ANAPHY - Digestive process

Pre-finals Reviewer

GI Tract

External environment for the digestive process

• Mechanical and chemical stimuli

• Extrinsic control by CNS centers

• Intrinsic control by local centers

Regulation of digestion in GI tract involves:

Mechanical and chemical stimuli

stretch receptors, osmolarity, and presence of substrate in the lumen

Mechano- and chemoreceptors

- respond to:

- Stretch, osmolarity, and pH

- Presence of substrate, and end products of digestion

- They initiate reflexes that:

- Activate or inhibit digestive glands

- Mix lumen contents and move them along

Intrinsic controls

- Nerve plexuses near the GI tract initiate short reflexes

- Short reflexes are mediated by local enteric plexuses (gut brain)

Extrinsic controls

- Long reflexes arising within or outside the GI tract

- Involve CNS centers and extrinsic autonomic nerves

Chyme

- Ingested food plus stomach secretions

Mucus

- surface and neck mucous cells

- Viscous and alkaline

- Protects from acidic chyme and enzyme pepsin

- Irritation of stomach mucosa causes greater mucus

Intrinsic factor

- parietal cells.

- Binds with vitamin B12 and helps it to be absorbed.

- B12 necessary for DNA synthesis

HCl

- parietal cells

- Kills bacteria

- Stops carbohydrate digestion by inactivating salivary amylase

- Denatures proteins

- Helps convert pepsinogen to pepsin

Pepsinogen

chief cells. Packaged in zymogen granules released by exocytosis. Pepsin catalyzes breaking of covalent bonds in proteins.

G-cells

- secrete the hormone gastrin which stimulates HCl secretion from parietal cells

Neural and hormonal mechanisms

- regulate the release of gastric juice

- Stimulatory and inhibitory events occur in three phases

CEPHALIC PHASE

- prior to food entry

The taste or smell of food, tactile sensations of food in the mouth, or even thoughts of food stimulate the medulla oblongata.

GASTRIC PHASE

- once food enters the stomach

1. Distention of the stomach activates the parasympathetic reflex. Action potentials are carried by the vagus nerves to the medulla oblongata.

2. Medulla oblongata stimulates further secretions of the stomach.

3. Distention also stimulates local reflexes that amplify stomach secretions.

INTESTINAL PHASE

- as partially digested food enters the duodenum

● Chyme in the duodenum with a pH less than 2 or containing lipids inhibits gastric

secretions by three mechanisms

1. Sensory input to the medulla from the duodenum inhibits the motor input from the

medulla to the stomach. Stops secretion of pepsin and HCl.

2. Local reflexes inhibit gastric secretion

Parasympathetic action potentials

are carried by the vagus nerves to the stomach

Preganglionic parasympathetic vagus nerve fibers

stimulate postganglionic neurons in the enteric plexus of the stomach

Postganglionic neurons

stimulate secretion by parietal and chief cells (HCl and pepsin) and stimulate the secretion of the hormone gastrin

Gastrin

is carried through the circulation back to the stomach where it stimulates further secretion of HCl and pepsin

Secretin, gastric inhibitory polypeptide, and cholecystokinin

produced by the duodenum inhibit gastric secretions in the stomach

- The neural enterogastric reflex

- Hormonal (enterogastrone) mechanisms

GASTRIC EMPTYING is regulated by?

Carbohydrate-rich chyme

- quickly moves through the duodenum

Fat-laden chyme

- is digested more slowly causing food to remain in the stomach longer

Connective tissue septa branch from the porta into the interior

- Divides liver into lobules

- Nerves, vessels and ducts follow the septa

Lobules

- portal triad at each corner

- Three vessels

- Central vein in center of lobule

- hepatic portal vein,

- hepatic artery

- bile duct (hepatic duct in diagram)

Three vessels of lobules:

Central veins

unite to form hepatic veins that exit liver and empty into inferior vena cava

Hepatic cords

- radiate out from the central vein.

- Composed of hepatocytes

Hepatic sinusoids

- Between cords, lined with endothelial cells and hepatic phagocytic (Kupffer) cells

Bile canaliculus

- between cells within cords

Bile production

- 600-1000 mL/day.

- Bile salts (bilirubin), cholesterol, fats, fat-soluble hormones, lecithin

- Neutralizes and dilutes stomach acid

- Bile salts emulsify fats.

- Most are reabsorbed in the ileum.

- Secretin (from the duodenum) stimulates bile secretions, increasing water and

bicarbonate ion content of the bile

Storage

- Glycogen, fat, vitamins, copper and iron.

- Hepatic portal blood comes to the liver from the small intestine.

Nutrient interconversion

- Amino acids to energy producing compounds

- Hydroxylation of vitamin D.

- Vitamin D then travels to kidney where it is hydroxylated again into its active form

Detoxification

- Hepatocytes remove ammonia and convert to urea

Phagocytosis

- Kupffer cells phagocytize worn-out and dying red and white blood cells, some bacteria

Synthesis

- Albumins, fibrinogen, globulins, heparin, clotting factors

BILE

- A yellow-green, alkaline solution containing

- bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes

Bile salts

are cholesterol derivatives that:

- Emulsify fat

- Facilitate fat and cholesterol absorption

- Help solubilize cholesterol

Enterohepatic circulation

recycles bile salts

bilirubin

what is the chief bile pigment that is a waste product of heme?

Endocrine

- pancreatic islets.

- Produce insulin, glucose, and somatostatin

Exocrine

- groups of acini (grape-like cluster) form lobules separated by septa.

hepatopancreatic ampulla; hepatopancreatic ampullar sphincter

Pancreatic duct joins common bile duct and enters duodenum at? _____ that is controlled by? ______

PANCREATIC JUICE

- Aqueous. Produced by columnar epithelium lining smaller ducts. Na+, K+, HCO3-, water.

- Bicarbonate lowers pH inhibiting pepsin and providing proper pH for enzymes

Enterokinase

from the duodenal mucosa and attached to the brush border activates trypsinogen to trypsin

Digestion

- Breakdown of food molecules for absorption into circulation

Mechanical digestion

- breaks large food particles to small

Chemical digestion

- breaking of covalent bonds by digestive enzymes

Lipids

- Include triglycerides, phospholipids, steroids, fat-soluble vitamins

- Bile salts surround fatty acid and glycerol to form micelles

Cholesterol

- 15% ingested

- 85% manufactured in liver and intestinal mucosa

lipids

have lower density than water

proteins

have higher density than water

Chylomicrons

- 99% lipid

- 1% protein (extremely low density)

- enter lymph

VLDL

- 92% lipid

- 8% protein

- Form in which lipids leave the liver

LDL

- 75% lipid

- 25% protein

- Transports cholesterol to cells

HDL

- 55% lipid

- 45% protein

- Transports excess cholesterol from cells to liver

proteins

- Pepsin breaks proteins into smaller polypeptide chains

- Proteolytic enzymes produce small peptide chains

- Dipeptides, tripeptides, amino acids

- After absorption, amino acids are are carried through the hepatic portal vein to the liver.

water

- can move in either direction across wall of small intestine depending on osmotic gradients

pancreatic ribonucleases and deoxynuclease

what enzymes are used in chemical digestion in the small intestine?