Digestive System Anatomy

The Digestive System Anatomy

• The digestive system includes the alimentary canal and its accessory organs and glands

Objectives

• Enumerate the digestive system's anatomical parts
• Discuss the anatomy of the alimentary canal and related organs, including salivary glands, teeth, the tongue and accessory organs
• Identify the liver's location, lobes, relations, and attachments
• Enumerate the parts of the biliary system
• Discuss the anatomy of the pancreas and its exocrine part
• Identify the anatomy of the spleen and its relations
• Mention clinically related notes about the digestive system

Organs of the Digestive System

• The main parts of the alimentary tract are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine
• The accessory organs and glands include the salivary glands, pancreas, and liver and biliary tract
• The alimentary tract traverses the head, neck, thorax, abdomen, and pelvis
• The alimentary tract is approximately 9 meters (30 feet) long in length, but shorter overall in a living person
• The tract starts at the mouth cavity and ends at the anus
• The spleen is anatomically related to the stomach but functionally related to the lymphatic system

The Mouth Cavity

• It represents the first part of the alimentary tract
• Located in the head, it is bounded anteriorly by the lips, laterally by the cheeks, and posteriorly by the oropharynx
• The mouth cavity is divided into 2 parts:
• The vestibule is a narrow slit between the lips/cheeks externally and the teeth/gums internally
• The oral cavity proper (buccal cavity) forming the rest of the mouth cavity, containing mainly the tongue
• It is roofed by the palate that separates it from the nasal cavity, with the uvula hanging from the posterior edge of the soft palate
• The ducts of salivary glands open into the mouth cavity
• The palatine and lingual tonsils are found at the posterior end of the oral cavity

The Tongue

• A muscular organ located in the floor of the oral cavity, associated with the functions of taste, speech, mastication, and deglutition
• It has intrinsic muscles covered by a specific mucous membrane, and is connected to surrounding bones by extrinsic muscles
• Parts: root, body and tip
• The root represents the posterior part and is connected to the mandible
• The tip is the anterior free part
• The body is its main bulk and has dorsal and ventral surfaces
• The dorsal surface is rough and characterized by lingual papillae, which contain taste buds for taste sensation
• The ventral surface is smooth and shows the median elevation (lingual frenulum) and lingual veins

The Teeth and Gums

• The upper and lower jaws are formed of bony arches containing sockets for teeth
• The roots of the teeth are covered by mucosa called the gingivae (gums)
• The number of teeth varies according to age
• Milky teeth (also known as deciduous teeth) number 20
• Permanent teeth number 32
• The teeth perform different functions such as cutting, tearing, crushing, and grinding

The Salivary Glands

• There are 2 types of salivary glands:
• Minor salivary glands: scattered in the oral cavity
• Major salivary glands:
  • 2 parotid glands: These glands are the largest salivary glands, They lie in front of the ear, and their ducts open into the vestibule of the oral cavity
  • 2 submandibular glands: Located in the floor of the mouth under the angle of the lower jaw, Its duct opens in the buccal part of the oral cavity
  • 2 sublingual glands: Located in the floor of the mouth below the tongue, Its duct opens in the buccal part of the oral cavity

The Pharynx

• The pharynx is a muscular tube and is subdivided into:
  • Nasopharynx: part of the respiratory passage way
  • Oropharynx: posterior to the oral cavity
  • Laryngopharynx: continuous with the esophagus inferiorly
• The walls of the pharynx contain two skeletal muscle layers:
  • Inner layer: runs longitudinally
  • Outer layer: constrictor muscles, run around the wall in a circular fashion
• Alternating contractions of both layers induce peristalsis

Histological Structure of Alimentary Tract

• The walls of alimentary canal organs from the esophagus to the large intestine are composed of the same four tissue layers, tunics:
  • Mucosa: the innermost layer
    • Surface epithelium: simple columnar except for the esophagus, which has stratified squamous epithelium for friction resistance
    • Connective tissue (lamina propria)
    • Scanty smooth muscle layer
  • Submucosa: it is a soft connective tissue containing blood vessels, nerve endings, mucosa-associated lymphoid tissue (MALT), and lymphatic vessels
  • Muscularis externa: a muscle layer made up of an inner circular layer and outer longitudinal layer of smooth muscle cells
  • Serosa: the outermost layer which represents the visceral peritoneum consisting of a single layer of flat serous fluid-producing cells
    • It is continuous with the slippery parietal peritoneum which lines the abdominopelvic cavity by membrane-like extensions, example the mesentery

Esophagus

• The continuation of the pharynx
• A muscular tube that passes in the neck, thorax, and abdomen
• Length: 25-30 cm (10 inches)
• Ends in the stomach after piercing the diaphragm

The Stomach

• The most dilated part of the gut which acts as a reservoir of food
• Lies in the upper left part (LT. hypochondrium) of the abdomen
• Size: variable in size, 15-25 cm (6-10 inches) in length and its diameter and volume depends on the amount of food it contains, when full it can hold about 4 liters (1 gallon)
• Shape: variable, when empty it takes the shape of a C or J
• Has anatomical features including:
  • 2 sphincters: cardiac between it and the esophagus, and pyloric between it and the duodenum
  • 2 borders: the right border, known as the lesser curvature and the left border, known as the greater curvature
  • 2 surfaces: the anterior that lies behind the anterior abdominal wall and the liver, and the posterior surface that lies on several structures that form the stomach bed
  • 2 parts: the cardiac part comprises the fundus, the body, and the pyloric part formed of the pyloric antrum and pyloric canal

Peritoneal Folds Related to Stomach

-  The lesser omentum is a double layer of peritoneum that extends from the liver to the lesser curvature of the stomach
-  The greater omentum is another extension of the peritoneum that drapes downward and covers the abdominal organs like a lacy apron before attaching to the posterior body wall

The Small Intestine

• A muscular tube extending from the Pyloric sphincter to the Large intestine
• The longest part of the gut tube: about 6 meters
• Occupies the middle part of the abdominal cavity
• Suspended from the posterior abdominal wall with the fan-shaped mesentery
• 3 parts:
  • The duodenum is the first part and has a C shape encircling the pancreas, and is considered the fixed part of the small intestine - Its length is about 25 cm and it receives the opening of bile and pancreatic ducts
  • The jejunum is the second part and represents the proximal 2/5 of the small intestine - Its length is about 250 cm
  • The ileum represents the distal 3/5 of the mobile part of the small intestine - With a length of 350 cm, it joins the large intestine at the ileocecal valve

The Large Intestine

• The second-longest part of the gut tube, approximately 1.5 meters (5 feet)
• It extends from the ileocecal valve to the anus
• Found along the margin of the abdominal cavity, framing the small intestine on three sides
• Formed from the cecum with the vermiform appendix, ascending colon, RT colic flexure, transverse colon, LT colic flexure, descending and sigmoid colon, and finally the rectum and the anal canal
• Characterized by its large caliber and sacculations, also, the presence of longitudinal bands of muscle fibers

The Anal Canal

• It ends at the anus which opens to the exterior
• It has two valves
  • The external anal sphincter is composed of skeletal muscle and is voluntary
  • The internal anal sphincter is formed by smooth muscle and is involuntary
• These sphincters act like purse strings to open and close the anus
• The anus is ordinarily closed except during defecation when feces are eliminated from the body

The Liver

• The largest gland performing many essential functions
• Occupies the upper right part (RT hypochondrium) of the abdomen below the diaphragm
• The weight in adults is about 1/50 of the body weight (1000-1500 gm)
• The shape has a wedge shape of 5 surfaces separated by ill-defined borders (anterior, superior, posterior, inferior and right lateral)
• The liver has four lobes:
  • Right lobe: large
  • Left lobe: smaller than the right lobe
  • Caudate lobe: present between the left lobe and inferior vena cava
  • Quadrate lobe: present between the left lobe and gallbladder
• It lies below the diaphragm, but in front of the stomach
• It is suspended from the diaphragm and abdominal wall by a delicate mesentery cord, the falciform ligament
• The liver has a digestive function, which is to produce bile
• Bile leaves the liver through the common hepatic duct and enters the duodenum through the bile duct

The Biliary System

• The biliary system consists of:
  • Intrahepatic system: bile canaliculi and ductules inside the liver
  • Extrahepatic system
• The right and left hepatic ducts join to form the common hepatic duct, which is joined by the cystic duct (from the gall bladder) to form the common bile duct
• Together with the main pancreatic duct, it opens into the duodenum
• The Gall Bladder:
  • Pear-shaped and is formed to include the fundus, body, and neck
  • Lies on the inferior surface of the liver
  • Its capacity is about 60 ml of bile

Pancreas

• A mixed gland with both exocrine for secreting pancreatic juices and endocrine function for secreting insulin and glucagon hormones
• Extends across the abdomen from the spleen to the duodenum
• Lies posterior to the peritoneum (retroperitoneal)
• Its head sits within the concavity of the duodenum, while the tail reaches the spleen

Parts of the Pancreas

• Head
• Neck
• Body
• Tail

The Spleen

• A special lymphoid organ that lies on the posterior aspect of the fundus of the stomach
• It separates the stomach from the diaphragm
• The size measures about the size of a closed fist
• It has a triangular shape with two borders, two surfaces and a middle hilum

Porto-Systemic Anastomosis

• Definition: Connections between the portal and systemic venous systems

• Sites of porto-systemic anastomosis:

  • Lower end of the esophagus: leading to esophageal varices
  • Around the umbilicus: caput medusae
  • Anal canal: hemorrhoids/piles
  • Bare area of the liver
  • Retroperitoneal

• Enlargement of Porto-systemic anastomosis occurs when there is an obstruction to the flow of blood in the portal vein such as in cases of alcoholic liver failure or cirrhosis

• The veins in these anastomotic sites become dilated and may bleed