Digestive Physiology
The primary function of the
digestive system is move nutrients
from the external environment into
the body
Outline
1)
2)
3)
4)
Digestion & Absorption of nutrients
Secretion of digestive chemicals
Regulation of Digestion
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: Stomach
2) Trypsin & chymotrypsin: SI
3) 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 diffusion
2) 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 glucose
2) Sucrose by Sucrase to glucose + fructose
3) Lactose to Lactase to glucose + galactose
Na+Glucose
Symporter
Glut5
4) Monosaccharides can FINALLY! be absorbed
1) Glucose & Galactose via Na+-glucose Symporter
Glut2
2) Fructose via Glut5 & Glut2 transporter
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
Acid Secretion
1) Protein in food activates
enteric nervous system
Surface Mucosa
2) Gastric enzymes activate a
variety of pathways
Gastric Glands
Vasculature
*
Muscle
3) Secretion of H+ &
Pepsinogen
Pepsinogen converted to
Pepsin by H+
*
Drugs for heartburn & ulcers (Zantac, Pepcid) block Histamine receptors in stomach!
Secretion: Pancreas
Pancreatic Duct
Small Intestine
Pancreas
Pancreas secretes
Trypsin & Trypsinogen
Exopeptidases
Amylase
Lipase
Acini Cells
Bicarbonate Ions (raise pH)
Pancreatic Enzymes work best at higher pH
Islet Cells
Secretion: Small Intestine
Villi
Small Intestine
Vasculature
Crypts with
glands
Muscle
Glands with crypts secrete:
Amylase, Maltase, Lactase, Sucrase
Peptidase
Enterokinase (activates trypsin!)
Secretion: Bile
Bile 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
Goblet Cell
Mucous Cells in Stomach
Goblet Cells in Small Intestine
Outline
1)
2)
3)
4)
Digestion & Absorption of nutrients
Secretion of digestive chemicals
Regulation of Digestion
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 selfregulating changes in digestive system
Neurons located in walls of digestive
system
3) Digestive Peptide (Hormones)
Autonomics work with the Enteric System to
coordinate Digestive Function
Digestive system release several
hormones to regulate itself and
communicate with brain
Digestive Peptides:
CCK, Gastrin, Secretin, GIP, Motilin, GLP-1
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 Lipoprotien
LDL = Low ‘’
IDL = Intermediate ‘’
HDL = High ‘’
Cholesterol Synthesis
‘Statins’ = Inhibit HMG CoA reductase
Reduce Total Cholesterol
*
Bile Acid Sequestrants
Bile Acids with Fats
REMAIN in Digestive
Tract
Acids + Cholesterol +
Triglycerides
are
Excreted via Feces!
Amino Acid Metabolism
-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 Cycle
2) Glutamate-Glutamine Transport
Kidney excretes urea and NH4 via urine
Amino Acid Metabolism
Urea, Glutamate and Glutamine carry Ammonia(ium) (NH3, NH4+) to kidney for excretion
Glutamine