Anatomy & physiology · Study page
Digestive system diagram and organ guide
The path of food organ by organ with a simple labeled schematic, what each organ does, the accessory organs, an enzymes table, a label-the-diagram exercise and 20 practice questions with every answer explained. Free, and for study only.
- 15 organs in order
- Enzymes table
- 20 practice questions
- No sign-up needed
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Part 1 · The path of food
The organs in order, mouth to anal canal
Food travels one route: mouth, pharynx, esophagus, stomach, small intestine, large intestine. The accessory organs (salivary glands, liver, gallbladder, pancreas) add their juices to that route but food never passes through them. The schematic is a route map, not an anatomical drawing, seen from the front: the person’s right side is on your left.
Numbers: the path of food. Letters: accessory organs. S: the spleen, a neighbor that is not a digestive organ. Dashed: the pancreas, which lies behind the stomach, and the spleen beside it. The small intestine passes behind the transverse colon.
Mouth to esophagus
Mouth
Food enters (ingestion). Teeth and tongue break it up, and saliva starts digesting starch.
Pharynx
The throat. Swallowing moves the bolus past the airway into the esophagus.
Esophagus
A muscular tube that carries the bolus to the stomach by peristalsis.
Stomach
Stomach
Churns the bolus with acid and pepsin into chyme, then releases it slowly into the duodenum.
Small intestine
Duodenum
The first, shortest part of the small intestine. Bile and pancreatic juice enter here.
Jejunum
The middle part of the small intestine, built for absorption.
Ileum
The last and longest part of the small intestine, ending at the ileocecal sphincter.
Large intestine
Cecum
A pouch below the ileocecal valve: the first part of the large intestine.
Appendix
A thin tube of lymphoid tissue hanging from the cecum.
Ascending colon
Runs up the right side of the abdomen to the bend below the liver (right colic or hepatic flexure).
Transverse colon
Crosses the abdomen from right to left, below the stomach, to the bend near the spleen (left colic or splenic flexure).
Descending colon
Runs down the left side of the abdomen toward the pelvis.
Sigmoid colon
The S-shaped last part of the colon, turning toward the midline to join the rectum.
Rectum
The last stretch of the canal, where feces are held before defecation.
Anal canal
The exit, closed by two sphincters.
Accessory organs
Salivary glands
Make saliva, which moistens food and carries salivary amylase for starch.
Liver
Makes bile, which emulsifies fats. Nutrient-rich blood from the gut passes through it first.
Gallbladder
Stores and concentrates bile, and releases it into the duodenum when CCK signals fat.
Pancreas
Makes pancreatic juice: bicarbonate plus the main enzymes for starch, fats, proteins and nucleic acids.
Spleen
Not a digestive organ: a lymphoid organ that filters the blood, lying against the left side of the stomach.
Part 2 · Organ by organ
What each organ does
Healthy anatomy only, in the order of the path, then the four accessory organs and the spleen. Each entry names the OpenStax section it was checked against. This is a study aid: nothing here is about a condition, a symptom or care.
Mouth
The teeth tear and grind the food and the tongue mixes it with saliva into a soft mass, the bolus. Salivary amylase begins breaking starch into smaller sugars, so chemical digestion of carbohydrates starts here. Lingual lipase, from glands on the tongue, travels down with the bolus and starts on fats once it reaches the stomach.
- Size:
- 20 deciduous teeth, 32 permanent teeth
- OpenStax:
- 23.3
Pharynx
Of its three parts, only the oropharynx and the laryngopharynx carry food; the nasopharynx is for air and speech. Swallowing (deglutition) has three phases: a voluntary (oral) phase in which the tongue pushes the bolus back, then an involuntary pharyngeal phase and an involuntary esophageal phase.
- OpenStax:
- 23.3
Esophagus
It runs through the chest to the stomach. The upper esophageal sphincter controls the entry of food from the pharynx. The lower esophageal sphincter (also called the gastroesophageal or cardiac sphincter) relaxes to let the bolus into the stomach and then closes, so stomach contents do not back up into the esophagus.
- Size:
- about 25.4 cm (10 in) long
- OpenStax:
- 23.3
Stomach
Four regions: the cardia, where the esophagus enters; the fundus, the dome above it; the body; and the pylorus, which ends at the pyloric sphincter. Three layers of smooth muscle (longitudinal, circular and an inner oblique layer) let it churn. In its gastric glands, parietal cells make hydrochloric acid and intrinsic factor (needed later to absorb vitamin B12); chief cells make pepsinogen, which the acid turns into the protein-digesting enzyme pepsin, and gastric lipase; and G cells release the hormone gastrin, which drives acid secretion. Protein digestion begins here. Little if any nutrient absorption happens in the stomach.
- Size:
- holds as much as 4 liters; gastric juice pH 1.5 to 3.5; a meal has emptied into the duodenum within 2 to 4 hours
- OpenStax:
- 23.4
Duodenum
It curves around the head of the pancreas. The common bile duct and the main pancreatic duct open into it together at the hepatopancreatic ampulla, so chemical digestion of every food group speeds up here. Duodenal (Brunner’s) glands in its wall make a bicarbonate-rich alkaline mucus that buffers the acidic chyme. Its lining releases the hormones secretin (which calls for bicarbonate from the pancreas), cholecystokinin or CCK (pancreatic enzymes and bile from the gallbladder) and gastric inhibitory peptide or GIP (which slows the stomach).
- Size:
- about 25.4 cm (10 in) long
- OpenStax:
- 23.5, 23.6, 23.2
Jejunum
Its Latin name means “empty”, because it is usually found empty after death. Like the rest of the small intestine, its wall is folded three ways to multiply the absorptive surface: circular folds, finger-like villi on the folds and microvilli on each lining cell (the brush border). Enzymes fixed to the microvilli finish digesting carbohydrates and proteins, and practically all absorption of nutrients happens across the small intestine.
- Size:
- about 0.9 m (3 ft) long in life
- OpenStax:
- 23.5
Ileum
Its lining holds Peyer’s patches, clusters of lymphoid tissue that help keep gut bacteria out of the bloodstream. Vitamin B12, bound to intrinsic factor from the stomach, is absorbed at its end. The ileocecal sphincter (valve) between the ileum and the cecum stays closed most of the time and relaxes when the ileum’s motility increases.
- Size:
- about 1.8 m (6 ft) long
- OpenStax:
- 23.5, 23.7
Cecum
The residue from the ileum arrives here and moves up into the ascending colon. The large intestine as a whole is about half as long as the small intestine and about 3 inches across.
- Size:
- about 6 cm (2.4 in) long
- OpenStax:
- 23.5
Appendix
It contains lymphoid tissue (MALT), which points to an immune role, but it is generally considered vestigial for digestion itself. One proposal is that it holds a reserve of normal gut bacteria that can repopulate the colon.
- Size:
- about 7.6 cm (3 in) long
- OpenStax:
- 23.5
Ascending colon
Along the colon, water and salts are absorbed, and resident bacteria make biotin, pantothenic acid and vitamin K, which are absorbed too. Three bands of longitudinal smooth muscle, the teniae coli, keep a steady tone that gathers the colon into pouches called haustra.
- OpenStax:
- 23.5
Transverse colon
It hangs from the mesocolon, the fold of peritoneum that anchors the transverse and sigmoid colon to the back wall of the abdomen.
- OpenStax:
- 23.5, 23.1
Descending colon
By now most of the water has been taken back: the small intestine absorbs about 90 percent of the water that enters it, and the colon takes most of the rest, turning the liquid residue into semisolid feces. Of about 500 mL that enter the cecum each day, about 150 mL leave as feces.
- OpenStax:
- 23.5
Sigmoid colon
It is the second part of the colon anchored by the mesocolon (with the transverse colon).
- OpenStax:
- 23.5, 23.1
Rectum
The rectum and the anal canal have no teniae coli and no haustra.
- Size:
- the final 20.3 cm (8 in) of the alimentary canal
- OpenStax:
- 23.5
Anal canal
The internal anal sphincter is smooth muscle and involuntary; the external anal sphincter is skeletal muscle under voluntary control. Both stay closed except during defecation.
- Size:
- 3.8 to 5 cm (1.5 to 2 in) long
- OpenStax:
- 23.5
The accessory organs
Four organs outside the route that make the juices the route depends on: saliva, bile and pancreatic juice.
Salivary glands
Three pairs of major glands (the parotid, submandibular and sublingual glands) plus many minor glands in the lining of the mouth secrete 1 to 1.5 liters of saliva a day. Saliva moistens and softens the food so it can be swallowed, and its salivary amylase starts breaking starch into smaller sugars. OpenStax groups the teeth and the tongue with the accessory structures too: they do the mechanical digestion in the mouth.
- Numbers:
- 1 to 1.5 liters of saliva a day
- OpenStax:
- 23.3, 23.1
Liver
The largest gland in the body, in the right upper quadrant just under the diaphragm, with a large right lobe and a smaller left lobe (the quadrate and caudate lobes are parts of the right lobe). Its hepatocytes, about 80 percent of its volume, make about one liter of bile a day. Bile is a mix of water, bile salts, bile pigments, phospholipids, electrolytes, cholesterol and triglycerides; the bile salts emulsify large fat globules into tiny droplets so pancreatic lipase can work on them, and they are recycled to the liver afterward. Bile collects in bile canaliculi between the hepatocytes, flows to the right and left hepatic ducts and on into the common hepatic duct. The liver gets two blood supplies: oxygen-rich blood through the hepatic artery, and nutrient-rich blood from the small intestine through the hepatic portal vein, so most newly absorbed nutrients reach the liver before the rest of the body.
- Numbers:
- about three pounds in an adult; about 1 liter of bile a day
- OpenStax:
- 23.6
Gallbladder
A muscular sac nested in a shallow hollow on the underside of the liver’s right lobe. It stores bile and concentrates it up to 10-fold by absorbing water and ions from it. When fatty chyme reaches the duodenum, the hormone cholecystokinin (CCK) from the duodenal lining calls for bile: the muscle fibers in the gallbladder wall contract and eject its contents through the cystic duct, which joins the common hepatic duct to form the common bile duct, and the bile enters the duodenum at the hepatopancreatic ampulla, where a smooth-muscle sphincter controls the flow.
- Numbers:
- 8 to 10 cm (3 to 4 in) long
- OpenStax:
- 23.6, 23.2
Pancreas
A soft gland behind the stomach, with its head in the C-shaped curve of the duodenum, its body extending about 15.2 cm (6 in) to the left and its tail reaching the spleen. Its acinar cells make more than a liter of pancreatic juice a day (pH 7.1 to 8.2): sodium bicarbonate to buffer the acid chyme, pancreatic amylase for starch, pancreatic lipase for fats, nucleases for DNA and RNA, and the protein-digesting trypsin, chymotrypsin and carboxypeptidase. The proteases are released in inactive forms (trypsinogen, chymotrypsinogen, procarboxypeptidase) and switched on only in the duodenum, where the brush border enzyme enteropeptidase activates trypsin. The juice reaches the duodenum through the pancreatic duct, which opens with the common bile duct at the hepatopancreatic ampulla, and an accessory duct. Its scattered islet cells are its endocrine side, releasing insulin and glucagon into the blood.
- Numbers:
- more than 1 liter of pancreatic juice a day
- OpenStax:
- 23.6
The spleen: the neighbor that is not a digestive organ
About 12 cm (5 in) long, attached to the lateral border of the stomach by the gastrosplenic ligament, dark red because it is so full of blood, and fragile because it has no strong capsule. It is a major secondary lymphoid organ. Its red pulp filters the blood: macrophages and dendritic cells there remove microbes, debris and old or damaged red blood cells. Its white pulp, around small arteries, holds B cells and T cells that mount immune responses to pathogens carried in the blood. The one link to digestion: bilirubin, the main bile pigment, is a waste product of the spleen’s breakdown of old red blood cells; the liver excretes it in bile, where it is the main pigment.
OpenStax: 21.1, 23.6
Part 3 · Enzymes and key numbers
The digestive enzymes
Every enzyme name ends in -ase, and the first part names what it acts on (the word-part decoder breaks these names down). The table follows OpenStax 23.7. On a phone, each enzyme is a card.
Carbohydrates
Salivary amylase
- Made by
- Salivary glands
- Acts on
- Polysaccharides (starch)
- Products
- Disaccharides and trisaccharides
Pancreatic amylase
- Made by
- Pancreas (acinar cells)
- Acts on
- Starch
- Products
- Alpha-dextrins, maltose, maltotriose
Alpha-dextrinase
- Made by
- Small intestine (brush border)
- Acts on
- Alpha-dextrins
- Products
- Glucose
Sucrase
- Made by
- Small intestine (brush border)
- Acts on
- Sucrose
- Products
- Glucose and fructose
Maltase
- Made by
- Small intestine (brush border)
- Acts on
- Maltose
- Products
- Glucose
Lactase
- Made by
- Small intestine (brush border)
- Acts on
- Lactose
- Products
- Glucose and galactose
Proteins
Pepsin
- Made by
- Stomach (chief cells)
- Acts on
- Proteins
- Products
- Peptides
- Note
- Secreted as pepsinogen; hydrochloric acid activates it.
Trypsin
- Made by
- Pancreas (acinar cells)
- Acts on
- Proteins
- Products
- Peptides
- Note
- Secreted as trypsinogen; enteropeptidase in the duodenum activates it.
Chymotrypsin
- Made by
- Pancreas (acinar cells)
- Acts on
- Proteins
- Products
- Peptides
- Note
- Secreted inactive and activated in the duodenum.
Elastase
- Made by
- Pancreas (acinar cells)
- Acts on
- Proteins
- Products
- Peptides
- Note
- Secreted inactive and activated in the duodenum.
Carboxypeptidase
- Made by
- Pancreas (acinar cells)
- Acts on
- Amino acids at the carboxyl end of peptides
- Products
- Amino acids and peptides
- Note
- Secreted inactive and activated in the duodenum.
Aminopeptidase
- Made by
- Small intestine (brush border)
- Acts on
- Amino acids at the amino end of peptides
- Products
- Amino acids and peptides
Dipeptidase
- Made by
- Small intestine (brush border)
- Acts on
- Dipeptides
- Products
- Amino acids
Enteropeptidase
- Made by
- Small intestine (brush border)
- Acts on
- Trypsinogen
- Products
- Trypsin
- Note
- The switch: active trypsin then activates the other pancreatic proteases.
Fats
Lingual lipase
- Made by
- Tongue (lingual glands)
- Acts on
- Triglycerides
- Products
- Free fatty acids, mono- and diglycerides
Gastric lipase
- Made by
- Stomach (chief cells)
- Acts on
- Triglycerides
- Products
- Fatty acids and monoacylglycerides
Pancreatic lipase
- Made by
- Pancreas (acinar cells)
- Acts on
- Triglycerides emulsified by bile salts
- Products
- Fatty acids and monoacylglycerides
Nucleic acids
Ribonuclease
- Made by
- Pancreas (acinar cells)
- Acts on
- RNA
- Products
- Nucleotides
Deoxyribonuclease
- Made by
- Pancreas (acinar cells)
- Acts on
- DNA
- Products
- Nucleotides
Nucleosidases and phosphatases
- Made by
- Small intestine (brush border)
- Acts on
- Nucleotides
- Products
- Pentoses, phosphates and nitrogenous bases
Where each kind of digestion happens
- Carbohydrates: Start in the mouth with salivary amylase. Pancreatic amylase does most of the work in the small intestine, and the brush border enzymes finish, leaving single sugars to absorb.
- Proteins: Start in the stomach, where hydrochloric acid unfolds them and pepsin cuts them into peptides. The pancreatic proteases and the brush border peptidases finish in the small intestine.
- Fats: Lingual and gastric lipases make a start, but the pancreas is the only consequential source of lipase, so virtually all fat digestion happens in the small intestine. Bile salts emulsify the fat first. Short-chain fatty acids pass straight into the lining cells and the blood; the long-chain products travel in micelles to the lining cells, are repackaged as chylomicrons and enter the lacteals (lymph), not the blood capillaries.
- Nucleic acids: Pancreatic nucleases cut DNA and RNA into nucleotides, and brush border enzymes take those apart into sugars, phosphates and bases.
The six processes
- 1Ingestion. Food enters the alimentary canal through the mouth.
- 2Propulsion. Food moves along the tract: swallowing, then peristalsis, the alternating waves of contraction and relaxation of the smooth muscle in the wall.
- 3Mechanical digestion. A purely physical breaking up of the food, which does not change its chemistry: chewing, the stomach’s churning, and segmentation, the back-and-forth mixing contractions of the small intestine.
- 4Chemical digestion. Digestive secretions break complex food molecules into their building blocks, such as proteins into amino acids.
- 5Absorption. Nutrients pass from the lumen of the canal into the blood or lymph, mostly across the lining of the small intestine.
- 6Defecation. Undigested material leaves the body as feces.
OpenStax 23.2
The wall, inside out
- 1Mucosa (the lining, facing the lumen)
- 2Submucosa
- 3Muscularis
- 4Serosa (the outer layer)
OpenStax 23.1
The hormones
Gastrin · Stomach (G cells)
Released when partially digested proteins are present; stimulates the parietal cells to secrete hydrochloric acid.
Secretin · Duodenum
Stimulates a watery, bicarbonate-rich secretion from the pancreas.
Cholecystokinin (CCK) · Duodenum
Released when fats and proteins arrive; stimulates the secretion of pancreatic enzymes and the release of bile into the duodenum.
Gastric inhibitory peptide (GIP) · Duodenum
Inhibits gastric secretion and slows gastric emptying and motility. Also called glucose-dependent insulinotropic peptide, because it stimulates the release of insulin.
OpenStax 23.2, 23.4
Key numbers
- Alimentary canal
- about 7.62 m (25 ft) long in life, nearer 10.67 m (35 ft) after death when the muscle loses its tone (OpenStax 23.1)
- Teeth
- 20 deciduous, 32 permanent (OpenStax 23.3)
- Saliva
- 1 to 1.5 liters a day (OpenStax 23.3)
- Esophagus
- about 25.4 cm (10 in) (OpenStax 23.3)
- Stomach
- holds as much as 4 liters; gastric juice pH 1.5 to 3.5; a meal has emptied within 2 to 4 hours (fatty chyme can take 6 hours or longer) (OpenStax 23.4)
- Small intestine
- about 3.05 m (10 ft) in life: duodenum 25.4 cm (10 in), jejunum about 0.9 m (3 ft), ileum about 1.8 m (6 ft); circular folds, villi and microvilli multiply its absorptive surface more than 600-fold, to about 200 m² (OpenStax 23.5)
- Water
- about nine liters of fluid enter the small intestine a day; about 90 percent is absorbed there, most of the rest in the colon (OpenStax 23.7, 23.5)
- Large intestine
- about half as long as the small intestine, about 3 in across; cecum about 6 cm (2.4 in); appendix about 7.6 cm (3 in); rectum 20.3 cm (8 in); anal canal 3.8 to 5 cm (1.5 to 2 in) (OpenStax 23.5)
- Feces
- of about 500 mL of residue entering the cecum a day, about 150 mL leaves as feces (OpenStax 23.5)
- Liver
- about three pounds; hepatocytes about 80 percent of its volume; about 1 liter of bile a day, pH 7.6 to 8.6 (OpenStax 23.6)
- Gallbladder
- 8 to 10 cm (3 to 4 in) long; concentrates bile up to 10-fold (OpenStax 23.6)
- Pancreas
- body extends about 15.2 cm (6 in) to the left; more than 1 liter of pancreatic juice a day, pH 7.1 to 8.2 (OpenStax 23.6)
- Spleen
- about 12 cm (5 in) long (OpenStax 21.1)
Part 4 · Label the diagram
Name the twelve blank marks
The same schematic, with twelve of its marks left blank in the legend. Choose the organ for each, then check. The marks that stay named are your anchors.
Mouth
Pharynx
Esophagus
Ascending colon
Descending colon
Anal canal
Salivary glands
Spleen (not a digestive organ)
Part 5 · Practice questions
20 practice questions
The path, the organs, the accessory organs, the enzymes and the spleen, multiple choice, one at a time. Every answer is explained with the OpenStax section it rests on, and your score is free: no sign-up needed.
Question 1 of 20
Which list gives the path of food in the right order?
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- Printable study sheet: the schematic with its legend, every organ on one line, the four accessory organs and the spleen, the enzymes table, the six processes, the hormones and the key numbers. It prints on its own, about two pages.
- 30 more practice questions, every answer explained with the OpenStax section it rests on.
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Sources
Where the facts come from
Everything on this page is reworded from OpenStax, Anatomy and Physiology 2e (an open textbook, CC BY 4.0). Each entry above names its section. Check anything important against the book and your program’s materials.
- OpenStax, Anatomy and Physiology 2e, 23.1 Overview of the Digestive System · the organs of the alimentary canal and the accessory organs, the wall layers, the peritoneal folds, the length of the canal
- OpenStax, 23.2 Digestive System Processes and Regulation · the six processes, peristalsis and segmentation, the hormones
- OpenStax, 23.3 The Mouth, Pharynx, and Esophagus · the salivary glands, teeth, swallowing, the esophagus and its sphincters
- OpenStax, 23.4 The Stomach · the regions, the gland cells and their secretions, capacity, emptying
- OpenStax, 23.5 The Small and Large Intestines · the three parts of the small intestine, the absorptive surface, the appendix, the colon, the rectum and the anal sphincters
- OpenStax, 23.6 Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder · the liver, bile and the bile ducts, the gallbladder, the pancreas and its juice
- OpenStax, 23.7 Chemical Digestion and Absorption: A Closer Look · the enzymes table, where each kind of digestion happens, micelles, chylomicrons and lacteals, water and vitamin absorption
- OpenStax, 21.1 Anatomy of the Lymphatic and Immune Systems · the spleen (red pulp, white pulp, what it filters) and mucosa-associated lymphoid tissue
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Questions
Common questions
Mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum) and large intestine (cecum with the appendix, then the ascending, transverse, descending and sigmoid colon, the rectum and the anal canal). The salivary glands, liver, gallbladder and pancreas are accessory organs: they add saliva, bile and pancreatic juice to that route, but food never passes through them.
The spleen is not a digestive organ, although it sits against the left side of the stomach. It is a lymphoid organ about 12 cm long: its red pulp filters the blood and removes microbes and old or damaged red blood cells, and its white pulp holds B and T cells that respond to pathogens carried in the blood. Its one link to digestion is bilirubin, the pigment left from those red blood cells, which the liver excretes in bile.
Two jobs. Its acinar cells make pancreatic juice, more than a liter a day, with bicarbonate to neutralize the acid chyme and the main enzymes for starch (amylase), fats (lipase), proteins (trypsin, chymotrypsin, carboxypeptidase, released inactive) and nucleic acids (nucleases); it reaches the duodenum through the pancreatic duct. Its islet cells release the hormones insulin and glucagon into the blood.
It makes bile, about a liter a day. Bile salts emulsify fats into tiny droplets so pancreatic lipase can digest them, and bile carries bilirubin out of the body. The liver also receives the nutrient-rich blood from the small intestine through the hepatic portal vein, so most newly absorbed nutrients reach it before the rest of the body. Its many other jobs in metabolism belong to other chapters; this page covers only its digestive role.
It stores bile from the liver and concentrates it up to 10-fold by absorbing water and ions. When fatty chyme reaches the duodenum, the hormone cholecystokinin (CCK) triggers the release of bile through the cystic duct into the common bile duct and on into the duodenum.
The appendix is a thin tube about 7.6 cm long hanging from the cecum. It contains lymphoid tissue, which points to an immune role, but it is generally considered vestigial for digestion itself. One proposal is that it holds a reserve of normal gut bacteria that can repopulate the colon.
No. This is a study aid for people in anatomy and physiology, nursing, medical assisting and other health programs. It describes healthy anatomy, reworded from an open textbook, and says nothing about conditions, symptoms or care. Your program’s materials and your instructor come first.
The path, the organ guide, the enzymes table, the label-the-diagram exercise and the 20 practice questions with your score are free with no sign-up. An email unlocks the bonus: a printable study sheet and 30 more questions.