14-34 Digestive System

Main Organs of the Digestive System

The digestive system consists of several key organs that work together to process food, absorb nutrients, and eliminate waste. The major organs include:

• Mouth: The entry point for food, where mechanical and chemical digestion begins.

• Esophagus: A muscular tube that transports food from the mouth to the stomach.

• Stomach: A hollow organ that further breaks down food using acids and enzymes.

• Small Intestine: Comprising the duodenum, jejunum, and ileum, it is where most nutrient absorption occurs.

• Large Intestine: Responsible for water absorption and the formation of feces; includes the cecum, colon, rectum, and anus.

• Liver: Produces bile, which aids in fat digestion and detoxifies various metabolites.

• Gallbladder: Stores and concentrates bile from the liver.

• Pancreas: Produces digestive enzymes and bicarbonate to neutralize stomach acid in the small intestine.

The Vestibule of the Mouth

The vestibule of the mouth is the space between the lips and cheeks externally and the gums and teeth internally. It plays a role in:

• Mechanical Digestion: Assists in holding food while chewing.

• Oral Hygiene: Helps in trapping food particles and bacteria.

Lips

The lips are highly mobile structures that form the anterior boundary of the oral cavity. Key features include:

• Structure: Composed of skin on the outside, mucous membrane on the inside, and muscle fibers (orbicularis oris) that allow for movement.

• Functions:

  • Aid in speech articulation.

  • Assist in food manipulation during chewing and swallowing.

  • Contribute to facial expressions

Cheeks

The cheeks form the lateral walls of the oral cavity. Their anatomy includes:

• Structure: Composed of skin, subcutaneous fat, buccinator muscles (which help in chewing), and mucous membranes.

• Functions:

  • Help keep food between the teeth during mastication.

  • Assist in forming sounds during speech.

Blood Supply of the Mouth, Lips and Cheeks

• External Carotid Artery:

  • Facial Artery: Supplies blood to the lips and superficial structures of the face.

  • Maxillary Artery: Supplies deeper structures including parts of the cheeks.

Nerve Supply of the Mouth, Lips and Cheeks

• Trigeminal Nerve (CN V):

  • Maxillary Division (V2): Provides sensory innervation to the upper lip and cheek area.

  • Mandibular Division (V3): Supplies sensory innervation to the lower lip and part of the cheek; also innervates muscles involved in mastication.

• Facial Nerve (CN VII):

  • Supplies motor innervation to muscles of facial expression, including those controlling lip movement.

Walls of the Oral Cavity Proper

The oral cavity proper is the space within the mouth that lies behind the gums and teeth. It is bounded by:

• Anterior Wall: Formed by the lips.

• Lateral Walls: Composed of the cheeks.

• Superior Wall: Consists of the hard palate and soft palate.

• Inferior Wall: Formed by the floor of the mouth, which includes the tongue and sublingual structures.

• Posterior Wall: Leads into the oropharynx

Oral Cavity - Hard and Soft Palate

• Hard Palate:

  • Composed of bone (palatine processes of the maxilla and horizontal plates of the palatine bones).

  • Provides a rigid surface against which the tongue can press food during mastication.

• Soft Palate:

  • Composed of muscle and connective tissue, covered by mucous membrane. It extends from the hard palate to the uvula.

  • Plays a crucial role in swallowing and speech; it elevates to close off the nasopharynx during swallowing to prevent food from entering the nasal cavity.

Oropharyngeal Isthmus

• The oropharyngeal isthmus is the narrow passage that connects the oral cavity to the oropharynx.

• It is bounded by:

  • Anteriorly: The palatoglossal arches (folds of tissue extending from the soft palate to the sides of the tongue).

  • Posteriorly: The palatopharyngeal arches.

• Function: Serves as a pathway for food and air, facilitating swallowing and breathing.

The Mucous Membrane of the Oral Cavity Proper

• Epithelium: Stratified squamous epithelium, providing protection against abrasion.

• Lamina Propria: A layer of connective tissue containing blood vessels, nerves, and glands.

• Function: The mucous membrane aids in lubrication, sensation, and protection against pathogens.

Oral Cavity Proper - Blood Supply

• External Carotid Artery:

  • Lingual Artery: Supplies blood to the floor of the mouth and tongue.

  • Facial Artery: Supplies blood to parts of the lips and cheeks.

  • Maxillary Artery: Supplies deeper structures, including parts of the hard palate.

Oral Cavity Proper - Nerve Supply

• Trigeminal Nerve (CN V):

  • Maxillary Division (V2): Provides sensory innervation to parts of the hard palate and upper teeth.

  • Mandibular Division (V3): Supplies sensory innervation to the floor of the mouth and lower teeth; also innervates muscles involved in mastication.

• Glossopharyngeal Nerve (CN IX):

  • Provides sensory innervation to parts of the oropharynx and contributes to taste sensation in posterior regions.

• Vagus Nerve (CN X):

  • Supplies motor innervation to muscles of the soft palate (except tensor veli palatini).

Teeth - Deciduous Dentition

• Also known as primary or baby teeth

• Typically consists of 20 teeth: 10 in the upper jaw and 10 in the lower jaw

• Erupts between 6 months to 3 years of age

• Includes incisors, canines, and molars, but no premolars

• 8 incisors (4 upper, 4 lower)

• 4 canines (2 upper, 2 lower)

• 8 molars (4 upper, 4 lower)

• These teeth begin to shed around age 6, making way for permanent teeth

Teeth - Permanent Dentition

• Also known as secondary teeth

• Comprises 32 teeth: 16 in the upper jaw and 16 in the lower jaw

• Erupts from around age 6 to late adolescence (typically by age 21)

• Includes incisors, canines, premolars, and molars

• 8 incisors (4 upper, 4 lower)

• 4 canines (2 upper, 2 lower)

• 8 premolars (4 upper, 4 lower)

• 12 molars (6 upper, 6 lower), including wisdom teeth

• The third molars, commonly known as wisdom teeth, may erupt later or may be absent in some individuals

Structure of the Tooth

• Crown:

  • The visible part of the tooth above the gum line, covered by enamel

• Root:

  • The portion embedded in the jawbone, anchoring the tooth in place

  • It is covered by cementum

• Enamel:

  • The hard outer layer of the crown made primarily of hydroxyapatite crystals

  • It is the hardest substance in the human body

• Dentin:

  • The layer beneath enamel and cementum is less hard than enamel but still provides support

  • It contains microscopic tubules that can transmit sensations

• Pulp:

  • The innermost part of the tooth contains nerves, blood vessels, and connective tissue

  • It nourishes the tooth and provides sensory functions.

Tongue - Parts

The tongue is a muscular organ involved in various functions, including taste, speech, and manipulation of food.

It can be divided into several parts:

• Body:

  • The anterior two-thirds of the tongue, which is visible in the oral cavity.

• Root:

  • The posterior one-third, which is anchored to the floor of the mouth and extends to the oropharynx.

• Apex:

  • The tip of the tongue is pointed and free-moving.

Tongue - Surfaces

• Dorsal Surface:

  • The upper surface, which is covered by a mucous membrane and features various papillae

• Ventral Surface:

  • The underside, which is smooth and contains blood vessels

  • It is attached to the floor of the mouth

Tongue - Mucous Membrane - Papillae

The mucous membrane covering the tongue contains specialized structures called papillae, which enhance its sensory functions:

• Filiform Papillae:

  • Most numerous and widely distributed

  • Conical or thread-like in shape

  • Primarily mechanical function, aiding in food manipulation

  • Do not contain taste buds

  • Contribute to the rough texture of the tongue

• Fungiform Papillae:

  • Mushroom-shaped structures

  • Scattered among filiform papillae, more concentrated at the tip and sides of the tongue

  • Contain taste buds

  • Involved in taste sensation

• Circumvallate Papillae:

  • Large, circular structures

  • Located at the back of the tongue, often arranged in a V-shape

  • Surrounded by a trench or moat

  • Contain numerous taste buds

  • Important for taste perception, especially bitter tastes

• Foliate Papillae:

  • Leaf-like or ridge-like structures

  • Found on the lateral posterior edges of the tongue

  • Contain taste buds

  • Less prominent in humans compared to some other mammals

Tongue - The Lingual Tonsil

• Location: Situated at the base (posterior third) of the tongue

• Structure:

  • Forms part of Waldeyer's ring, a ring of lymphoid tissue in the pharynx

  • Composed of numerous small, rounded masses of lymphoid tissue

  • Lacks a distinct capsule, unlike palatine tonsils

  • Covered by stratified squamous epithelium

  • Contains crypts (small pits or invaginations) that increase surface area

• Lymphatic Drainage: Drains to the deep cervical lymph nodes

• Function:

  • Part of the immune system, particularly mucosal immunity

  • Helps protect against pathogens entering through the mouth and pharynx

  • Produces antibodies and lymphocytes

  • Forming an important part of the mucosa-associated lymphoid tissue (MALT) system.

The Lingual Muscles - Types and Subtypes

• Intrinsic Lingual Muscles:

  • These muscles are entirely contained within the tongue and do not attach to any bones.

  • They are responsible for changing the shape of the tongue.

  • Superior longitudinal muscle

  • Inferior longitudinal muscle

  • Transverse muscle

  • Vertical muscle

• Extrinsic Lingual Muscles:

  • These muscles originate from structures outside the tongue and insert into it.

  • They are responsible for larger movements of the tongue.

  • Genioglossus: The largest extrinsic muscle, it protrudes and depresses the tongue

  • Hyoglossus: Depresses and retracts the tongue

  • Styloglossus: Retracts and elevates the sides of the tongue

  • Palatoglossus: Elevates the back of the tongue

• The extrinsic muscles work in conjunction with the intrinsic muscles to produce the wide range of tongue movements necessary for speech, swallowing, and food manipulation.

• The majority of the lingual muscles are innervated by the hypoglossal nerve (CN XII), with the exception of the palatoglossus, which is innervated by the vagus nerve (CN X).

• The lingual artery, a branch of the external carotid artery, provides the primary blood supply to the tongue and its muscles.

Tongue - Blood Supply

• Lingual Artery:

  • A branch of the external carotid artery that provides oxygenated blood to both the anterior and posterior parts of the tongue.

Tongue - Nerve Supply

• Hypoglossal Nerve (CN XII): Provides motor innervation to all intrinsic and extrinsic muscles (except palatoglossus)

• Lingual Nerve (from CN V3): Supplies general sensory innervation to the anterior two-thirds

• Glossopharyngeal Nerve (CN IX): Provides taste sensation from the posterior one-third

• Vagus Nerve (CN X): Supplies taste sensation from the epiglottis region

Tongue - Lymphatic Drainage

• Submandibular Lymph Nodes: Drain lymph from most of the anterior two-thirds.

• Deep Cervical Lymph Nodes: Drain lymph from the posterior one-third and lingual tonsils.

Major Salivary Glands

• The major salivary glands include three pairs of glands that produce saliva, essential for digestion, oral hygiene, and taste.

• These glands are:

  • Parotid Glands

  • Submandibular Glands

  • Sublingual Glands

Major Salivary Glands - Parotid Gland - Position, Structure, Blood Supply, Lymph Drainage and Nerve

• Position: Located anterior to the ear, extending from the zygomatic arch to the angle of the mandible.

• Structure:

  • Largest of the salivary glands

  • Composed primarily of serous cells, producing a watery, enzyme-rich saliva

  • The parotid duct (Stensen's duct) opens into the oral cavity opposite the second upper molar

• Blood Supply: Supplied by branches of the external carotid artery, particularly the superficial temporal and maxillary arteries.

• Lymph Drainage: Drains into the parotid lymph nodes, which then drain into the deep cervical lymph nodes.

• Nerve Supply: Innervated by the glossopharyngeal nerve (CN IX), which stimulates saliva production through parasympathetic fibers.

Major Salivary Glands - Submandibular Gland - Position, Structure, Blood Supply, Lymph Drainage and Nerve Supply

• Position: Located beneath the floor of the mouth, medial to the mandible.

• Structure:

  • Composed of both serous and mucous cells, producing a mixed saliva.

  • The submandibular duct (Wharton's duct) opens into the oral cavity at the sublingual caruncle.

• Blood Supply: Supplied by branches of the facial artery and lingual artery.

• Lymph Drainage: Drains into submandibular lymph nodes, which then drain into deep cervical lymph nodes.

• Nerve Supply: Innervated by the facial nerve (CN VII) through its chorda tympani branch, which carries parasympathetic fibers to stimulate saliva secretion.

Major Salivary Glands - Sublingual Gland - Position, Structure, Blood Supply, Lymph Drainage and Nerve Supply

• Position: Located beneath the tongue, in the floor of the mouth.

• Structure:

  • Smallest of the major salivary glands.

  • Primarily composed of mucous cells, producing a thick, viscous saliva.

  • Multiple small ducts (Bartholin's duct) open directly into the oral cavity along the sublingual fold.

• Blood Supply: Supplied mainly by branches of the lingual artery and submental artery (a branch of the facial artery).

• Lymph Drainage: Drains into submandibular lymph nodes and then into deep cervical lymph nodes.

• Nerve Supply: Innervated by the facial nerve (CN VII) via its chorda tympani branch, similar to the submandibular gland.

Pharynx - Position

• Located behind the nose, mouth, and larynx

• Extends from the base of the skull to the level of the 6th cervical vertebra (C6)

• Connects the nasal and oral cavities to the larynx and esophagus

Pharynx - Shape

• Funnel-shaped

• Wider at the top

• Narrower at the bottom

• Approximately 12-14 cm long in adults

Pharynx - Parts

• Nasopharynx

  • Superior portion, located behind the nasal cavity

  • Extends from the base of the skull to the soft palate

  • Contains the pharyngeal tonsil (adenoids) on its posterior wall

  • Houses the openings of the Eustachian tubes, which connect to the middle ear

• Oropharynx

  • Middle portion, located behind the oral cavity

  • Extends from the soft palate to the level of the hyoid bone

  • Contains the palatine tonsils and lingual tonsils

  • Posterior one-third of the tongue forms its anterior wall

• Laryngopharynx (Hypopharynx)

  • Inferior portion, located behind the larynx

  • Extends from the hyoid bone to the lower border of the cricoid cartilage

  • Contains the piriform sinuses on either side of the laryngeal inlet

Pharynx - Description

• Anterior wall:

  • Incomplete and differs for each part

  • Nasopharynx: choanae (posterior nasal apertures)

  • Oropharynx: isthmus of the fauces and base of the tongue

  • Laryngopharynx: posterior surface of the larynx

• Posterior wall:

  • Continuous from the base of the skull to the esophagus

  • Composed of the pharyngeal constrictor muscles

• Lateral walls:

  • Contain the palatine tonsils in the oropharynx

  • House the pharyngeal openings of the Eustachian tubes in the nasopharynx

Structure of the Pharyngeal Wall

• Mucosa: The innermost layer lined with respiratory epithelium in the nasopharynx and stratified squamous epithelium in the oropharynx and laryngopharynx.

• Submucosa: Contains connective tissue, blood vessels, and glands.

• Muscular Layer: Composed of two muscle groups:

  • Constrictor Muscles (superior, middle, and inferior): Contract sequentially during swallowing to propel food into the esophagus.

  • Longitudinal Muscles (stylopharyngeus, salpingopharyngeus, palatopharyngeus): Elevate the pharynx during swallowing.

Pharynx - Blood Supply

The blood supply of the pharynx is primarily derived from branches of the external carotid artery. The main arteries supplying the pharynx are:

• Ascending pharyngeal artery

  • A major source of blood supply to the pharynx

  • Provides branches to the posterior and lateral pharyngeal walls

• Ascending palatine artery

  • A branch of the facial artery

  • Supplies the soft palate and contributes to the pharyngeal blood supply

• Tonsillar branch of the facial artery

  • Supplies the palatine tonsils and surrounding pharyngeal tissues

• Descending palatine artery

  • A branch of the maxillary artery

  • Contributes to the blood supply of the soft palate and pharynx

• Superior thyroid artery

  • Provides branches to the inferior part of the pharynx

Pharynx - Lymphatic Drainage

• Deep Cervical Lymph Nodes: Primarily drain lymph from the pharyngeal region.

• Jugulodigastric Node: Often referred to as the "tonsillar node," it receives lymph from the palatine tonsils.

Pharynx - Nerve Supply

• Vagus Nerve (CN X): Supplies motor innervation to most of the pharyngeal muscles (except stylopharyngeus).

• Glossopharyngeal Nerve (CN IX): Provides sensory innervation to the oropharynx and motor innervation to the stylopharyngeus.

• Trigeminal Nerve (CN V): The maxillary division provides sensory input from the nasopharynx.

Oesophagus - Position

• Extends from the lower border of the cricoid cartilage (C6 vertebral level) to the cardiac orifice of the stomach (T11 vertebral level)

• Passes through the neck, thorax, and abdomen

• It runs posterior to the trachea and heart

Oesophagus - Parts

1. Cervical portion

   • The upper portion located in the neck

   • Lies posterior to the trachea, anterior to the vertebral column

2. Thoracic portion

   • The middle section that runs through the thoracic cavity

   • Passes through the superior and posterior mediastinum

3. Abdominal portion

   • The short segment that passes through the diaphragm into the abdomen before connecting to the stomach

   • Enters the abdomen through the esophageal hiatus of the diaphragm

Oesophagus - Description + Layers

• Approximately 25-30 cm long in adults

• Diameter is about 2 cm

• Diameter varies, with narrowest points at the cricoid cartilage, where it crosses the aortic arch, and at the diaphragmatic hiatus

• Upper and lower esophageal sphincters regulate food passage

• Transverse esophageal folds present, particularly in the distal esophagus

• Layers:

  • Mucosa: Lined with stratified squamous epithelium

  • Submucosa: Contains blood vessels and mucous glands

  • Muscularis externa: Inner circular and outer longitudinal muscle layers

  • Adventitia: Loose connective tissue (replaced by serosa in the abdominal portion)

Structure of the Pharyngeal Wall

1. Mucosa

   • Innermost layer

   • Lined with stratified squamous epithelium in the oropharynx and laryngopharynx

   • Lined with respiratory epithelium (ciliated pseudostratified columnar) in the nasopharynx

2. Submucosa

   • Contains blood vessels, lymphatics, and mucous glands

3. Muscularis

   • Composed of two groups of muscles:

     • Circular constrictor muscles (superior, middle, and inferior pharyngeal constrictors)

     • Longitudinal muscles (stylopharyngeus, salpingopharyngeus, and palatopharyngeus)

   • aid in swallowing

   • facilitate peristalsis = wave-like contractions that move food

Oesophagus - Blood Supply

• Cervical Region: Supplied by branches of the inferior thyroid artery.

• Thoracic Region: Supplied by branches of the aorta (esophageal arteries).

• Abdominal Region: Supplied by branches of the left gastric artery and inferior phrenic artery.

Oesophagus - Lymphatic Drainage

1. Cervical Esophagus (Upper Third)

   • Drains primarily to the deep cervical lymph nodes

   • Some lymphatics drain to the supraclavicular and paratracheal lymph nodes

2. Thoracic Esophagus (Middle Third)

   • Drains to the mediastinal lymph nodes, including:

     • Paratracheal nodes

     • Tracheobronchial nodes

     • Posterior mediastinal nodes

3. Abdominal Esophagus (Lower Third)

   • Drains to the left gastric (cardiac) lymph nodes

   • Some lymphatics drain to the celiac lymph nodes

Oesophagus - Nerve Supply

• Parasympathetic

  • Upper third: Vagus nerve (CN X) via the recurrent laryngeal nerves

  • Middle and lower thirds: Vagus nerve (CN X) directly

  • Functions: Stimulates peristalsis and secretion from esophageal glands

• Sympathetic

  • Derived from the sympathetic trunk (T1-T10 segments)

  • Upper third: Superior cervical ganglion

  • Middle third: Middle and inferior cervical ganglia, upper thoracic ganglia

  • Lower third: Greater splanchnic nerves

  • Functions: Inhibits peristalsis and secretion, regulates blood flow

• Somatic Motor

  • Upper third (striated muscle): Vagus nerve (CN X) via the recurrent laryngeal nerves

  • Functions: Controls voluntary swallowing initiation

• Sensory

  • General sensation: Vagus nerve (CN X)

  • Upper third also receives sensory fibers from the glossopharyngeal nerve (CN IX)

  • Functions: Mediates pain, temperature, and stretch sensations

• Intrinsic

  • Myenteric (Auerbach's) plexus: Between muscle layers, controls motility

  • Submucosal (Meissner's) plexus: In submucosa, regulates secretion and blood flow

Stomach - Position

• Situated between the esophagus and the small intestine

• Lies primarily in the left upper quadrant of the abdomen

• Extends from the left hypochondriac region into the epigastric region

• It lies anterior to the pancreas and posterior to the liver and spleen.

Stomach - Shape

• J-shaped or hook-shaped

• Has a greater curvature (left side) and a lesser curvature (right side)

• Divided into four main regions:

  • Cardia: Where the esophagus connects to the stomach

  • Fundus: The upper curved portion

  • Body: The main central portion

  • Pylorus: The narrower lower portion that connects to the duodenum

Stomach - Size

• Varies depending on factors such as body size, amount of food consumed, and individual variation

• When empty, the average adult stomach is about 12 inches (30 cm) long

• Can expand to hold approximately 1-1.5 liters of food and fluid

• Capacity can increase up to 4 liters in some individuals

• Its dimensions vary but typically measures around 10 cm (4 inches) in width at its widest point.

Peritoneal Relation of the Stomach

• The stomach is almost entirely covered by peritoneum and it is considered an intraperitoneal organ

• Lesser Sac (Omental Bursa)

  • The posterior surface of the stomach forms the anterior wall of the lesser sac

  • This potential space lies behind the stomach and extends to the spleen

• Greater Omentum

  • Descends from the greater curvature of the stomach

  • Forms a large, fat-filled peritoneal fold that hangs over the intestines

• Lesser Omentum

  • Extends from the lesser curvature of the stomach to the liver

  • Consists of the Hepatogastric ligament: Connects stomach to the liver

  • And the Hepatoduodenal ligament: Contains the portal triad (hepatic artery, portal vein, bile duct)

• Gastrosplenic Ligament

  • Connects the greater curvature of the stomach to the spleen

  • Contains short gastric vessels

• Gastrophrenic Ligament

  • Connects the upper part of the stomach (fundus) to the diaphragm

• Bare Area

  • A small area on the posterior surface of the cardia lacks peritoneal covering

  • This is where the stomach is directly fixed to the diaphragm

• Anterior surface: Completely covered by peritoneum

• Posterior surface: Mostly covered, except for a small bare area near the cardia

• Gastrocolic Ligament

  • The lower part of the greater omentum connecting the stomach to the transverse colon

Stomach - Blood Supply

• Left Gastric Artery

  • Arises directly from the celiac trunk

  • Supplies the lesser curvature of the stomach

  • Anastomoses with the right gastric artery

• Right Gastric Artery

  • Usually a branch of the proper hepatic artery

  • Supplies the pyloric region and lesser curvature

  • Anastomoses with the left gastric artery

• Right Gastroepiploic Artery

  • Branch of the gastroduodenal artery (from hepatic artery)

  • Runs along the greater curvature

  • Supplies the lower part of the stomach

  • Anastomoses with the left gastroepiploic artery

• Left Gastroepiploic Artery

  • Branch of the splenic artery

  • Runs along the greater curvature

  • Supplies the lower left part of the stomach

  • Anastomoses with the right gastroepiploic artery

• Short Gastric Arteries

  • 5-7 small branches from the splenic artery

  • Supply the fundus of the stomach

Stomach - Lymphatic Drainage

• Cardia and Fundus

  • Drain primarily to left gastric and splenic nodes

• Body

  • Lesser curvature drains to left gastric nodes

  • Greater curvature drains to splenic and greater curvature nodes

• Pylorus

  • Drains to hepatic, suprapyloric, and infrapyloric nodes

Stomach - Nerve Supply

Parasympathetic:

• Primarily supplied by the vagus nerve (CN X)

• Vagal fibers reach the stomach through two main pathways:

  • Left vagus: Supplies the anterior surface of the stomach

  • Right vagus: Supplies the posterior surface of the stomach

• These nerves form an extensive network around the stomach, including:

  • Myenteric plexus (between muscle layers)

  • Submucosal plexus

• Functions: Stimulates gastric motility, acid secretion, and release of gastrin

Sympathetic:

• Originates from the thoracic spinal segments (T6-T10)

• Fibers pass through the celiac and superior mesenteric ganglia

• Forms a network around the arteries supplying the stomach

• Functions: Inhibits gastric motility and secretion, regulates blood flow

The stomach's nerve supply allows for:

• Coordination of gastric motility

• Regulation of gastric secretions

• Sensory feedback (e.g., fullness, pain)

• Integration with other digestive processes

Stomach - Parts

• Cardia

  • The uppermost part of the stomach

  • Located immediately below the gastroesophageal junction

  • Contains the cardiac glands that secrete mucus

  • Acts as a sphincter to prevent reflux of stomach contents into the esophagus

• Fundus

  • The dome-shaped upper portion of the stomach, above and to the left of the cardia

  • Often contains a gas bubble visible on X-rays

  • Contains parietal cells that secrete hydrochloric acid and intrinsic factor

• Body (Corpus)

  • The largest part of the stomach, between the fundus and the pyloric antrum

  • Main site for gastric glands that secrete pepsinogen, hydrochloric acid, and intrinsic factor

  • Responsible for mixing and churning food

• Pyloric Antrum

  • The lower portion of the stomach, leading to the pylorus

  • Contains G cells that secrete the hormone gastrin

  • Involved in grinding and mixing food with gastric secretions

• Pylorus

  • The narrow, distal end of the stomach

  • Contains the pyloric sphincter, which regulates the passage of chyme into the duodenum

  • Has thick muscular walls to aid in grinding food

• Greater Curvature

  • The long, convex left border of the stomach

  • Attachment site for the greater omentum

  • Contains the gastroepiploic vessels

• Lesser Curvature

  • The shorter, concave right border of the stomach

  • Attachment site for the lesser omentum

  • Contains the left and right gastric vessels

• Incisura Angularis

  • A notch on the lesser curvature marks the junction between the body and the pyloric antrum

Structure of the Stomach Wall

Mucosa (innermost layer)

• Surface epithelium: Simple columnar epithelium

• Gastric pits: Invaginations of the surface epithelium

• Gastric glands: Located in the lamina propria, containing various cell types:

  • Parietal cells: Secrete hydrochloric acid and intrinsic factor

  • Chief cells: Secrete pepsinogen

  • Mucous neck cells: Secrete mucus

  • G cells: Secrete gastrin (mainly in the pyloric region)

  • Enterochromaffin-like (ECL) cells: Secrete histamine

• Lamina propria: Connective tissue supporting the epithelium and glands

• Muscularis mucosae: Thin layer of smooth muscle

Submucosa

• Loose connective tissue

• Contains blood vessels, lymphatics, and nerve plexuses (Meissner's plexus)

• Provides support and nutrition to the mucosa

Muscularis propria (Muscularis externa)

• Three layers of smooth muscle:

  • Inner oblique layer: Present mainly in the body and fundus

  • Middle circular layer: Thickens at the pylorus to form the pyloric sphincter

  • Outer longitudinal layer: Continuous with the esophageal muscles

• Contains the myenteric (Auerbach's) plexus between muscle layers

Serosa (outermost layer)

• Visceral peritoneum covering most of the stomach

• Consists of a single layer of mesothelial cells and underlying connective tissue

Stomach - Glands

Gastric glands are located in the mucosa of the stomach and are composed of different cell types:

• Parietal (Oxyntic) Cells

  • Secrete hydrochloric acid and intrinsic factor

  • Essential for protein digestion and vitamin B12 absorption

• Chief (Zymogenic) Cells

  • Produce and secrete pepsinogen, the precursor to pepsin

  • Pepsin is crucial for protein digestion

• Mucous Neck Cells

  • Secrete mucus to protect the stomach lining

• Enteroendocrine Cells

  • Include G cells (produce gastrin) and D cells (produce somatostatin)

  • Regulate gastric secretion and motility

Types of Glands:

• Cardiac Glands: Located near the esophageal opening, mainly secrete mucus

• Fundic Glands: Found in the fundus and body, contain all cell types

• Pyloric Glands: Located in the antrum, primarily contain mucous and G cells

The gastric glands play a vital role in:

• Producing gastric juice for food digestion

• Protecting the stomach lining from acid and enzymes

• Regulating stomach functions through hormonal secretions

Duodenum - Position

• The duodenum is positioned in the upper abdomen, extending from the pylorus of the stomach to the jejunum.

• It forms a C-shaped curve around the head of the pancreas

• The duodenum-jejunum bend, also known as the ligament of Treitz, is typically located in the ante-centric position of the spine and corresponds with the plane of the 11th and 12th thoracic vertebrae

Duodenum - Parts

• First part (superior part): Begins at the pylorus and extends for about 5 cm.

• Second part (descending part): Runs vertically downward for about 7-10 cm.

• Third part (horizontal part): Crosses from right to left for about 10 cm.

• Fourth part (ascending part): Ascends to the left of the aorta and ends at the duodenojejunal flexure

Duodenum - Description

• Most of the duodenum (except for a small portion of the first part) is retroperitoneal, meaning it lies behind the peritoneum

• Lumen: The inner surface of the duodenum contains circular folds (plicae circulares) that increase the surface area for absorption

• It curves around the head of the pancreas

• The second part of the duodenum is where the common bile duct and pancreatic duct typically enter through the major duodenal papilla (ampulla of Vater)

• The superior portion is closely related to the liver and gallbladder

Duodenum - Peritoneal Relation

• First Part (Superior Part)

  • The first part of the duodenum, also known as the duodenal bulb, is mostly intraperitoneal.

  • This means it is almost entirely covered by peritoneum and is relatively mobile.

  • The peritoneum forms a complete covering around this segment, except for a small area posteriorly where the common bile duct passes behind it.

• Second Part (Descending Part)

  • The second part of the duodenum becomes retroperitoneal as it descends.

  • It is only covered by peritoneum on its anterior surface, while its posterior surface is directly adjacent to retroperitoneal structures.

  • This retroperitoneal position is important for its close relationship with the head of the pancreas and the common bile duct.

• Third Part (Horizontal Part)

  • The third part of the duodenum is also retroperitoneal.

  • It lies transversely across the posterior abdominal wall, covered by the peritoneum only on its anterior surface.

  • This segment passes in front of important retroperitoneal structures such as the inferior vena cava and the aorta.

• Fourth Part (Ascending Part)

  • The fourth and final part of the duodenum continues to be retroperitoneal until it reaches the duodenojejunal flexure.

  • At this point, it transitions back to an intraperitoneal position as it becomes the jejunum.

  • The peritoneum covers its anterior and left lateral surfaces.

Duodenum - Structure of the Wall

Mucosa

• The mucosa is the innermost layer of the duodenum wall, lined by simple columnar epithelium

• Villi: Finger-like projections that increase the surface area for absorption

• Absorptive cells (enterocytes): Responsible for nutrient absorption

• Goblet cells: Secrete mucus to protect the intestinal lining

• Enteroendocrine cells: Located mainly in the crypts, these cells produce hormones

Submucosa

• The submucosa is a layer of loose connective tissue beneath the mucosa

• Brunner's glands: These are unique to the duodenum and secrete an alkaline mucus that helps neutralize stomach acid

Muscularis

• The muscular layer of the duodenum wall consists of two distinct layers of smooth muscle:

  • Inner circular layer

  • Outer longitudinal layer

• These muscle layers are responsible for peristalsis, the wave-like contractions that move food through the digestive tract.

Serosa

• The serosa is the outermost layer of the duodenum wall.

• It appears as a thin layer of loose connective tissue

Duodenum - Blood Supply

Celiac Trunk

• Gastroduodenal artery: This artery descends behind the first part of the duodenum and divides into two branches:

  • Anterior superior pancreaticoduodenal artery

  • Posterior superior pancreaticoduodenal artery

• Common hepatic artery: It gives rise to the right gastric artery, which supplies the proximal part of the duodenum.

Superior Mesenteric Artery

• Inferior pancreaticoduodenal artery: This artery divides into anterior and posterior branches.

The superior and inferior pancreaticoduodenal arteries form an arterial arcade around the head of the pancreas, ensuring a consistent blood supply to the duodenum

Duodenum - Lymph Drainage

• Superior (First) part: Drains primarily to the pancreaticoduodenal and hepatoduodenal ligament lymph nodes.

• Descending (Second) part: Mainly drains to the pancreaticoduodenal lymph nodes.

• Horizontal (Third) and Ascending (Fourth) parts: These parts drain predominantly to the superior mesenteric lymph nodes.

Duodenum - Nerve Supply

Intrinsic Innervation

• The intrinsic innervation of the duodenum is provided by the enteric nervous system (ENS), which is often referred to as the "brain of the gut".

• The ENS consists of two major plexuses:

1. Myenteric plexus (Auerbach's plexus): Located between the longitudinal and circular muscle layers, it primarily controls motility.

2. Submucosal plexus (Meissner's plexus): Found in the submucosa, it regulates secretion and local blood flow.

Extrinsic Innervation

• Parasympathetic Innervation

  • The duodenum receives parasympathetic fibers from the vagus nerve (cranial nerve X).

  • These fibers synapse with neurons in the enteric plexuses and generally promote digestive activities, including increased motility and secretion.

• Sympathetic Innervation

  • Prevertebral ganglia: Sympathetic innervation originates from the celiac and superior mesenteric ganglia.

  • These fibers typically inhibit digestive activities and regulate blood flow.

Jejunum - Position

• The jejunum is located in the upper left portion of the abdominal cavity

• It begins at the duodenojejunal flexure, near the ligament of Treitz, and continues downwards as a loosely supported structure

• The jejunum forms multiple loops within the abdominal cavity, held in place by the mesentery

Jejunum - Peritoneal Relation

• The jejunum is an intraperitoneal organ, meaning it is almost completely surrounded by peritoneum

• It is suspended by the mesentery, a fan-shaped fold of peritoneum that attaches the jejunum to the posterior abdominal wall

• This mesenteric attachment allows for considerable mobility of the jejunal loops within the abdominal cavity

Jejunum - Structure of the Wall

• Mucosa: The innermost layer, which is highly specialized for absorption.

• Submucosa: A layer of connective tissue supporting the mucosa.

• Muscularis propria: Contains circular and longitudinal muscle layers responsible for peristalsis.

• Serosa: The outermost layer, formed by peritoneum.

• The jejunal wall is typically thinner compared to other parts of the small intestine

• The average wall thickness of the jejunum is approximately 1-2 mm, although this can vary depending on the individual and the degree of distension

• The inner surface of the jejunum is characterized by circular folds called plicae circulares, which increase the surface area for absorption

• These folds are more prominent in the jejunum compared to other parts of the small intestine.

Jejunum - Blood Supply

The primary blood supply to the jejunum comes from the superior mesenteric artery, a branch of the abdominal aorta

• Vasa recta: These vessels divide into two equal branches that encircle the gut, anastomosing with each other across the anti-mesenteric border

• Submucosal plexus: A rich arterial network in the submucosa provides small branches to supply the mucous membrane

• Villus vasculature: Each villus in the jejunum has a specific vascular arrangement to facilitate nutrient absorption

The mesentery, a double fold of peritoneum, contains the blood vessels that supply the jejunum, anchoring it to the posterior abdominal wall

Jejunum - Lymphatic Drainage

• Drains into superior mesenteric lymph nodes, which then transport lymph to larger lymphatic trunks before entering the bloodstream

Jejunum - Innervation

• The jejunum receives both sympathetic and parasympathetic innervation

• The parasympathetic supply comes from the vagus nerve

• Sympathetic innervation is derived from the superior mesenteric plexus

• An intrinsic nervous system, known as the enteric nervous system, is present within the intestinal wall.

• This system includes the submucosal (Meissner's) plexus and the myenteric (Auerbach's) plexus

Ileum - Position

• The ileum is the final segment of the small intestine, extending from the jejunum to the cecum.

• It typically occupies the lower right quadrant of the abdomen, with its terminal part located in the right iliac fossa.

• In some cases, the terminal ileum may be retroperitoneal, ascending vertically to connect with the cecum, which can complicate surgical procedures and diagnosis

Ileum - Peritoneal Relation

• The ileum is primarily intraperitoneal, meaning it is suspended within the peritoneal cavity by a fold of peritoneum known as the mesentery.

• The mesentery contains blood vessels, nerves, and lymphatics that supply the ileum.

• The peritoneal relation allows for mobility and flexibility of the ileum during digestive processes

Ileum - Structure of the Wall

• Mucosa: This innermost layer features villi and microvilli that increase surface area for absorption. The mucosa contains specialized cells such as enterocytes and Goblet cells.

• Submucosa: Composed of connective tissue, this layer houses blood vessels, lymphatics, and nerve fibers.

• Muscularis: This layer consists of two muscle layers (inner circular and outer longitudinal) that facilitate peristalsis.

• Serosa: The outermost layer is a smooth membrane that covers the ileum and is continuous with the mesentery

Ileum - Blood Supply

• The blood supply to the ileum is primarily provided by branches from the superior mesenteric artery (SMA). These branches include:

  • Ileal arteries: These are several small arteries that arise from the SMA and supply blood directly to the ileum.

  • Ileocolic artery: This artery supplies both the terminal ileum and cecum.

• Venous drainage occurs via the superior mesenteric vein, which drains into the portal vein

Ileum - Lymphatic Drainage

• Mesenteric lymph nodes: Located throughout the mesentery. Drains into the superior mesenteric lymph nodes.

• Celiac nodes: Drain into larger lymphatic trunks that eventually empty into the thoracic duct.

Ileum - Innervation

• Parasympathetic innervation: Primarily from the vagus nerve, stimulating digestive activity

• Sympathetic innervation: Arising from thoracic splanchnic nerves, which inhibit peristalsis and reduce blood flow during stress

Large Intestine - Parts

• Cecum:

  • The first part of the large intestine

  • Located in the right lower quadrant

  • Connects to the ileum of the small intestine via the ileocecal valve

• Appendix:

  • A small, tubular structure attached to the cecum

  • It is considered a vestigial organ but may have immune functions.

• Colon: Divided into four segments:

  • Ascending Colon: Extends upward from the cecum to the hepatic flexure.

  • Transverse Colon: Runs horizontally across the abdomen from the hepatic flexure to the splenic flexure.

  • Descending Colon: Travels downward from the splenic flexure to the sigmoid colon.

  • Sigmoid Colon: An S-shaped segment that connects to the rectum.

• Rectum: The final section that stores feces before elimination.

Large Intestine - Position

• The large intestine is positioned in the abdominal cavity

• It is generally located around the periphery of the small intestine

• It starts in the right lower quadrant with the cecum and extends upward (ascending colon), across (transverse colon), downward (descending colon), and finally into the pelvis (sigmoid colon and rectum).

Large Intestine - External Features

• Haustra: Sacculations formed by contractions of the Taeniae coli, giving it a segmented appearance.

• Teniae Coli: Three longitudinal bands of smooth muscle that run along the length of the colon, responsible for its characteristic shape and motility.

• Epiploic/Omental Appendages: Small pouches of visceral peritoneum filled with fat that hang from the surface of the colon.

• The large intestine has a larger lumen compared to the small intestine, which facilitates its role in water absorption.

Large Intestine - Peritoneal Relation

• Intraperitoneal:

  • Ceceum

  • Appendix (usually)

  • Transverse Colon (suspended by mesocolon)

  • Sigmoid Colon

• Retroperitoneal:

  • Ascending Colon (fixed to posterior abdominal wall)

  • Descending Colon

  • Rectum

Large Intestine - Blood Supply

The blood supply to the large intestine comes from branches of the superior mesenteric artery (SMA) and inferior mesenteric artery (IMA):

• Cecum and Appendix: Supplied by ileocolic artery (branch of SMA).

• Ascending Colon: Supplied by right colic artery (branch of SMA).

• Transverse Colon: Supplied by middle colic artery (branch of SMA) and anastomoses with branches from IMA.

• Descending Colon: Supplied by left colic artery (branch of IMA).

• Sigmoid Colon: Supplied by sigmoid arteries (branches of IMA).

• Rectum: Supplied by superior rectal artery (continuation of IMA) and inferior rectal artery (branch of internal pudendal artery).

Large Intestine - Lymph Drainage

• Lymph from the cecum and appendix drains into ileocolic lymph nodes.

• Lymph from the ascending and transverse colon drains into right colic and middle colic lymph nodes, respectively.

• Lymph from the descending colon drains into left colic lymph nodes.

• Lymph from sigmoid colon drains into sigmoid lymph nodes before reaching inferior mesenteric lymph nodes.

Large Intestine - Innervation

• Parasympathetic Innervation:

  • From vagus nerve for proximal two-thirds (cecum to transverse colon).

  • From pelvic splanchnic nerves (S2-S4) for distal one-third (descending colon to rectum).

• Sympathetic Innervation:

  • From Thoracic splanchnic nerves for proximal parts.

  • From Lumbar splanchnic nerves for distal parts.

Colon - Parts

• Cecum: The initial segment, connecting to the ileum of the small intestine.

• Ascending Colon: Travels upward on the right side of the abdomen.

• Transverse Colon: Runs horizontally across the abdomen.

• Descending Colon: Moves downward on the left side.

• Sigmoid Colon: Forms an S-shaped curve leading to the rectum.

Colon - Peritoneal Relation

• Intraperitoneal: The cecum, transverse colon, and sigmoid colon are intraperitoneal, meaning they are covered by peritoneum and have a mesentery.

• Retroperitoneal: The ascending and descending colons are retroperitoneal, lying behind the peritoneum and attached to the posterior abdominal wall.

Colon - Structure of the Wall

• Mucosa: The innermost layer featuring a simple columnar epithelium with goblet cells for mucus secretion.

• Submucosa: Contains connective tissue, blood vessels, and nerves.

• Muscularis Externa: Composed of two layers of smooth muscle (inner circular and outer longitudinal), which facilitate peristalsis.

• Serosa: The outermost layer, a smooth membrane covering the colon.

Colon - Blood Supply

The blood supply to the colon is primarily derived from branches of the superior mesenteric artery (SMA) and inferior mesenteric artery (IMA):

• Cecum and Ascending Colon: Supplied by the ileocolic artery and right colic artery from SMA.

• Transverse Colon: Supplied by middle colic artery from SMA.

• Descending Colon: Supplied by left colic artery from IMA.

• Sigmoid Colon: Supplied by sigmoid arteries from IMA.

Colon - Lymphatic Drainage

• Lymph from the cecum and ascending colon drains into the right colic lymph nodes.

• Lymph from the transverse colon drains into the middle colic lymph nodes.

• Lymph from the descending colon drains into the left colic lymph nodes.

• Lymph from the sigmoid colon drains into sigmoid lymph nodes, eventually leading to inferior mesenteric lymph nodes.

Colon - Innervation

• Sympathetic Innervation: Primarily from the thoracic splanchnic nerves and lumbar splanchnic nerves, which inhibit peristalsis and reduce secretions.

• Parasympathetic Innervation: Provided by the vagus nerve (for proximal colon) and sacral spinal nerves (for distal colon), stimulating peristalsis and secretion.

Vermiform Appendix - Position, Shape and Size

• The vermiform appendix is a small, tubular structure attached to the cecum of the large intestine.

• Retrocecal: This is the most frequently observed position, where the appendix is located behind the cecum

• The vermiform appendix is generally described as a worm-like structure, hence its name "vermiform" (meaning worm-shaped).

• The appendix typically has a narrow lumen and can vary in diameter but is generally consistent in being tubular.

• The length of the appendix can range from approximately 3 cm to 10 cm, with an average length reported at about 7.37 cm

Caecum - Structure of the Wall

• The caecum is a pouch-like structure located at the junction of the small and large intestines, specifically at the ileocaecal junction.

• Its wall is composed of three primary layers:

  • Mucosa: The innermost layer, which contains epithelial cells that facilitate absorption and secretion.

  • Muscularis (Musculosa): This middle layer consists of smooth muscle fibers responsible for peristalsis and movement of contents within the caecum.

  • Serosa: The outermost layer, which is a protective membrane covering the caecum

Caecum - Peritoneal Relation

• The caecum is situated in the right lower quadrant of the abdomen and is covered by peritoneum.

• It is typically intraperitoneal, meaning it is suspended by a fold of peritoneum called the mesentery, which allows for mobility while providing blood supply and nerve connections.

• The caecum can have various positions, including retrocaecal or pelvic, depending on individual anatomical variations

Caecum - Blood Supply

• The blood supply to the caecum primarily comes from branches of the superior mesenteric artery (SMA):

  • Ileocolic artery: This artery branches off from the SMA and supplies blood to both the caecum and the terminal ileum.

  • Appendicular artery: A branch that supplies blood to the appendix, which is attached to the caecum.

• Venous drainage occurs through the ileocolic vein, which drains into the superior mesenteric vein, eventually leading to the portal vein

Caecum - Lymphatic Drainage

• Ileocolic lymph nodes: These nodes receive lymph from both the caecum and appendix.

• Superior mesenteric lymph nodes: Further drainage occurs through these nodes, which are associated with larger vessels in the mesentery

Caecum - Innervation

• Autonomic Innervation

  • The caecum receives parasympathetic innervation from the vagus nerve, promoting digestive functions.

  • Sympathetic innervation comes from thoracic splanchnic nerves via the superior mesenteric ganglia, which modulate blood flow and digestive activity.

• Enteric Nervous System: Within its wall, there are networks of neurons (Auerbach's plexus and Meissner's plexus) that control local gut motility and secretion independently of central nervous system input

Rectum - Position

• The rectum is located in the pelvic cavity, extending from the sigmoid colon to the anal canal.

• It begins at the level of the third sacral vertebra (S3) and typically ends at the anal verge, which is about 12-15 cm in length.

• The rectum is positioned posterior to the bladder and prostate in males and posterior to the vagina and uterus in females.

Rectum - Shape

The rectum has a somewhat cylindrical shape, but it is not perfectly straight. It exhibits three distinct curves:

• Sacral flexure: This curve follows the curvature of the sacrum.

• Perineal flexure: This occurs as the rectum approaches the anal canal.

• Transverse folds: The rectal wall contains transverse folds known as the rectal valves, which help support fecal material.

Rectum - Size

• The size of the rectum varies among individuals but generally measures about 12-15 cm in length.

• Its diameter can range from 2.5 to 4 cm, depending on factors such as distension and individual anatomy.

Rectum - Peritoneal Relation

• The rectum is primarily retroperitoneal, meaning it lies behind the peritoneum.

• The upper third of the rectum is covered anteriorly by the peritoneum, while the middle and lower thirds are not.

• In females, the rectum is separated from the vagina by a connective tissue layer called the rectovaginal septum

• In males, it is separated from the prostate gland.

Rectum - Structure of the Wall

• Mucosa: The innermost layer features a columnar epithelium with numerous goblet cells that secrete mucus for lubrication.

• Submucosa: This layer contains connective tissue, blood vessels, lymphatics, and nerves.

• Muscularis: Composed of an inner circular layer and an outer longitudinal layer of smooth muscle, this layer facilitates peristalsis.

• Adventitia: The outermost layer consists of connective tissue that anchors the rectum to surrounding structures.

Rectum - Blood Supply

• Superior rectal artery: A continuation of the inferior mesenteric artery that supplies the upper part.

• Middle rectal artery: Branches from the internal iliac artery supplying the middle part.

• Inferior rectal artery: A branch of the internal pudendal artery that supplies the lower part.

• The superior rectal vein into the inferior mesenteric vein

• The middle rectal vein into the internal iliac vein

• The inferior rectal vein into the internal pudendal vein

Rectum - Lymphatic Drainage

• Upper part drains into inferior mesenteric lymph nodes

• Middle part drains into internal iliac lymph nodes

• Lower part drains into superficial inguinal lymph nodes

Rectum - Innervation

• Sympathetic innervation: Comes from lumbar splanchnic nerves (T10-L2) via hypogastric plexus.

• Parasympathetic innervation: Arises from sacral splanchnic nerves (S2-S4), which promote peristalsis and relaxation of internal anal sphincter.

• Somatic innervation: Provided by inferior rectal nerve (a branch of pudendal nerve) for sensation and voluntary control over external anal sphincter.

Liver - Position, Shape and Size

• The liver is a large, vital organ located in the upper right quadrant of the abdominal cavity

• Its position is primarily in the right hypochondrium, extending across the epigastrium into the left hypochondrium

• The liver's shape is roughly wedge-like or pyramidal, with a convex superior (diaphragmatic) surface and a concave (visceral) inferior surface

• The size of an adult liver is considerable, weighing approximately 1.2-1.5 kg and measuring about 20-22 cm across at its widest point, 15-17 cm at its greatest vertical height, and 10-12 cm from front to back