DigestiveSystem

Gastrointestinal Tract 1. Mouth 2. Pharynx

3. Esophagus 4. Stomach 5. Small Intestine 6. Large Intestine

Accessory Structures 1. Teeth 2. Tongue 3. Salivary Glands 4. Liver 5. Gallbladder

6. Pancreas

The Processes of Digestion1. Ingestion

taking food into the mouth 2. Secretion

GI tract and accessory cells secrete water, acid, buffers, and enzymes into lumen 3. Mixing and Propulsion

alternating contraction and relaxation of smooth muscles in the walls of the GI tract 4. Digestion

Breaking down of larger food particles into smaller molecules Mechanical Digestion

Chemical Digestion 5. Absorption

small molecules pass from the lumen into the blood and lymph 6. Defecation

elimination of indigestible substances and bacteria from the GI tract

Mouth (Oral Cavity)

Mechanical Digestion

mastication (chewing) tongue manipulates food teeth grind food saliva mixes so that food isreduced to a soft flexiblemass called a bolusthat is easily swallowed

Salivary Glands

Chemical Digestion

salivary amylase initiates the breakdown ofstarch ingested disaccharides and starches must be broken down into monosaccharides

lingual lipase initiates digestion of triglycerides into fatty acids and monoglycerides

Esophagus and Swallowing

Stomachconnects the esophagus to the duodenum (first part of the small intestine)

Functions:

1. serves as a mixing area for saliva, food and gastric juice

2. serves as a reservoir for holding food before release into the SI

3. secretes gastric juice (HCL, pepsin, intrinsic factor, gastric lipase) HCL kills bacteria, denatures protein pepsin begins digestion of proteins intrinsic factor aids absorption of vitamin B12 gastric lipase aids digestion of triglycerides

4. secretes gastrin (digestive hormone) into the blood

Physiology of Digestion and Absorption in the Stomach

Mechanical Digestion after food enters the stomach, gentlemixing waves pass over the stomach every 15-25 seconds it macerates the food and mixes it with gastric juice, forming chyme the chyme moves back and forthand keeps mixing until it is sentthrough the pyloric sphincter intothe duodenum

Chemical Digestion lingual lipase and salivary amylase continue to break down fats and carbohydrates respectively pepsin and HCL break down proteinsgastric lipase splits triglycerides into fatty acids and monoglycerides

Peristalsis

Segmentation

Regulation of Gastric Secretion and Motility• Cephalic Phase:

– Sight, smell, taste, and thought initiate reflex• Cortex and hypothalamus send impulses to medulla• Parasympathetic (Vagus nerve) impulses from medulla promote:

– Peristalsis in stomach– Gastric glands to secrete:

» Pepsinogen» HCL» Mucus» gastrin

• Gastric Phase:– Stretch receptors detect distention of stomach– Chemoreceptors detect increased pH of gastric juice

• Stimulates stomach churning and secreting– ACH from parasympathetic activity stimulates gastrin secretion

• Stimulates gastric juice secretion• Stimulates stomach motility

• Intestinal Phase– Stretch receptors detect distention of duodenum– Chemoreceptors detect fatty acids and glucose in duodenum

• Impulses from small intestine inhibit stomach secretions and motility• Stimulates release of:

– Gastric inhibitory peptide (GIP)» Inhibits gastric secretions and gastric motility in stomach

– Secretin» Decreases gastric secretions

– Cholecystokinin (CCK)» Inhibits stomach emptying» Inhibits gastric secretions and gastric motility in stomach

Stomach Cell Layers

Stomach Cells

Small Intestines

Functions: 1. mixes chyme with digestive

juices and brings food in contact with surface mucosa for absorption

2. completes digestion of carbohydrates, proteins, & lipidsbegins & completes digestion of nucleic acids

3. is responsible for the absorption of about 90% of all nutrients

source of the major events of digestion and absorption its long length (over 10 feet) is to increase the surface area it has many circular folds, villi and microvilli

Small IntestinesPhysiology

Digestionchyme remains in SI for 3-4 hours Mechanical Digestion

mixes chyme with digestive enzymesundergoes peristalsis

Chemical Digestion pancreatic juice assists in digestion

Absorptionfood products pass through mucosa absorptive cellspassage of these molecules from the GI tract to the blood or lymph is called absorption

Small Intestine Cell Layers

Small Intestine Cells

Liver and Gall Bladder

Brush Border Enzymes

• Carbohydrate digesting:– Alpha-dextrinase– Maltase– Sucrase– Lactase

• Protein digesting:– Aminopeptidase– Dipeptidase

• Nucleic Acid digesting– Nucleosidase– Phosphatase

GlucoseAbsorption

Carbohydrates arebroken down into monosaccharides likeglucose

Glucose passes throughSI Absorptive cells viafacilitative diffusion,actually with the help of a Na+ co-transporter

Glucose then passively diffuses into the blood

ProteinAbsorption

Proteins are broken down intoamino acids

Amino acids and the slightlylarger di-peptides passively diffuse across the membranesof the SI Absorptive cells

Amino acids then passivelydiffuse into the blood

Pancreatic Juice Enzymes• Carbohydrate digesting:

– Pancreatic amylase

• Protein digesting:– Trypsin– Chymotrypsin– Carboxypeptidase– Elastace

• Lipid digesting– Pancreatic lipase

• Nucleic Acid digesting– Ribonuclease– Deoxyribonuclease

FatAbsorption

Large fat droplets are emulsifiedinto monoglycerides and fatty acids by bile salts from the liver and are packaged into micelles

Micelles merge with SI Absorptivecells and via a type of endocytosisdeposit the fat fragments in the cells

The fat fragments are then reassembled into triglycerides in thecell and packaged in chylomicrons

Chylomicrons then exit the cell andenter the lymph on their way to the liver for final digestion and storage

Large Intestine (Colon)

Functions: 1. completion of absorption 2. manufactures vitamins

(some B’s & K)3. formation of the feces 4. expulsion of the feces

Structures: cecumascending colon transverse colon descending colon sigmoid colon rectum anal canal anus anal sphincters

Rectumand

Anus

Internal Anal Sphincter

External Anal Sphincter

Larger Intestine Cell Layers

Large Intestine Cells