Gastrointestinal System - Stomach, Intestines, and Accessory Organs

Stomach (1)

- Dilated portion of the digestive tract

- Varies significantly between species depending on their diet

- Carnivores: small, simple stomach

- Herbivores: may have a small, simple stomach or a large, complex stomach, depending on what part of their GI tract breaks down complex carbohydrates (vegetation)

- Distal to the esophagus, proximal to the small intestine

Simple (Monogastric) Stomach

- Fully within the ribcage

- Receives food from the esophagus

- Area where the esophagus opens into: cardia (1)

- Typically toward the left side of the body

- Large blind sac portion that rises above the body: fundus (2)

- Main area from the cardia toward the "bend": body (3)

- Tubular part: pyloric portion (4)

- Typically toward the right side of the body

Layers of the simple Stomach Wall

Very similar to the esophageal layers

- Serosa: outermost layer

- Smooth muscle

- Three overlapping layers of muscle in different directions

- Submucosa

- Aids in creating the characteristic mucosal folds

- Muscularis mucosa (very thin)

- Mucosa: innermost layer

Mucosal Layer of Simple Stomach

- Contains 3 types of glands

- Cardiac - produce mucus to protect the mucosa

- Proper gastric - produce hydrochloric acid for digestion

- Pyloric - produce mucus to protect the mucosa

- The location of the glands do NOT always correspond with their namesake

Inside the Simple Stomach

- 2 muscular sphincters to control flow in/out of the stomach

- Cardiac sphincter - controls flow associated with the esophagus; "entry"

- Pyloric sphincter - controls flow associated with the small intestine; "exit"

- Rugae = folds

- Provide extra surface area for secretion and digestion

- Stretch out when the stomach is fully distended

The Intestine

- Begins at the pylorus and ends at the anus

- Shorter in carnivorous animals, and longer in herbivores

- Small intestine: proximal portion, small in diameter but very long

- Duodenum: most proximal

- Jejunum• Ileum: most distal

- Large intestine: distal portion, large in diameter but often shorter

- Cecum

- Colon

- Rectum

Small Intestine

- Primary function is to be the location of enzymatic digestion and nutrient absorption

- Has the usual 4 layers of tissue

- Serosa (4)

- Muscular - 2 layers, running indifferent directions (3)

- Submucosa (2)

- Mucosa (1)

- Both the liver and the pancreas have ducts that enter the small intestine to aid with digestion

Small Intestinal Mucosa

- Contains many intestinal villi (finger- like projections) that greatly increase the surface area available for nutrient absorption

-Also contains crypts (glands that open up into the surface of lumen) that secrete mucous and enzymes to aid in digestion

Sections of the Small Intestine

- Duodenum (2-4)

- Relatively short

- A continuation of the pylorus of the stomach

- Jejunum (5)

- Very long and relatively free in the abdomen -very long mesenteric attachment

- Ileum (6)

- Mostly a direct path from the jejunum to the large intestine

Large Intestine

- Divided into the cecum, colon, and rectum (in that order)

Cecum

- Blind-ended sac protruding from the continuous tube of the large intestine

- First portion of the large intestine

- In carnivores & omnivores it is relatively small and unimpressive

- In the ruminant, it is larger and is continuous with the colon

- Division of the cecum & colon is marked by the entrance of the ileum

Colon

- Generally has a larger diameter than the small intestine, and the mucosa is smooth(no villi)

- Suspended within the abdomen and receives its blood supply via the mesocolon(mesentery specifically for the colon)

- Both an ileal orifice (4) and a ceco-colic orifice (5) open into the colon from their respective structures

- Where exactly these openings are relative to one another depends on the species

Colon

- Divided into ascending (5),transverse (6), and descending (7) parts, based on location in the abdomen