Digestive System — revision sheet

The digestive system is a continuous muscular tube, the alimentary canal, running from the mouth to the anus, together with the accessory organs that empty into it: teeth, tongue, salivary glands, liver, gallbladder and pancreas. It turns food into molecules small enough to absorb, takes up nutrients, water and electrolytes, and gets rid of indigestible residue. It is also the body's largest endocrine organ and its largest immune interface.

Length of alimentary canal: ≈5 m in life; ≈7–9 m at autopsyFluid entering the gut lumen: ≈9 L/day (≈2 L ingested + ≈7 L secreted)Water lost in faeces: ≈100–200 mL/daySmall-intestinal mucosal surface: ≈30 m² (modern estimate; older texts ≈200 m²)Stomach capacity: ≈50 mL empty; ≈1–1.5 L after a meal; up to ≈4 LGastric pH: ≈1.5–3.5Liver mass and blood flow: ≈1.4–1.5 kg; ≈1.5 L/min (≈25% of cardiac output)Daily secretions: Saliva ≈1–1.5 L · gastric juice ≈1.5–2 L · bile ≈0.6–1 L · pancreatic juice ≈1–1.5 LTransit times: Stomach 2–4 h · small intestine 3–5 h · colon 12–48 hDentition: 20 deciduous teeth · 32 permanent teethEnteric neurons: ≈200–600 million (more than in the spinal cord)

Oral cavity & teeth

Oral cavity Region: The oral cavity is the first part of the digestive tract, running from the lips to the oropharyngeal isthmus. Food is taken in, tasted, chewed and mixed with saliva here before it is swallowed.
Tongue Organ: The tongue is a mobile mass of skeletal muscle covered by specialised mucosa. It moves food during chewing and swallowing, carries the taste buds and shapes speech sounds.
Hard palate Organ part: The hard palate is the bony anterior two-thirds of the roof of the mouth. It separates the oral and nasal cavities and gives the tongue a rigid surface to press food against.
Soft palate Organ part: The soft palate is a mobile fibromuscular flap hanging from the back of the hard palate and ending in the uvula. During swallowing and speech it rises to seal the nasopharynx off from the oropharynx.
Tooth Organ: Teeth are calcified organs held in the alveolar sockets of the jaws that cut, tear and grind food. Humans have two sets: 20 deciduous (primary) teeth and 32 permanent teeth.
Enamel Tissue: Enamel is the hardest substance in the body. It is an acellular, about 96% mineral coat of hydroxyapatite covering the crown of each tooth.
Dentin Tissue: Dentin is the yellowish, bone-like calcified tissue that forms most of the tooth. It is penetrated by microscopic tubules containing the processes of odontoblasts.
Dental pulp Tissue: The dental pulp is the soft connective-tissue core of the tooth. It contains odontoblasts, vessels and nerves and fills the pulp chamber and root canals.
Cementum Tissue: Cementum is a thin, bone-like calcified layer covering the root dentin. Periodontal ligament fibres are embedded in it, anchoring the tooth.
Periodontal ligament Ligament: The periodontal ligament is a thin layer of dense fibrous connective tissue that suspends each tooth in its bony socket. It forms the gomphosis joint and acts as a shock absorber and pressure sensor.
Gingiva Tissue: The gingiva is the firm, keratinised masticatory mucosa that covers the alveolar processes and surrounds the necks of the teeth. It seals the tooth–gum junction against bacteria.

Salivary glands

Salivary glands Gland: Three pairs of major salivary glands (parotid, submandibular and sublingual) and hundreds of minor glands secrete about 1–1.5 L of saliva a day into the mouth. The autonomic nervous system controls them almost entirely.
Parotid gland Gland: The parotid is the largest salivary gland and is purely serous. It lies in front of the ear and wraps around the ramus of the mandible. The facial nerve, the retromandibular vein and the external carotid artery all pass through it.
Submandibular gland Gland: The submandibular gland is a walnut-sized mixed (mainly serous) gland in the submandibular triangle. It produces about two-thirds of unstimulated saliva.
Sublingual gland Gland: The sublingual gland is the smallest major salivary gland and is mainly mucous. It lies under the mucosa of the floor of the mouth and drains through many small ducts.
Saliva Fluid: Saliva is a hypotonic, slightly acidic to neutral fluid secreted at about 1–1.5 L a day. It lubricates food, starts starch digestion and protects the mouth and teeth.

Oesophagus

Oesophagus Organ: The oesophagus is a muscular tube about 25 cm long that carries food from the pharynx to the stomach. It runs through the neck, the posterior mediastinum and the oesophageal hiatus of the diaphragm.
Lower oesophageal sphincter Organ part: The lower oesophageal sphincter is a 3–4 cm high-pressure zone at the gastro-oesophageal junction. It stays tonically closed to prevent reflux and relaxes when a swallow arrives.

Digestive physiology

Deglutition (swallowing) Physiological process: Deglutition moves a bolus from the mouth to the stomach. It has a voluntary oral phase followed by reflex pharyngeal and oesophageal phases, coordinated by a swallowing centre in the medulla.
Gastric acid secretion and its phases Physiological process: Acid secretion is driven by three stimulants that reinforce each other: acetylcholine, gastrin and histamine. It is braked by somatostatin, and it runs in cephalic, gastric and intestinal phases.
Gastrointestinal motility Physiological process: GI motility is the coordinated contraction of gut smooth muscle that mixes and propels contents. It includes peristalsis, segmentation, the migrating motor complex and colonic mass movements, and it is paced by interstitial cells of Cajal and run by the enteric nervous system.
Carbohydrate digestion and absorption Physiological process: Starch and disaccharides are broken down by salivary and pancreatic amylase and then by brush-border enzymes into glucose, galactose and fructose. These are absorbed by SGLT1 and GLUT5 and leave the enterocyte by GLUT2 into the portal blood.
Protein digestion and absorption Physiological process: Proteins are denatured by gastric acid and hydrolysed in turn by pepsin, then by pancreatic proteases activated through enteropeptidase and trypsin, and finally by brush-border peptidases. The products are absorbed as amino acids and di- and tripeptides.
Lipid digestion and absorption Physiological process: Fat is emulsified by bile salts, hydrolysed by pancreatic lipase and colipase, and carried to the enterocyte in micelles. Inside, it is re-esterified and packaged into chylomicrons, which enter the lacteals and reach the blood through the thoracic duct.
Absorption of vitamins, iron and calcium Physiological process: Micronutrients are absorbed by dedicated mechanisms at specific sites. Vitamin B12 is absorbed bound to intrinsic factor in the terminal ileum. Iron and active calcium uptake occur in the duodenum. Folate is absorbed in the jejunum, and the fat-soluble vitamins travel with micelles.
Water and electrolyte transport Physiological process: About 9 L of fluid enters the gut lumen each day, and only 100–200 mL leaves in the faeces. Water is absorbed passively, following Na⁺ and other solutes, through several segment-specific transport mechanisms. Crypts secrete Cl⁻ and fluid in the opposite direction.
Enterohepatic circulation of bile acids Physiological process: Bile acids secreted in bile are about 95% reabsorbed in the terminal ileum and returned to the liver in the portal blood. A pool of only about 3 g therefore recycles several times a day.
Defecation and continence Physiological process: Defecation is a spinal reflex triggered by rectal distension, which relaxes the internal anal sphincter and contracts the rectum. Voluntary control of the external sphincter and puborectalis decides whether it goes ahead or is postponed.

Stomach

Stomach Organ: The stomach is the J-shaped, most dilated part of the alimentary canal, between the oesophagus and the duodenum. It stores the meal, churns it with acid and pepsin into chyme, and releases it to the duodenum at a controlled rate.
Gastric glands Gland: Gastric glands are simple branched tubular glands that open into the gastric pits. Their cell types secrete acid, intrinsic factor, pepsinogen, mucus and regulatory peptides.
Parietal cell Cell: Parietal cells are large, eosinophilic gland cells in the gastric body and fundus. They secrete hydrochloric acid using the H⁺/K⁺-ATPase proton pump, and they also secrete intrinsic factor.
Gastric chief cell Cell: Chief cells are basophilic, protein-secreting cells at the base of the oxyntic glands. They release pepsinogen and gastric lipase.
Gastric juice Fluid: Gastric juice is the strongly acidic secretion of the gastric glands, about 1.5–2 L a day. It contains HCl, pepsinogen, gastric lipase, mucus and intrinsic factor.

Small intestine

Small intestine Organ: The small intestine is the longest part of the gut, running from the pylorus to the ileocaecal junction. Most chemical digestion and almost all nutrient absorption happen here, across a mucosa enlarged by folds, villi and microvilli.
Duodenum Organ part: The duodenum is the first, shortest and most fixed part of the small intestine. It is a C-shaped tube about 25 cm long that wraps around the head of the pancreas and receives bile and pancreatic juice at the major duodenal papilla.
Jejunum Organ part: The jejunum is the proximal two-fifths of the mesenteric small intestine. It is wide, thick-walled and very vascular, and it is the main site of nutrient absorption.
Ileum Organ part: The ileum is the distal three-fifths of the mesenteric small intestine, ending at the ileocaecal valve. It specialises in absorbing vitamin B12 and bile salts and is rich in Peyer patches.
Plicae circulares Organ part: Plicae circulares are permanent, crescent-shaped folds of mucosa and submucosa in the small intestine. They roughly triple the absorptive surface and slow the passage of chyme.
Intestinal villi Organ part: Villi are finger- or leaf-shaped projections of the small-intestinal mucosa, about 0.5–1 mm tall. Each holds a capillary loop and a central lacteal under a brush-bordered epithelium, and together they form the absorptive surface of the gut.
Intestinal crypts Gland: Intestinal crypts (of Lieberkühn) are simple tubular glands that dip from the mucosal surface down to the muscularis mucosae. They hold the stem cells that renew the epithelium, and they secrete fluid, mucus and antimicrobial proteins.
Enterocyte Cell: Enterocytes are the tall columnar absorptive cells of the intestinal epithelium. Their microvillous brush border carries digestive enzymes and nutrient transporters.
Paneth cell Cell: Paneth cells are secretory cells at the base of the small-intestinal crypts, packed with large eosinophilic granules. They release antimicrobial peptides and support the intestinal stem-cell niche.
Central lacteal Lymphatic vessel: The central lacteal is a blind-ended lymphatic capillary in the core of each intestinal villus. It takes up chylomicrons and other large particles that cannot enter blood capillaries.
Ileocaecal valve Organ part: The ileocaecal valve is the junction where the terminal ileum projects into the caecum. A thickened circular muscle forms a sphincter that controls ileal emptying and limits backflow of colonic contents.

Large intestine

Large intestine Organ: The large intestine is the final segment of the gut, about 1.5 m long, running from the ileocaecal junction to the anus. It absorbs water and electrolytes, hosts the fermenting microbiota, and stores and expels faeces.
Caecum Organ part: The caecum is the blind-ended pouch at the start of the large intestine, lying in the right iliac fossa below the ileocaecal junction. The appendix arises from it.
Vermiform appendix Organ part: The vermiform appendix is a narrow, blind, worm-like tube about 6–10 cm long arising from the caecum. Its wall is packed with lymphoid tissue, and acute appendicitis is the commonest abdominal surgical emergency.
Ascending colon Organ part: The ascending colon runs up the right flank from the caecum to the underside of the liver, where it turns left at the right colic (hepatic) flexure. It is secondarily retroperitoneal.
Transverse colon Organ part: The transverse colon is the longest and most mobile part of the large intestine. It hangs on the transverse mesocolon between the hepatic and splenic flexures.
Descending colon Organ part: The descending colon runs down the left flank from the splenic flexure to the left iliac fossa, where it becomes the sigmoid colon. It is secondarily retroperitoneal and part of the hindgut.
Sigmoid colon Organ part: The sigmoid colon is a mobile, S-shaped loop of variable length (about 40 cm) that links the descending colon to the rectum at the level of S3. It hangs on the sigmoid mesocolon.
Rectum Organ part: The rectum is the 12–15 cm pelvic segment of the large intestine between the sigmoid colon (at S3) and the anal canal. It follows the curve of the sacrum and stores faeces until defecation.
Anal canal Organ part: The anal canal is the terminal 3–4 cm of the gut, running from the anorectal junction to the anus. It is surrounded by the internal and external anal sphincters and divided by the pectinate line into embryologically distinct upper and lower parts.
Pectinate line Region: The pectinate line is the scalloped line formed by the anal valves about halfway down the anal canal. It roughly marks the junction of endodermal hindgut and ectodermal proctodeum, so epithelium, blood supply, lymph drainage and nerve supply all change across it.
Internal anal sphincter Muscle: The internal anal sphincter is the involuntary smooth-muscle thickening of the inner circular layer around the upper two-thirds of the anal canal. Its constant tone provides most of the resting anal pressure.N Sympathetic (L1–L2, via the hypogastric plexuses) contracts it. Parasympathetic (S2–S4, pelvic splanchnic nerves) and enteric inhibitory…
Taeniae coli Organ part: The taeniae coli are three narrow longitudinal bands of smooth muscle on the caecum and colon. Because they are shorter than the bowel wall, they gather it into sacculations called haustra.

Liver

Liver Organ: The liver is the largest gland and largest internal organ, weighing about 1.5 kg. It has a dual blood supply from the hepatic portal vein and the hepatic artery, and it processes everything absorbed from the gut. It makes bile and plasma proteins, controls fuel metabolism and detoxifies drugs and waste.
Liver lobule Tissue: The classic liver lobule is a hexagonal prism of hepatocyte plates and sinusoids, about 1–2 mm across. It is centred on a central vein, with portal triads at its corners. Blood flows inward to the central vein and bile flows outward to the portal triads.
Portal triad Region: A portal triad is a connective-tissue tract at the corner of a liver lobule containing a branch of the hepatic portal vein, a branch of the hepatic artery and an interlobular bile duct. Lymphatics and nerves accompany them.
Hepatocyte Cell: Hepatocytes are the large polyhedral parenchymal cells of the liver, making up about 80% of its volume. They carry out nearly all of its metabolic, synthetic, detoxifying and bile-secreting work.
Kupffer cell Cell: Kupffer cells are the resident macrophages of the liver sinusoids. They are the largest fixed population of tissue macrophages in the body and act as a filter for portal blood.
Hepatic stellate cell Cell: Hepatic stellate (Ito) cells are pericyte-like cells in the space of Disse that store vitamin A. When the liver is injured they become collagen-producing myofibroblasts and drive fibrosis.

Gallbladder & biliary tree

Bile canaliculi Duct: Bile canaliculi are the smallest bile channels, about 1 µm wide. They are formed by grooves in the membranes of neighbouring hepatocytes and sealed by tight junctions, and they are where the biliary tree begins.
Gallbladder Organ: The gallbladder is a pear-shaped sac about 7–10 cm long that sits in a fossa on the underside of the liver. It stores and concentrates bile between meals and contracts in response to CCK to deliver it to the duodenum.
Common hepatic duct Duct: The common hepatic duct is formed at the porta hepatis by the union of the right and left hepatic ducts. It runs down about 3–4 cm in the hepatoduodenal ligament until the cystic duct joins it to form the bile duct.
Cystic duct Duct: The cystic duct is a short, tortuous duct (about 3–4 cm) that connects the neck of the gallbladder to the common hepatic duct. Its mucosa forms a spiral fold.
Common bile duct Duct: The common bile duct is formed by the union of the cystic and common hepatic ducts. It runs about 6–8 cm behind the duodenum and through the head of the pancreas, then joins the pancreatic duct to open at the major duodenal papilla.
Hepatopancreatic ampulla and sphincter of Oddi Duct: The hepatopancreatic ampulla is the short dilated common channel formed when the bile duct and main pancreatic duct unite in the duodenal wall. It opens on the major duodenal papilla, and the smooth-muscle sphincter of Oddi controls its outflow.
Bile Fluid: Bile is the alkaline, yellow-green secretion of the liver, about 0.6–1 L a day. It contains bile salts, phospholipid, cholesterol and conjugated bilirubin. It emulsifies fats and is the main route for excreting cholesterol and bilirubin.

Pancreas

Pancreas Gland: The pancreas is an elongated mixed gland lying across the posterior abdominal wall behind the stomach. About 98–99% of it is exocrine and secretes about 1–1.5 L of alkaline, enzyme-rich juice a day. The remaining 1–2% is the endocrine islets.
Pancreatic ducts Duct: The main pancreatic duct runs the length of the gland and joins the bile duct at the hepatopancreatic ampulla. The accessory duct drains the upper head into the minor duodenal papilla.
Pancreatic acini Gland: Pancreatic acini are berry-like clusters of serous acinar cells that make and secrete the digestive enzymes of pancreatic juice. Most of these enzymes are released as inactive zymogens.
Pancreatic juice Fluid: Pancreatic juice is the alkaline exocrine secretion of the pancreas, about 1–1.5 L a day at pH ≈8. It contains the enzymes that digest all major nutrients and enough bicarbonate to neutralise gastric acid.

GI wall

Wall of the alimentary canal Tissue: From the oesophagus to the anal canal the gut wall has the same four concentric layers: mucosa, submucosa, muscularis externa and serosa or adventitia. Two enteric nerve plexuses lie within it.
Mucosa (GI tract) Tissue: The mucosa is the innermost layer of the gut wall. It consists of an epithelium, a connective-tissue lamina propria and a thin muscularis mucosae, and it is the site of secretion, absorption and barrier defence.
Muscularis mucosae Tissue: The muscularis mucosae is a thin layer of smooth muscle at the deep border of the mucosa, separating it from the submucosa. It produces small local movements of the mucosa.
Submucosa (GI tract) Tissue: The submucosa is a layer of dense irregular connective tissue binding the mucosa to the muscularis externa. It carries the larger blood vessels, lymphatics and the submucosal (Meissner) nerve plexus.
Muscularis externa Tissue: The muscularis externa is the main muscle coat of the gut. An inner circular and an outer longitudinal layer of smooth muscle, with the myenteric plexus between them, mix and propel the contents.
Serosa and adventitia (GI tract) Membrane: The outermost layer of the gut is a serosa (visceral peritoneum) where the bowel lies free in the peritoneal cavity, or a fibrous adventitia where it is fixed to surrounding structures.

Peritoneum & blood supply

Peritoneum Membrane: The peritoneum is the largest serous membrane in the body, with an area of about 1.8 m². It lines the abdominopelvic walls as parietal peritoneum and covers the viscera as visceral peritoneum. Between the two is the potential peritoneal cavity, which holds a thin film of fluid.
Greater omentum Membrane: The greater omentum is a large, fatty, four-layered fold of peritoneum that hangs like an apron from the greater curvature of the stomach over the intestines, then folds back up to attach to the transverse colon.
Lesser omentum Membrane: The lesser omentum is a double layer of peritoneum joining the lesser curvature of the stomach and the first part of the duodenum to the liver. Its thickened free right edge carries the portal triad.
Mesentery Membrane: The mesentery is the fan-shaped double fold of peritoneum that suspends the jejunum and ileum from the posterior abdominal wall. It carries their blood vessels, lymphatics, lymph nodes and nerves.
Omental bursa (lesser sac) Space: The omental bursa (lesser sac) is the part of the peritoneal cavity behind the stomach and lesser omentum. It communicates with the greater sac only through the omental (epiploic) foramen.
Retroperitoneal space Space: The retroperitoneum is the space between the posterior parietal peritoneum and the posterior abdominal wall. It holds the great vessels, kidneys and several digestive organs that are fixed behind the peritoneum.
Arterial supply of the gut (foregut, midgut, hindgut) Vessel: Three unpaired branches of the abdominal aorta each supply one embryological region of the gut: the coeliac trunk (foregut), the superior mesenteric artery (midgut) and the inferior mesenteric artery (hindgut). Venous blood returns through the hepatic portal vein.

Mnemonics

Retroperitoneal organs: SAD PUCKER: Suprarenal (adrenal) glands · Aorta and IVC · Duodenum (2nd–4th parts) · Pancreas (except the tail) · Ureters · Colon (ascending and descending) · Kidneys · Esophagus (thoracic and most of the abdominal part) · Rectum (middle and lower thirds).
Meckel diverticulum: rule of 2s: Found in about 2% of people · about 2 feet (60 cm) proximal to the ileocaecal valve · about 2 inches (5 cm) long · 2 types of ectopic mucosa (gastric and pancreatic) · usually presents before age 2 · about 2:1 male predominance. It is a true diverticulum on the antimesenteric border.
Causes of acute pancreatitis: I GET SMASHED: Idiopathic · Gallstones · Ethanol · Trauma · Steroids · Mumps (and other viruses) · Autoimmune · Scorpion venom · Hypercalcaemia / Hypertriglyceridaemia · ERCP · Drugs (e.g. azathioprine, valproate, thiazides).
Gallstone risk factors: the 5 Fs: Female · Fat (obese) · Fertile (pregnancy, oestrogens) · Forty (age over 40) · Fair. It is a crude but memorable summary of the risk factors for cholesterol stones.
Facial nerve branches in the parotid gland: "To Zanzibar By Motor Car": Temporal · Zygomatic · Buccal · Marginal mandibular · Cervical.
Dental formula (per quadrant): Permanent: 2-1-2-3 (incisors, canine, premolars, molars) × 4 = 32. Deciduous: 2-1-0-2 × 4 = 20, with no premolars in the deciduous set.
Unpaired gut arteries: 12-1-3: Coeliac trunk at T12 (foregut) · superior mesenteric artery at L1 (midgut) · inferior mesenteric artery at L3 (hindgut). Each supplies one embryological gut segment and matches a site of referred pain: epigastric, umbilical and hypogastric.
Inguinal hernias: MDs don't LIe: Medial to the inferior epigastric vessels = Direct; Lateral = Indirect.

From the Corpus anatomy atlas — for education only.