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