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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
*
*
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 ‘’
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