The Small Intestine

The Small Intestine

• The small intestine is the longest section of the alimentary canal

• It is the major organ of digestion and absorption

LE 41-19

Stomach

Pancreas

Liver

Gall-bladder

Duodenum ofsmall intestine

Intestinaljuice

Bile

Acid chyme

Pancreatic juice

• The pancreas produces proteases, protein-digesting enzymes that are activated after entering the duodenum

LE 41-20

Pancreas

Lumen of duodenum

Membrane-boundenteropeptidase

Inactivetrypsinogen

Other inactiveproteases Active

proteases

Trypsin

• The liver produces bile, which aids in digestion and absorption of fats

• The epithelial lining of the duodenum, called the brush border, produces several digestive enzymes

LE 41-21

Oral cavity,pharynx,esophagus

Stomach

Lumen ofsmall intes-tine

Smaller polysac-charides, maltose

Polysaccharides DisaccharidesCarbohydrate digestion

Salivary amylase

PolysaccharidesPancreatic amylases

Maltose and otherdisaccharides

Epitheliumof smallintestine(brushborder)

Disaccharidases

Monosaccharides

Protein digestion Nucleic acid digestion

Proteins

Fat digestion

Fat globulesDNA, RNA

Fat dropletsNucleotides

Bile salts

Pancreatic lipase

Pancreaticnucleases

Nucleotidases

Glycerol, fattyacids, glycerides

Nitrogenous bases,sugars, phosphates

NucleosidesNucleosidasesand phosphatases

Dipeptidases, carboxy-peptidase, and aminopeptidase

Amino acids

Amino acids

Small peptides

Pancreatic carboxypeptidase

Pancreatic trypsinand chymotrypsin

Smaller polypeptides

Polypeptides

Small polypeptidesPepsin

LE 41-22

Pancreas

Stomach

Entero-gastrone

Gall-bladder

Liver

DuodenumSecretin

CCK

CCK

Stimulation

Inhibition

Gastrin

Key

Absorption of Nutrients

• The small intestine has a huge surface area, due to villi and microvilli that are exposed to the intestinal lumen

• The enormous microvillar surface greatly increases the rate of nutrient absorption

LE 41-23

Key

Nutrientabsorption

Microvilli(brush border)

Epithelial cellsLacteal

Lymphvessel

Villi

Largecircularfolds

Epithelialcells

Bloodcapillaries

Vein carrying bloodto hepatic portalvessel

Muscle layers

Villi

Intestinal wall

• Amino acids and sugars pass through the epithelium of the small intestine and enter the bloodstream

• After glycerol and fatty acids are absorbed by epithelial cells, they are recombined into fats within these cells

Fat dropletscoated withbile salts

Fat globule

Bile salts

Micelles madeup of fatty acids,monoglycerides,and bile salts

Epitheliumof smallintestine

Epitheliumof lacteal

Lacteal

The Large Intestine

• The large intestine, or colon, is connected to the small intestine

• Its major function is to recover water that has entered the alimentary canal

• The colon houses strains of the bacterium Escherichia coli, some of which produce vitamins

Concept 41.5: Evolutionary adaptations of vertebrate digestive systems are often associated with diet

• Digestive systems of vertebrates are variations on a common plan

• However, there are intriguing adaptations, often related to diet

Some Dental Adaptations

• Dentition, an animal’s assortment of teeth, is one example of structural variation reflecting diet

• Mammals have specialized dentition that best enables them to ingest their usual diet

LE 41-26

Incisors

Carnivore

CaninesPremolars

Molars

Herbivore

Omnivore

Stomach and Intestinal Adaptations

• Herbivores generally have longer alimentary canals than carnivores, reflecting the longer time needed to digest vegetation

LE 41-27

Smallintestine

Stomach

Cecum

Colon(largeintestine)

Carnivore Herbivore

Small intestine

Symbiotic Adaptations

• Many herbivores have fermentation chambers, where symbiotic microorganisms digest cellulose

• The most elaborate adaptations for an herbivorous diet have evolved in the animals called ruminants

LE 41-28

Intestine Rumen

Reticulum

Omasum

EsophagusAbomasum