Chapter 23C Digestive System Slides by Barbara Heard and W. Rose. figures from Marieb & Hoehn 9 th ed. Portions copyright Pearson Education.

Chapter 23C

Digestive System

Slides by Barbara Heard and W. Rose.

figures from Marieb & Hoehn 9th ed.

Portions copyright Pearson Education

Digestive SystemIntroduction/Overview

Functional Anatomy

Physiology of Digestion & AbsorptionChemical Digestion

Absorption

© 2013 Pearson Education, Inc.

Digestion

• Digestion– Catabolic; macromolecules monomers

small enough for absorption

• Enzymes– Intrinsic and accessory gland enzymes break

down food

• Hydrolysis– Water is added to break bonds


Digestion of Carbohydrates

• Only monosaccharides can be absorbed

• Monosaccharides absorbed as ingested– Glucose, fructose, galactose

• Digestive enzymes– Salivary amylase, pancreatic amylase, and

brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase)

– Break down disaccharides sucrose, lactose, maltose; polysaccharides glycogen and starch


Digestion of Carbohydrates

• Starch digestion– Salivary amylase (saliva) oligosaccharides

at pH 6.75 – 7.00– Pancreatic amylase (small intestine)

breaks down any that escaped salivary amylase oligosaccharides

– Brush border enzymes (dextrinase, glucoamylase, lactase, maltase, sucrase) oligosaccharides monosaccharides


Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (1 of 4)

Foodstuff Enzyme(s) and source Site of action Path of absorption

Starch and disaccharides

Oligosaccharidesand disaccharides

Carbohydratedigestion

Lactose Maltose Sucrose

Galactose Glucose Fructose

Salivary amylase

Pancreatic amylase

Brush border enzymes in small intestine(dextrinase, gluco-amylase, lactase, maltase, and sucrase)

Mouth

Small intestine

Small intestine

• Glucose and galactose are absorbed via cotransport with sodium ions.• Fructose passes via facilitated diffusion.• All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.


Digestion of Proteins

• Source is dietary, digestive enzymes, mucosal cells; digested to amino acid monomers

• Begins with pepsin in stomach at pH 1.5 – 2.5– Inactive in high pH of duodenum

• Pancreatic proteases– Trypsin, chymotrypsin, and carboxypeptidase

• Brush border enzymes– Aminopeptidases, carboxypeptidases, and

dipeptidases


Figure 23.33 Protein digestion and absorption in the small intestine. Slide 1Lumen of intestine

Pancreaticproteases

Amino acids of protein fragments

Brush border enzymes

Na+

Absorptiveepithelialcell

Apical membrane (microvilli)

Aminoacidcarrier

Capillary

Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxypeptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase)of mucosal cells.

The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side.

The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary viaintercellular clefts.

Na+

1

2

3



Pancreaticproteases



Na+



Capillary


Na+

1



Pancreaticproteases



Na+



Aminoacidcarrier

Capillary



Na+

1

2



Pancreaticproteases



Na+



Aminoacidcarrier

Capillary



The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary viaintercellular clefts.

Na+

1

2

3



Proteindigestion

Proteins

Large polypeptides

Small polypeptides,small peptides

Amino acids(some dipeptidesand tripeptides)

Pepsin (stomach glands)in presence of HCl

Pancreaticenzymes (trypsin, chymotrypsin,carboxypeptidase)

Brush border enzymes(aminopeptidase,carboxypeptidase,and dipeptidase)

Stomach

Small intestine

Small intestine

• Amino acids are absorbed via cotransport with sodium ions.• Some dipeptides and tripeptides are absorbed via cotransport with H+ and hydrolyzed to amino acids within the cells.• Infrequently, transcytosis of small peptides occurs.• Amino acids leave the epithelial cells by facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.



Digestion of Lipids

• Pre-treatment—emulsification by bile salts– Does not break bonds

• Enzymes—pancreatic lipases Fatty acids and monoglycerides


Fat globule

Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets).

Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa.

Micelles made up of fatty acids,monoglycerides, and bile salts

Bile salts

Fat dropletscoated withbile salts

Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other fatty substances and proteins to form chylomicrons.

Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals and are carried away from the intestine in lymph.

Lacteal

Epithelialcells ofsmallintestine

1

2

3

4

Figure 23.34 Emulsification, digestion, and absorption of fats. Slide 1


Fat globule


Bile salts


1



Fat globule




Bile salts


1

2



Fat globule




Bile salts



Lacteal


1

2

3



Fat globule




Bile salts



Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals and are carried away from the intestine in lymph.

Lacteal


1

2

3

4




Fat digestion

Unemulsified triglycerides

Lingual lipase

Gastric lipase

Emulsification by the detergent action of bile salts ductedin from the liver

Pancreatic lipases

Monoglycerides (or diglycerideswith gastric lipase) and fatty acids

Mouth

Stomach

Small intestine

Small intestine

• Fatty acids and monoglycerides enter the intestinal cells via diffusion. • Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells. The resulting chylomicrons are extruded by exocytosis.• The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct.• Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein.



Digestion of Nucleic Acids

• Enzymes– Pancreatic ribonuclease and

deoxyribonuclease nucleotide monomers– Brush border enzyme nucleosidases and

phosphatases free bases, pentose sugars, phosphate ions



Nucleic aciddigestion

Nucleic acids

Pentose sugars, N-containing bases,

phosphate ions

Pancreatic ribo-nuclease and deoxyribonuclease

Brush borderenzymes(nucleosidasesand phosphatases)

Small intestine

Small intestine

• Units enter intestinal cells by active transport via membrane carriers.• Units are absorbed into capillary blood in the villi and transported to the liver via the hepatic portal vein.



Absorption

• ~ All food; 80% electrolytes; most water absorbed in small intestine– Most prior to ileum

• Ileum reclaims bile salts

• Most absorbed by active transport blood– Exception - lipids


Absorption of Carbohydrates

• Glucose and galactose– Secondary active transport (cotransport) with

Na+ epithelial cells– Move out of epithelial cells by facilitated

diffusion capillary beds in villi

• Fructose– Facilitated diffusion to enter and exit cells


Absorption of Carbohydrates

• Glucose and galactose– Secondary active transport (cotransport) with

Na+ epithelial cells– Move out of epithelial cells by facilitated

diffusion capillary beds in villi

• Fructose– Facilitated diffusion to enter and exit cells


Absorption of Protein

• Amino acids transported by several types of carriers– Most coupled to active transport of Na+

• Dipeptides and tripeptides actively absorbed by H+-dependent cotransport; digested to amino acids within epithelial cells

• Enter capillary blood by diffusion


Homeostatic Imbalance

• Whole proteins not usually absorbed

• Can be taken up by endocytosis/exocytosis– Most common in newborns food allergies

• Usually disappear with mucosa maturation

– Allows IgA antibodies in breast milk to reach infant's bloodstream passive immunity


Absorption of Lipids

• Absorption of monoglycerides and fatty acids– Cluster with bile salts and lecithin to form micelles– Released by micelles to diffuse into epithelial cells– Combined with lecithin, phospholipids, cholesterol, &

coated with proteins to form chylomicrons– Enter lacteals; transported to systemic circulation– Hydrolyzed to free fatty acids and glycerol by

lipoprotein lipase of capillary endothelium• Cells can use for energy or stored fat

• Absorption of short chain fatty acids– Diffuse into portal blood for distribution


Absorption of Nucleic Acids

• Absorption– Active transport across epithelium

bloodstream


Absorption of Vitamins

• In small intestine– Fat-soluble vitamins (A, D, E, and K) carried

by micelles; diffuse into absorptive cells– Water-soluble vitamins (vitamin C and B

vitamins) absorbed by diffusion or by passive or active transporters.

– Vitamin B12 (large, charged molecule) binds with intrinsic factor, and is absorbed by endocytosis


Absorption of Vitamins

• In large intestine– Vitamin K and B vitamins from bacterial

metabolism are absorbed


Absorption of Electrolytes

• Most ions actively along length of small intestine• Iron and calcium are absorbed in duodenum • Na+ coupled with active absorption of glucose

and amino acids• Cl– transported actively• K+ diffuses in response to osmotic gradients; lost

if poor water absorption• Usually amount in intestine is amount absorbed


Absorption of Electrolytes

• Iron and calcium absorption related to need– Ionic iron stored in mucosal cells with ferritin– When needed, transported in blood by

transferrin

• Ca2+ absorption regulated by vitamin D and parathyroid hormone (PTH)


Absorption of Water

• 9 L water, most from GI tract secretions, enter small intestine– 95% absorbed in the small intestine by

osmosis– Most of rest absorbed in large intestine

• Net osmosis occurs if concentration gradient established by active transport of solutes

• Water uptake coupled with solute uptake


Malabsorption of Nutrients

• Causes– Anything that interferes with delivery of bile or

pancreatic juice – Damaged intestinal mucosa (e.g., bacterial

infection; some antibiotics)


Malabsorption of Nutrients

• Gluten-sensitive enteropathy (celiac disease)– Immune reaction to gluten– Gluten causes immune cell damage to

intestinal villi and brush border– Treated by eliminating gluten from diet (all

grains but rice and corn)


Developmental Aspects

• Oral membrane mouth opening

• Cloacal membrane anus

• By week 5 alimentary canal continuous tube from mouth to anus

• Shortly after, accessory organs bud from mucosa


Figure 23.36 Embryonic development of the digestive system.

Brain

Oral membrane

Heart

Yolk sac

Cloacalmembrane

Body stalk

Endoderm

Proctodeum

Hindgut

Spinal cord

Midgut

Site of liverdevelopment

Foregut

Stomodeum

Lung bud

Liver Stomach

Bileduct

Cystic duct

Gall-bladder

Ventral pancreatic bud

Dorsalpancreaticbud

Duodenum


Homeostatic Imbalance

• Cleft palate and cleft lip

• Tracheoesophageal fistula– Opening between esophagus and trachea

• Cystic fibrosis– Genetic disease thick mucus can block

pancreatic duct



• Fetal nutrition via placenta, but GI tract stimulated to mature by amniotic fluid swallowed in utero

• Newborn's rooting reflex helps infant find nipple; sucking reflex aids in swallowing

• Newborns double birth weight in six months; adult diet by 2 years

• Cholecystitis, ulcers – problems of middle age



• During old age – GI tract activity declines, less digestive juice,

absorption less efficient, peristalsis slows less frequent bowel movements

– Taste/smell less acute; periodontal disease often develops

– Diverticulosis, fecal incontinence, and cancer of GI tract fairly common


Cancer

• Stomach and colon cancers rarely have early signs or symptoms

• Metastasized colon cancers frequently cause secondary liver cancer

• Prevention– Regular dental and medical examination

Chapter 23C Digestive System Slides by Barbara Heard and W. Rose. figures from Marieb & Hoehn 9 th ed. Portions copyright Pearson Education.

Documents

protein digestion

starch slide

absorption enzymes intrinsic

foodstuff enzymes

flowchart of digestion

dipeptidases slide

absorption starch

digestion of proteins