A&P II- Digestion I

The Digestive System

Structure

• Gastrointestinal/digestive system,

• Alimentary canal: extends from the mouth to the anus through the ventral body cavity (approximately 9 m, or 30 ft.).

• Accessory organs: teeth, tongue, salivary glands, liver, gallbladder, and pancreas

Function

ABSORPTION.

 Digestion

◦ Chemical

◦ Mechanical

 ingestion,

 secretion,

 mixing and propulsion,

 defecation

Role in metabolic processes.

 Catabolism: Larger molecules are broken into smaller molecules (mouth, stomach, duodenum).

◦ In the GI tract, this is called Digestion and can occur by either mechanical or chemical means.

 Anabolism: Smaller molecules are used as building blocks for larger molecules (liver)

Digestion

Mechanical digestion: all Movements that facilitate catabolic processes

 Mastication

 Mixing

◦ Increase contact of food with digestive chemicals

Other movements:

 Swallowing

 Peristalsis

◦ Movement of muscles within the GI tract that facilitates movement of food

Chemical digestion: breaking large molecule into smaller ones, mainly through hydrolysis.

 Fats -> fatty acids and glycerol.

 Carbohydrates: polysaccharides -> monosaccharides.

 Proteins-> polypeptides -> amino acids.

Requires specific enzymes

GI Tract Anatomy Overview

Four layers of tissues

• Mucosa (deepest), Submucosa, Muscularis, Serosa/adventitia (superficial) lumen is the inside of the tube.

Mucosa:

• mucous membrane

• Epithelial tissue

  • Stratified squamous (mouth, pharynx, esophagus, anus)

  • Simple columnar (stomach and intestines)

  • Includes glandular cells that secrete mucus and fluid into the lumen

• Areolar connective tissue (lamina propria.)

• Smooth muscle (muscularis mucosae)

• Lamina propria contains “MALT”, mucosa-associated lymphatic tissue that protect against disease.

• Muscularis mucosae: creates folds in the lining of the stomach and small intestines

Submucosa

• irregular connective tissue that binds the mucosa to the muscularis.

  • blood and lymphatic vessels (to receive absorbed substances)

  • submucosal plexus: a network of neurons

Muscularis:

• mouth, pharynx, superior and middle parts of the esophagus, and anal sphincter contain skeletal muscle

• Rest of the tract: smooth muscle

• Inner circular sheet

• Outer longitudinal sheets, myenteric nerve plexus between them

Serosa/adventitia

• is the outermost layer.

• adventitia - fibrous connective tissue attached to surrounding tissues (e.g. esophagus).

• serosa - fibrous connective tissue in the peritoneal cavity, with a mesothelium surface layer

  • Serosa covers the intra-abdominal organs as the visceral peritoneum

Anatomy Overview

Peritoneum

• the body’s largest serous membrane, wraps around most abdominopelvic organs.

  • Visceral peritoneum: the serosa of the alimentary canal and covers other intra- abdominal organs.

  • Parietal peritoneum: the abdominal wall, connects to visceral peritoneum.

Retroperitoneal organs

• are covered by visceral peritoneum only on their anterior surfaces. The portion of the organ that lies behind the peritoneum is said to be “retroperitoneal”.

• Organs in the retroperitoneal space include:

  • The kidneys and ureters

  • Most of the pancreas

  • The adrenal glands

  • The aorta and inferior vena cava

Peritoneal Folds

five major peritoneal folds that bind the organs to one another and to the cavity walls

Greater omentum

• largest peritoneal fold.

• Connects the stomach to the transverse colon

• Drapes over the transverse colon and the anterior coils of the small intestine.

• Contains: lymph nodes and a large amount of adipose tissue that can greatly expand

Falciform ligament

• attaches the liver to the anterior abdominal wall and diaphragm.

Lesser omentum

• suspends the stomach and duodenum from the inferior edge of the liver.

  • pathway for blood vessels to enter the liver

  • contains the common bile duct

Mesentery (small) and Mesocolon (large)

• attach intestine to posterior abdominal wall

• Loose attachment so muscular contractions can mix and move the contents along the GI tract

Physiology Overview

Digestive activities are controlled in three overlapping phases:

1. The cephalic phase

2. The gastric phase

3. The intestinal phase

Cephalic phase:

• smell, sight, thought, or initial taste of food neural centers in the CNS to prepare for digestion.

• stimulate secretion of saliva and gastric juice

Gastric phase:

• food enters the stomach.

• Nervous and endocrine systems, gastrin is a key hormone

• Promotes secretion of gastric juice and gastric motility.

Intestinal phase:

• acidic food enters the small intestine.

• Neural response decreases gastric motility

• Hormones (secretin, CCK) increases intestinal secretions and decreases gastric secretions and motility

The Mouth

oral or buccal cavity:

• formed by the cheeks, hard and soft palates, and the tongue.

• Mechanical digestion: mastication (chewing)

• Saliva

  • mixes with food to soften it so it can be easily swallowed.

• Starts the process of Chemical digestion

  • Carbohydrates: salivary amylase

  • Fats: lingual lipase active in acidic stomach (limited activity)

Salivary regulation is under the control of the ANS

• Parasympathetic stimulation promotes secretion

  • This is enhanced by the cephalic phase.

• Sympathetic stimulation decreases saliva secretions.

The tongue

• Moves food in the mouth for chewing and swallowing (deglutition)

• Provides a sense of taste

• composed of skeletal muscle

Teeth

Teeth or dentes:

• located in sockets of the mandible and maxillae.

  • Periodontal ligament - a dense fibrous connective tissue that anchors the teeth.

• three major external regions: the crown, root, and neck.

  • The neck of each tooth is covered by the gingivae , or gums, which extend slightly into each socket.

Dentin:

• calcified connective tissue that forms most of the tooth.

  • Enamel: harder-than-bone calcified non-living material, covers the crown

  • Pulp provides nutrition

  • 20 dentitions as a child

  • 32 as an adult

Deglutition

Deglutition:

• swallowing food.

• Making sure food moves from the mouth through the pharynx to the esophagus and NOT the airway/nasal cavity.

• 3 stages:

  • Voluntary: tongue pushing bolus of food into the pharynx,

  • Pharyngeal: palate rises and epiglottis lowers involuntarily due to presence of food

  • Esophageal: peristalsis moves the bolus down toward the stomach

Peristalsis: a progression of coordinated contractions and relaxations of the circular and longitudinal layers of the muscularis, push the bolus onward.

The Esophagus

Esophagus:

• Propulsion is only function (moving food into the stomach).

• Connects laryngopharynx to stomach posterior to the trachea.

• traverses the posterior mediastinum pierces the diaphragm

Mucosa: nonkeratinzed stratified squamous epithelium

Muscularis:

• superior 1/3: skeletal muscle

• intermediate 1/3: skeletal and smooth muscle

• inferior 1/3: smooth muscle

Upper and lower esophageal sphincters (UES and LES) at each end of the tube.

• The LES regulates the movement of food from the esophagus into the stomach