GI physiology

Monogastric

Single chambered stomach

Ruminants

Rumen, reticulum, omasum, then “stomach” = abomasum

Hind-gut fermentation

• Monogastrics but specialized for fermentation in large intestine

• Ex: Horses and rabbits

Animals that have a forestomach

Ruminants and camelids

Which species is an obligate carnivore?

Cat

Which species is a facultative carnivore?

Dogs

Flow of feed

1. Mouth

2. Esophagus

3. Stomach (± forestomach)

4. Small intestine

5. Large intestine

6. Rectum/anus

Order from stomach to anus

1. Stomach

2. Duodenum

3. Jejunum

4. Ileum

5. Cecum

6. Ascending colon

7. Transverse colon

8. Descending colon

9. Rectum

10. Anus

Major functions of the stomach

• Mechanical digestion

• Acidic digestion

• Some enzymatic digestion

Major functions of the small intestine

• Most similar among species

• Enzymatic digestion

• Absorption

• Immune functions

Major functions of the large intestine

• Fermentation

• Absorption

Major function of the cecum

• Between the ileum and colon

• Fermentation

Function of Descending Colon/Rectum/Anus

• Absorption

• Transport

• Storage

What is in red?

Luminal

What is in blue?

Mural

Mucosa

The Epithelium that lines the lumen of the GI tract

Submucosa

Underlying connective tissue- Collagen with blood vessels and nerves (provides structural integrity)

Muscularis

• Smooth muscle that produces wave-like contractions (peristalsis)

• Peristalsis- progression of coordinated contractions and relaxations of the smooth muscles of the GI tract to propel food

Serosa

Connective tissue; outermost covering (visceral peritoneum is on top of the serosa)

Where does the blood supply and innervation from the GI system come from?

It comes through the mesentery (the sheet of fat that organs hang by)

The GI tubular system is covered by two layers, what are they?

• Serosa

• Muscularis

  • Outer longitudinal

  • Inner circular

Enteric Nervous System

• Independent and intrinsic to the GI tract

• Controls nearly all aspects of GI function

  • Motility

  • Secretions

  • Blood flow

• Second Brain

  • Can operate independently of CNS, but normally does receive input from autonomic nervous system

  • Complex- the number of neurons = number of neurons in spinal cord

The enteric nervous system two interconnected systems of ganglia

• Myenteric (Auerbach)

  • Between the muscle layers (longitudinal and circular)

  • Regulates motility

• Submucosal (Meissner)

  • Within the submucosa

  • Regulates secretions from mucosa

  • Sensory neurons too

• Interneurons

  • Connect myenteric and submucosal ganglia

Parasympathetic input on ENS

• Vagus nerve

• Pelvic splanchnic nerve

• Gives input for rest and digest

• Promotes motility and secretions

• ACH is the neurotransmitter

Sympathetic input on ENS

• Thoracic/lumbar splanchnic nerves

• Stops motility, decreases blood flow, stops secretions

• Fight or Flight response

• Postganglionic fibers

• Norepinephrine is the neurotransmitter

Gut Brain Endocrine Axis

• Stress can impact the GI system

• The GI tract makes serotonin

Sphincters

A ring of muscle surrounding and serving to guard or close an opening or tube

The 6 sphincters of the GI tract

• Upper esophageal sphincter

• Lower esophageal sphincter

• Pyloric sphincter

• Ileocecal valve/sphincter (dogs and cats ileocolic junction)

• Internal anal sphincter

• External anal sphincter (voluntary control through the pudendal nerve)

Intestinal barrier importance

• Poop is bad if it is in the blood

• Luminal contents contains

  • Bacteria and bacterial products (LPS- massive inflammatory response)

  • Digestive enzymes capable of tissue injury (acids, biles and reactive metabolites)

  • Food antigens and toxins

• The mucosal barrier prevents these from entering the patient’s system

  • Even small breaches (ulceration) → massive inflammation

  • Translocation of bacteria or endotoxin → systemic inflammatory response, sepsis, shock, death

Intestinal mucosa layers (from luminal to mural)

• Epithelium

  • 1 later of cells

• Lamina propria

  • Loose collagen

  • Immune cell rich

• Muscularis mucosa

  • Thin layer of smooth muscle

Intestinal epithelial cells

• Simple columnar

  • Absorptive epithelial cells (absorb nutrients or water/electrolytes)

    • Enterocytes

    • Colonocytes

  • Other types of epithelial cells

    • Goblet cells- Mucin for protection and lubrication

    • Paneth cells- immune cell that secretes antimicrobial peptides and growth factors

    • Stem cells- progenitor cells

    • Enteroendocrine and Neuroendocrine- releases hormones regulating motility, secretion, and appetite

    • Many others

Crypts vs villi

• Both small and large intestine have crypts

• The bottom of crypts contain critical cells (stem cells)

• Stem cells at bottom of the crypt migrate towards the lumen, differentiate into other cell types, migrate, die, and slough (3-5 days)

• Only small intestine has villi

• Villi provide an increased surface area

Mucosal blood flow

• Tremendous metabolic demand

• Tremendous blood flow

  • around 25% of cardiac output at rest to GI tract

    • 75% of that to mucosa

• Homeostasis

• Mucosal surface is hypoxic (tip of villi have low oxygen to accommodate for anaerobic microbes)

• Counter current blood flow

• Needed for microbes in lumen

  • Anaerobic environment

Intestinal barrier, mucosal layer cell specifics

• Epithelial cells

  • Tight junctions between cells

  • Barrier

  • Controlled transport for absorption and enzymes

• Mucus layers

  • Produced by goblet cells (mucin)

  • Physical barrier

  • Food for beneficial bacteria

• In the crypt

  • Antimicrobial peptides made by paneth cells

• Lamina propria

  • Immune cell rich

  • Last line of defense

Layers of mucosal barrier from lumen to tissue

1. Microbiota

2. Mucus

3. Antimicrobial peptides and immunoglobulins

4. Epithelial cells

5. Lamina propria (immune cells)

Visceral pain

• Abdominal organs

• Pathway- visceral afferent neurons

• Character- dull and vague (poorly localized/referred)

• Stimuli/cause

  • Stretch of mesentery, distension of bowel

    • Mechanoreceptors

  • Inflammation

    • Chemoreceptors

Somatic pain

• Parietal peritoneum/body wall

• Pathway- somatic spinal nerves

• Character- sharp, localized, intense

• Causes/stimuli

  • Mechanical, thermo, stretch, chemical, etc.

GI organs are relatively insensitive to:

• Cutting

• Crushing

• Burning

• Touching the mucosa

GI organs are relatively sensitive to:

• Distension and excessive stretch

• Traction of the mesentery

• inflammation/ischemia

• Forceful smooth-muscle contraction

• Chemical (bile peritonitis)

Infarction

Tissue death due to loss of blood supply

Peritoneal fluid

• On ultrasound fluid is black

• Normal amount is small (<3mm depth)

• Functions for lubrication

• Normal parameters

  • Clear (yellow/straw colored)

  • Can read through it

  • On cytology low cell and protein count

Gut microbiota

• Collection of micro-organisms

  • bacteria, fungi, viruses, and archaea

• Colon has the greatest number of bacteria

• Ruminants have the greatest number of microorganisms

• Horses have the greatest number of species

• These organisms are anaerobic and have specific requirements so they are difficult to culture

• 20-60% not cultivable

pH order of organs from most acidic to most basic

1. Stomach

2. Large intestine

3. Small intestine

Gut microbiota, what do they do?

• Part of the intestinal barrier

• Provide competition with pathogens

• GI motility

  • Serotonin and other neurotransmitters are made by some bacteria

Healthy vs. Unhealthy microbiome

• Healthy- is diverse and prevents infection from enteric pathogens

• Unhealthy- Has reduced diversity of microbiota

GALT

• Responsible for differentiating between nutrients on the one hand and potential harmful substances

• Largest immune organ in the body

• Where are these cells:

  • Individual cells throughout

  • Lymphoid follicle throughout (cluster of immune cells usually B cells in mucosa of intestine

  • Peyer’s patches (ileum) (large aggregation of lymphoid tissue in the ileum and sometimes jejunum)

  • Lymphatics to mesenteric lymph nodes (lymph fluid then goes to thoracic duct)

• Immune cells get out of GALT and into circulation through the thoracic duct

• Specialized cell types

  • Paneth cells - AMPs

  • M- cells- Antigen sampling

The importance of GALT

• Mucosa constantly exposed to antigens

• Food allergies

• IBD

• Enteric pathogens

• Regulated response (Regulatory T cells)