How Does Digestion Work? — A Tube Through the Body, and a Wall Only Small Pieces Can Cross

When Does Swallowed Rice Enter the Body?

For lunch, you eat a spoonful of rice topped with a fried egg. When you swallow, the rice goes down your throat and into your belly. Normally, you feel like the food has entered your body at this moment.

The time when the grains of rice truly enter the body is long after that. It is when they are broken down into very tiny pieces inside the long tube passing through the belly, and those pieces cross the wall of the tube to enter the body. Until then, the rice is inside the tube passing through the body.

This article follows that tube from start to finish. We look at how food is crushed and cut, where rice, eggs, and oil are each broken down and by what, where the broken pieces cross the wall and where they go first, and who times that sequence. What the stomach and esophagus, the end of the tube and the microbes living inside it, and the liver do are covered more deeply one by one in other parts of this series.

From Mouth to Anus, a Pipe That Pierces the Body

The digestive tract is a single long tube running from the mouth to the anus, a path where food passes, is broken down, and is absorbed. The mouth, esophagus, stomach, the thinner bowel below it, and the thicker bowel connect in turn to form a single line. Because both ends of the tube open to the outside, the inside of the tube is an outside space passing right through the body.

What divides the inside of the tube from the inside of the body is the wall of the tube. The cell layer lining the inside of the intestine is a selective membrane that lets nutrients and water pass while blocking microbes and toxins. This balance between absorption and defense is essential for living (PMID 42330921).

Thus, the work of digestion converges on one thing: making food small enough for that wall to let it pass. The starch in rice, the protein in eggs, and the oil in food are all large molecules, so they cannot cross the wall as they are.

There are two ways of breaking them down. One is breaking the lumps apart through the force of chewing, mashing, and mixing, and the other is cutting the molecules themselves. Enzymes take charge of the side that cuts molecules. Digestive enzymes are proteins that cut large molecules in food into small pieces that the body can absorb. Digestive enzymes come out in the mouth, in the stomach, and in the tube below them (PMID 41008568).

Digestion is the work of breaking food down small enough for the wall of the tube to pass it, and what enters the inside of the body are the pieces that have crossed that wall.

Breaking, Mixing, and Pushing Along

Let's look at chewing first. The jaw and tongue move in rhythm to crush food between the teeth. Saliva wets the broken pieces, binds them into a single mass, and makes them easy to swallow (PMID 16564557). Enzymes in saliva already begin chopping up starch at this point (PMID 25841068).

The act of swallowing is executed in a set sequence by neural circuits in the lower brain (PMID 1389633). That junction where the food path and airway split is covered separately in "Swallowing, Speech and Language — The Crossing Where the Food Path Meets the Airway".

Food that passes through the esophagus reaches the stomach and stays there briefly. Gastric acid is a strong acid released by cells in the stomach wall that prepares proteins for digestion and reduces bacteria that entered with the food. While staying in the stomach, the food mass is soaked in this acid (PMID 21889024). Gastric acid helps digest proteins, aids in the absorption of iron, calcium, and vitamin B12, and kills swallowed microorganisms (PMID 24100729). The way the stomach wall protects itself from being harmed by its own acid is handled by the installment of this series that covers the stomach and esophagus.

The stomach also kneads. Peristalsis is the movement in which the muscles of a tube contract in turn and push contents forward like a wave. When you eat, these waves in the stomach arise at a pace of about three times per minute (PMID 42018791).

The main job of the stomach is to prepare food so it can be digested and absorbed in the lower intestines (PMID 21889024). Food that has finished preparation goes down little by little. Gastric emptying is the process by which the stomach sends food down into the intestines.

The tubes above and below the stomach do not rest either. They stretch, contract, and fold during meals and between meals alike. While the waves of peristalsis push the contents from the stomach toward the anus, segments of the tube tighten and loosen in place to break up and mix the food mass so that it touches the walls better (PMID 42330921).

The mouth and stomach break, wet, and soak food in acid, turning it into a state that the lower tube can receive and break down further.

The Small Intestine, Where Nutrients Cross the Wall

The small intestine is a narrow, long tube between the stomach and the large intestine where most of the food's nutrients are absorbed. The small intestine extracts amino acids, sugars, fats, vitamins, and minerals from digested food (PMID 42330921).

To absorb a lot, the wall must be wide. The small intestine is a very long tube, and its inner wall is folded once more into villi and even smaller microvilli (PMID 27381224). Villi are small projections that rise densely like fingers on the inner wall of the small intestine, expanding the surface where nutrients make contact. Inside the small intestine, these villi are lined up densely, and each single villus works as a unit of absorption (PMID 29513926).

Inside a single villus are fine blood vessels, lymphatic vessels, nerves, and muscles (PMID 29513926).

What has passed entirely through the small intestine moves on to the large intestine. The large intestine is a thick tube that follows the small intestine, gathering water and salts from what remains and forming stool to send out. Above the small intestine, a large amount of salt pours in carried by saliva, pancreatic juice, and digestive juices from the liver, and that much is gathered back up by the end of the small intestine and the beginning of the large intestine (PMID 8125391). Some of the starch that passes over without being digested in the small intestine is broken down together by bacteria living in the large intestine (PMID 31624005). The work done by the large intestine and its microbes is continued in the large intestine and gut microbes installment of this series.

The place where most of the food's nutrients enter inside the body is the widely folded wall of the small intestine.

Two Liquids Poured Nearby

At the front of the small intestine, two organs outside the tube pour in liquids. One is the pancreas. The pancreas is an organ lying horizontally behind the stomach that simultaneously makes digestive enzymes to send to the intestine and releases hormones into the blood that handle blood sugar. A hormone is a substance made in one part of the body that travels through the blood to a distant spot and changes what the cells there do. The part responsible for digestion makes digestive enzymes and sends them to the front part of the small intestine for digestion (PMID 40530519). Enzymes also exist in the mouth and stomach, but the most important digestive enzymes are made by the pancreas (PMID 41008568). The side of the work dealing with blood sugar can be seen in "How Blood Sugar Stays Steady — A Letter to Store, a Letter to Release".

The other comes from the liver. The liver is the largest organ inside the body, located in the upper right side of the belly, and it receives blood coming from the intestine first to store, alter, and send out nutrients. Bile is a liquid made by the liver and sent to the intestine that breaks fats down finely so that digestive enzymes can reach them. Bile is made by the liver cells, gathers along narrow tubes, and flows into the gallbladder and the intestine. The vast majority of bile is water, and the substances dissolved in it are around 5% (PMID 23897680).

The gallbladder is a small pouch attached beneath the liver that stores bile and squeezes it out when food enters. What bile does becomes clear in front of fat. Fat does not mix with water and clumps together. When bile breaks that fat down finely, enzymes can reach the fat better and cut it up (PMID 38429963).

The pancreas pours cutting enzymes, and the liver and gallbladder pour fat-breaking liquids into the small intestine.

Where Rice, Eggs and Oil Are Broken Down

The starch in rice is a long chain of glucose molecules linked together (PMID 31624005). Enzymes in saliva begin to chop this chain, but most of the starch-cleaving enzymes come from the pancreas. The chopped starch becomes two-glucose pieces, and these pieces are split once more into single glucose units (PMID 27640169). This final cut is made by enzymes attached to the surface of the cells lining the small intestine. Enzymes that split the milk sugar lactose and table sugar into single sugars are also on this surface, and the wall's cells load those single sugars inside (PMID 16586532).

The protein in an egg begins to be broken down in the stomach. Enzymes in the stomach start chopping the protein into smaller pieces, and this beginning amplifies the further cutting that follows below (PMID 41008568). In the small intestine, pancreatic enzymes continue the cutting, and processes taking place at the small intestine wall also contribute a significant share to protein digestion. However, the protein-cleving enzymes on the small intestine wall may do the work of transmitting signals rather than cutting food (PMID 41008568). Proteins finally cross the wall as single amino acids, the small units that make them up, or as pieces made of two or three amino acids. This absorption is mostly finished by the time food reaches the end of the small intestine (PMID 37285555).

Oil begins to be chopped a little by enzymes in the tongue and stomach. Most of it is broken down in the small intestine through the combined work of pancreatic enzymes and bile. Once bile disperses the oil, enzymes cut the oil into small pieces like fatty acids, and the intestine's cells absorb those pieces (PMID 38429963). One pancreatic enzyme that cleaves the main components of dietary oil handles 50 to 70% of that component in food (PMID 38081381).

Rice, eggs and oil all begin to be broken down in the mouth or stomach, and are chopped in earnest when they meet pancreatic enzymes in the small intestine. The stomach is one of the places where digestion begins, and the greater part of digestion takes place below it.

After Crossing the Wall, Straight to the Liver

Sugars and amino acids that have crossed the wall are loaded into the blood. The first place that blood reaches is the liver. The hepatic portal vein is a blood vessel that sends blood carrying what was absorbed in the stomach and intestines to the liver before sending it to the heart. Sugars and amino acids absorbed by the small intestine are sent to the liver to undergo the next processing (PMID 38886098). Hormones from the intestines and pancreas also follow this path, reaching the liver in a concentrated form before they dilute into the blood of the whole body (PMID 41208029). The liver receives blood coming from the intestines before any other part of the body.

Pieces derived from fats take a different route. After being bundled back into large fat packages inside the cells of the intestine, they leave via lymphatic vessels instead of the hepatic portal vein (PMID 11229876). These are the lymphatic vessels that run alongside the blood vessels inside the villi.

How the liver stores, transforms, and sends out what it receives is continued in another part of this series, "What Does the Liver Do, and What Do Liver Tests Measure? — The Factory at the Gate, and the Signal That Leaks Through the Wall". What fat packages become in the blood continues into "What Cholesterol Is and Why We Measure It — Four Lines on a Blood Test, and the Packages They Name".

Sugars and amino acids that have crossed the intestinal wall reach the liver first along the hepatic portal vein, while fat pieces take a different path called lymphatic vessels.

Nerves and Hormones That Keep the Order

Just smelling something delicious makes your mouth water. Saliva starts to flow simply from the anticipation of eating food (PMID 31940641). The first signals of digestion go out before food even enters the mouth.

Signals are transmitted by two means: nerves and hormones. Russian physiologist Ivan Pavlov explained the secretion of pancreatic juice solely through nerve reflexes. Later, it was discovered that hormones released by the wall of the small intestine also stimulate the pancreas. The secretion of pancreatic juice after a meal is guided together by nerves and hormones. Intestinal hormones can also stimulate nearby sensory nerves, moving the pancreas through nerve reflexes (PMID 14566070).

On the nerve side, there is a neural network embedded within the tube itself. The enteric nervous system is a network of nerve cells spread like a web inside the wall of the digestive tract, locally controlling how the tract moves and secretes juices. The enteric nervous system is at the center of regulating digestion and defense in the digestive tract (PMID 36521049). Digestion is regulated jointly by signals from the enteric nervous system and the brain and spinal cord, and how much of the work each takes on varies greatly depending on the location in the tube. Nerve regulation and hormone regulation are closely intertwined with each other (PMID 22392290). Research showing that the gut and brain send signals back and forth while microbes living in the gut intervene is continued in the colon and gut microbes parts of this series (PMID 39743581).

Let us look at the hormone side. Cells scattered throughout the gut wall release hormones into the blood (PMID 30760847). With over 30 known gut hormone genes, the gut is considered the largest hormone-producing organ in the body (PMID 11762354).

One of these coordinates the sequence of digestion. Cholecystokinin is a hormone released into the blood by cells in the upper small intestine when food enters, which causes the gallbladder to contract and summons digestive juices from the pancreas. This hormone also regulates the speed of stomach emptying and induces a feeling of fullness. A single hormone interlocks and regulates eating, breaking down, and absorbing (PMID 9074762).

Speed regulation is attached to stomach emptying. When glucose, fat, or amino acids touch the wall of the upper small intestine, the speed at which the stomach sends food down slows down (PMID 2454545). Half-digested nutrients send a signal back from the small intestine to the stomach to "send it slowly," delaying the descent of the next food, and contents must pass at an appropriate speed so there is time to use for digestion and absorption (PMID 8125391).

This speed is also connected to blood sugar. The speed at which glucose descends into the small intestine determines the pattern of post-meal blood sugar rises and affects how much insulin is released (PMID 2454545). How gut hormones handle blood sugar is continued in "How Blood Sugar Stays Steady — A Letter to Store, a Letter to Release" and the story of drugs resembling gut hormones is continued in "What Are Obesity and GLP-1 Drugs? — A Letter That Lingers, a Weight That Wants to Return"

The opposite end of the tube also responds to meals. Usually within a few minutes after eating, the movement of the large intestine increases, and this response occurs through nerves and hormones (PMID 36324042).

The sequence of digestion is coordinated together by the nerves in the tract wall, the nerves in the brain, and the hormones of the gut.

Following a Spoonful of Lunch

We return to our starting lunch. A spoonful holds rice, a fried egg, and the oil used to fry it.

The moment you smell it, saliva begins to gather in your mouth. Once the spoon enters your mouth, your jaw and tongue crush the rice grains and the egg, and saliva wets the pieces to clump them into a single mass. Enzymes in the saliva begin to chop up the starch at this point. The mass passes down the throat in the sequence ordered by the brain's circuits.

The mass that reaches the stomach is submerged in acid and kneaded by waves arriving roughly three times a minute. The protein and oil of the egg begin to be chopped here a little at a time, but the stomach's main job is to prepare this mass so the intestine can receive it. The prepared mass descends into the small intestine little by little.

When nutrients touch the front wall of the small intestine, the cells there release cholecystokinin. This hormone squeezes the gallbladder, calls forth the digestive juices of the pancreas, and regulates the speed of stomach emptying. When glucose, fat, and amino acids touch the wall of the small intestine, the speed at which the stomach sends them down slows.

Inside the small intestine, the three nutrients are chopped in their respective places. The starch of the rice is chopped by pancreatic enzymes, and finally chopped by enzymes on the wall surface to become single units of glucose. The protein of the egg becomes single amino acids or pieces of two or three. The oil is dissolved by bile and then chopped by pancreatic enzymes.

The chopped pieces cross the wall, which is densely folded with villi. Blood carrying glucose and amino acids reaches the liver first along the hepatic portal vein, and oil pieces are bundled into packages and go out into lymphatic vessels. Some starch that passes through the small intestine without being chopped is handled by bacteria in the large intestine, and the large intestine gathers water and salt from what remains. Usually within minutes after eating, at the end of the tract, the movement of the large intestine has already increased.

Where Else to Look

There are also unanswered questions. Because the starch enzyme in saliva is a small share of the starch enzymes in the whole body, it is not yet clear why that enzyme exists separately (PMID 27640169).

Other parts of this series take over each section of this tract. Stomach acid and the stomach's defense, and the boundary with the esophagus, are handled by "What Do the Stomach and Esophagus Do? — A Pouch That Holds Acid, and Its Two Doors", the large intestine and the microbes inside it by "What Do Gut Microbes Do? — Guests Who Eat the Leftovers, Guests Who Hold the Room", how the liver handles what comes in through the portal vein and the liver tests on a checkup sheet by "What Does the Liver Do, and What Do Liver Tests Measure? — The Factory at the Gate, and the Signal That Leaks Through the Wall", and the process in which the liver hardens from long-term damage by "How Do Fatty Liver, Hepatitis and Cirrhosis Connect? — Ground That Hardens as Scars Build Up, and the Stretch Where You Can Still Turn Back".

It also connects to previously published articles. Gut hormones and how the pancreas holds blood sugar steady can be seen in "How Blood Sugar Stays Steady — A Letter to Store, a Letter to Release", and drugs resembling gut hormones in "What Are Obesity and GLP-1 Drugs? — A Letter That Lingers, a Weight That Wants to Return" and "What Do GLP-1 Drugs Do Beyond Weight? — Signal, Association, and Confirmation". The very front of the tract, where the food path and airway split, is covered by "Swallowing, Speech and Language — The Crossing Where the Food Path Meets the Airway", and the rest of the story of the fat packages that went out into the lymphatic vessels by "What Cholesterol Is and Why We Measure It — Four Lines on a Blood Test, and the Packages They Name".

Source Papers

• "Gut mechanics." (2026) — PMID 42330921 — The main job of the gut is nutrient absorption and protection against harmful things; the small intestine extracts amino acids, sugars, lipids, vitamins, and minerals; the large intestine reclaims water and electrolytes, and compacts the leftovers for elimination; the epithelium acts as a selective barrier that lets nutrients and water pass while blocking microbes, antigens, and toxins; the gut moves constantly both during and between meals, with peristaltic waves pushing contents from the stomach to the anus mixed with segmental contractions.
• "Non-Pancreatic Digestive Enzymes." (2025) — PMID 41008568 — The pancreas makes the most important digestive enzymes, but there are other contributors: salivary amylase (carbohydrates), lingual and gastric lipase (fat), gastric pepsin begins protein digestion and drives downstream proteolysis, and processing by the intestinal mucosa is an important contributor to protein digestion.
• "Oral physiology and mastication." (2006) — PMID 16564557 — Chewing is a sensory-motor activity that prepares food for swallowing, breaking food down between the teeth via jaw and tongue movements, while saliva wets it and forms a bolus to aid swallowing.
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• "Gastric secretion." (2013) — PMID 24100729 — Stomach acid helps with protein digestion and the absorption of iron, calcium, vitamin B12, and some drugs, and kills swallowed microbes to prevent bacterial overgrowth and gut infections.
• "Gastric electrophysiology." (2026) — PMID 42018791 — The mechanical activity of the stomach is mediated by electrical signals, and a test meal triggers peristaltic contractions at a rate of 3 per minute (with rhythm abnormalities seen in conditions like gastroparesis).
• "Generation of intestinal surface: an absorbing tale." (2016) — PMID 27381224 — The small intestine needs a huge surface area for nutrient absorption, which it achieves through a very long tube and folding of the surface into villi and microvilli.
• "Blueprint for an intestinal villus: Species-specific assembly required." (2018) — PMID 29513926 — Villi are finger-like projections that expand the nutrient contact surface, with each villus serving as a functional absorptive unit containing blood vessels, lymphatic vessels (lacteals), nerves, smooth muscle, and immune cells (this is a mammalian and avian developmental biology review; the figure of "billions" is species-nonspecific so it is not used in the main text).
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• "Starch Digestion by Gut Bacteria: Crowdsourcing for Carbs." (2020) — PMID 31624005 — Starch is a glucose polymer; resistant starch escapes small intestinal enzymes and travels to the large intestine where various gut bacteria break it down and ferment it together (increasing short-chain fatty acids).
• "Nutrition and the pancreas." (2026) — PMID 40530519 — The pancreas is a complex organ responsible for digestion, absorption, and blood sugar homeostasis; its exocrine part makes digestive enzymes and bicarbonate used for digestion in the duodenum and jejunum, while its endocrine cells make blood sugar-regulating hormones, and nutritional status affects the prognosis of pancreatic disease.
• "Bile formation and secretion." (2013) — PMID 23897680 — Bile is an aqueous secretion made by hepatocytes and refined by bile duct epithelium, with about 5% organic and inorganic solutes, flowing from canaliculi to bile ducts to the gallbladder and common bile duct to the intestine, and most of its determining factors have been identified at the molecular level in animal models and humans.
• "Fat digestion and absorption: Normal physiology and pathophysiology of malabsorption, including diagnostic testing." (2024) — PMID 38429963 — Fat digestion starts with gastric lipase, but mostly takes place in the small intestine via pancreatic enzymes and bile acid salts; emulsification by bile acids helps enzyme action to break down triglycerides into free fatty acids and monoglycerides for absorption by enterocytes.
• "Salivary Amylase: Digestion and Metabolic Syndrome." (2016) — PMID 27640169 — Salivary amylase accounts for a small share of total amylase (most is pancreatic), turning starch into maltose and two glucoses (maltase), though why salivary amylase exists and its evolutionary advantage remain unclear.
• "Intestinal sugar transport." (2006) — PMID 16586532 — Most carbohydrates are digested by salivary and pancreatic amylases, and brush border enzymes on enterocytes (such as lactase and sucrase) break them down into simple sugars, which transporters in mature enterocytes then move into the cells.
• "Intestinal Amino Acid Transport and Metabolic Health." (2023) — PMID 37285555 — Amino acid absorption is mediated by amino acid transporters and di- and tripeptide transport, with absorption mostly completed by the end of the small intestine, and the large intestine absorbing amino acids derived from bacterial metabolism.
• "Pancreatic lipase and its related proteins: where are we now?" (2024) — PMID 38081381 — Pancreatic lipase hydrolyzes 50 to 70% of triglycerides in food.
• "Digestive dynamics: Unveiling interplay between the gut microbiota and the liver in macronutrient metabolism and hepatic metabolic health." (2024) — PMID 38886098 — All nutrients absorbed by the small intestine are delivered to the liver for further metabolism, and microbial metabolites from the large intestine also reach the liver via the portal circulation.
• "Peptide hormones and bile acids shaping immune tolerance of the liver: implications and applications." (2026) — PMID 41208029 — Noting that postprandial gut and pancreatic endocrine factors reach the liver via the portal vein at high concentrations before being diluted in the systemic circulation, it is proposed that this contributes to hepatic immune tolerance (this article is cited only for the portal transit part).
• "Intestinal lipid absorption and transport." (2001) — PMID 11229876 — Triglyceride digestion starts with gastric lipase and proceeds mostly in the intestinal lumen via pancreatic lipase, after which enterocytes absorb the digestion products, resynthesize complex lipids, package them into chylomicrons, and send them out to the lymphatic system.
• "Chapter 14: Impact of Oral Health on Diet/Nutrition." (2020) — PMID 31940641 — Saliva secretion is stimulated in anticipation of food intake and during chewing, which starts the action of digestive enzymes in the mouth and stomach and affects appetite and bioavailability.
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• "The enteric nervous system." (2023) — PMID 36521049 — The enteric nervous system is a complex network of neurons and glial cells within the gut wall that serves as the center for regulating digestive and defensive functions, interacting with the immune system, gut microbiota, and epithelium.
• "The enteric nervous system and neurogastroenterology." (2012) — PMID 22392290 — Digestion is regulated by the integration of signals from the enteric nervous system and the central nervous system, with neural and endocrine regulation closely intertwined, and the respective regulatory shares of the enteric and central nervous systems varying greatly across different sites of the digestive tract.
• "Microbiota-neuroepithelial signalling across the gut-brain axis." (2025) — PMID 39743581 — A two-way communication where gut microbes regulate brain activity and behavior while brain signals affect microbes, with the gut sensory system at the center.
• "Function and mechanisms of enteroendocrine cells and gut hormones in metabolism." (2019) — PMID 30760847 — Gut hormones are made by cells scattered throughout the intestinal epithelium and regulate gut function, insulin secretion, nutrient assimilation, and feeding, serving as the basis for treatments like GLP-1 analogs.
• "The gut as the largest endocrine organ in the body." (2001) — PMID 11762354 — With over 30 gut hormone genes and numerous bioactive peptides, the gut is the largest endocrine organ in the body, and can also be viewed as a sensory organ operating through neural, endocrine, and immune cells.
• "Cholecystokinin cells." (1997) — PMID 9074762 — CCK cells are concentrated in the proximal small intestine and secreted into the blood upon food intake, stimulating pancreatic secretion and gallbladder contraction, regulating gastric emptying, and inducing satiety to coordinate intake, digestion, and absorption; they are produced by two cell types, enteroendocrine cells and neurons.
• "Control of gastric emptying by regulatory peptides." (1988) — PMID 2454545 — When glucose, fat, and amino acids touch the duodenal mucosa, inhibitory mechanisms slow gastric emptying; CCK reduces glucose delivery at physiological concentrations, and the rate of glucose delivery to the duodenum determines the shape of the blood glucose curve and the amount of insulin secreted.
• "Gastrocolonic Response." (2022) — PMID 36324042 — The gastrocolonic response is a physiological increase in colon motility usually within a few minutes after a meal, triggered by gastric distension, calories, and fat, and mediated by vagal and neurohumoral mechanisms.