The Digestive System. Digestive System Overview Known as gastrointestinal (GI) tract or alimentary...
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Transcript of The Digestive System. Digestive System Overview Known as gastrointestinal (GI) tract or alimentary...
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.