L4: Regulation of Digestion

Functions gastrointestinal tract

Digestion

Absorption

Secretion

Motility

Digestion

Breakdown of large macro-nutrients

Absorption

Absorption digested nutrients in blood

Secretion

Endocrine and exocrine secretion

Motility

Movement of food through gastrointestinal tract

General anatomy gastrointesinal wall

Mucosa - surround intestinal lumen

Submucosa - connective tissue

Muscularis externa - smooth muscle

Serosa - connective tissue

Enteric nervous system

Mucosa consists of

Mucous membrane

Lamina propria

Muscularis mucosae

Muscularis externa consists of

Circular muscle

Longitudinal muscle

Enteric nervous system consists of

Submucosal plexus

Myentreic plexus

Anatomy gastro tract

Fundus

ody

Antrum

Fundus: thin flexible (stretch) wall (secretion)

Body: rugged wall (secretion)

Antrum: muscular wall (knead, propel)

Fundus+ body: gastric pits (secretion)

Exocrine

• Chief cells, pepsinogen

• Parietal cells, HCl

Endocrine

• Gastric cells, gastrin

Secretion gastric juice

2-4L/day, pH2

• anti-bacterial

• protein denaturation

• activation pepsinogen

  • pH-optimum pepsin

what casues the low pH in gastric juice

Parietal cells

• H+ and HCO3- production (carbon anhydrase, CA)

  • Net result: H+ and Cl- in lumen, HCO3- in interstitial tissue

Anatomy intestinal tract

Duodenum

Jejunum

Ileum

Colon

Intestianl tract - function

Absorption nutrients and water

Intestinal tract - increase surface area

Vili in mucusa

Microvilli, brush border (enzymes), (blood and lymph vessels in vili)

Accessory glands

Salivary glands

Liver

pancreas

Salivary glands - function

HCO3-

Mucus (lubricates food)

Amylase (digestion starch)

Lysozyme (killing bacteria)

Pancreas

Can be either endocrine or exocrineP

Pancreas - endocrine

Islet cells

• secretion hormones

  • Insulin

  • Glucagon

Exocrine pancreas

Pancreatic juice secretion

Acini: H20, electrolytes, enzymes (primary secretion)

Ducts (converging): increase volume, bicarbonate (secondary secretion)

Pancreas exocrine (acini)c

Secretion bicarbonate and digestion enzymes:

• amylase (starch)

• lipase (fat)

• proteases (protein)

  • nucleases (nucleic acids)

Liver function production and secretion bile

contains (a.o. salt, bicarbonate)

continuous production in liver, periodic secretion in duodenum

storage in gallbladder

(regulation via sphincter of oddi)

function liver

metabolic conversion nutrients

absorptive phase

Glucose	→	Glycogen
Amino acid	→	Fatty acids
Triglyceride, cholesterol	→	Lipoproteins

postabsorptive phase

Glycogen	→	Glucose
Fatty acids	→	Ketone bodies
Gluconeogenesis

Digestion macronutrients

Function

To go from poly to monomersD

Digestion macronutrients

Digestion enzymes

Mostly hydrolases

Amylase - carbohydrate

Peptidase - protein

Lipase -fat

Digestion macronutrients - regulation enzymes

pH-optimum

T-optimum

Carbohydrates; digestion

Carbohydrates (sugars, saccharides)

Polymers

• polysaccharide

  • starch (plant)
    glycogen (animal)

• disaccharides

  • maltose

  • sucrose

  • lactose

only monomers are absorbed

Carbohydrates - what is the route of digestion

Luminal (mouth, intestine)

• Amylase: polymers → oligo, dimers

Cell-surface (brush-border intestine)

• Oligo, dimers → monomers

  • maltase

  • sucrase

  • lactase

Carbohydrates - absorption

to portal blood (liver)

monomers via carriers (secondary active transport, facilitated diffusion)

Proteins: digestion

Proteins

Polymers

• di, tri, oligo, polypeptides

Monomers

• amino acid

monomers and oligomers are absorbed

Proteins - digestion - enzymes

Peptidase, protease

• endo-peptidase

• exo-peptidase

zymogen, activation needed (HCl, enterokinase)

Proteins: what is the route of digestion

luminal (stomach, intestine)

• polymers → oligo, tri, di, monomers

cell-sruface (brush border intestine)

• oligo, tri, dimers → tri, di, monomers

intracellular (intestine)

• tri, dimers → monomers

Fat digestion

fats and oils (lipids)

• triglycerides

• cholesterol, fatty acids

digestion

• fat - hydrophobic, lipase - hydrophilic

  • bile acid: emulsification → increase surface availability for lipase

fat digestion enzymes

lipase (saliva, stomach, pancreas)

fat - absorption

diffusion in the enterocytes (lipophilic)

repackaging in triglycerides

packed in chylomicrons (lipoproteins)

exocytosis to interstitial fluid

uptake in lymphatic system (lacteals)

regualtion gastrointestinal function

maximizing digestion and absorption (function of gastrointestinal tract)

• neuronal and hormonal)

Neuronal regulation gastrointestinal tract

1. plexus of meissner or submucosal plexus

2. plexus of auerbach or meyenteric plexus

neuronal regulation - nerves

input

• receptors gastrointestinal tract (mechano, osmo, chemoreceptors)

• autonomic nervous system

output

• plexus of Meissner; exocrine and endocrine glands, blood flow

  • plexus of Auerbach; smooth muscle → motility

plexus of meissner - function

exocrine and endorcine glands, blood flowp

plexus of auerbach function

smooth muscle → motility

neuronal regulation function

regulates motility, secretion

externally independent (some autonomic regulation)

regulation gastrointestinal tract: reflexes

phases

cephalic

gastric

intetsinal

cephalic phase

stimuli arising from the head

gastric phase

stimuli arising form stomach (CNS, ENS or endocrine)

intestinal phase

stimuli arising from intestine (CNS, ENS, or endocrine)

Regulating secretion saliva

taste and texture food → mechano and taste receptors in mouth → salviary centre in brain stem → activating autonomic nerve system: cortex

cephalic regulation

parasympatic, increase watery saliva

sympaticL increase viscous, portein rich saliva

Regulating secretion gastric juice

HCl and pepsinogen secretion

parasympathetic and gastrin (G-cells)

cephalic and gastric stimuli: increase

intestinal stimuli decrease

regulation of secretion pancreatic juice primarily by

intestinal stimuli

regulating secretion pancreatic juice

acid, protein, fat (chemo), chyme (mechano)

CCK: stimulation acini cells (primary secretion: enzymes etc)
Secretin: stimulation duct cells (secondary secretion: bicarbonate etc)