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5 Tasks of Digestive Systems 4. Absorption of nutrients and fluids 5. Elimination of wastes and residues
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Two Types of Systems
• Incomplete digestive system– One-way, saclike
digestive cavity
• Complete digestive system– Tube with opening at
each end
5 Tasks of Digestive Systems
1. Mechanical processing- break up, mix, and move food material
2. Secretion- of enzymes into tube where digestion
occurs
3. Digestion- break down of food particles into
smaller molecules
5 Tasks of Digestive Systems
4. Absorptionof nutrients and fluids
5. Eliminationof wastes and residues
Human Digestive System
• Complete system with many specialized accessory glands and organs
• About 10 meters long
• Lined with mucus-secreting epithelium
• Movement is one way, from mouth to anus
Major Components
• Mouth (oral cavity)
• Pharynx (throat)
• Esophagus
• Gut– Stomach– Small intestine– Large intestine– Rectum – Anus
Accessory Organs
• Salivary glands– Secrete saliva
• Liver – Secretes bile
• Gallbladder– Stores and concentrates bile
• Pancreas– Secretes digestive enzymes
Saliva
• Produced by salivary glands at back of mouth and under tongue
• Saliva includes– Salivary amylase (enzyme)– Bicarbonate (buffer)– Mucins (bind food into bolus)– Water
Structure of Stomach
• J-shaped organ lies below the diaphragm
• Sphincters at both ends
• Outer serosa covers smooth muscle layers
• Inner layer of glandular epithelium faces lumen
mucosa
sphincters
muscle
serosa
Stomach Secretions
• Secreted into lumen (gastric fluid)– Hydrochloric acid (HCl) – Mucus (protective)– Protein-digesting enzymes
• Stomach cells also secrete the hormone gastrin into the bloodstream
Small Intestine
• Longest segment of digestive tract
• Receives chyme from stomach
• Receives secretions from liver, gallbladder, and pancreas
• Digests food into absorbable subunits
Walls of Small Intestine
• Projections into the intestinal lumen increase the surface area available for absorption
one villus
mucosa(inner lining)
Fig. 24-6a, p.407
microvilli at the free surface of an absorptive cell (brush border cell)
cytoplasm
mucus secretion
hormone secretion
lysozyme secretion
nutrient absorption
Fig. 24-6b2, p.407
Nutrient Absorption
• Passage of molecules into internal environment
• Occurs mainly in small intestine
• Various methods of absorption– Osmosis, transport proteins, diffusion
Large Intestine (Colon)
• Concentrates and stores feces
• Actively transports sodium ions out of lumen; water follows
• Resident bacteria produce vitamins
• Large and small intestines reclaim 90% of water that enters alimentary
Dietary Recommendations
• Whole foods help prevent chronic disease
Vitamins and Minerals
• Vitamins– Essential organic substances
• Minerals– Essential inorganic substances