Lesson # 15 The Digestive System 1Chapter 24Objectives:1- To explain the functions of the digestive system. 2- To describe the structure of the parietal and visceral peritoneum.3- To describe the functional histology of the digestive tract.4- To explaining how food travels through the alimentary canal.5- To explain the mechanisms that control the digestive function.

It is the chemical breakdown of food into small organic fragments for absorption by the digestive epithelium.

Functions of the Digestive System 1- Ingestion:

2- Mechanical processing: It occurs when materials enter the digestive tract via the mouth.

It is the crushing and shearing that make materials easier to propel along the digestive tract.

3- Digestion:

4- Secretion:It is the release of water, acids, enzymes, buffers, and salts by the epithelium of the digestive tract and by the glandular organs.

5- Absorption: It is the movement of organic substrates, electrolytes, vitamins, and water across the digestive epithelium into interstitial fluid of the digestive tract.

6- Excretion: It is the removal of waste products from the body fluids.

Digestive System

Digestive tract

Accessory organs

- Oral cavity- Pharynx- Esophagus- Stomach- Small intestine- Large intestine

- Teeth- Tongue- Salivary glands- Liver

- Pancreas- Gallbladder

The Components of the Digestive System

Mouth

Anus

The Digestive Organs and the Peritoneum

Parietal peritoneumVisceral peritoneum

Peritoneal cavityIt contains the peritoneal fluid which provides lubrication. Accumulation of fluid is called ascites. Infection is called peritonitis.

Mesenteries:They are sheets of serous membrane that connect the visceral peritoneum of different organs and the visceral peritoneum to the parietal peritoneum.

It attaches the stomach to the liver.

Falciform ligamentIt attaches the liver to the diaphragm and the abdominal wall.

It is a pouch that hanks like an apron from the lateral and inferior borders of the stomach. It provides padding, protection, and energy reserve

Mesentery properIt attaches and suspends the small intestine to the posterior wall of the abdomen.

PancreasDuodenum

Retroperitoneal organs:It supports the transverse colon.

Sigmoid mesocolon

It supports the sigmoid colon.

Lesser omentum

Transverse mesocolon

Greater omentum

Greater omentum

Histological Organization of the Digestive Tract

4- Serosa

EpitheliumStratified squamous (oral cavity, pharynx and esophagus).Simple columnar (stomach, small intestine, and almost the entire large intestine).

Lamina propria

Areolar connective tissue with blood vessels, sensory nerve endings, lymphatic vessels, and lymphoid tissue. It supports, nourishes and protects the epithelium.

Muscularis mucosa

Smooth muscle and elastic fibers between the mucosa and submucosa.

Dense irregular connective tissue, containing large blood and lymphatic vessels, exocrine glands, and a network of nerve fibers and neurons of the autonomic system called submucosal plexus or Meissner plexus.

3- Muscularis externa:Two or three layers of smooth muscle tissue for mechanical processing and movement of material along the digestive tract. It contains the myenteric plexus or Auerbach plexus. Parasympathetic: Increases muscle tone and activity.Sympathetic: Decreases muscle tone and activity.

1- Mucosa:

2- Submucosa:

Small Intestine

DiaphragmEsophagus

Stomach

Large Intestine

Changes in the Mucosa and the Number of Layers of the Muscularis Externa. The folds.

Stratified squamous epithelium(also in oral cavity, pharynx and esophagus).

Two layers of smooth muscle in the muscularis externa: outer longitudinal and inner circular.

Simple columnar epithelium (it contains gastric pits and gastric glands)

Three layers of smooth muscle in the muscularis externa: outer longitudinal, middle circular, and inner oblique.Folds of the mucosa called rugae.

Longitudinal folds of the mucosa that allow for expansion

Simple columnar epithelium with microvilli (it contains crypts of Lieberkuhn and intestinal glands)

Transverse folds of the mucosa called plicae circulares, and fingerlike projections called villi.

Two layers of smooth muscle in the muscularis externa: outer longitudinal and inner circular.

Simple columnar epithelium without villi (it is dominated by mucous cells)

Transverse folds of the wall called haustra.

Two layers of smooth muscle in the muscularis externa: outer longitudinal reduced to the taeniae coli, and inner circular.

Taeniae coli

The Movement of Digestive MaterialsThe smooth muscle of the muscularis externa is called visceral smooth muscle. Some cells of the visceral smooth muscle called pacesetter cells undergo spontaneous depolarization that triggers waves of contraction through the entire muscular sheet.

Waves of contraction PeristalsisSegmentation

Peristalsis:

Waves of contractions propel material from one portion of the digestive tract to another.

Waves of contractions does not propel material, but churns, fragments, and mixes the bolus with intestinal secretions.

Bolus Bolus

Bolus Bolus

Control of Digestive Functions

2- Neural Mechanisms

3- Hormonal Mechanisms

1- Local Factors

a- Short reflexes

b- Long reflexes

Prostaglandins, histamine, and other chemicals released into interstitial fluid, may affect adjacent cells within small segment of digestive tract, coordinating response to changing conditions (variations in local pH, chemical, or physical stimuli).

Local factors affect only a portion of tract.

1- Local Factors

1- Short reflexes:They are responsible for local reflexes that control small segments of digestive tract.They operate entirely outside of CNS control.

They control the movement of materials along digestive tract, and secretory functions.

Motor neurons located in myenteric plexus control smooth muscle contraction and glandular secretion.

Stretching of the stomach activates a short reflex that increases gastric secretions.

2- Long reflexes:

Acid and semi-digested fats in the duodenum trigger a long reflex. The medulla oblongata inhibits vagal nuclei, reducing vagal stimulation of the stomach.

They control large-scale peristaltic waves that move material from one region of the digestive system to another.

They provide a higher level control that involves interneurons and motor neurons in CNS.

2- Neural Mechanisms

They reach target organs after distribution in bloodstream.

3- Hormonal MechanismsAt least 18 peptide hormones affect most aspects of digestive function and some of them also affect activities of other systems.

The hormones gastrin, secretin, and other; are produced by enteroendocrine cells in the digestive tract.

Ex: Chyme also stimulates duodenal enteroendocrine cells to release secretin and cholecystokinin (CCK) that suppress gastric secretion and motility.

Neural Control Mechanisms

Local Factors

Hormonal Control Mechanisms

Longreflex

Shortreflex

Stretch receptors,chemoreceptors

CNS

Myentericplexus

Secretorycells

Enteroendocrinecells

Hormonesreleased

Via thebloodstream

Peristalsis andsegmentationmovements

Buffers, acids,enzymes released

Local factors are the primary stimulus fordigestive activities. They coordinate theresponses to:

The digestive tract produces at least 18 hormones that affectalmost every aspect of digestivefunction, and some of themalso affect the activities ofother systems.

The movement of materials along the digestive tract, as well as many secretoryfunctions, is primarily controlled by local factors. These reflexes are:

1- Changes in the pH of the contents of the lumen.2- Physical distortion of the wall of the digestive tract.3- The presence of chemicals—either specific nutrients or chemical messengers released by cells of the mucosa.

1- Short reflexes are triggered by chemoreceptors or stretch receptors in the walls of the digestive tract; the controlling neurons are located in the myenteric plexus.

2- Long reflexes involving interneurons and motor neurons in the CNS provide a higher level of control over digestive and glandular activities, generally controlling large-scale peristaltic waves that move materials from one region of the digestive tract to another.

These hormones are peptides produced by enteroendocrine cells, endocrine cells in the epithelium of the digestive tract.

Control of Digestive Functions

1- Carbohydrates

Oligosaccharides

Disaccharides

MonosaccharidesThey are absorbed immediately

Amylases Amylases Amylases

Starch is the most digestible carbohydrate

2- Proteins

Protein

Polypeptides

Aminoacids

Peptidases Peptidases

They are absorbed immediately

Overview of Chemical Digestion

Monoglycerides

+

Fatty acids

3- Lipids

Triglycerides

+

Glycerol Fatty acids

Lipases

Lipases

Upper lip

Lower lip

CheekVestibule

Soft palate Uvula

The Oral Cavity, the Tongue, the Salivary Glands, and the Teeth

Tongue

Lingual frenulum

The Oral CavityThe oral cavity is lined by the oral mucosa, which has a stratified squamous epithelium.

Hard palate

1- Sensory analysis of material before swallowing.2- Mechanical processing through actions of teeth, tongue, and palatal surfaces.3- Lubrication by mixing with mucus and salivary gland secretions.4- Limited digestion of carbohydrates and lipids.

Functions:

Gingiva

It is the space between the cheek (or lips) and the teeth .

Palatine process of maxilla

Palatine bone

The Tongue

5- Helping in speech.

Functions:

They provide friction

They contain

taste buds

They contain

taste buds

Body or Oral

portion

Root or Pharyngeal

portion

1- Mechanical processing by compression, abrasion, and distortion.2- Manipulation of food to assist in chewing and swallowing.3- Sensory analysis of food by touch, temperature, and taste receptors. 4- Secretion of mucus and lingual lipase.

Parotid gland

Parotid duct

Sublingual gland

Submandibular gland

Duct

They produce mucins

Functions of Saliva:

Salivary Glands

They produce enzymes

Serous cells

Mucous cells

1- Cleanses the mouth and protects the mouth (it contains antibodies and lysozyme).2- Lubricates the mouth.3- Moistens and lubricates materials (food) in the mouth.4- Dissolves chemicals that stimulate taste buds. 5- Initiates the digestion of carbohydrates (salivary amylase)

The Teeth

Functions:Mechanical break down of food (chewing or mastication)

Crown

Neck

Root

Enamel

Pulp cavity

Dentin

Periodontal ligament

Cementum

Root canal

Blood vessels and nerve

Gingiva or gum

Types of Teeth

Upper dental arch

Lower dental arch

Central incisorLateral incisor

(Blade-shape teeth: Clipping and cutting)

Cuspid or canine

(Conical with a sharp ridgeline and a pointed tip: Tearing or slashing) Bicuspids or

premolars(Flattened crown with prominent ridges: Crushing, smashing and grinding)

Molars(Very large flattened crowns with prominent ridges: Crushing and grinding)

Total: 32 permanent or secondary teeth

Total: 20 primary, temporary or

deciduous teeth

EsophagusTrachea

Larynx

Posterior nasal

aperture

Nasopharynx

Oropharynx

Laryngopharynx

Pharynx:

It is a passageway for air.

It is a passageway for air, food and drink.

It is a passageway for air, food and drink.

It is lined by a pseudostratified columnar epithelium.

It is lined by a stratified squamous epithelium.

It is lined by a stratified squamous epithelium.

The PharynxFunctions:Air passageway between nasal cavity and the larynx.Food passageway between the oral cavity and the esophagus.

The EsophagusIt transports solids and liquids from the pharynx to the stomach. Diaphragm

Stratified squamous epithelium(also in oral cavity, pharynx and esophagus).

Two layers of smooth muscle in the muscularis externa: outer longitudinal and inner circular.

Longitudinal folds of the mucosa that allow for expansion.

Mucosa

Serosa: The peritoneum is a serosa. The visceral layer lines several organs of the digestive system (stomach, small intestine, liver).

Adventitia: There is no serosa in oral cavity, pharynx, esophagus, and rectum. They have adventitia, a fibrous sheath of collagen fiber that attaches them to the adjacent structures.

Esophageal hiatus

Function:

Swallowing or Deglutition

It is divided into three phases:

Swallowing, also called deglutition, is a complex process that can be initiated voluntarily but proceeds automatically once it begins.

The buccal phase is strictly voluntary. It begins with the compression of the bolus against the hard palate and the elevation of the soft palate. The bolus enters the oropharynx.

When the bolus enters the oropharynx, the swallowing reflex is triggered and the process becomes involuntary. The epiglottis closes the larynx, and the soft palate and the uvula close the nasopharynx.The pharyngeal muscles propel the bolus into the esophagus in less than one second.

The esophageal phase begins as the contraction of the esophageal muscles forces the bolus through the entrance of the esophagus.Once in the esophagus, the bolus is pushed toward the stomach by a peristaltic wave.The approach of the bolus triggers the opening of the lower esophageal sphincter, and the bolus enters the stomach.

2- Pharyngeal phase3- Esophageal phase

1- Buccal phase