The Digestive System I

Anatomy

The digestive system

• The organs of the digestive system can be separated into 2 main groups:

1. Alimentary canal: digests food and absorbs the digested fragments through its lining into the blood

2. The accessary digestive organs: assist the process of digestive breakdown in various ways

Organs of the Alimentary Canal• It is also called the gastrointestinal (GI) tract• It is a continuous, coiled, hollow, muscular tube

that pass through the ventral body cavity and is open at both ends

• Its organs are: - The mouth- Pharynx- Esophagus- Stomach- Small intestine- Large intestine

• In a cadaver, the alimentary canal is approximately 9 meters long, but in a living person, it is considerably shorter because of its relatively constant muscle tone

• Food material within this tube is technically outside the body, because it has contact only with cells lining the tract and is open to the external environment at both ends

Organs of the Alimentary Canal

Mouth

• Mouth: oral cavity, a mucous membrane-lined cavity

• Lips: Labia, protect the mouth’s anterior opening• Cheeks: Form the mouth’s lateral wall• Hard palate: Forms the mouth’s anterior roof• Soft palate: Forms the mouth’s posterior roof• Uvula: A fleshy fingerlike projection of the soft

palate, which extends downward from its posterior edge

Mouth

• Vestibule: The space between the lips and cheeks externally and the teeth and gums internally

Tongue• The muscular tongue occupies

the floor of the mouth• The tongue has several bony

attachments:1. 2 hyoid bones2. The styloid process of the skull• The frenulum, a fold of mucous

membrane, secures the tongue to the floor of the mouth and limits its posterior movements

• The Papillae on the tongue containing taste buds, or taste receptors

Pharynx• From the mouth, food

passes posteriorly into the oropharynx and laryngopharynx: Both of Which are common passageways for food, fluids and air

• Oropharynx: posterior to the oral cavity

• Laryngopharynx: continuous with the esophagus below

Pharynx

• The walls of the pharynx contain two skeletal muscle layers:

- The cells of the inner layer run longitudinally- The cells of the outer layer run around the

wall in a circular fashion- Alternating contractions of these two muscle

layers propel food through the pharynx into the esophagus

Esophagus

• The esophagus, or gullet, runs from the pharynx through the diaphragm to the stomach

• The mechanism that esophagus propel food down is called peristalsis

Walls of the alimentary canal organs• The walls of the esophagus to the large intestine are

made up of the same four basic tissue layers or tunics:1. The mucosa: The inner most layer, a moist membrane

that lines the cavity or lumen of the organ

Walls of the alimentary canal organs

2. The submucosa: Is found just beneath the mucosa. It is a soft connective tissue layer containing blood vessels, nerve endings, lymph nodes and lymphatic vessels

Walls of the alimentary canal organs

3. The muscularis externa: A muscle layer typically made up of an inner circular layer and an outer longitudinal layer of smooth muscle cells

Walls of the alimentary canal organs

4. The serosa: the outermost layer of the wall. It consists of a single layer of flat serous fluid-producing cells, the visceral peritoneum

Nerve Plexus

• The alimentary canal wall contains 2 important nerve plexus (a branching network of axons outside of the central nervous system): Submucosal nerve plexus and the mysenteric nerve plexus

• They help regulate the mobility and secretory activity of GI tract organs

Stomach• The C shaped

stomach is on the left side of the abdominal cavity

• The stomach has 2 sphincters:

- Esophageal sphincter (Superior)

- Pyloric sphincter (Inferior)

Stomach

• The fundus is the expanded part of the stomach

• The body is the mid-portion

• The funnel shaped pylorus is the terminal part

Stomach• The stomach is approximately 25 cm long, but

its diameter depends on how much food it contains:

- When its full, it holds about 4 liters of food- When its empty, it collapses inward on itself

and its mucosa is thrown into large folds called rugae (roo’ge)

Stomach

• The lateral convex surface of the stomach is called the greater curvature

• The medial surface is the lesser curvature

Stomach• The lesser omentum is a

double layer of peritoneum extends from the liver to the lesser curvature

• The greater omentum drapes downward and covers the abdominal organs like a lacy apron

• The infection of peritoneum is called peritonitis : the peritoneal membranes tend to stick together around the infection site

Stomach

• The stomach acts as a temporary “storage tank” for food as well as a site for food breakdown:

1. Physical break down:- Besides the usual longitudinal and circular

layers, its wall contains a third obliquely arranged layer of muscle in the muscularis externa

- This muscle allows the stomach to churn, mix and physically breaking the food down

Stomach

2. Chemical breakdown:- The mucosa of the stomach is dotted with millions of

deep gastric pits, which lead into gastric glands that secrete the solution called gastric juice

- The chief cells produce protein-digesting enzymes, mostly pepsinogens

- The parietal cells produce corrosive hydrochloric acid- The mucous neck cells produce a sticky alkaline

mucus, which clings to the stomach mucosa and protects the stomach wall itself from being damaged

Stomach• Most digestive activity occurs in the pyloric

region of the stomach• After food has been processed in the stomach,

it resembles heavy cream and is called chyme• The chyme enters the small intestine through

the pyloric sphincter

Small Intestine

• It is the body’s major digestive organ

• It is the longest section of the alimentary tube (avg. 2m)

• The small intestine is a convoluted tube extending from the pyloric sphincter to the ileocecal valve

Small intestine• The small intestine has 3 subdivisions:- duodenum: beginning- Jejunum: middle- Ileum: terminal

Small intestine• The small intestine is able to process only a

small amount of food at one time• The pyloric sphincter controls food movement

into the small intestine from the stomach

Small intestine• Enzymes produced by the

intestinal cells and by the pancreas (more important) are ducted into the duodenum through the pancreatic duct

• Bile (produced by the liver, stored in the gallbladder) also enters the duodenum through the bile duct

• The pancreatic and bile duct join at the duodenum, to form the flask-like hepatopancratic ampulla

Small intestine

• The wall of the small intestine has 3 structures that increase the absorptive surface:

- Microvilli: tiny projections of the plasma membrane of the mucosa cell, sometimes it is referred to as the brush border

- Villi: fingerlike projection of the mucosa - Circular folds: deep folds of both mucosa and

submucosa layers

Villi and microvilli

Villi• Within each villus is a

rich capillary bed and a modified lymphatic capillary called a lacteal

• The digested food are absorbed through the mucosa cells into both the capillary and the lacteal

Circular folds

• Unlike the rugae folds of the stomach, the circular folds do not disappear when food fills the small intestine

• The number of folds decrease toward the end of the small intestine

• Payer’s patch (lymphatic tissue) increase toward the end of the small intestine

Large Intestine

• The large intestine is about 1.5 m long

• Its major functions are to dry out the indigestible food residue by absorbing water and eliminate these residues from the body as feces

Large intestine

• It frames the small intestine on three sides and has the following subdivisions:

- Cecum- Appendix- Colon- Rectum- Anal canal

Colon

• The colon is divided into several distinct regions:

- The ascending colon- The transverse colon- The descending colon- Sigmoid colon

Anal Canal• The anal canal has an external voluntary

sphincter formed by skeletal muscle and internal involuntary sphincter formed by smooth muscle

Large intestine• Because most nutrient absorption has

occurred before the large intestine, no villi are seen in the large intestine

• There are tremendous numbers of goblet cells in its mucosa that produce mucus

• The pocket like sacs on the large intestine are called haustra