Physiology of the Normal Equine GI Tract (Hostnik)

Question 1

What is the name of the junction between the glandular and non-glandular regions of the equine stomach?

Answer 1

Margo plicatus

Question 2

What is the acid-secreting region of the equine stomach?

Answer 2

Glandular
- ulceration are more common

Question 3

What is the approximate volume of ingesta the equine cecum can hold (in the typical, adult, light-breed horse)?

Answer 3

30 liters

Question 4

(T/F) Except for its origin and termination, most of the equine ascending colon is freely movable within the abdominal cavity

Answer 4

True

Question 5

Which of the following are points of decrease in diameter within the large colon?

Answer 5

• Pelvic flexure
• Right dorsal colon transverse colon transition

Question 6

List other hindgut fermenters:

Answer 6

• Rabbit
• Chinchilla
• Koala
• Elephant
• Rhinoceros

Question 7

Oral Cavity:
- Mechanical breakdown of plant cell walls

Answer 7

Mastication

Question 8

• Mostly water (99%) -> hypotonic
• Also contains calcium, chloride, bicarbonate, sodium
• Secrete 35-40 liters/day
• pH 8.6-9.1
• Very little digestive enzymes (amylase)
• Acts as a lubricant and buffer
• Forage intake stimulates a larger volume of salivary secretion

Answer 8

Saliva

Question 9

Esophagus/Lower Esophageal Sphincter:
- In the cranial 2/3 of the esophagus

Answer 9

Skeletal muscle

Question 10

Esophagus/Lower Esophageal Sphincter:
- In the caudal 1/3 of the esophagus

Answer 10

Smooth muscle

Question 11

(T/F) Muscle bundles increase towards the cranial esophagus

Answer 11

False, towards the caudal esophagus

Question 12

Esophagus/Lower Esophageal Sphincter:
Thickened inner circular muscle = ______________

Answer 12

Lower esophageal sphincter

Question 13

What structures make it extremely difficult for horses to vomit?

Answer 13

• Thickened inner circular muscle = lower esophageal sphincter
• Oblique angle at the junction with cardia
  (gastric rupture is a major concern in horses showing signs of abdominal pain (colic))

Question 14

How are Gastric decompression done?

Answer 14

• Nasogastric intubation – very important in horses with abdominal pain to prevent fatal gastric rupture
  - Should be prioritized in horses with significant tachycardia (> 60 bpm) or severe pain

Question 15

What separates the oral cavity from the upper airway/esophagus?

Answer 15

Positioning of the epiglottis over the caudal rim of the soft palate

Question 16

(T/F) Horses are obligate nasal breathers

Answer 16

True

Question 17

What is the approximate capacity of ingesta the equine stomach can hold ( adult horse)?

Answer 17

8-10 liters
(can be more with distention)

Question 18

Where is the horse stomach located?

Answer 18

Within the Left abdomen, just caudal to the diaphragm/liver between the 9th and 15th ribs

Question 19

Equine Stomach:
- More susceptible to acid damage
- Protective Factors:
- Tight junctions between cells
- Hydrophobic phospholipid bilayer
- Saliva (buffer)

Answer 19

Dorsal Fundus = Nonglandular + stratified squamous epithelium

Question 20

Equine Stomach:
Contains:
- Parietal cells (acid-secreting)
- Chief cells (pepsinogen-secreting)
- Enterichromaffin cells (histamine-secreting)
- D cells (somatostatin secreting)

Answer 20

Ventral (body, fundus) of the stomach; Proper gastric gland region = glandular

Question 21

Equine Stomach:
Contains:
- Goblets cells (mucus/mucin-secreting cells)
- G cells (gastrin-secreting)
- Enteroendocrine cells

Answer 21

Antrum & Pyloric gland region of the stomach = glandular

Question 22

Protective factors:
Bicarbonate-rich and phospholipid-rich mucus layer

Question 23

Equine Stomach:
Horses have continuous secretion of what?

Answer 23

Gastric Acid
- even when not actively eating (evolved to continuously graze) = “basal secretion”

Question 24

Equine Stomach:
Who is more prone to gastric ulcerations and why?

Answer 24

Foals/Neonates may be more susceptible because they have thinner squamous mucosa and gastric acid secretion occurs early in life

Question 25

Equine Stomach: When forage is available, the area just beyond the lower esophageal sphincter carries a (lower/higher)__________ pH

Answer 25

higher

Question 26

Equine Stomach: The lower, glandular region, containing mostly liquid, remains at a (lower/higher) __________ pH

Answer 26

lower

Question 27

Equine Stomach: Protective Mechanisms within the Glandular Mucosa What is expressed when the enteric nervous system senses a drop in pH?

Answer 27

Prostaglandin E2 (PGE2) - Promotes an increase in mucosal blood flow, increased mucus, and bicarbonate secretion (these maintain pH near neutral at the mucosa within the glandular region) - PGE2 also directly inhibits acid secretion by parietal cells

Question 28

What digestive enzymes are included in the Equine stomach?

Answer 28

- *Pepsin* (pepsinogen converted to pepsin in an acidic enviroment) - *Lipase* (also produced by zymogen cells in the fundus) *Relative importance of these enzymes in digestion in horses is unknown*

Question 29

Equine Stomach: - Large, diverse bacterial community present within the equine stomach - Similar numbers of anaerobic bacteria in cecum and colon - High proportion of *amylolytic bacteria* (starch digesting) - Negligible cellulolytic bacteria - Some volatile fatty acids (VFAs) are produced here -- *how much is absorbed through the gastric wall is unclear*

Answer 29

Microbial Fermentation

Question 30

- 50-70 feet in length in adult horses - *about 50 liters* - Digestion/absorption -> primarily in the *proximal half* - *Intestinal villi* -> perform digestive/absorptive functions - *Crypts* -> secretory function, contain intestinal epithelial stem cells (for self-renewal)

Answer 30

Equine Small Intestine

Question 31

Equine Small Intestine: On what side of the horse can the *duodenum* be visualized *ultrasonographically*?

Answer 31

On the right side - *Ventral* to the right kidney - Between the *right liver lobe* and the *right dorsal colon* (12th - 14th ICS)

Question 32

Equine Small Intestine: Hydrogen ions stimulate the duodenum to release _____ from S cells

Answer 32

Secretin - stimulates *bicarbonate secretion* from the *pancreas* and *liver* (neutralize acid, a protective mechanism, maintain appropriate pH for digestion)

Question 33

Equine Small Intestine: Protein and fats that appear within the duodenum stimulate the release of ____________

Answer 33

Cholecystokinin (CCK) - stimulates the release of *digestive enzymes* from the *pancreas* - Trypsinogen (activated by enterokinase in the brush border to trypsin)

Question 34

Definition: Activates other pancreatic enzymes for digestion/absorption

Answer 34

Trypsin

Question 35

(T/F) Pancreatic secretions are *continuous*, but increase rapidly within feeding

Answer 35

True

Question 36

Equine Small Intestine: Bile duct and pancreatic duct open into duodenum at ____________

Answer 36

major duodenal papilla

Question 37

Equine Small Intestine: Carbohydrate digestion & absorption What is breaking down *Polysaccharide* -> *Disaccharide*?

Answer 37

Pancreatic amylase

Question 38

Equine Small Intestine: Carbohydrate digestion & absorption Disaccharides -> monosaccharides

Answer 38

Brush border enzymes: - Sucrase (horses have a decent amount in the proximal small intestine) - Maltase (evenly distributed, high activity) - Lactase (horses' lactase activity decreases with time)

Question 39

Equine Small Intestine: Protein digestion & absorption Protein -> Polypeptide

Answer 39

- Trypsin - Others (activated by trypsin) - Chymotrypsin - Carboxypeptidase - Elastase

Question 40

Equine Small Intestine: Protein digestion & absorption Peptide -> amino acids, di- and tri-peptides

Answer 40

Brush border oligopeptidases

Question 41

Equine Small Intestine: With damage to intestinal villi (from disease), what can happen?

Answer 41

Brush border enzymes may be destroyed: - Maldigestion - Malabsorption ->> leading to Osmotic Diarrhea

Question 42

What is a major site of *glucose* absorption?

Answer 42

The proximal small intestine

Question 43

SGLT1 expression increases in response to what?

Answer 43

Increased glucose exposure in the small intestine

Question 44

(T/F) Sodium-dependent and independent absorption of monosaccharides

Answer 44

True

Question 45

(T/F) Fructose does not require sodium

Answer 45

True

Question 46

Equine Small Intestine: (T/F) The *small* intestine does the majority of the water absorption

Answer 46

False, Large intestine does the majority

Question 47

Where do most of the water absorption occur in the small intestines?

Answer 47

Distal third

Question 48

Equine Small Intestine: Water absorption is ______(a)______ (primarily paracellular) and follows ____(b)_____

Answer 48

(a) passive (b) electrolyte transport

Question 49

- Water content about 86-95% - pH 6-7.3 (higher in the ileum) - higher than in the stomach because it is buffered - Bacteria present (lactobacilli, streptococci, and lactate-utilizing bacteria) - Liquid transit through the small intestine about 60-90 minutes - Solid transit between 4-7 hours

Answer 49

Equine Small Intestine

Question 50

Equine large intestine: What two structures appear gas-filled on abdominal ultrasound?

Answer 50

- Cecum - Colon (large colon) *generally only distinguishable by location*

Question 51

Is especially adapted (large ascending colon & cecum) for hindgut fermentation, providing a large portion of energy from the horse's diet Functions: - Fermentation - Volatile Fatty Acid (VFA) *production & absorption* - Water and electrolyte absorption

Answer 51

Equine Large Intestine

Question 52

(T/F) Rabbits are similar to horses but primarily utilize the cecum for fermentation and the ascending colon to a lesser extent

Answer 52

True