anatomy final - digestive

Functions of the digestive system

1. Ingestion & motility

2. Secretion of enzymes, acids, buffers

3. Mixing & propulsion

4. Digestion (mechanical + chemical)

5. Absorption into blood/lymph

6. Defecation

Defense mechanisms

taste/smell, vomit reflex, stomach acid, intestinal flora, lymphatic tissue

GI tract (macroscopic)

mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus

Accessory organs

teeth, tongue, salivary glands, liver, gallbladder, pancreas

GI tract (microscopic)

from outer to inner

1. serosa

2. longitudinal muscle

3. circular muscle

4. submucosa

5. muscularis mucosae

6. mucosa

Smooth muscle feature

High contractility (up to 80% length change)

Enteric nervous system

-”brain of the gut” - 100 million neurons

-two major plexuses:

myenteric

submucosal

-Can function independently but is modulated by ANS

Myenteric

-controls motility

-strength and frequency on contraction

-can function independently

Submucosal

Controls secretion and blood flow

Autonomic nervous system

-Parasympathetic (vagus +pelvic nerves): increases secretion, increases motility

-Sympathetic: decreases secretion, decreases motility, contracts sphincters

Two types of electrical activity

1. Slow waves

2. Spike potentials (action potentials)

Slow waves

-Spontaneous rhythmic depolarizations

-Set frequency of contractions (3-12/min depending on region

-Generated by interstitial cells of cajal

-electrical pacemakers for smooth muscle cells

-propagate through smooth muscle cells by gap junctions

Spike potentials

-Occur when slow waves reach threshold

-Cause actual muscle contraction

-More spike - stronger contraction

GI contractions

-MMC (migrating motor complex): fasting, cleaning waves

-Peristalsis: propulsion

-Segmentation: Mixing

Chewing

-Voluntary but regulated by reflexes

Functions: break food, mix with saliva, prepare for swallowing

Tongue

-Skeletal muscle

-Papillae with taste buds

Salivary glands

-Parotid: serous - amylase

-Submandibular: mixed (mostly serous)

-Sublingual: mostly mucous

Saliva components

-99.5% water

-Electrolytes (Na+, K+, Cl-, HCO3-)

-a-amylase, lingual lipase

-Mucins

-IgA, lysozyme, lactoferrin

-chloride ions activate salivary amylase

-bicarbonate and phosphate ions buffer acidic food

Functions of saliva

Lubrication, taste, initial digestion, oral defense, hydration

Regulation of saliva

-Parasympathetic: increase in watery saliva

-Sympathetic: decrease in volume, thicker mucus

-Dehydration - glands stop secreting

Pharynx

-Nasopharynx (respiratory), oropharynx, larynpharynx

-Skeletal muscle - voluntary + vagus nerve

Esophagus

-25 cm tube, no digestion/absorption

-Moves bolus via peristalsis

Swallowing phases

1. Voluntary oral

2. Pharyngeal (involuntary)

3. Esophageal (involuntary)

Motor functions of stomach

1. Storage (fundus expands; food stored - 1 hour)

2. Mixing - chyme formation

3. Propulsion and retropulsion (antral pump)

4. Gastric emptying (2-4 hours)

Regulation of motor functions

-Stimulate emptying: gastric volume, gastrin

-Inhibit emptying: CCK, secretin, GIP, low pH, high fat

Secretory cells

-Surface mucus cells

-Mucus neck cells

-Parietal cells

-Chief cells

-G cells

Surface mucus cells

thick alkaline mucus (protective)

Mucus neck cells

mucus

Parietal cells

HCL + intrinsic factor

Chief cells

Pepsinogen + gastric lipase

G cells

gastrin

Gastric Juice

-Water, HCL, pepsinogen, intrinsic factor, mucus

-Functions: protein denaturation, kill bacteria, activate enzymes, trigger secretions

Digestion in stomach

-Carbs: salivary amylase (brief)

-Lipids: lingual and gastric lipase

-Proteins: pepsin (optimal pH 1.8-3.5)

-digestion and starch and triglycerides continues, digestion of proteins begins

-semisolid bolus is converted to a liquid

Motor functions of small intestine

-Segmentation (mixing)

-Peristalsis (propulsion)

-Regulated by: gastrin, CCK, secretin, motilin, ENS reflexes

Secretory functions of small intestine

-Glands of Brunner

-Crypts of Lieberkuhn

Glands of Brunner

-secretion of neutral and alkaline glycoproteins and bicarbonate ions

- protects duodenal wall

Crypts of Lieberkuhn

-Intestinal juice

-Enterokinase activate trypsinogen - trypsin

Pancreas exocrine secretion

-Bicarbonate-rich fluid

-Digestive enzymes (as zymogens)

amylase trypsin, chymotrypsin, elastase

lipase nucleases

Pancreas endocrine secretion

-Insulin, glucagon, somatostatin, pancreatic polypeptide

-secretes hormones

Liver functions

-Metabolism (carbs, lipids, proteins)

-Detoxification

-Produce bile

-Store vitamins/minerals

-Make plasma proteins

-Phagocytosis (Kupffer cells)

Bile

-Water, bile salts, bile pigments, cholesterol, lecithin

-Functions:

Emulsify fats

Form micelles

Neutralize acid

Excrete wate (bilirubin)

-Produced by hepatocytes

Gallbladder

-Stores and concentrates bile 10x

-CCK stimulates contraction and bile release

Digestion in the small intestine

-Carbs: disaccharides (brush border)

-Proteins: peptidases + pancreatic enzymes

-Lipids: pancreatic lipase + bile micelles

-Nucleic acids: nucleosidases, phosphatases

Absorption in small intestine

-Enhanced by:

Circular folds

Villi

Microvilli (brush border)

-Fats absorbed as:

1. Micelles

2. Diffuse into cells

3. Packaged as chylomicrons

4. Enter lymph (lacteals)

Large intestine motor functions

-Haustral contractions (mixing)

-Mass movements (1-3/day)

-Gastrocolic reflex (triggered by eating)

Large intestine secretory functions

-Crypts of Lieberkuhn - mucus

-Functions:

Protect wall

Lubricate stool

Protect from bacteria and acids

Large intestine digestive functions

-Bacterial fermentation of:

Undigestible carbs - gas

Proteins - skatole, indole, ammonia

-Bacteria produce: vitamin K, B12, thiamine, ribofllavin

Large intestine absorptive functions

-Water and electrolytes

-Formation of feces (3/4 water, ¼ solids)

what vagus nerves supply

parasympathetic fibers

salivation

secretion of saliva controlled by the ANS

Nasopharynx

respiration

Oropharynx

digestion and respiration

Laryngopharynx

digestion and respiration

Storage in stomach

1. The fundus is capable of expanding to accumulate food from the esophagus

2. The food is stored for 1 hour

3. During this time the food is separated by density

Mixing and propulsion of food

-The middle part starts with weak peristaltic contractions = wave mixing

-Mixture of food with gastric secretions: chyme

-As waves approach the pylorus, they increase in speed and strength and become more powerful peristaltic contractions = peristaltic constriction rings

-PSN, gastrin and motilin increase the frequency of action potential and the force of contractions

-SNS, secretin, CCK, GIP decrease it

CCK

-fatty acids

-digestive hormone released with secretin when food from the stomach reaches the first part of the small intestine

-stimulates the gallbladder to contract and release stored bile into the intestine. It also stimulates the secretion of pancreatic juice and may induce satiety

-reduces gastric acid secretion and inhibit gastric emptying

GIP

glucose and fatty acids

Secretin

-Chyme is acid

-duodenum and jejunum - S cells

-Target: pancreas, stomach

-Inhibits gastric acid secretion, reduces gastric emptying

-stimulates pancreatic juice and bile

Bicarbonate

secreted from the epithelium into the mucus layer, where it neutralizes acid that is back-diffused from the lumen of the stomach and forms a pH gradient, with a higher pH at epithelial cell surface

Fundus and body gastric glands

80%

Mucus neck cells

Parietal cells

Chief cells

Piloric antrum piloric glands

20%

Mucus neck cells

G cells

Cephalic phase

via vagus

Gastric phase

1. Local nervous secretory reflexes

2. Vagal reflexes

3. Gastrin-histamine stimulation

Intestinal phase

1. Nervous mechanisms

2. Hormonal mechanisms

Nerve signals

gastroenteric reflex

Hormonal signals

gastrin, CCK, insulin, motilin, secretin

Absorptive cell

digests and absorbs nutrients

Goblet cell

secrets mucus

Enteroendocrine cell

secretes the hormone secretin, cholecystokinin, or GIP

Paneth cell

secrets lysozyme and is capable of pahgocytosis

Lecitin

phospholipids that solubilize cholesterol

Emulsification

Bile breaks down fat into small droplets within the small intestine

Hydrolysis of triglycerides

In emulsified fat droplets into fatty acid and monoglycerides

Pancreatic lipase

main fat-digesting enzyme

gastrin

-g cells

-increases gastric acid secretion and gastric motility

GIP

-duodenum and jejunum - k cells

-releasing stimuli: glucose, fatty foods

-reduces gastric acid secretion. decreases gastric emptying

Somatostatin

-D cells which are located throughout the GI tract

-releasing stimuli: eating fatty foods

-reduces gastrin and stomach acid secretion

Motilin

-secreted by small intestine, colon

-releasing stimuli: fasting

-Controls peristalsis by stimulating smooth muscle contraction and relaxation to coordinate the movement of food through the gut.

VIP

-secreted by nerve fibers supplying all parts of GI tract

-vasodilator - increases blood flow to the gut

Grehlin

-secreted by stomach

-secretion stimulated by fasting or starvation and suppressed by eating food

-Stimulates appetite

Huastra

saclike segments, appear after contractions of the large intestine

Proximal colon

-where water absorption occurs

-ileocecal sphincter contracts to prevent reflux into the ileum

-segmentation contractions

mass movements 1/3 times a day

Distal colon

-fecal material turns semisolid and moves slowly

Rectum/anal canal

-Intra-abdominal pressure is increased by expiring against a closed glottis

Gastrocolic reflex

-The presence of food in the stomach increases the motility of the colon and increases the frequency of mass movements.

-a rapid parasympathetic component