The Digestive System

Digestive System Overview

• Known as gastrointestinal (GI) tract or alimentary canal.

• Open at both ends to the outside world.

• Consists of long convoluted tube with accessory organs.

Steps in Digestion

1. Ingestion

2. Digestion

3. Absorption

4. Elimination

Types of Digestion

Mechanical• Food mass is shredded, torn & churned.• Occurs in mouth & stomach.

Chemical• Chemicals & enzymes break down food.• Occurs in mouth, stomach & small

intestine.

Organs of the Digestive System

• Oral Cavity

• Esophagus

• Stomach

• Small Intestine

• Large Intestine

Oral Cavity

• Lips, tongue, teeth & jaw muscles.

• Increases surface area of food.

• Food mixes with saliva and amylase.

• A bolus forms and is swallowed.

• The bolus passes through the pharynx, past the epiglottis & through the esophagus.

Esophagus

• Muscular tube that connects pharynx & stomach.

• The cardiac or lower esophageal sphincter connects the esophagus to the stomach (prevents b).

• Peristalsis is a series of coordinated muscular contractions that propels food along the digestive tract.

Stomach

• J-shaped stretchable organ.• Acts as a reservoir for food (1.5 L).• Movement of food controlled by two sphincters:

– Cardiac sphincter: Between the stomach & the esophagus.

– Pyloric sphincter: Between the stomach & the small intestine.

Stomach

• Folds (rugae) allow for expansion.

• Mechanical digestion: Walls churn & squeeze.

• Chemical digestion: Gastric juices.– Hydrochloric acid secreted by gastric

glands.– Pepsinogen released & converts to pepsin.

• Bolus becomes a liquefied paste (chyme).

Stomach

• Mucus cells secrete mucus to line & protect stomach from HCl (aq).

• Ulcers: HCl (aq) burns a hole through the mucus, irritating the stomach cells below.

Small Intestine

• Major site of digestion & absorption (80%).

• 6 m long & has a SMALLER diameter than the large intestine.

• Lined with tiny finger-like projections called villi, which project into the lumen.

• Microvilli line the villi.

• Villi & microvilli increase the surface area for absorption.

Small Intestine

• Consists of three sections:1. Duodenum

2. Jejunum

3. Ileum.

Small Intestine

• Mechanical digestion via muscular contractions.

• Chemical digestion via intestinal and pancreatic secretions.

• Villi and microvilli responsible for absorption.

• Absorbed nutrients transported to the liver.

Large Intestine

• Reabsorbs water, salt & some vitamins.• Holds & compacts unabsorbed material.• 1.5 m long & has a LARGER diameter than the

small intestine.• Consists of four sections:

1. Caecum2. Colon3. Rectum4. Anus.

Large Intestine

• Chyme enters caecum, through the ileocaecal valve.

• Waste products compacted into feces (3/4 water, 1/4 solid matter).

• Feces pass through the rectum & exit the body through the anus.

• Appendix attached to caecum, exact function is unknown.

Large Intestine

• Defecation controlled by two sphincters:– Rectal sphincter: Between the large intestine

& the rectum.– Anal sphincter: Between the rectum & the

anus.