Digestive System II: Digestive Activities

Digestive System II: Digestive Activities

Digestive Activities in the Mouth, Swallowing

Digestive Activities in the Stomach

• Gastric juice

• Hormones that stimulate stomach acid and enzyme secretion

Contributions of the Liver in Digestion

Digestion in the Small Intestine

• Pancreatic juice

• Hormonal stimulation of pancreas

• Methods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acids

Absorption in the Large Intestine and Feces Formation

Salivary Glands Extrinsic salivary glands (parotid,

submandibular, and sublingual)

Intrinsic (buccal) salivary glands are scattered in the oral mucosa

Secretion (saliva)

• Cleanses the mouth

• Moistens and dissolves food chemicals

• Aids in bolus formation

• Contains enzymes that begin the breakdown of starch

Composition

• 97–99.5% water, slightly acidic solution containing:

• Electrolytes—Na+, K+, Cl–, PO4 2–, HCO3

–

• Salivary amylase and lingual lipase; mucin;

• Metabolic wastes—urea and uric acid; lysozyme, IgA, defensins, and a cyanide compound protect against microorganisms

Control of Digestive Activity and Mouth Mostly controlled by reflexes via

the parasympathetic division

Enteric Nervous System

• (ENS) Intrinsic nerve supply of the alimentary canal

o Sympathetic impulses inhibit secretion and motility

o Parasympathetic impulses stimulate

Chemical and mechanical receptors are located in organ walls that trigger reflexes




• Gastric juice








Food Breakdown in the Stomach

Gastric juice is regulated by neural and hormonal factors

Presence of food or falling pH causes the release of gastrin

Gastrin causes stomach glands (chief cells) to produce protein-digesting enzymes (pepsinogen)

Hydrocholoric acid from parietal cells makes the stomach contents very acidic, kills bacteria

Heartburn from Acid Reflux

Acid reflux animation online

Ulcer formation movie online

Chief cellpepsinogen

Digestion and Absorption in the Stomach Protein digestion enzymes

(proteases)

Stomach proteins are exported into the stomach as zymogens (extra long, inactive polypeptide chains)

• Pepsin – an active protein digesting enzyme (converted from pepsinogen through action of HCl and active pepsin)

• Rennin (chymosin)– works on digesting milk casein protein in infants

The only absorption that occurs in the stomach is of alcohol and aspirin

Conversion of pepsinogen to pepsin

The action of chymosin on casein

Figure 23.17

Presence of lowpH, partially digested foods, fats, or hypertonic solution in duodenum when stomach begins to empty

Distension;presence offatty, acidic,partiallydigested foodin theduodenum

Briefeffect

Intestinal(enteric)gastrinreleaseto blood

Entero-gastricreflex

Release of intestinalhormones (secretin,cholecystokinin, vasoactiveintestinal peptide)

Localreflexes

Vagalnucleiin medulla

Pyloricsphincter

Stimulate

Inhibit

1

1

2

Stomachsecretoryactivity

Sight and thoughtof food

Stomachdistensionactivatesstretchreceptors

Stimulation oftaste and smellreceptors

Food chemicals(especially peptides and caffeine) and rising pHactivate chemoreceptors

Loss ofappetite,depression

Emotionalupset

Lack ofstimulatoryimpulses toparasym-patheticcenter

Cerebralcortex

Cerebral cortexConditioned reflex

Vagovagalreflexes

Localreflexes

Medulla

G cells

Hypothalamusand medullaoblongata

Vagusnerve

Vagusnerve

Gastrinreleaseto blood

Gastrinsecretiondeclines

G cells

Overridesparasym-patheticcontrols

Sympatheticnervoussystemactivation

1

11

1

2

2

2

Stimulatory events Inhibitory events

Cephalicphase

Gastricphase

Intestinalphase

Excessiveacidity (pH <2) in stomach

Distension of duodenum; presence of fatty, acidic, hypertonic chyme, and/or irritants in the duodenum

Regulation of Stomach Activity




• Gastric juice








Liver Largest gland in the body

Connected to the gall bladder via the common hepatic duct

Bile Production in the Liver

• Composition

o Bile salts, bile pigment (broken down hemoglobin, cholesterol, phospholipids

Bile is introduced into the duodenum in the presence of fatty food to cause emulsification

Emulsification solubilizes lipids so that they can come in contact with water-soluble lipases and be broken down into fatty acids and glycerol.

Role of the Liver in Metabolism

Produces blood proteins (albumin, clotting proteins) and lipoproteins Degrades hormones

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•

•

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•

•

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Figure 23.25a, b

(a) (b)Lobule Central vein Connectivetissue septum

The Liver Processes/Detoxifies Absorbed Food in Lobules




• Gastric juice








Digestion in the Small Intestine Enzymes from the brush border

• Break double sugars into simple sugars (carbohydrases)

• Complete some protein digestion (proteases)

Pancreatic enzymes play the major digestive function; usually zymogens

• Help complete digestion of starch (pancreatic amylase)

• Carry out about half of all protein digestion (trypsin, etc.)

• Responsible for fat digestion (lipase)

• Digest nucleic acids (nucleases)

• Bicarbonate ion neutralizes acidic chyme, raises pH to 8

Chemical Digestion : Pancreas

Figure 14.6

Produces a wide spectrum of digestive enzymes that break down all categories of food

Acinar cells produce enzymes that are secreted into the duodenum

Alkaline fluid introduced with enzymes neutralizes acidic chyme

Stimulation of the Release of Pancreatic Juice Vagus nerve: secretion of enzymes,

weak release of bile

Local hormones from duodenal mucosa

• Secretin (enzyme release, bile production, gastrin & stomach activities)

• Cholecystokinin (enzyme and bicarbonate release, bile release)

Liver +




• Gastric juice








Other Activities in the Small Intestine Water absorption

Nutrient absorption by mucosal cells via active or passive transport

Lipids are absorbed by diffusion into lacteals

Substances are transported to the liver by the hepatic portal vein or lymph (lacteal)

Figure 23.32 (1 of 4)

Carbohydrate digestion

• 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.

Starch and disaccharides

Oligosaccharidesand disaccharides

Lactose Maltose Sucrose

Glucose Fructose

Salivaryamylase

Mouth

Pancreaticamylase

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

Smallintestine

Smallintestine

Foodstuff

Galactose

Path of absorptionEnzyme(s)and source

Site ofaction

Summary of Carbohydrate Digestion

(monosaccharides)

Figure 23.32 (2 of 4)

Protein digestion

• Amino acids are absorbed by cotransport with sodium ions.• Some dipeptides and tripeptides are absorbed via cotransport with H+

and hydrolyzed to amino acids within the cells.

+

• 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.

Smallintestine

Smallintestine

Stomach

Foodstuff

Protein

Large polypeptides

Pepsin(stomach glands)in presence of HCl

Small polypeptides,small peptides

Pancreaticenzymes (trypsin, chymotrypsin,carboxypeptidase)

Amino acids(some dipeptidesand tripeptides)

Brush border enzymes(aminopeptidase,carboxypeptidase,and dipeptidase)


Site ofaction

Summary of Protein Digestion

Figure 23.34

Epithelialcells ofsmallintestine

Fat dropletscoated withbile salts

Fat globule

Lacteal

Bile salts

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

1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum.

2 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.

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

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

Lipid Digestion and Fatty Acid/Glycerol Import

Figure 23.32 (3 of 4)

Fat digestion

Small intestine

Small intestine

Foodstuff

Unemulsifiedfats

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

Pancreatic lipases

Monoglyceridesand fatty acids

Glyceroland

fatty acids


Site ofaction

• 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, and 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.

Summary of Lipid Digestion

Figure 23.32 (4 of 4)

Nucleic acid digestion

• 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.

Smallintestine

Smallintestine

Foodstuff

Nucleic acids

Pancreatic ribo-nuclease and deoxyribonuclease

Brush borderenzymes(nucleosidasesand phosphatases)

Nucleotides


Site ofaction

Summary of Nucleic Acid Digestion




• Gastric juice








Processes in the Large Intestine No digestive enzymes are

produced

Resident bacteria digest remaining nutrients

• Produce some vitamin K and B

• Release methane and hydrogen sulfide gases

Water and vitamins K and B are absorbed

Remaining materials are eliminated via feces

CH4 H2S

Vitamin KVitamin B

Propulsion in the Large Intestine (Colon) Sluggish peristalsis

Mass movements

• Slow, powerful movements

• (Three to four times per day)

Lack of fiber and weakening of colon walls cause diverticula

Presence of feces in the rectum causes defecation reflex

• Internal anal sphincter is relaxed

• Defecation occurs with relaxation of the voluntary external anal sphincter

Neural control of the rectum and anal sphicters




• Gastric juice








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Digestive System II: Digestive Activities