PowerPoint file for IIT

Digestive System 1 (upper digestive system)

Presented by: Leo Cheung Chi Man

Objectives: Identify the organs of the digestive tract and the

accessory organs of digestion List the six main functions of the digestive

system

Explain how ingested materials are propelled through the digestive tract

Describe the mechanism of fluid secretions within the stomach, small and large intestines, including an understanding of how these secretions can be modified by intrinsic and extrinsic regulation

Digestive System: Overview

Digestive Process

• There are six essential activities:

1) Ingestion

2) Propulsion

3) Mechanical digestion

4) Chemical digestion

5) Absorption

6) Defecation

Part1: The Mouth (Oral or Buccal Cavity)

Part1: The Mouth (Oral or Buccal Cavity)

Its functions include:• Analysis of material before

swallowing• Mechanical processing by the

teeth, tongue, and palatal surfaces

• Lubrication• Limited digestion

Part1: The Mouth (Oral or Buccal Cavity)

Source and composition of

saliva: Secreted from cells of salivary glands A 97 - 99.5% water, hypotonic, slightly

acidic solution containing

• Electrolytes: Na+, K+, Cl–, PO42–,

HCO3–

• Digestive enzyme: salivary amylase• Proteins: RNAase, DNAase,

lysozyme, peroxidase, and IgA etc…• Metabolic wastes: urea and uric acid

Part1: The Mouth (Oral or Buccal Cavity)

Functions of saliva:• Cleanses the mouth• Moistens and dissolves food chemicals • Aids in bolus formation• Contains a-amylase that breakdown

starch

Part1: The Mouth (Oral or Buccal Cavity)

Carbohydrate Digestion • Begins in the mouth

• Salivary amylase

• Video 1 (http://www.youtube.com/watch?v=vBtJ1D3S_p8&p=1282F776C2E3DA12&playnext=1&index=12)

Part2: The Pharynx and esophagus

Functions:• They contribute nothing to digestion but

provide the pathway by which ingested materials reach stomach

• The muscles (smooth and skeletal) in the walls of these segments control swallowing

• Video 2 (http://www.youtube.com/watch?

v=h0E9ITyRlh0&feature=related )

Part3: The Stomach

Muscularis – has an additional oblique layer that• Allows the stomach

to churn, mix and pummel food physically

• Breaks down food into smaller fragments

Part3: The Stomach Functions:

• They Bulk storage of undigested food• Mechanical breakdown of food• Disruption of chemical bonds via acids

and enzymes• Preliminary digestion of proteins with

Pepsin• Production of intrinsic factor required for

absorption of vitamin B12

Part3: The Stomach

Epithelial lining is composed of :• Goblet cells that

produce a coat of alkaline mucus

Gastric pits containing gastric glands that secrete:

• Gastric juice • Mucus • Gastrin

The Stomach Lining

Mucous cells – secrete mucus Parietal (oxyntic) cells – secrete

HCl and intrinsic factor Chief (zymogenic/peptic) cells –

produce pepsinogen Pepsinogen is activated to

pepsin by:• HCl in the stomach• Pepsin itself by a positive

feedback mechanism G cells – secrete gastrin

Part3: The Stomach

Gastric Secretory Cells

Part3: The Stomach Protein Digestion

• pepsin in the stomach

• Video 3 (http://www.youtube.com/watch?v=Q-n_Q0qKXzg&feature=related )

Neural and hormonal mechanisms regulate the release of gastric juice

Stimulatory and inhibitory events occur in three phases• Cephalic (reflex) phase: prior to food entry• Gastric phase: once food enters the

stomach• Intestinal phase: as partially digested food

enters the duodenum

Part3: The Stomach

Regulation of Gastric Secretion