epithelium of mucous cells producing a bicarbonate-rich alkaline mucus that shields the wall from acid and enzymes.
• Gastric glands. The lining is dotted with millions of gastric pits leading into gastric glands that secrete gastric juice.
• Intrinsic factor. Some stomach cells produce intrinsic factor, needed to absorb vitamin b12 in the small intestine.
• Chief cells. Produce protein-digesting enzymes, mostly pepsinogens.
• Parietal cells. Produce hydrochloric acid, which acidifies stomach contents and activates the enzymes.
• Enteroendocrine cells. Produce local hormones such as gastrin.
• Chyme. Processed food that resembles heavy cream.

Small Intestine

The body's major digestive organ.

• Location. A muscular tube from the pyloric sphincter to the large intestine.
• Size. The longest section of the alimentary tube, averaging 2.5 to 7 m (8 to 20 feet) in a living person.
• Subdivisions. Three: the duodenum, jejunum, and ileum, contributing 5 percent, nearly 40 percent, and almost 60 percent of its length, respectively.
• Ileocecal valve. Where the ileum meets the large intestine.
• Hepatopancreatic ampulla. The main pancreatic and bile ducts join at the duodenum to form this flasklike structure.
• Duodenal papilla. Bile and pancreatic juice travel through it into the duodenum together.
• Microvilli. Tiny projections of the mucosal cells' plasma membrane (the brush border). They carry brush border enzymes that finish digesting proteins and carbohydrates.
• Villi. Fingerlike mucosal projections giving a velvety surface.
• Lacteal. Each villus holds a rich capillary bed and a modified lymphatic capillary, the lacteal.
• Circular folds (plicae circulares). Deep folds of mucosa and submucosa that do not flatten when food fills the intestine.
• Peyer's patches. Collections of lymphatic tissue in the submucosa, increasing toward the end of the small intestine.

Large Intestine

Larger in diameter than the small intestine but shorter.

• Size. About 1.5 m (5 feet) long, from the ileocecal valve to the anus.
• Functions. Dry out indigestible residue by absorbing water, and eliminate it as feces.
• Subdivisions. Frames the small intestine on three sides: cecum, appendix, colon, rectum, and anal canal.
• Cecum. The saclike first part.
• Appendix. The wormlike projection off the cecum; a trouble spot where bacteria can accumulate and multiply.
• Ascending colon. Travels up the right abdomen, then turns at the right colic (hepatic) flexure.
• Transverse colon. Crosses the abdominal cavity.
• Descending colon. Turns at the left colic (splenic) flexure and runs down the left side.
• Sigmoid colon. Enters the pelvis as the S-shaped sigmoid.
• Anal canal. Ends at the anus, which opens to the exterior.
• External anal sphincter. Voluntary, skeletal muscle.
• Internal anal sphincter. Involuntary, smooth muscle.

Accessory Digestive Organs

Teeth

We masticate (chew) by opening and closing the jaws and moving them side to side while the tongue repositions food.

• Function. Tear and grind food into smaller fragments.
• Deciduous teeth. The first set (baby or milk teeth) begin erupting around 6 months; a baby has a full set (20 teeth) by age 2 years.
• Permanent teeth. As the deeper permanent teeth develop, the milk-tooth roots are reabsorbed; between ages 6 to 12 years they loosen and fall out.
• Incisors. Chisel-shaped, for cutting.
• Canines. Fanglike, for tearing and piercing.
• Premolars and molars. Broad crowns with rounded cusps, for grinding.
• Crown. The enamel-covered exposed part above the gingiva (gum).
• Enamel. The hardest substance in the body, brittle because it is heavily mineralized with calcium salts.
• Root. Covered by cementum, which anchors the tooth to the periodontal membrane (ligament).
• Dentin. A bonelike material under the enamel forming the bulk of the tooth.
• Pulp cavity. Surrounds the central pulp (connective tissue, blood vessels, nerve fibers).
• Root canal. Where the pulp cavity extends into the root, carrying vessels and nerves into the tooth.

Salivary Glands

Three pairs empty into the mouth.

• Parotid glands. Lie anterior to the ears.
• Submandibular and sublingual glands. Empty into the floor of the mouth through tiny ducts.
• Saliva. A mixture of mucus and serous fluids.
• Salivary amylase. The serous portion's enzyme, in a bicarbonate-rich juice, starts starch digestion in the mouth.

Pancreas

The only organ producing enzymes that break down all categories of digestible food.

• Location. A soft, pink triangular gland from the spleen to the duodenum. Most of it lies posterior to the parietal peritoneum, so it is retroperitoneal.
• Pancreatic enzymes. Secreted into the duodenum in an alkaline fluid that neutralizes acidic chyme from the stomach.
• Endocrine function. Also produces the hormones insulin and glucagon.

Liver

The largest gland in the body.

• Location. Under the diaphragm, more to the right; overlies and nearly covers the stomach.
• Falciform ligament. The liver has four lobes and hangs from the diaphragm and abdominal wall by this mesentery cord.
• Function. Its digestive role is producing bile.
• Bile. A yellow-to-green watery solution of bile salts, bile pigments, cholesterol, phospholipids, and electrolytes.
• Bile salts. Bile has no enzymes, but its salts emulsify fats, breaking large globules into smaller ones to give fat-digesting enzymes more surface area.

Gallbladder

Bile is concentrated here by removing water.

• Location. A small, thin-walled green sac in a shallow fossa on the inferior liver.
• Cystic duct. When digestion is not occurring, bile backs up the cystic duct into the gallbladder for storage.

Physiology of the Digestive System

The system ingests food, digests it physically and chemically into nutrient molecules, absorbs the nutrients into the blood, and defecates the indigestible remains.

Mouth, Pharynx, and Esophagus

Ingestion, breakdown, and propulsion happen here.

Ingestion and Breakdown

• Physical breakdown. Chewing breaks food into smaller particles.
• Chemical breakdown. Mixed with saliva, salivary amylase begins digesting starch into maltose.
• Saliva. Food in the mouth triggers large amounts of saliva; simple pressure from chewing also stimulates release.
• Passageways. The pharynx and esophagus have no digestive function. They carry food to the stomach.

Propulsion: Swallowing and Peristalsis

• Deglutition (swallowing). A complex process coordinating the tongue, soft palate, pharynx, and esophagus.
• Buccal phase. Voluntary, in the mouth. Once chewed and mixed with saliva, the bolus is forced into the pharynx by the tongue.
• Pharyngeal-esophageal phase. Involuntary; moves food through the pharynx and esophagus. The parasympathetic division controls it and drives motility from here on.
• Blocked routes. Every route except the path down the digestive tract is sealed: the tongue blocks the mouth, the soft palate closes the nasal passages, and the larynx rises so the epiglottis covers its opening.
• Stomach entrance. At the distal esophagus, food presses on the cardioesophageal sphincter, which opens to let food into the stomach.

Activities of the Stomach

Food Breakdown

The sight, smell, and taste of food trigger parasympathetic reflexes that increase gastric juice secretion.

• Gastric juice. Regulated by both neural and hormonal factors.
• Gastrin. Food and a rising pH stimulate stomach cells to release gastrin, which drives more pepsinogen, mucus, and hydrochloric acid.
• Pepsinogen. Hydrochloric acid creates the acidic environment that activates pepsinogen to pepsin, the active protein-digesting enzyme.
• Rennin. The stomach's second protein-digesting enzyme; works on milk protein, converting it to a sour-milk-like substance.
• Food entry. As food fills the stomach, the wall stretches while gastric juices are secreted.
• Wall activation. The three muscle layers compress and pummel the food, breaking it apart and mixing it with gastric juice to form semifluid chyme.

Food Propulsion

• Peristalsis. Once food is well mixed, rippling peristalsis begins in the upper stomach, contractions growing stronger toward the pyloric valve.
• Pyloric passage. The pylorus holds about 30 ml of chyme and meters its passage. Because the pyloric sphincter barely opens, each stomach contraction squirts 3 ml or less of chyme into the small intestine.
• Enterogastric reflex. When the duodenum fills and its wall stretches, this reflex puts the brakes on gastric activity, slowing stomach emptying by inhibiting the vagus nerves and tightening the pyloric sphincter so intestinal processing can catch up.

Activities of the Small Intestine

Breakdown and Absorption

• Digestion. Food arriving here is only partially digested: carbohydrate and protein digestion has started, but virtually no fat.
• Brush border enzymes. The microvilli carry enzymes that break double sugars into simple sugars and complete protein digestion.
• Pancreatic juice. Foods are deluged with enzyme-rich pancreatic juice from the pancreas plus bile from the liver. Pancreatic juice, with brush border enzymes, completes starch digestion, handles about half of protein digestion, and is fully responsible for fat and nucleic acid digestion.
• Hormone release. Chyme entering the small intestine stimulates the mucosa to produce hormones; secretin and cholecystokinin influence the release of pancreatic juice and bile.
• Absorption. Water and end products are absorbed all along the small intestine, most by active transport across the intestinal cell membranes.
• Diffusion. Lipids are absorbed passively by diffusion.
• Debris. At the end of the ileum, only water, indigestible material, and large amounts of bacteria remain; this passes through the ileocecal valve into the large intestine.

Propulsion

• Peristalsis. Moves food through the small intestine like toothpaste squeezed from a tube.
• Segmentation. Rhythmic segmental contractions produce local constrictions that mix chyme with digestive juices and help propel it.

Activities of the Large Intestine

Breakdown and Absorption

What reaches the large intestine holds few nutrients, but the residue still spends another 12 to 24 hours there.

• Metabolism. Resident bacteria metabolize remaining nutrients, releasing gases (methane and hydrogen sulfide) that contribute to fecal odor.
• Flatus. About 50 ml of gas is produced daily, much more with certain carbohydrate-rich foods.
• Absorption. Limited to vitamin K, some B vitamins, some ions, and most of the remaining water.
• Feces. The semisolid product delivered to the rectum: undigested residue, mucus, millions of bacteria, and just enough water for smooth passage.

Residue Propulsion and Defecation

• Haustral contractions. Slow segmenting movements lasting about one minute, occurring every 30 minutes or so.
• Propulsion. As a haustrum fills, distension stimulates its muscle to contract, pushing contents into the next haustrum.
• Mass movements. Long, slow, powerful waves that sweep large areas of the colon three or four times daily, forcing contents toward the rectum.
• Rectum. Usually empty. When mass movements force feces in and the wall stretches, the defecation reflex fires.
• Defecation reflex. A spinal (sacral) reflex that contracts the sigmoid colon and rectum walls and relaxes the anal sphincters.
• Voluntary control. As feces enter the anal canal, messages reach the brain, giving time to decide whether the external voluntary sphincter stays open or constricts.
• Reset. Within seconds the reflex contractions end and the rectal walls relax; the next mass movement starts the reflex again.

Age-Related Changes

Aging brings reduced saliva, decreased esophageal and stomach motility, decreased stomach emptying time, decreased intrinsic factor, and decreased intestinal absorption, motility, and blood flow. Tooth enamel becomes harder and more brittle, raising fracture risk.

Health-promotion teaching for older adults covers preventive dental care and oral hygiene, appropriate diet and adequate fluids, regular bowel maintenance, and colorectal cancer screening.