Digestive System Anatomy

Digestive System Overview

• The digestive system spans from the mouth to the anus
• Primary organs include the mouth, pharynx, esophagus, stomach, small and large intestines (duodenum, jejunum, ileum, colon, rectum, and anal canal)
• Accessory organs include salivary glands, liver, gall bladder, and pancreas

Mouth Anatomy

• The mouth's anatomy encompasses lips, cheeks, teeth, gums, tongue, hard palate, soft palate, and the floor of the mouth
• The mouth is divided into the vestibule (space between teeth, lips, and cheeks) and the oral cavity proper

Mouth Details

• The mouth contains teeth, salivary glands, and the tongue
• Teeth are categorized as incisors (cutting), canines (tearing), premolars, and molars (grinding)
• The tongue consists of intrinsic and extrinsic muscles
• Taste buds facilitate gustatory sensation

Pharynx

• The pharynx is a muscular tube connecting the oral and nasal cavities to the larynx and oesophagus
• It starts at the base of the skull and ends at the inferior border of the cricoid cartilage (C6)
• The pharynx has three parts: nasopharynx, oropharynx, and laryngopharynx (superior to inferior)

Esophagus

• The esophagus is a fibromuscular tube
• It begins at the inferior border of the cricoid cartilage (C6) and extends to the cardiac orifice of the stomach (T11)
• The esophagus is 25 cm long
• It pierces the diaphragm at the tenth thoracic vertebra (T10)
• The abdominal part of the esophagus is the remaining 2.5 cm
• The esophagus consists of cervical, thoracic, and abdominal components

Stomach

• The stomach is an intraperitoneal digestive organ located between the oesophagus and the duodenum
• It has a 'J' shape with lesser and greater curvatures
• The anterior and posterior surfaces are smoothly rounded with a peritoneal covering
• Four main divisions exist: cardia, fundus, body, and pylorus
• The cardia surrounds the superior opening of the stomach at the T11 level
• The fundus is rounded, often gas-filled, and located superior and to the left of the cardia
• The body is the large central portion inferior to the fundus
• The pylorus connects the stomach to the duodenum, divided into the pyloric antrum, canal, and sphincter
• The pyloric sphincter marks the transpyloric plane at L1

Small Intestines

• The small intestines extend from the pylorus of the stomach to the ileocecal junction
• They are approximately 6 to 6.5 m long
• The parts are the duodenum, jejunum, and ileum
• The duodenum is about 10 inches
• The jejunum is about 100+ inches
• The ileum is about 150+ inches

Duodenum Details

• The duodenum marks the beginning of the small intestines
• It is the shortest and widest part of the small intestines, wider than the jejunum
• The duodenum is 25 cm (10 inches) long, extending from the pylorus to the ligament of Treitz
• The duodenum can be subdivided into four parts
• The first part is 2 inches long
• The second part is 3 inches long
• The third part is 4 inches long
• The fourth part is 1 inch long

Jejunum and Ileum

• The jejunum's diameter is greater than the ileum's
• The walls of the jejunum appear thicker than the ileum
• The jejunum is more vascular than the ileum
• The jejunum’s luminal surface has plicae circulares
• The ileum has more mucosa-associated lymphoid tissue (MALT) than the jejunum
• The ileum terminates at the ileocecal valve
• The ileocecal valve prevents reflux of the bolus from the large intestines

Jejunum vs. Ileum

• The jejunum is located in the upper left quadrant, while the ileum is in the lower right quadrant
• The jejunum has a thick intestinal wall, and the ileum has a thin intestinal wall
• The jejunum has longer vasa recta (straight arteries), while the ileum has shorter vasa recta
• The jejunum has less arcades ( arterial loops), the ileum has more
• The jejunum's color is red, while the ileum's is pink

Large Intestines

• The large intestine has a wide diameter and less length than the small intestine
• Special features include sacculations (haustrations), appendices epiploica, and three bands of taenia coli
• Parts include the cecum, ascending colon, ileum, and appendix
• The cecum is 6 X 7.5 cm
• It has three communications

Appendix and Colon

• The appendix is a narrow, blind-ended tube
• The appendix lies in the right iliac fossa, is 2-15 cm long, and has a 5mm diameter
• The colon is the distal part of the gastrointestinal tract, extending from the cecum to the anal canal
• The colon averages 150 cm
• The colon can be divided into ascending, transverse, descending, and sigmoid sections
• These form an arch encircling the small intestine

Colon Divisions

• The ascending colon is 5 inches long, from the cecum to the hepatic flexure and is retroperitoneal
• The transverse colon is 20 inches long, extending from the hepatic flexure to the splenic flexure, is not retroperitoneal, has a transverse mesocolon, and is mobile
• The descending colon is 10 inches long, from the splenic flexure to the sigmoid colon, is retroperitoneal, and is fixed

Sigmoid Colon and Rectum

• The sigmoid colon is 15 inches long and S-shaped
• It extends from the descending colon to the rectum
• It is completely covered by the peritoneum and terminates at the level of the third sacral vertebra (S3)
• The rectum is the distal part of the large gut
• It is located between the sigmoid colon and anal canal
• The rectumis 12 cm long
• It is located in the lesser pelvis, in front of the lower 3 pieces of the sacrum and coccyx
• The rectum lacks sacculations, taeniae coli, and appendices epiploicae

Anal Canal

• The anal canal is the last part of the gastrointestinal tract
• It is surrounded by internal and external anal sphincters
• The anal canal is about 3 to 4 cm long
• It begins at the anorectal junction distally and ends at the anus
• It is divided into columnar, intermediate and cutaneous anatomical zones
• The pectinate (dentate) line divides it into upper (two-thirds) and lower (one-third) parts

Salivary Glands

• The salivary glands are exocrine glands
• Types of glands: parotid, submandibular, and sublingual
• The parotid gland is located between the ramus of the mandible and sternocleidomastoid muscle, its excretory duct is Stensen's duct (opens on the buccal wall at the level of the upper second molar)
• The submandibular gland is located beneath the tongue and its excretory duct is Wharton's duct (opens at sublingual papilla under the tongue
• The sublingual gland is located beneath the sublingual fold and has multiple ducts that open along sublingual folds

Liver

• The liver is the largest gland in the human body
• The liver weighs approximately 1.5 kilograms
• The liver has right, left, caudate, and quadrate lobes
• The liver’s surfaces include diaphragmatic and visceral
• Ligaments include coronary, left triangular, falciform, round ligament, and venous
• Fissures and recesses include the porta hepatis, subphrenic recess, and hepatorenal recess

Gallbladder

• The gallbladder is a pear-shaped sac that communicates with the common hepatic ducts via the cystic duct
• It appears grey-blue in vivo
• It is 7.5 – 12 cm long
• It is found on the inferior aspect of the anatomical right lobe of the liver
• It accommodates 25 – 30 mL of bile under normal circumstances, but can expand up to 50 mL
• Parts include fundus, body, and neck

Pancreas

• The pancreas is approximately 15 cm
• The pancreas is an accessory and exocrine digestive gland and an endocrine gland producing hormones
• It is retroperitoneal with five parts and an internal duct system
• It lies obliquely across the posterior abdominal wall at the L1 and L2 vertebral bodies
• External parts include head, uncinate process, neck, body, and tail
• Internal parts consist of the main and accessory pancreatic ducts