12: Digestive System
- Page ID
- 100178
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- 12.1: Overview of the Digestive System
- The function of the digestive system is to break down the foods you eat, release their nutrients, and absorb those nutrients into the body. Although the small intestine is the workhorse of the system, where the majority of digestion occurs, and where most of the released nutrients are absorbed into the blood or lymph, each of the digestive system organs makes a vital contribution to this process.
- 12.2: Liver, Pancreas, and Gallbladder — Accessory Digestive System Organs
- Chemical digestion in the small intestine relies on the activities of three accessory digestive organs: the liver, pancreas, and gallbladder. The liver produces bile and performs extensive metabolic and detoxifying functions, the gallbladder stores and concentrates bile for release during meals, and the pancreas provides digestive enzymes and bicarbonate while also releasing hormones that regulate blood glucose.
- 12.3: Structural Organization of the Intestinal Wall and Peritoneum
- The wall of the digestive tract is organized into four layers — mucosa, submucosa, muscularis, and serosa or adventitia — each specialized to support protection, secretion, absorption, mixing, and propulsion. These layers vary by region to match function, such as churning in the stomach and absorption in the small intestine. The peritoneum surrounds and supports many abdominal organs, reducing friction and providing pathways for blood vessels, lymphatics, and nerves.
- 12.4: From the Mouth to the Stomach
- The mouth, pharynx, esophagus, and stomach coordinate the early stages of digestion by mechanically breaking down food, initiating chemical digestion, and safely moving a bolus toward the stomach. Saliva, teeth, and swallowing reflexes prepare food for transport, while peristalsis delivers it to the stomach. The stomach then acts as a temporary storage and mixing chamber, using strong muscular contractions and gastric secretions to convert food into chyme before controlled release into the small
- 12.5: Anatomy of the Small Intestine
- The small intestine is the primary site of digestion and absorption, structured as a long, highly folded tube divided into the duodenum, jejunum, and ileum. Its internal surface is amplified by circular folds, villi, and microvilli, creating an enormous absorptive area that maximizes nutrient uptake into blood and lymph. Supported by the peritoneum and mesentery, it efficiently completes chemical digestion and transfers nearly all usable nutrients to the body before contents move into the large
- 12.6: Activities of the Small Intestine
- The small intestine completes chemical digestion using pancreatic enzymes, bile, and brush border enzymes to break carbohydrates, proteins, and lipids into absorbable units. Monosaccharides and amino acids enter the bloodstream directly, while most lipids are packaged into chylomicrons and transported through the lymphatic system before reaching the blood. Together, these coordinated processes ensure that nutrients are efficiently absorbed and delivered to the liver and tissues for energy, stora
- 12.7: Anatomy of the Large Intestine
- The large intestine consists of the cecum, colon, rectum, and anal canal, and features teniae coli, haustra, and numerous goblet cells that facilitate lubrication and the movement of waste. Beyond its role in digestion and elimination, it is clinically important because most colorectal cancers develop from polyps in its mucosa, making regular screening critical for prevention and early detection.
- 12.8: Activities of the Large Intestine
- The large intestine completes digestion by absorbing remaining water and electrolytes, compacting residue into feces, and coordinating elimination through the defecation reflex. Its resident gut microbiome ferments undigested carbohydrates, produces vitamins such as vitamin K and certain B vitamins, and supports immune and barrier functions. Together, colonic motility, microbial activity, and fluid regulation maintain bowel health and overall homeostasis.
- 12.9: Lactose Intolerance
- Lactose intolerance results from insufficient lactase production in the small intestine, preventing the digestion of lactose into absorbable sugars. Undigested lactose passes into the large intestine, where bacterial fermentation produces gas, bloating, cramping, and diarrhea. Unlike milk allergy, it is not immune-mediated and is typically managed through dietary modification, lactose-free products, or lactase supplements.
- 12.10: Peptic Ulcer Disease
- Peptic ulcer disease is a breakdown of the protective mucosal barrier of the stomach or duodenum, allowing acid to erode the underlying tissue. Most ulcers are caused by Helicobacter pylori infection or long-term NSAID use, both of which weaken normal defenses against gastric acid. Treatment typically includes acid suppression and, when H. pylori is present, combination antibiotic therapy, while complications such as bleeding or perforation require urgent medical care.