Digestion and Absorption

You should be able to:

  1. Identify gastrointestinal and accessory organs that plays a role in digestion and absorption.

  2. Learn about key phases, cells, enzymes, and hormones within the G.I. tract.

  3. Examine how macronutrients and micronutrients are absorbed within the small intestine into circulation.

The Process of Digestion and Absorption

How does the food we eat transform into nutrients used throughout the body? Food consumed must undergo several physical and chemical changes to be absorbed into the bloodstream. This guide will dive into the process by evaluating the functions of both gastrointestinal (GI) organs and accessory organs that participate in digestion and absorption along with key phases, cells, enzymes, and hormones. 

Cephalic Phase

The cephalic phase is initiated by the sight, smell, thought, or the first taste of food to prepare the body for digestion.  The brain stem activates the facial (VII), vagus, and glossopharyngeal nerves.¹ The vagus nerve stimulates gastric glands to release gastric juice while the facial and glossopharyngeal nerves aid in the production of saliva.¹ 

Mouth

The mouth consists of the teeth and tongue which helps in chewing and swallowing. The process of chewing is physically digesting the food by breaking down the food into smaller pieces. Minor and major glands in the mouth secrete saliva, including the parotid, submandibular, and sublingual glands.¹ Saliva is a mixture of water and ions such as sodium, potassium, and chloride.¹ The process of swallowing food into the pharynx (throat) is not only a function performed by the tongue; the uvula (dangling muscular structure) and soft palate (back of the mouth) close off the nasopharynx so food does not go into the nasal cavity. A structure known as the epiglottis covers the laryngopharynx or windpipe.  

Mechanical Digestion: Chewing, or mastication, is a form of mechanical digestion that breaks down food into a smaller mass called a bolus. Saliva mixes with food, causing salivary amylase to break down the starchy foods into simpler disaccharides and monosaccharides (absorbed in the bloodstream).¹ 

Chemical Digestion: Salivary amylase is found within the parotid gland’s secretions. It is a digestive enzyme activated by chloride ions found in the saliva and breaks down starch into maltose, maltotriose, and a-dextrin within the mouth.¹

Lysozyme is a bactericidal enzyme that kills microbes but is not found in a large quantity within the oral cavity. 

Lingual lipase is secreted from minor salivary glands found on the tongue that convert triglycerides in the diet (fat) into diglycerides and simple fatty acids.¹ It does not work within the mouth and is activated by stomach acid.

Pharynx

The pharynx can be divided into the nasopharynx, oropharynx, and laryngopharynx. During swallowing, food passes into the oropharynx (uvula, soft palate, and epiglottis prevent food from entering the nose and windpipe). No mechanical or chemical digestion occurs in this area. 

Esophagus

The esophagus is a muscular tube that connects to the pharynx and transports the bolus to the stomach. The muscular layer consists of circular and longitudinal muscles. The upper esophageal sphincter transports food from the pharynx to the stomach, and the lower esophageal sphincter transports food from the esophagus to the stomach.¹ During the esophageal phase of swallowing, peristalsis (coordinated contractions and relaxations of the muscular layer) occurs once the bolus enters the esophagus. Peristalsis begins with circular muscle contractions to push the bolus down, and then the longitudinal muscles contract below the bolus to push it downwards. This continues until the lower esophageal sphincter opens to release the bolus. 

Gastric Phase

The gastric phase begins when food enters the stomach. The stomach has stretch receptors and chemoreceptors (monitor the pH of stomach chyme). Food entering the stomach causes stretching and an increase in the pH of hydrochloric acid (stomach acid) due to protein buffering. Nerve impulses from the stretch receptors and chemoreceptors relay this information, and the submucosal neural plexus sends nerve impulses that cause parietal cells in the stomach to release stomach acid and perform peristalsis.¹ Once stomach acid mixes with the chyme and releases a portion into the small intestine, the stomach contractions lessen, and pH increases to suppress the secretion of stomach acid until finished. Hormones, such as gastrin, stimulate gastric glands to secrete gastric juice, prevent reflux, increase movement, and relax the pyloric sphincter allowing for gastric emptying. 

Stomach

The stomach is a J-shaped tube connected to the esophagus and the duodenum, the first section of the small intestine. The lower esophagus to the anal canal consists of four tissue layers; from superficial to deep, the layers are mucosa (epithelial layer, lamina propria [areolar connective tissue with MALT, or mucosa-associated lymphoid tissue], and muscularis mucosae), submucosa, muscular layer, and serosa.¹ 

Key cells found within the stomach include gastric glands containing exocrine cells such as mucous neck cells, parietal cells, and chief cells. Gastric juice is composed of the secretions of these cells:

Mucous neck cells - secrete mucus.

Parietal cells - secrete hydrochloric acid (HCl) and provide an intrinsic factor for absorption of vitamin B12

Chief cells - secrete enzymes pepsinogen (breaks down polypeptide into amino acids) and gastric lipase (breaks down triglycerides into diglycerides and monoglycerides). 

G cells - enteroendocrine cell that secretes gastrin hormone into the bloodstream. 

Mechanical Digestion: Once food enters the stomach, peristalsis occurs and moves the bolus into the lower stomach. With the pyloric sphincter slightly open, particles that are small enough are able to pass and move into the duodenum. Large particles are retropulsed and undergo peristalsis until they can pass.¹ The liquid mixture of food and gastric juice combined is known as chyme. 

Chemical Digestion: Parietal cells secrete hydrogen and chloride ions separately into the lumen of the stomach which combine to form HCl.¹ The enzyme carbonic anhydrase found in parietal cells combines water and carbon dioxide to form carbonic acid (H2CO3). Carbonic acid dissociates into hydrogen and bicarbonate. The bicarbonate is exchanged for chloride ions which enter the lumen, combining with hydrogen to form HCl. The bicarbonate is taken up by blood capillaries for excretion.¹

The enzyme pepsin is secreted by chief cells and denatures dietary proteins by breaking peptide bonds into amino acids or shorter peptides. Pepsin is only activated within the stomach acid. 

The enzyme gastric lipase, secreted from the minor salivary glands of the tongue, activates within the stomach acid and breaks down dietary fats into fatty acids and monoglycerides.¹ It is not a major fat-digesting enzyme compared to pancreatic lipase. 

Chyme is emptied into the duodenum, the first portion of the small intestine, after 2-4 hours.¹ 

Pancreas 

The pancreas is a retroperitoneal (behind the peritoneum, or the membrane that covers the front of most abdominal organs) organ that is not part of the G.I. tract. The pancreas has endocrine and exocrine functions. Endocrine functions of the pancreas arise from the pancreatic islets and produce hormones such as insulin, glucagon, somatostatin (stops gastrin), and pancreatic polypeptide that travels through the bloodstream to the targeted areas.¹ 

Chemical Digestion: The exocrine function plays more of a role in the chemical digestion of food. The exocrine functions constitute 99% of the glandular clusters of the pancreas and are known as the pancreatic acini.¹These cells secrete pancreatic juice which is composed of water, enzymes, sodium bicarbonate, and salts. Sodium bicarbonate has a higher pH (more basic) and neutralizes the acidic chyme to prevent intestinal damage; it also allows for digestive enzymes in the intestines to initiate reactions while halting the activity of the enzyme pepsin.

Enzymes in the pancreatic juice consist of:

Pancreatic amylase - breaks down starch into monosaccharides 

Trypsin, chymotrypsin, carboxypeptidase, elastase - breaks down peptides (proteins) into amino acids

Pancreatic lipase - a principal triglyceride-digestive enzyme in adults breaks down triglycerides into mono and di-glycerides. 

Ribonuclease, deoxyribonuclease - digests RNA and DNA into nucleotides. 

Liver

The liver is the largest gland in the body composed of hepatocytes, or specialized hexagonal epithelial cells that secretes bile that serves both secretory and excretory functions. The bile canaliculi in between the hepatocytes collects bile which makes its way through a series of ducts into the common hepatic duct.¹ From this point, the common hepatic duct joins the cystic duct into the bile duct within the gallbladder and releases bile into the duodenum.¹

Role in Chemical Digestion: Bile consists of water, bile salts (potassium and sodium salts), cholesterol, ions, bile pigments, and phospholipids known as lecithin.¹ Bilirubin is a bile pigment derived from heme of red blood cells that are broken down in the intestines, producing stercobilin that makes feces brown. The bile salts emulsify fat (turning large lipid molecules into smaller globules) and help pancreatic lipase work at a faster absorption rate. 

The liver also plays a role in the metabolism of macronutrients, storage of vitamins/activating vitamin D, creation of bile salts, and processing of drugs and hormones. 

Gallbladder

The gallbladder is a sac-like organ positioned under the liver with a mucosa and smooth muscle layer. 

Role in Chemical Digestion: The major function of the gallbladder is to store and release bile produced by the liver. The gallbladder also concentrates the liquid bile and absorbs some water and ions.¹

Intestinal Phase

When chyme exits the stomach into the small intestine, the intestinal phase of digestion is initiated. The enterogastric reflex causes stretch receptors in the duodenum to send nerve impulses to the medulla oblongata which stops gastric motility and contracts the pyloric sphincter in order not to overload the small intestine.¹

Hormones cholecystokinin (CCK) and secretin play important roles within the intestinal phase. Cholecystokinin from CCK cells is prompted by partially digested amino acids and triglycerides, causing the secretion of pancreatic juice and bile while inhibiting gastric emptying and producing the feeling of fullness.¹ Secretin, stimulated by acidic chyme entering the duodenum, results in the release of pancreatic juice to neutralize acid and stops the formation of gastric juice. 

Small Intestine

The small intestine is a 16-foot-long folded tube divided into three sections beginning from the stomach and ending at the large intestine: duodenum, jejunum, and ileum.¹ The small intestine is primarily responsible for digestion and absorption of nutrients. Specialized cells within the small intestine include:

  • Absorptive cells: found in the mucosa’s epithelium containing digestive enzymes and intestinal microvilli that help absorb nutrients

  • Goblet cells: found in the mucosa’s epithelium and produce mucus 

  • Paneth cells: found within intestinal glands and produce lysozyme, a bactericidal enzyme to destroy microbes

  • Enteroendocrine cells: found within intestinal glands and secretes secretin, cholecystokinin (CCK), and glucose-dependent insulinotropic peptide (GIP) from S, CCK, and K cells. 

The small intestine also contains mucosa-associated lymphoid tissue (MALT) (within the mucosa’s lamina propria), Peyer’s patches (aggregated lymphoid nodules in the ileum), and duodenal glands (secrete alkaline mucus to neutralize chyme). 

The small intestine contains specialized structures, including:

Circular folds - mucosa and submucosa folds to enhance absorption.¹

Intestinal villi - tiny projections from mucosa that increase absorption and contain blood and lymph capillaries responsible for transporting absorbed nutrients to plasma.¹

Microvilli - tiny projections from absorptive cells’ membranes forming a ‘microvillous brush border’ containing enzymes and increasing surface area.¹

Microvillous Brush Border 

The microvillous brush border contains several enzymes that digest at the absorptive cell level. Includes, but is not limited to, :

Carb-digesting enzymes: a-dextrinase, maltase, sucrase, lactase

Protein digesting enzymes: aminopeptidase and dipeptidase

Nucleotide digesting enzymes: nucleosidases and phosphatases

Intestinal juice is also found within the lumen of the small intestine with a slightly alkaline pH to neutralize acidic chyme and is paired with pancreatic juice to digest what is remaining.¹

Mechanical Digestion: The small intestine uses segmentations (localized, mixing contractions) to mix chyme with digestive juices and does not propagate chyme through the intestine. After segmentation, a form of peristalsis known as the migrating motility complex pushes chyme forward.¹

Chemical Digestion: Carbohydrates, proteins, and fats have been partially or mostly digested within the GI tract but may still require further breakdown. 

Carbohydrates - Pancreatic amylase and a-dextrinase break down any undigested or partially digested starch into a-dextrin glucose units.¹ 

Sucrase, lactase, and maltase break down sucrose, lactose, and maltose into monosaccharides. 

Cellulose, or indigestible plant fiber, is not broken down by enzymes.

Protein/Nucleotides - Pancreatic juice enzymes, trypsin, chymotrypsin, carboxypeptidase, and elastase, break down protein into peptides.

Brush border enzymes, aminopeptidase and dipeptidase, work together to further break down protein into amino acids. 

Pancreatic juice enzymes, ribonuclease and deoxyribonuclease, digest DNA and RNA within the stomach and small intestine.

Brush border enzymes, nucleosidases and phosphatases, break down nucleotides into pentoses, phosphates, and nitrogenous bases.¹

Lipids - Pancreatic lipase breaks down triglycerides into fatty acids and monoglycerides after triglycerides are emulsified by bile salts. 

Absorption within Small intestine

The digested nutrients must be transported into intestinal absorptive cells. After moving from the absorptive cells, nutrients enter the blood or lymph plasma to be used by cells.¹

Carbohydrates: Monosaccharides are the only form of carbohydrates that can be absorbed. 

Glucose and galactose are transported by secondary active transport with sodium into enterocytes. 

Fructose is transported by facilitated diffusion into enterocytes. 

The monosaccharides undergo facilitated diffusion into the blood capillaries of intestinal villi and then enter hepatocytes in the liver via the hepatic portal vein.¹ If not removed, it enters general circulation. 

Cellulose and fibers are not absorbed. 

Protein: Amino acids are transported into enterocytes via active transport or secondary active transport and arise from digested food, sloughed cells, and digestive juices. Amino acids diffuse into the intestinal villi and follow the same pathway as monosaccharides.¹

Lipids: Fatty acids and monoglycerides from the diet are absorbed by simple diffusion into enterocytes. Short-chain fatty acids (10-12 carbon atoms) pass from absorptive cells of the intestine using simple diffusion since it is more water soluble, following the same path as carbs (monosaccharides) and protein (amino acids).¹ Large short-chain fatty acids (>10-12 carbons), long-chain fatty acids, and monoglycerides are bundled into micelles (spherical bile salts surrounding fatty acids, monoglycerides, fat-soluble vitamins, and cholesterol) and enter absorptive cells through diffusion.¹ Chylomicrons are formed by combining long-chain fatty acids and monoglycerides back into triglycerides.¹ The triglycerides form a spherical shape along with phospholipids, cholesterol, and a coating of proteins. Chylomicrons exit the absorptive cells into lymphatic capillaries via exocytosis due to their larger size and then enter blood capillaries. An enzyme known as lipoprotein lipase breaks down chylomicrons back into monoglycerides and fatty acids within hepatocytes and adipose tissue to once again form triglycerides. Bile salts are reabsorbed in the ileum and recycled when transported back to the liver via the hepatic portal system - the process is known as enterohepatic circulation.¹

Electrolytes: Substances that dissociate (gain or lose electrons) into charged particles (ions) are termed electrolytes. Sodium enters intestinal absorptive cells during absorption (such as with monosaccharides and amino acids) but is removed by Na+-K+ ATPases out of cells into the bloodstream.

Bicarbonate, chloride, iodide, and nitrate ions are negatively charged and follow sodium or are actively transported. 

Hormone calcitriol actively transports calcium. Potassium, magnesium, and phosphate ions are actively transported.¹

Vitamins: Fat-soluble vitamins, A, D, E, and K, are included in micelles and absorbed by simple diffusion into enterocytes, packed into chylomicrons, and exit via lymph circulation. Vitamin B12 combines with the intrinsic factor found within the stomach and is actively transported within the small intestine.¹ Other water-soluble vitamins are generally absorbed by simple diffusion. 

Water: Water and fluids during the process of digestion and absorption are reabsorbed in the small intestine and large intestine via osmosis and can move into blood cells or stay in the intestinal lumen depending on the concentration needed to maintain osmotic balance.¹

Chyme is processed within the small intestine for 3-5 hours.¹

Large Intestine

The large intestine is a five-foot-long tube that ultimately serves for feces formation and defecation but also plays a role in absorption. From beginning to end, the large intestine can be divided into the cecum, colon (ascending, transverse, descending, and sigmoid) and rectum. The rectum connects to the anal canal where feces are moved and expelled by control of the internal and external anal sphincter.¹

Specialized cells found within the large intestine include: 

Intestinal glands - contain absorptive cells with microvilli and goblet cells that secrete mucus

Tenia coli - three thickened bands of longitudinal muscles within the muscular layer that are found along the length of the large intestine

Haustra - contractions of tenia coli forming pouches within the colon 

Mechanical Digestion - A gastroileal reflex causes peristalsis to intensify after a meal, leading to the slow emptying of chyme from the ileum (last part of the small intestine) to the cecum (beginning of the large intestine) through the ileal orifice.¹

Haustral churning is a movement specific to the large intestine where chyme fills the relaxed haustra (pouches) and contracts when full, moving the chyme into the next haustra. Mass peristalsis, or a large peristaltic wave, occurs in the transverse colon to drive the chyme from the transverse colon into the rectum and is initiated by food in the stomach.¹

Chemical Digestion - Bacteria ferment any undigested carbohydrates, such as cellulose, releasing carbon dioxide, methane gasses, and hydrogen that contribute to flatulence (or farts)! The fermentation of cellulose also produces short-chain fatty acids used for energy in GI cells and serves roles in aiding healthy gut bacteria. The remaining protein is converted to amino acids by bacteria; end products result in the feces’ smell or are broken down into simpler compounds in the liver to pass through urine.¹ Bilirubin from bile is converted into compounds such as stercobilin (which makes feces brown). Bacteria in the large intestine also absorb vitamin K and B vitamins. 

Absorption - Chyme remains in the large intestine anywhere between 3-10 hours before being considered feces.¹ Water is absorbed from chyme during the passage through the large intestine, resulting in feces. Feces’ composition includes but is not limited to, dead cells, indigestible fibers, water, inorganic salts, and bacteria.¹

Defecation - Defecation occurs when feces pass from the rectum to the anus. When feces move into the rectum, stretch receptors send nerve impulses to the sacral spinal cord which then sends motor impulses for the colon to contract.¹ The contraction results in pressure which opens the internal anal sphincter. The external anal sphincter is voluntarily controlled in adults and defecation occurs when ready. If feces are not passed, the feces back up into the sigmoid colon until the next peristaltic wave. Diarrhea occurs due to increased motility and less absorption within the small intestine, causing increased fluid, volume, and movement of feces. Constipation occurs due to decreased motility and excessive water reabsorption in the large intestine, resulting in infrequent, hard, and dry stools. 

Other Notable Hormones within the G.I. Tract

Several hormones play a role during digestion and absorption in the G.I. tract, including:

Ghrelin - increases appetite.

Glucose-dependent insulinotropic peptide (GIP)/Glucagon-like peptide (GLP) - stimulates the release of insulin from the pancreas.

Source(s):

1. Jerry Tortora and Bryan Dickerson, Principles of Anatomy & Physiology, 16th ed. (New Jersey: John Wiley & Sons, 2021).