15.10: The Small Intestine — Read

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Figure 23 | Small Intestine | ³¹ The small intestine is located inferior to the stomach. The three regions of the small intestine are the duodenum, jejunum, and ileum. The ileum continues into the large intestine.

The term "intestine," a Latin word signifying "internal," refers to the small and large intestines collectively occupying a substantial portion of the abdominal cavity. These parts form the most massive and extended segments of the alimentary canal. With the exception of ingestion, the small and large bowel undertake all digestive system functions.

Following the release of chyme from the stomach, the small intestine emerges as the primary digestive organ in the body. Not only does it serve as the primary site for most digestion processes, but it also stands as the central hub for virtually all absorption activities. The small intestine, the longest segment of the alimentary canal, measures approximately 3 to 5 meters (20 to 22 feet) in a living person. Interestingly, its length appears doubled in a cadaver due to the loss of muscle tone. The nomenclature "small intestine" reflects its relatively smaller diameter, measuring about 2.54 cm (1 in), compared to the larger 7.62 cm (3 in) diameter of the large intestine.

Beyond its impressive length, the small intestine's efficiency is accentuated by the intricate folds and projections of its lining, providing an expansive surface area. This enormous surface area, approximately 200 m², surpasses the skin's surface area by more than 100 times. Such a vast expanse is indispensable for the intricate processes of digestion and absorption that unfold within the small intestine.

The small intestine is divided into three distinct regions, each with unique anatomical and functional characteristics: the duodenum, jejunum, and ileum. The duodenum is the first and shortest segment of the small intestine, originating from the stomach's pyloric sphincter. It spans approximately 25 to 30 centimeters (about 10 to 12 inches) in length. The duodenum is marked by the presence of the duodenal papilla, through which bile and pancreatic secretions enter. It also forms a C-shaped loop around the head of the pancreas. In addition to receiving chyme from the stomach, the duodenum is a crucial site for digestion. Here, bile from the liver and pancreatic enzymes aid in the breakdown of macronutrients.

Following the duodenum, the jejunum is the middle segment of the small intestine. It constitutes most of the small intestine, extending approximately 2.5 meters (about 8 feet). The jejunum is characterized by a thicker wall than the other small intestine segments, with a more prominent vascular network. The primary role of the jejunum is nutrient absorption. It is particularly involved in absorbing carbohydrates and proteins, contributing significantly to the body's nutritional intake.

The ileum forms the terminal part of the small intestine, connecting to the large intestine at the ileocecal valve. It spans around 3.5 meters (about 11 feet), making it the longest section among the three. The ileum exhibits a thinner wall and a higher number of lymphoid nodules, contributing to its role in immune surveillance. The ileum continues the absorption of nutrients, mainly the uptake of bile salts and vitamin B12. It also plays a crucial role in the reabsorption of water and electrolytes.

Histology of the Small Intestine

The small intestine is endowed with specialized structures that significantly enhance its absorptive and digestive functions. Two key features are villi, microscopic finger-like projections, and microvilli, tiny hair-like structures covering the surface of absorptive cells. Villi are finger-like projections that protrude from the inner lining of the small intestine, specifically from the mucosa. Their purpose is to amplify the surface area available for nutrient absorption. This increased surface area, in turn, facilitates the efficient absorption of digested nutrients into the bloodstream. Each villus contains a network of blood vessels and a central lacteal, a lymphatic capillary. The blood vessels transport absorbed nutrients such as carbohydrates and amino acids to the rest of the body, while the lacteal is instrumental in absorbing dietary fats.³

Figure 24 | Histology of the Small Intestine | ³² (a) The absorptive surface of the small intestine is vastly enlarged by the presence of circular folds, villi, and microvilli. (b) Micrograph of the circular folds. (c) Micrograph of the villi. (d) Electron micrograph of the microvilli.

Microvilli are microscopic extensions of the plasma membrane of absorptive cells that cover the surface of villi. This structure forms the "brush border" due to its appearance under a microscope. Microvilli further increase the absorptive surface area and contain enzymes involved in the final stages of digestion. The microvilli play a pivotal role in the absorption of nutrients. They are densely packed with transport proteins that facilitate the uptake of nutrients, such as amino acids, carbohydrates, and electrolytes, into the absorptive cells.

The mucosal surface between the villi reveals intricate features marked by deep crevices, each leading into tubular structures known as intestinal glands or crypts of Lieberkühn (Figure 24). These glands are formed by cells lining the crevices and play a vital role in the secretion of intestinal juice—a slightly alkaline mixture with a pH ranging from 7.4 to 7.8—composed of water and mucus.

On a daily basis, these intestinal glands contribute to the secretion of approximately 0.95 to 1.9 liters of intestinal juice. This secretion is triggered either by the distention of the small intestine or in response to the irritating effects of chyme on the intestinal mucosa. Notably, this fluid aids in the lubrication and facilitation of digestive processes within the small intestine.

In the submucosa within the duodenum, a distinctive feature is the presence of specialized mucus-secreting glands called Brunner's glands. These duodenal glands are unique to the duodenum and contribute to the secretion of bicarbonate-rich alkaline mucus. This alkaline mucus serves as a buffering agent, neutralizing the acidic nature of chyme as it arrives from the stomach. Brunner's glands play a crucial role in creating an optimal environment for subsequent digestive processes in the small intestine. The pancreas contributes pancreatic juice rich in digestive enzymes and bicarbonate ions. This secretion not only aids in the breakdown of macronutrients but also helps in maintaining an optimal pH environment for efficient digestive processes in the small intestine.

Mechanical Digestion in the Small Intestine

The coordination of contractions associated with small intestinal motility is governed by a phenomenon known as slow wave activity. This process involves rhythmic, spontaneous electrical depolarizations across the smooth muscle cells of the small intestine. These depolarizations generate slow waves, which set the groundwork for subsequent contractions. The duodenum, the initial part of the small intestine, exhibits a relatively higher frequency of slow waves, typically around 12 per minute. The jejunum demonstrates a slightly slower frequency, averaging around ten slow waves per minute. The ileum, which constitutes the terminal segment of the small intestine, experiences a further decrease in slow wave frequency, with an average of around 8 per minute.

In addition to slow wave activity, the initiation of contractions requires the presence of spike potentials. These spike potentials are transient bursts of electrical activity that occur on top of the slow waves. They play a crucial role in surpassing the threshold for muscle contraction and are integral to the generation of peristaltic waves.

The small intestinal motility is closely linked to pancreatic contribution. Pancreatic secretion aids in the breakdown of complex nutrients and contributes to the regulation of the small intestine's motility. Small intestinal motility is intricately regulated by neural and hormonal inputs. The enteric nervous system coordinates local reflexes, while motilin influences the pace and coordination of contractions. These regulatory mechanisms use slow wave activity and spike potentials to maintain optimal motility patterns. Epinephrine inhibits small intestinal motility, potentially contributing to the digestive slowdown during fight-or-flight responses. Motilin plays a key role in coordinating MMCs, rhythmic contractions that aid in cleansing the small intestine between meals. Serotonin and prostaglandins can stimulate small intestinal motility, influencing the contraction patterns. Hormones such as gastrin, CCK, and insulin contribute to the stimulation of small intestinal contractions, facilitating the movement of chyme. Conversely, hormones like secretin and glucagon inhibit small intestinal contractions, playing a role in regulating the overall motility.

The movement of intestinal smooth muscles involves a process known as segmentation, distinct from the peristaltic mixing waves observed in the stomach. Segmentation in the small intestine manifests as incremental back-and-forth shoving of the intestinal contents, facilitated by the rhythmic contraction and relaxation of rings of smooth muscle. Unlike peristalsis, segmentation does not propel chyme forcefully through the tract. Instead, it serves to mix chyme with digestive juices and pushes food particles against the mucosa, facilitating absorption. The duodenum exhibits the most rapid segmentation, occurring at a rate of approximately 12 contractions per minute, while in the ileum, segmentations occur around eight times per minute (Figure 25).

As chyme absorption progresses and the small intestinal wall becomes less distended, the localized segmentation process transitions to transport movements. The duodenal mucosa secretes the hormone motilin, initiating contractions. These complexes commence in the duodenum, propelling chyme through a short section of the small intestine before coming to a temporary halt. Subsequent contractions initiate a bit farther down the intestine, repeating the process of propelling chyme and momentarily stopping. These complexes traverse the small intestine gradually, taking approximately 90 to 120 minutes to reach the end of the ileum. Upon reaching the ileum, the cycle begins again in the duodenum.³³

Figure 25 | Segmentation | ³⁴ This illustration depicts the process of segmentation, a type of contraction in the small intestine that involves mixing rather than propelling contents forward. The leftmost part shows large, distinct sections (colored in yellow and blue), which represent different portions of intestinal content before segmentation. The middle section shows alternating contractions of the smooth muscle (indicated by arrows pointing toward each other within sections), dividing the contents into smaller, mixed segments. This mixing allows the content to move back and forth, promoting effective blending of digestive enzymes and nutrients. The rightmost section displays uniformly mixed contents (in green), indicating that the segments have been thoroughly combined. This mixed state ensures that nutrients are more accessible for absorption as they continue along the digestive tract.

The ileocecal valve, functioning as a sphincter, typically maintains a constricted state. However, when motility increases in the ileum, this sphincter relaxes, facilitating the entry of food residue into the initial segment of the large intestine, cecum. The relaxation of the ileocecal sphincter is a regulated process involving both nervous and hormonal control mechanisms. On average, it takes approximately 3 to 5 hours for all chyme to traverse the small intestine, emphasizing the coordinated regulation of sphincter activity and motility for optimal digestive function.

Chemical Digestion and Absorption in the Small Intestine

The small intestine secretes a variety of digestive enzymes. Enteropeptidase, also known as enterokinase, is an enzyme produced by the small intestine, specifically by the duodenal mucosa. Its primary role is to activate trypsinogen, an inactive precursor enzyme secreted by the pancreas, into its active form called trypsin. This activation is a crucial step in the digestion of proteins within the small intestine. Trypsin activates other pancreatic zymogens like chymotrypsinogen and procarboxypeptidase. This creates a cascade of enzymatic activations that play a crucial role in breaking down dietary proteins into smaller peptides and amino acids. The activation of trypsinogen by enteropeptidase is a tightly regulated process. It ensures that the activation occurs in the appropriate location (duodenum) and at the right time during the digestive process.

Lactase, Sucrase, and Maltase are brush border enzymes that digest specific disaccharides, lactose, sucrose, and maltose. Peptidases are found on the brush border of the microvilli. These enzymes are associated with the absorptive cells of the small intestine's mucosa.

The small intestine serves as a crucial site for the completion of digestion and absorption of major nutrients. While lipids enter the small intestine largely undigested, the primary focus shifts to lipid digestion, a process facilitated by bile and the enzyme pancreatic lipase. The small intestine takes center stage in water absorption through osmosis. The absorptive cells of the small intestine contribute to digestive processes by synthesizing digestive enzymes and embedding them in the plasma membranes of microvilli. This feature distinguishes the small intestine from the stomach, highlighting that enzymatic digestion occurs not only within the lumen but also on the luminal surfaces of mucosal cells.

For optimal chemical digestion, the delivery of chyme from the stomach to the small intestine must occur gradually and in modest amounts. This approach is imperative because chyme exiting the stomach is typically hypertonic. If large quantities were forcefully introduced into the small intestine, the resulting osmotic water loss from the blood into the intestinal lumen could lead to potentially life-threatening low blood volume. Additionally, the sustained digestion process necessitates an elevation of the low pH characteristic of stomach chyme, alongside the thorough mixing of chyme with bile and pancreatic juices. Both processes demand time, underscoring the importance of carefully controlling the pumping action of the pylorus to prevent the duodenum from being inundated with chyme.

The absorption of nutrients in the small intestine involves five main mechanisms: active transport, passive diffusion, facilitated diffusion, co-transport, and endocytosis. The composition of the cell's plasma membrane, consisting of hydrophobic phospholipids, poses a challenge for water-soluble nutrients. To overcome this, transport molecules embedded in the membrane facilitate the entry of water-soluble nutrients into cells. Tight junctions between epithelial cells in the intestinal mucosa prevent substances from passing between cells. Consequently, the only route for substances to enter blood capillaries is through the apical surfaces of epithelial cells, allowing water-soluble nutrients to enter the capillary blood in the villi. These nutrients then travel to the liver via the hepatic portal vein, ensuring an efficient and controlled absorption process in the small intestine.

Digestion and Absorption of Major Nutrients

Substantial food molecules, such as proteins, lipids, nucleic acids, and starches, necessitate breakdown into smaller subunits to enable absorption through the lining of the alimentary canal. This intricate process is achieved by enzymes through hydrolysis, catalyzing the cleavage of large molecules into their constituent parts. Table 6 encapsulates the multifaceted enzymatic activities crucial for the efficient breakdown of complex nutrients into absorbable forms.⁹

**Table 6** | Digestive Enzymes |
Enzyme Category	Enzyme Name	Source	Substrate	Product
Salivary Enzymes	Lingual lipase	Lingual glands	Triglycerides	Free fatty acids, and mono and diglycerides
Salivary Enzymes	Salivary amylase	Salivary glands	Polysaccharides	Disaccharides and trisaccharides
Gastric Enzymes	Gastric lipase	Chief cells	Triglycerides	Fatty acids and monoacylglycerides
Gastric Enzymes	Pepsin	Chief cells	Proteins	Peptides
Brush Border Enzymes	Enteropeptidase	Small intestine	Trypsinogen	Trypsin
Brush Border Enzymes	Lactase	Small intestine	Lactose	Glucose and galactose
Brush Border Enzymes	Maltase	Small intestine	Maltose	Glucose
Brush Border Enzymes	Nucleosidase and phosphatases	Small intestine	Nucleotides	Phosphates, nitrogenous bases, and pentoses
Brush Border Enzymes	Peptidases	Small intestine	Aminopeptidase; amino acids at the amino end of peptides Dipeptidase; dipeptides	Aminopeptidase; amino acids at the amino end of peptides Dipeptidase; dipeptides
Pancreatic Enzymes	Carboxypeptidase	Pancreatic acinar cells	Amino acids at the carboxyl end of peptides	Amino acids and peptides
Pancreatic Enzymes	Chymotrypsin	Pancreatic acinar cells	Proteins	Peptides
Pancreatic Enzymes	Elastase	Pancreatic acinar cells	Proteins	Peptides
Pancreatic Enzymes	Nucleases	Pancreatic acinar cells	Ribonuclease; ribonucleic acids Deoxyribonucleic acids	Nucleotides
Pancreatic Enzymes	Pancreatic amylase	Pancreatic acinar cells	Polysaccharides (starches)	𝛼-Dextrins, disaccharides (maltose), trisaccharides (maltotriose)
Pancreatic Enzymes	Trypsin	Pancreatic acinar cells	Proteins	Peptides
Pancreatic Enzymes	Pancreatic lipase	Pancreatic acinar cells	Lipids that are emulsified	Monoglycerides and fatty acids

Digestion and Absorption of Carbohydrates

The typical American diet comprises roughly 50 percent carbohydrates. Among the commonly consumed and readily absorbed monosaccharides are glucose, galactose, and fructose. The digestive system is proficient in breaking down various carbohydrates, including the disaccharides sucrose (found in regular table sugar, composed of glucose and fructose), lactose (present in milk, consisting of glucose and galactose), and maltose (common in grains, made up of two glucose units) and polysaccharides, glycogen and starch. Salivary amylase initiates carbohydrate digestion in the mouth, but its activity is inhibited due to the stomach's acidic environment. The pancreas plays a key role in carbohydrate digestion. Pancreatic amylase, released into the small intestine, digests polysaccharides and partially digested carbohydrates (Figure 26). Subsequently, brush border enzymes—sucrase, maltase, and lactase—hydrolyze disaccharides into monosaccharides. Sucrase breaks down sucrose into fructose and glucose, maltase cleaves maltose into two glucose molecules, and lactase digests lactose into glucose and galactose. Although indigestible polysaccharides, such as cellulose, offer no nutritional value, they contribute to dietary fiber, promoting the propulsion of food through the alimentary canal.

Figure 26 | Carbohydrate Digestion | ³⁵ Carbohydrates are broken down into their monomers in a series of steps. Carbohydrate digestion begins with the breakdown of starch and glycogen by salivary amylase, followed by further hydrolysis by pancreatic amylase. The resulting disaccharides—maltose, sucrose, and lactose—are cleaved into monosaccharides by enzymes (maltase, sucrase, and lactase), producing glucose, fructose, and galactose for absorption.

The absorption of carbohydrates occurs exclusively in the form of monosaccharides, and the small intestine demonstrates remarkable efficiency in this process, with an estimated absorption rate of 120 grams per hour. All dietary carbohydrates that undergo normal digestion are absorbed, while indigestible fibers are excreted in the feces. The monosaccharides glucose and galactose utilize common protein carriers referred to as sodium-glucose transporters (SGLT) for transport into epithelial cells, employing secondary active transport in conjunction with sodium ions. Subsequently, these monosaccharides exit the cells through facilitated diffusion, gaining entry into capillaries through intercellular clefts. Conversely, fructose, a monosaccharide present in fruit, undergoes absorption and transport solely through facilitated diffusion.⁹

Digestion and Absorption of Proteins

Proteins undergo digestion to reduce them to their fundamental amino acid components. To sustain normal body structures, an average requirement of approximately one gram of protein per pound of body weight is necessary each day. The initial phase of protein digestion unfolds in the stomach, where hydrochloric acid and pepsin collaborate to break down proteins into smaller polypeptides. Subsequently, these polypeptides proceed to the small intestine.

Figure 27 | Digestion of Proteins | In the stomach, proteins are successively broken down into their amino acid components by the activation of pepsinogen into pepsin by hydrochloric acid (HCl). In the duodenum, enterokinase activates trypsinogen into trypsin, which in turn activates chymotrypsinogen into chymotrypsin. Trypsin and chymotrypsin further hydrolyze peptides and polypeptides into dipeptides. Finally, peptidase enzymes break dipeptides into individual amino acids for absorption.

Within the small intestine, the process of chemical digestion continues with the aid of pancreatic enzymes, each specifically targeting bonds within amino acid sequences. Simultaneously, the cells lining the brush border release enzymes such as aminopeptidase and dipeptidase, further dismantling peptide chains (Figures 27). This breakdown results in the formation of molecules small enough to traverse the intestinal lining and enter the bloodstream (Figure 28).^{3, 9}

Figure 28 | Digestion and Absorption of Proteins | ³⁶ As shown in the illustration, the breakdown of proteins commences in the stomach and is completed within the small intestine. Essential to this process are the secretions of hydrochloric acid from the stomach, along with a suite of protein-digesting enzymes from the stomach, pancreas, and the small intestine's own lining. Although gastric digestion begins the process, the small intestine is where most significant protein digestion takes place. The final products, individual amino acids, are then absorbed into the bloodstream. From there, they travel to the liver for either general circulation to supply body tissues or for metabolic processing and storage.

Active transport mechanisms, primarily concentrated in the duodenum and jejunum of the small intestine, play a pivotal role in the absorption of proteins, predominantly in the form of their breakdown products—amino acids. Remarkably, the small intestine achieves the digestion and absorption of almost all (95 to 98 percent) ingested protein. The carriers facilitating amino acid transport exhibit variability, with most being intricately linked to the active transport of sodium ions. Notably, short chains of two amino acids (dipeptides) or three amino acids (tripeptides) are also subject to active transport mechanisms. However, upon entering the absorptive epithelial cells, these peptide chains undergo subsequent breakdown into their constituent amino acids. Following this intracellular process, the liberated amino acids exit the cell and enter the capillary blood through diffusion. This orchestrated sequence ensures the efficient absorption of amino acids from the digestive milieu into the bloodstream.³

Digestion and Absorption of Lipids

Dietary fat undergoes limited digestion until it reaches the small intestine, specifically the duodenum. In this segment of the digestive tract, fats are subjected to emulsification facilitated by bile salts. Emulsification is a crucial process involving the fragmentation of large fat molecules into smaller fragments (Figure 29). Bile, originating from the liver and stored in the gallbladder, is released into the duodenum in response to a lipid-rich meal, primarily serving the function of emulsification.

Once fats are emulsified, further digestion ensues with the assistance of pancreatic lipase. As implied by its name, pancreatic lipase is produced in the pancreas and then released into the duodenum through the pancreatic duct. Under the action of pancreatic lipase, fat molecules undergo breakdown into monoglycerides and fatty acids. To facilitate their transport within the lumen of the small intestine, these digestion products bind to a plasma protein, forming a structure known as a micelle. This micelle efficiently transports monoglycerides and fatty acids into the intestinal cells, specifically the enterocytes. This orchestrated process ensures the effective absorption of fat components into the enterocytes within the small intestine.

Figure 29 | Digestion of Lipids | Large fat globules are broken down into smaller fat droplets through the action of bile salts.

Once they are inside the enterocyte, fatty acids and monoglycerides undergo reconstruction into triglyceride molecules. These newly formed triglycerides are then bundled together with dietary cholesterol and phospholipids to create particles known as chylomicrons, a type of lipoproteins which play a vital role in facilitating the movement of fats and cholesterol within aqueous environments, such as the lymph and blood.

Chylomicrons exit the enterocytes through the process of exocytosis, entering the lymphatic system via lymph capillaries (lacteals) located in the villi of the small intestine (Figure 30). Subsequently, chylomicrons are transported through the lymphatic system and eventually enter the bloodstream. This route through the lymphatics represents the primary pathway through which dietary lipids gain access to the blood. However, a minor fraction of fatty acids, particularly short-chain fatty acids, are absorbed into the hepatic portal system, allowing them to reach the liver directly. Upon entering circulation, chylomicrons undergo the enzyme lipoprotein lipase action, releasing some fatty acids into tissues. The remaining chylomicron structure, referred to as a chylomicron remnant, is approximately half the size of chylomicrons and relatively rich in cholesterol and cholesteryl esters. These remnants reach hepatocytes, where the liver reutilizes them to convert lipids from chylomicrons into various lipid-containing products. Particularly during periods between meals, fasting, and starvation, the liver exhibits heightened activity in producing lipoproteins to sustain circulating lipid levels.⁹

Figure 30 | Lipid Absorption | ³⁷ This diagram illustrates the process of lipid absorption in the small intestine. Fatty acids and monoglycerides, resulting from fat digestion, enter the epithelial cells by micelles. Fatty acids then re-form into triglycerides, which are then combined with proteins in the Golgi apparatus to form chylomicrons. Chylomicrons are then extruded from the epithelial cell and enter a lacteal (lymph capillary), which transports them away from the intestine in the lymph.

Digestion and Absorption of Nucleic Acids

The nucleic acids DNA and RNA are prevalent components in most of the foods. The digestion of these nucleic acids is facilitated by two types of pancreatic nucleases: deoxyribonuclease, responsible for digesting DNA, and ribonuclease, which digests RNA. The resulting nucleotides from this digestion are further broken down by two intestinal brush border enzymes, nucleosidase and phosphatase. These enzymes break down nucleotides into pentoses, phosphates, and nitrogenous bases. These smaller components are then absorbed through the wall of the alimentary canal. Table 7 provides the list of large food molecules and their essential subunits.

The products of nucleic acid digestion, including pentose sugars, nitrogenous bases, and phosphate ions, are transported across the villus epithelium through active transport mechanisms facilitated by carriers. Following this transport, these products enter the bloodstream, where the body can further utilize them.⁹

**Table 7** | Macromolecules and their simpler forms |
Macromolecule	Simpler Form
Carbohydrates	Monosaccharides: glucose, galactose, and fructose
Proteins	Single amino acids, dipeptides, and tripeptides
Triglycerides	Monoglycerides, glycerol, and free fatty acids
Nucleic acids	Pentose sugars, phosphates, and nitrogenous bases

Electrolyte absorption in the small intestine involves both gastrointestinal secretions and ingested foods. Most electrolytes are absorbed via active transport throughout the small intestine. Co-transport mechanisms result in sodium ion accumulation inside cells, while anti-port mechanisms reduce potassium ion concentration inside cells. To restore the sodium-potassium gradient across the cell membrane, an ATP-dependent sodium-potassium pump pumps sodium out and potassium in. All minerals entering the intestine are generally absorbed except iron and calcium. Ionic iron needed for hemoglobin production is actively transported into mucosal cells. Inside mucosal cells, iron binds to ferritin, creating iron-ferritin complexes for storage. When the body has sufficient iron, most stored iron is lost during the shedding of worn-out epithelial cells. Increased iron uptake occurs during iron deficiency, such as during bleeding. Women experiencing significant iron loss during menstruation have more iron transport proteins in intestinal epithelial cells than men.

The small intestine absorbs naturally occurring vitamins from food and supplements. Fat-Soluble Vitamins (A, D, E, K) are absorbed with dietary lipids in micelles through simple diffusion. Most water-soluble vitamins (B Vitamins, Vitamin C) are absorbed via simple diffusion, except for vitamin B12. Intrinsic factor, secreted in the stomach, binds to vitamin B12, preventing its digestion. The complex binds to mucosal receptors in the terminal ileum, where it is endocytosed for absorption.⁹

15.10: The Small Intestine is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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