Stomach, Sm Intestine

GI Tract histology: 4 layers of the gut wall

outer — inner

1. Mucosa → secrete mucus and fluid into the lumen

2. Submucosa → includes blood and lymphatic vessels that transport absorbed nutrients and a scattering of submucosal glands that release digestive secretions.

3. Muscularis → promote mechanical digestion, expose more of the food to digestive chemicals, and move the food along the canal.

4. Serosa → protective layer for internal organs, lubcrication

Stomach

lined with simple columnar: tunica muscularis had a 3rd layer (oblique) inside the usual circular and longitudinal muscle layers

• watery contents = chyme

• 4 regions(in order): Cardia, fundus, body, and pyloris

Gastric glands

secretory cells within gastric pits

• Parietal cells: HCI and intrinsic factor—B12 binding

• Chief cells: pepsinogen

• G cells: enteroendocrine cells

→ secrete gastrin which stimulates above cells

Small intestine

• 20 ft long!

• 90% of nutrients are absorbed here due to the high amounts of mucosal surface area

Small intestine: Plicae circularis

permanent internal ridges visible to the naked eye.

Villi

small, finger-like projections — lined with simple columnar epithelium

• microvilli on apical surfaces

Lacteals

part of the lymph system that absorbs lipids

Intestinal glands (Crypts of Lieberkuhn)

found near the base of intestinal villi; secreting hormones and enzymes

• Goblet cells: secrete protective mucus

Digestive enzymes

from the pancreas and intestinal glands

• digest all classes of molecules

What protects the lining from being digested?

• Goblet cells (part of endothelium)

• Brunner’s glands (deep into endothelium)

→ Both produce alkaline mucous

Segmentation

a movement characteristic of the small intestine

• a churning motion that does NOT produce any net movement down the GI tract

• mixes chyme with intestinal secretions

Peristalsis

starts in the esophagus, net movement down the GI tract

Peristaltic rush

diarrhea

Duodenum

the first region begins after the pyloric sphincter and ends at the duodenojejunal flexure

• is the shortest (~10in long) and widest segment of small intestine

• forms a loop around the pancreas

Duodenum: Hepatopancreatic ampulla

where the common bile duct fuses w/ pancreatic duct

• wide variation

Duodenal papilla w/ hepatopancreatic sphincter

opens to allow juices out, but keeps chyme from reversing back up

Jejunum

~8ft long; supported by mesentery

• the bulk of chemical digestion and nutrient absorption occurs here

ileum

longest ~12ftl

• continues absorption

• ends at the ileocecal valve (controls rate of emptying)

• large amount of MALT to counteract bacteria found in the large intestine → toward the end of small intestine

→ Mucosa Associated Lymphoid Tissue

• MALT aggregates to form Peyer’s patches in the terminal region of the ileum