Digestive Physiology

The primary function of the digestive system is move nutrients from the external environment into the body

Outline

1) Digestion & Absorption of nutrients

2) Secretion of digestive chemicals

3) Regulation of Digestion

4) Regulation of Metabolism

Digestion & Absorption of Protiens

1) Proteins digested in stomach & small intestine

2) Proteins are broken into smaller peptides & amino acids by:

1) Pepsin: Stomach2) Trypsin & chymotrypsin: SI3) Exopeptidases: SI

3) Peptides/Amino Acids are absorbed in SI by:

1) Cotransport then Antiport with H+2) Cotransport then Antiport with Na+3) Transcytosis via vesicles

Digestion & Absorption of Fats

Emulsified fats are digested by Lipase & Colipase

4) End products move into SI wall 1) Monoglycerides & fatty acids: simple

diffusion2) Cholesterol: NPC1L1 transporter

5) End products are re-comined in absoptive cells to

form Chylomicrons

6) Chylomicrons travels through lymph to body

Digestion & Absorption of Carbohydrates

1) Carbs digested in mouth & SI

2) Complex Carbs are digested into Disaccharides by amalyase & alpha-amylase

Maltose, Sucrose, Lactose

3) Disaccharides are digested into Monosacch. By:

1) Maltose by Maltase to glucose2) Sucrose by Sucrase to glucose + fructose3) Lactose to Lactase to glucose + galactose

4) Monosaccharides can FINALLY! be absorbed

1) Glucose & Galactose via Na+-glucose Symporter

2) Fructose via Glut5 & Glut2 transporter

Glut5

Glut2

Na+-Glucose Symporter

Absorption of Vitamins, Ions, Minerals, Water

Vitamins: Fat-Soluble (A, D, E, K) – transported with fats Water-Soluble (C, B) – mediated transport

Minerals: Ca2+ (hormonally controlled ion channels/Active transport) – used everywhere Iron (???)- needed for RBC

Ions: Na+ - active transport, symport, antiport

Water: Follows Na+ & other absorbed solutes by Osmosis

Secretion: Salivary Glands

Parotid, Sublingual, & Submandibular glands secrete Saliva

Saliva contains Alpha-amylase (carbs), lingual lipase (fats)

Secretion: Stomach

Drugs for heartburn & ulcers (Zantac, Pepcid) block Histamine receptors in stomach!

Muscle

Vasculature

Gastric Glands

Surface Mucosa

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1) Protein in food activates enteric nervous system

2) Gastric enzymes activate a variety of pathways

3) Secretion of H+ & Pepsinogen

Pepsinogen converted to Pepsin by H+

Acid Secretion

Secretion: Pancreas

Pancreas

Small IntestinePancreatic Duct

Acini Cells

Islet Cells

Pancreas secretes

Trypsin & TrypsinogenExopeptidases

Amylase

Lipase

Bicarbonate Ions (raise pH)

Pancreatic Enzymes work best at higher pH

Secretion: Small Intestine

Villi

Crypts with glands

Muscle

VasculatureSmall Intestine

Glands with crypts secrete:

Amylase, Maltase, Lactase, SucrasePeptidaseEnterokinase (activates trypsin!)

Secretion: BileBile is secreted by the LIVER

Stored in Gall Bladder

Without Bile….fat digestion is very slow

Why doesn’t the digestive system digest itself?

Digestive tract is made of cells that can be digested by enzymes!

Mucus forms a protective barrier between intestinal walls &

acids/bases, enzymes

Mucous Cells in StomachGoblet Cells in Small Intestine

Goblet Cell

Outline

1) Digestion & Absorption of nutrients

2) Secretion of digestive chemicals

3) Regulation of Digestion

4) Regulation of Metabolism

Regulation of Digestion: Thinking with your stomach

1) Long Reflexes – CNS

Parasympathetic- Increase Digestion

Sympathetic – Slow Digestion

2) Short Reflexes – Enteric Nervous System

Ingested food and pH trigger self-regulating changes in digestive system

Neurons located in walls of digestive system

3) Digestive Peptide (Hormones)

Digestive system release several hormones to regulate itself and communicate with brain

Digestive Peptides:

CCK, Gastrin, Secretin, GIP, Motilin, GLP-1

Autonomics work with the Enteric System to coordinate Digestive Function

The Liver

1) Bile Production: Fat Emulsification

2) Processes & Detoxifies Absorbed Products

Hepatic Portal System

carries products to and from liver

3) Storage: Liver is a major storage site of glucose & some vitamins

Cholesterol Metabolism

LPL = Lipoprotien Lipase (Endothelial Enzyme)CE = Cholesteryl Esters

VLDL = Very Low Density LipoprotienLDL = Low ‘’IDL = Intermediate ‘’HDL = High ‘’

Cholesterol Synthesis

‘Statins’ = Inhibit HMG CoA reductaseReduce Total Cholesterol

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Bile Acid Sequestrants

Bile Acids with Fats REMAIN in Digestive

Tract

Acids + Cholesterol + Triglycerides

areExcreted via Feces!

-Some AA are used by body-Excess is metabolized by the liver

-End product is NH3 or NH4 = TOXIC

-NH3 and NH4 repackaged for transport and excretion

1) Urea Cycle2) Glutamate-Glutamine Transport

Amino Acid Metabolism

Kidney excretes urea and NH4 via urine

Amino Acid Metabolism

Glutamine

Urea, Glutamate and Glutamine carry Ammonia(ium) (NH3, NH4+) to kidney for excretion