digestive system

What are the four layers of the digestive system

Mucosa, submucosa, serosa, muscularis externa

Explain the three layers of the mucosa

Mucosal membrane – epithelial cells that come in contact with food. Endocrine cells that secrete hormones into the blood and exocrine cells that secrete juices into the lumen

Lamina propria – connective tissue and lymphatic tissue (macrophages and lymphocytes)

Muscular mucosa – layer of smooth muscle

Explain the submucosa

Second layer – Connective tissue, blood vessels, lymph vessels, lymphoid tissue

plexus of Meisner - network of nerves that control secretions and peristalsis

What type of muscle does the muscularis external have?

Circular, smooth muscle and long smooth muscle

Which plexus does muscularis externa have

Myentric Plexus– controls the frequency and strength of contractions

Saliva contains

Water, electrolytes, mucus, enzymes, and antibacterial proteins

Explain the serosa

– Continuous with the peritoneum

– Connective tissue (outer cover that protects the G.I. tract)

What neural and hormonal system prevents gastroesophageal reflux?

The tonic pressure of the sphincter is higher than the intragastric pressure that keeps it closed

What are the functions of the gastric juice

1. convert pepsinogen to pepsin

2. denaturing proteins

3. release nutrients/minerals for absorption

4. bactericide agent

explain the enzymes in gastric juice

1. pepsin: protease (hydrolyze protein) and endopeptidase (hydrolyze peptide bonds in proteins)

2. gastric lipase (hydrolyze fatty acids from glycerol)

3. a-amylase (starch breakdown)

explain the first phase of gastric secretions

1. stimulation: eating, tasting, thinking, seeing, smelling food

2. submucosal plexus: acetylcholine secretes and enhances release of gastric from g cells —>

3. acetylcholine triggers paracrine histamine and HCl secretes

4. chief cells release enzymes

explain the second phase of gastric secretions

1. stimulation: food

2. distension and protein presence enhances gastric secretions (by submucosal plexus, gastrin, parasym NS

explain the third phase of gastric secretions

stimulation: food in small intestine

2. reduced chyme and pH gastric juice triggers somatostatin in pancreas, antrum, duodenum

3. presence of chyme in duod: release of secretin and CCK (reduce gastric juice, peristalsis, gastric emptying

parts of the small intestine

1. kerckring folds

2. villi: has enterocytes, capillaries, lacteal vessels for nutrient transport

3. microvilli: has glycocalyx (hydrolyze partially dig nutr)

4. crypts of lieberkuhn: stem cells for enterocytes

explain the absorptive process

• either diffusion, facilitated diffusion, active transport, or pinocytosis

• nutrients absorbed on brush border to basolateral membrane OR through tight junctions

• depends on solubility/ electric gradient/ size

what does the colon do for absorption

• liquid content get mixed by contractions

• proximal colon mucosal cells absorb Na, Cl, water.

  • sodium absorbed by active transport and ADH

what are colonic secretions

goblet cells secrete mucus (lubricant for feces and protect mucus cells from gut bacteria)

bicarbonate secreted and trades with Cl (which is absorbed)

• provide alkaline environment that helps neutralize acids produced by gut microbiome

what is the role of the pancreas in digestion

islets of langerhans:

• a cells: glucagon

• b cells: insulin

• d cells: somatostatin

acinar exocrine cells produce and package digestive enzymes and alkaline rich juice to buffer chyme (water, cations and anions, bicarb)

stimuli for release of pancreatic juice and enzymes are

secretin: in response to acidic chyme

cholecystokinin: presense of fat and partial digested proteins

what is bile made of

made in the liver, it has bile acids, salts, cholesterol, phospholipids, bile pigments dissolved in alkaline juice

why is bile important

it emulsifies large fat globules into small micelles for easy lipid absorption

how are gallstones formed

when bile becomes super saturated with cholesterol and crystallizes

explain hypercholesterolemia and how to treat it

• def: high blood cholesterol concentations

• resin medication binds to bile and excretes it from the body

• plant stanols/sterols can also bind bile and excrete it WHICH CAUSES decreased recirculation of bile and absorption of cholesterol so the body uses cholesterol to make more bile, decreasing the concentration

peptic ulcer disease

caused by H pylori, alcoholism, NSAIDS (pain meds_. treated by H2 receptor blockers (pepcid)

pancreatitis

• occur with alcoholism, hypertriglyceridemia, duct blockage,

• Zymogens become activated and digest pancreatic tissue

• patients need lipase or partially hydrolyzed nutrients

what are some regulatory hormones that control digestion/absorption

• gastrin (stimulate HCl and Pepsin)

• CCK (bile release into small intestine)

• secretin (pancreatic juice to neutralize)

• peptide YY (fat causes decrease appetite and gastric acid/emptying)

• motilin (stim by acetylcholine, controls motility bwn meals)

what are some regulatory paracrines that control digestion/absorption

• somatostatin: inhibit gastrin, gastric empty

• histamine: stimulate parietal cells to secrete HCl

• glucagon like peptides: release insulin and inhibit glucagon

• insulin like growth factor 1: increase proliferation of GI