• Most animals ingest chunks of food

Figure 21.1E

21

21.2 Overview: Food processing occurs in four stages

Figure 21.2

1 INGESTION 2 DIGESTION 3 ABSORPTION 4 ELIMINATION

Food

Mechanicalbreakdown

Piecesof food

Chemical breakdown(enzymatic hydrolysis)

Smallmolecules

Nutrientmoleculesenter bodycells Undigested

material

Figure 21.5

TEETH

Incisors

Canine

Premolars

Molars

“Wisdom”tooth

Tongue

Salivaryglands

Opening of asalivary gland duct

• When food is swallowed, it is moved through the alimentary canal by peristalsis– Peristalsis is rhythmic muscle contraction in the

walls of the digestive tract– Ringlike sphincter muscles regulate the passage

of food

21.4 The human digestive system consists of an alimentary canal and

accessory glands

HUMAN DIGESTIVE SYSTEM

Figure 21.4

Mouth

Tongue

Salivaryglands

Oral cavity

Pharynx

Esophagus

Stomach

Liver

Gall-bladder

PancreasSmallintestine

Largeintestine

Smallintestine

Stomach

Pyloricsphincter

Anus

Rectum

• The stomach mixes food with gastric juice– The gastric juice contains pepsin, which begins

the hydrolysis of protein

21.8 The stomach stores food and breaks it down with acid

and enzymes

21

21

• Alkaline pancreatic juice neutralizes stomach acids– Its enzymes digest polysaccharides, proteins,

nucleic acids, and fats

• Bile emulsifies fat droplets for attack by pancreatic enzymes– It is made in the liver and stored in the gall

bladder

21.10 The small intestine is the major organ of chemical

digestion and nutrient absorption

Figure 21.10A

LiverBile

Gall-bladder

Bile

Duodenum ofsmall intestine

Acid chyme

Stomach

Pancreas

• The lining of the small intestine is folded and covered with tiny, fingerlike villi

– Villi increase the absorptive surface

• Nutrients pass through the epithelium of the villi and into the blood– The blood flows to the liver– The liver can store nutrients and convert them to

other substances the body can use

• Undigested material passes to the large intestine, or colon– Water is

absorbed – Feces are

produced

21.11 The large intestine reclaims water

Figure 21.11

Largeintestine(colon)

Endof smallintestine

Nutrientflow

Appendix

Cecum

Anus

Rectum

Smallintestine

Figure 21.10B

Musclelayers

Circular folds

Villi

INTESTINAL WALL

Blood vesselwith blooden route tothe liver

INTERIOR OF INTESTINE

Nutrientabsorption

Epithelialcells

Lumen

Bloodcapillaries

Lymphvessel

Nutrientabsorption

VILLI

Nutrientabsorption

Microvilli

EPITHELIALCELLS