Lesson 79: Structure and Function of the Small Intestine in Domestic Animals

The average length of the small intestine in various animals

➢ Dog – 4 m (13 feet)

➢ Cat – 1.72 m (5.6 feet)

➢ Horse – 22 m (73 feet)

➢ Pig – 18 m. (59 feet)

➢ Bovine – 46 m. (150 feet)

➢ Small ruminant – 26 m (85 feet)

Function of the SI: Digestion of Food

Enzymatic Breakdown: Digestive enzymes from the pancreas and intestinal mucosa break down carbohydrates, proteins, and fats.

In ruminants, the small intestine continues the digestion of microbial protein from the rumen

Function of the SI: absorption of carbs

Broken down into monosaccharides (glucose, fructose) and absorbed into the bloodstream

Function of the SI: absorption of proteins and lipids

Digested into amino acids and peptides for absorption.

Lipids: Emulsified by bile, broken down by lipases, and absorbed as fatty acids and monoglycerides

Function of the SI: absorption of vitamins and minerals

Fat-soluble vitamins (A, D, E, K) require bile for absorption.

Water-soluble vitamins (B-complex, C) are absorbed directly.

Minerals (Ca, P, Na, K) are absorbed through active and passive transport.

Function of the SI: Movement of Chyme

Peristalsis: Coordinated contractions move food along the intestine.

Segmentation: Mixing contractions help enhance digestion and absorption.

Function of the SI: Immune function

Gut-associated lymphoid tissue (GALT):

Peyer’s patches (especially in the ileum) help detect and respond to pathogens.

Secretory IgA and mucosal immunity prevent infections

Function of the SI: water and electrolyte balance

absorbs a large portion of water from digestive secretions.

Electrolyte Transport: Sodium, chloride, and potassium balance is maintained for cellular function.

Function of the SI: species specific adaptations

Ruminants: The small intestine absorbs microbial protein and volatile fatty acids.

Horses: The small intestine is crucial for protein and fat absorption, as the hindgut

ferments fiber.

Carnivores (Dogs & Cats): Highly adapted for protein and fat digestion.

Pigs & Omnivores: Efficient digestion of both plant and animal matter

The duodenum

The proximal part of the small intestine → Fairly fixed and attached to body wall

Connecting peritoneum is called mesoduodenum

Three segments: Descending duodenum, Transverse duodenum, Ascending duodenum

Anatomy of the duodenum

Descending duodenum

Proximal and the most fixed part of the duodenum and small intestine.

On the right of the median plane/mesentery

Contacts right abdominal wall

Proximal ‘S’ shaped OR Cranial duodenal flexure

Major and Minor duodenal papillae

Descending duodenum: Major and Minor duodenal papillae

Major duodenal papilla – Opening of the common bile duct and major/main pancreatic duct

Minor Duodenal papilla – Opening of the lesser/accessory pancreatic duct

The Minor Duodenal papilla is?

Inconstant in cats(Present only in 20% cats)

Transverse Duodenum

➢ Also called caudal flexure of the duodenum

➢ Crosses the median plane

➢ Mostly along the roof

Ascending Duodenum

On the left side, Mostly along the roof

Duodenojejunal flexure: A sharp bend where it becomes the jejunum

➢ Duodeno-colic fold

Small intestine - The Jejunum

From ascending duodenum to the Ileum

Long, coiled loops with a thin, flexible wall

Very flexuous portion, remains free in the abdominal cavity and forms bulk of the small intestine

Connecting peritoneum of the jejunum

Mesojejunum /Jejunal mesentery/ mesentery is very long

• Occupies most of the abdominal space in the dog and cat

Cranioventral and caudoventral

The Mesentery/Great mesentery” and “Root of the mesentery”

Small intestine - The Ileum

Shortest portion of the small intestine, located in the caudal abdominal cavity.

Slightly thicker wall than the jejunum.

Suspended by the mesentery and connected to the cecum via the ileocecocolic junction

The antimesenteric artery can be used to?

demarcate the ileum from the jejunum

The ileocolic junction (Ileocecocolic junction) → Ileum opens into the ascending colon

The ileal (ileocolic) orifice, The ileal papilla, The Ileocolic sphincter

The duodenum - blood supply

Cranial part: Hepatic a. → Gastroduodenal a. → Cranial pancreaticoduodenal a.

Caudal Part: Cranial Mesenteric Artery → caudal pancreaticoduodenal a.

Cranial mesenteric artery (#2) supplies almost entire small intestine and major part of the large intestine

Jejunal aa. #9 (Multiple)

Common trunk aka Ileocolic a. (#3)

Mesenteric ileal a. (#7)

Antimesenteric ileal a. (#8)

SI: venous return

Hepatic Portal vein: Gastroduodenal vein (#3), Cranial pancreaticoduodenal vein (#8)

Hepatic Portal vein: The Cranial mesenteric vein (#4)

Semischematic dorsal view of the formation of the portal vein (dog).

1, Portal vein;

2, splenic vein;

3, gastroduodenal vein;

4, cranial mesenteric vein;

5, caudal mesenteric vein;

5′, ileocolic vein;

5″, middle colic vein;

6, left gastric vein;

7, right gastroepiploic vein;

8, cranial pancreaticoduodenal vein

SI: sympathetic innervation

originates from the thoracic spinal cord (T5–T12)

Travel via the greater splanchnic nerve (T5–T9) and lesser splanchnic nerve (T10–T12)

Motor and sensory

SI: pre and postganglionic fibers

The preganglionic fibers synapse in the Prevertebral ganglia

• the celiac ganglia, and cranial mesenteric ganglion

The postganglionic fibers travel along the branches of hepatic artery (for duodenum) and cranial mesenteric artery (jejunum and ileum) to reach the organs.

SI: Parasympathetic innervation

Originates from the dorsal motor nucleus of the Vagus n. in the medulla oblongata of the brainstem

Dorsal and Ventral Vagal Trunks – Plexuses

Ventral Vagal trunk – Duodenum and pancreas

Dorsal Vagal trunk - Jejunoileum

Hernia/Incarceration: Through natural/pathological openings in the body wall/ mesentery

Intussusception – telescoping(one part of intestine sliding into another)

➢ Intussusceptum (inner)

➢ Intussuscipiens (Outer)

The Small intestine – Common procedures: Intestinal anastomosis (End to end anastomosis)