The Human Digestive System

Vocabulary flashcards covering the human digestive system, including digestive tract structures, associated organs, tissue layers, stomach phases, secretions, and intestinal histology.

Digestion

The breakdown of large organic molecules into smaller molecules that can be absorbed by the body.

Digestive Tract

Also called the gastrointestinal (GI) tract; a continuous muscular tube extending from the mouth to the anus through which food travels.

Associated Organs

Organs not located directly along the digestive tract that deliver secretions via ducts, including the salivary glands, liver, gallbladder, and pancreas.

Mucosa

The innermost layer (tunic) of the digestive tract wall that secretes mucus and contains epithelium, lamina propria, and muscularis mucosae.

Submucosa

The connective tissue layer located above the mucosa that contains blood vessels, nerves, lymphatic nodules, and small glands.

Muscularis

The muscle layer of the tract wall consisting of circular and longitudinal smooth muscle layers (and oblique muscle in the stomach).

Serosa (Adventitia)

The outermost layer of the digestive tract wall consisting of a connective tissue layer and simple squamous epithelium; also known as the visceral peritoneum.

Layers of the Digestive Tract Wall

The four fundamental tunics forming the wall: the mucosa, submucosa, muscularis, and serosa.

Visceral Peritoneum

The smooth serous membrane that covers all the abdominal organs.

Parietal Peritoneum

The serous membrane that lines the internal walls of the abdominal cavity.

Mesentery

A continuous fold of peritoneal tissue that attaches the intestines to the posterior abdominal wall, supplying them with blood vessels, lymphatics, nerves, and fat.

Lesser Omentum

A peritoneal fold connecting the lesser curvature of the stomach to the liver and diaphragm.

Greater Omentum

A large peritoneal fold extending from the greater curvature of the stomach to attach to the transverse colon and posterior body wall.

Parotid Glands

Salivary glands situated in front of the ears that produce a thin, watery fluid rich in digestive enzymes.

Submandibular Glands

Salivary glands located beneath the lower jawbone that secrete a balanced mixture of fluid and mucus, maintaining oral moisture when not eating.

Sublingual Glands

Salivary glands situated directly beneath the tongue that secrete a thick, viscous mucus designed for lubrication.

Salivary Amylase

A digestive enzyme present in saliva that initiates the chemical breakdown of carbohydrates.

Lysozyme

An enzyme present in saliva with antibacterial properties that protects against oral bacteria.

Enamel

The hardest substance in the human body, forming the protective outer layer of the tooth crown.

Hard Palate

The anterior, bony portion of the oral cavity roof that provides a solid surface against which the tongue presses food.

Soft Palate

The posterior, muscular part of the palate that closes off the nasal cavity during swallowing.

Peristalsis

Coordinated, wave-like smooth muscle contractions that propel food and chyme through the digestive tract.

Heartburn

A burning sensation caused by the regurgitation of acidic gastric juices from the stomach into the esophagus.

Rugae

Large internal folds of the stomach wall that unfold to allow the organ to expand as food enters.

Chyme

A semifluid, paste-like mixture of partially digested food particles and gastric secretions formed in the stomach.

Muscular Layers of the Stomach

Three distinct smooth muscle layers—outer longitudinal (shortens stomach), middle circular (constricts to mix), and inner oblique (churns food).

Pyloric Sphincter

A thick ring of smooth muscle located at the pyloric opening that regulates the exit of chyme into the duodenum.

Cephalic Phase

The first phase of gastric secretion control, triggered by the sight, smell, taste, or thought of food prior to ingestion via vagus nerve stimulation.

Gastric Phase

The second phase of gastric secretion control, initiated by stomach distention and partially digested proteins, prompting gastrin release and acid secretion.

Intestinal Phase

The third phase of gastric control, triggered by acidic chyme ($\text{pH } < 2$) or lipids entering the duodenum, which inhibits gastric secretions and slows stomach emptying.

Pancreatic Islets

Clusters of endocrine cells in the pancreas that produce and secrete the hormones insulin and glucagon.

Portal Triad

A vascular and ductal arrangement situated at the corners of liver lobules, consisting of a hepatic artery branch, hepatic portal vein branch, and hepatic duct.

Hepatic Sinusoids

Enlarged capillary spaces between liver cords containing phagocytic cells that filter blood and remove foreign particles.

Common Bile Duct

The duct formed by the confluence of the common hepatic duct and the cystic duct from the gallbladder.

Bile

An alkaline fluid secreted by hepatocytes ($\sim 700\,\text{mL/day}$) that neutralizes acidic gastric juice and emulsifies fats.

Cholecystokinin (CCK)

A duodenal hormone released in response to chyme that stimulates gallbladder contraction and relaxes hepatopancreatic sphincters to release bile.

Duodenum

The first segment of the small intestine ($25\,\text{cm}$ in length) containing circular folds, villi, and openings for bile and pancreatic secretions.

Crypts of Lieberkôhn

Tubular mucosal glands (intestinal glands) located at the bases of villi housing granular cells and hormone-secreting endocrine cells.

Segmental Contractions

Localized, alternating contractions of short segments of the small intestine that churn and mix digestive contents with secretions without forward propulsion.

Ileocecal Valve

A one-way valve at the ileocecal junction that prevents the retrograde movement of fecal matter from the cecum into the ileum.

Cecum

The blind pouch at the beginning of the large intestine that connects to the ileum and gives rise to the appendix.

Large Intestine

The terminal portion of the digestive tract consisting of the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal.

Absorption

The movement of digested nutrients and electrolytes across the intestinal mucosal epithelium into the blood or lymphatic vessels.

Transportation

The systemic circulation of absorbed nutrients via the bloodstream and lymphatic vessels to various tissues and cells for energy, repair, and storage.