Physiology: DIGESTION

What is the digestive system?

a network of organs that mediate the injestion and digestion of food as well as absorption of nutrients and elimination of waste

What is the alimentary canal?

organs that transport food. They are digestion, absorption, and elimination sites.

• food physically passes through these organs

Ex.

• oral cavity

• esophagus

• pharynx

• stomach

• small intestine

• large intestine

• rectum

What are accessory organs?

orangs that aid in digestion and absorption

food does not directly pass through.

direct contact with food examples:

• teeth

• tongue

indirect contact with food examples:

• salivary glands

• livier

• gall bladder

• pancreas

What is mechanical digestion?

mediated by tearing, grinding, and crushing

ex. chewing by teeth and tongue

What is chemical digestion?

mediated by chemicals (acids, enzymes, etc.) secreted by accessory organs

ex. secretion of enzymes in saliva

Digestion in the oral cavity?

Mechanical: mastication (teeth and tongue)

Chemical: mediated by saliva (99% water, 1% numerous dissolved eliments)

• lubricate food to help with swallowing

• delivers enzymes to breakdown fats (lingual lipase) and breakdown of carbohydrates (salivary amylase)

What is bolus?

food difested at the oral cavity

What are acinar cells?

create and secrete saliva in response to external stimuli

How does parasympathetic stimulation affect salivation

stimulates salivation

1. ion secretion into the duct lumen

2. h2o diffusion into the lumen

3. ion reabsorption to maintain hypotonicity

(cell secrete ions, water follows, ions are reabsorbed)

How does sympathetic stimulation affect salivation?

inhibits salivation

What is peristalsis?

movement. wave like sequence of smooth muscle contractions and relaxation

What are circular muscles? (peristalsis)

form upper and lower esophageal sphincters; contract behind bolus to move it forward

• wraps around tube like a belt, contracts to narrow tube, like a drawstring on a bag

What are longitudinal muscles? (peristalsis)

contracts to stablize esophagus

• runs along the length of the tube. when contract, shortens section of the tube

How does chemical digestion take place in the stomach?

Mediated by gastric juice: composed of HCL and digestive enzymes

What is pepsinogen? (chemical digestion in the stomach)

protease precursor secreted by chief cells

• inactive form of pepsin

  • pepsin: an enzyme that breaks down proteins

What secretes HCl? (chemical digestion in the stomach)

secreted by parietal cells

• weakens hydrogen bonds

• mediates conversion of pepsinogen to pepsin

• secretes in parts: hydrogen and chlorine

What is chyme? (chemical digestion in the stomach)

food mixed w/ gastric juice

What do goblet cells secrete? (stomach)

mucus that protects the stomach lining

(Goblins Prefer Chocolate Donuts Glazed)

What do parietal cells secrete? (stomach)

Gastric acid (HCl)

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What do chief cells secrete? (stomach)

pepsinogen (protease precursor)

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What do D-cells secrete? (stomach)

somatostatin: inhibits acid secretion

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What do G-cells secrete? (stomach)

Gastrin: stimulates acid secretion

(Goblins Prefer Chocolate Donuts Glazed)

Steps of chemical digestion in the stomach

1. Chyme stimulates G cells which secrete gastrin

2. Gastrin stimulates parietal cells which secrete HCl. Gastrin indirectly promotes histamine production

3. HCl stimulates goblet cells which secrete mucus lining to protect stomach

4. Low pH promotes pepsinogen conversion to pepsin

How the stomach keeps making acid (HCl)

1. chloride swaps places with bicarbonate (passive), goes into stomach

2. H+ swaps places with K+ (active) at the lumen

3. Stomach keeps making bicarbonate to keep both of these swaps going

What are ICC in stomach?

Interstitial Cells of Cajal

• pacemaker cells of digestive organs, stomach and intestines

How does mechanical digestion in the stomach occur?

gastric peristalsis promotes mixing and propulsion of chyme

1. Stomach pacemaker cells (ICC) depolarize

2. signal moves downward in a slow wave passing from one group of ICC cells to the next

3. smooth muscles contract to squeeze and mix food

4. depolarization stops at the plyoric sphincter, the muscle that controles food leaving the stomach

5. a new depolarization wave starts

When chyme forms…

chemoreceptors and stretch receptors initiate a positive feedback mechanism that further stimulates

• cells of the gastric pits

  • gastrin pathway enhances positive feedback mechanism

  • ICC cells, promoting high frequency depolarization

When chyme enters the duodenum

concentration of digested molecules increases triggering the release of

• cholecystokinin (CKK)

• gastric inhibitory peptid (GIP)

Neutralization of chyme pH triggers the release of…

secretin

What is the duodenum?

first part of the small intestine, right after stomach

• receives chyme from the stomach

• mixes it with digestive juices from the pancreas (enzyme) and liver/gallbladder (bile)

• neutralizes stomach acid so it doesn’t harm intestines

• begins nutrient absorption

What is CCK (Cholecystokinin)?

made in the duodenum

• tells gallbladder to release bile (to help digest fats)

• tells pancreas to release enzymes (help digest fats and proteins

• slows down stomach emptying so digestion isnt rushed

What is GIP (Gastric Inhibitory peptide)?

made in the duodenum

• tells stomach to slow down acid and movement

• tells the pancreas to release insulin to manage rising blood sugar

What is secretin?

made by the duodenum

• tells pancreas to release bicarbonate to neutralize stomahc acid

• slows down stomach activity

• helps liver make more bile

What is segmentation? (mechanical digestion in the small intestine)

moves chyme back and forth to mix it

• ring like smooth muscle contractions

• not as precise as esophageal peristalsis.

• 3-5 hours to clear contents

What is chemical digestion in the small intestine mediated by?

pancreatic juice and bile

What is pancreatic juice?

secreted by the pancreas

• composed to HCO3- and numerous digestive enzymes

  • bicarbonate is a buffer against acidity to chyme

What is bile?

secreted by the liver

stored in the gall bladder

• hypotonic solution of water, ions, and organic substances like bile salts (help breakdown fats)

How does absorption occur in the stomach?

minimal absorption. only absorb:

• alcohol

• aspirin

• penicillin (certain types)

the effect of substances absorbed at the stomach may be apparent before those that are absorbed at the small intestine

How does absorption occur at the small intestine?

primary state of nutrient absorption

• peptides and amino acids

• glucose and fructose

• fats

• water

• minerals and vitamisn

absorption of each molecule is mediated by a different physiological mechanism

What optimizes nutrient absorption in the small intestine?

• circular folds, villi, and microvilli increase surface area for absorption

• length of the small intestine prolongs exposure to the villus

• villus circulation proximity to capillaries and lacteals facilitates nutrient absorption

What are glucose transports (GLUT) (glucose absorption)

family of substrate specific proteins that bind to glucose to mediate its transport

What is SGLT1? (glucose transporter, glucose absorption)

transports glucose/Na+ from the intestinal lumen into the enterocyte (small intestine cell)

• Na gradient is maintained by Na+ / K+ pump (secondary active transport)

What is GLUT 2

transports glucose from enterocyte to the bloodstream

How is the pancreas both an exocrine and endocrine gland?

exocrine - pancreatic juice (has digestive enzymes

• exo = exit → sends substances out through ducts to help difest food

endocrine - insulin and glucagon secretion

• endo = in → sends hormones into blood to control body functions

• increase in blood glucose → triggers beta cells → insulin release

• decrease in blood glucose → triggers alpha cells → glucagon release

How does the pancreas promote glucose uptake?

beta cells → insulin released → triggers GLUT4 → glucose uptake

How is glucose produced?

low glucose consentration

• inhibits beta cells

• alpha cells → glucagon

glucagon activates adenylate cyclase. 2nd messengers cAMP and Ca+2 inhibit glycolysis and glycogenesis

• inhibit glycolysis, slow down using up glucose so that blood sugar gets raised

• inhibit glycogenesis. prevents glucose from being stored, encouraging it to stay in blood stream instead

How does glucose affect insulin?

glucagon does NOT DIRECTLY inhibit insulin

1. surge in glucagon → somatostatin secreted by D cells

2. somatostatin inhibit beta cells, decrease insulin production

What is hyperglycemia?

excess glucose damages blood vessels around the body

What is Type 1 diabetes

immune system destroys beta cells, trouble producing insulin (autoimmune condition).

• rely on supplemental insulin

What is Type 2 diabetes?

body cells are not as responsive to insulin

• reversed through lifestyle changes

• may require supplemental insulin if untreated

How are amino acids absorbed?

• amino acids passively transported into the enterocyte w/ Na+

• amino acids passively diffuse into the bloodstream via protein carriers

Na+ gradient is maintaine dby Na+/K+ pump (secondary active transport)

What does exogenous mean?

comes from outside the body

What does endogenous mean?

made inside the body

What are emulsion droplets? (fat emulsification and transport)

fat droplet fragment from emulsifaction by bile

What are micelle? (fat emulsification and transport)

aggregation of lipids and bile acids. hydrophobic portions are isolated from aqueous environment

• tiny fat transport bubbles that make fat more soluble in watery environments for easier transport

What are lipoproteins? (fat emulsification and transport)

family of lipid transport complexes

What are chylomicrons? (fat emulsification and transport)

lipoprotein formed by dietary lipids in smooth ER

• carry dietary fat from intestines → body

How are lipids digested and absorbed?

1. bile salts partially cover fat droplets

2. pancreatic lipase hydrolyzes (breaks up) triglycerides (fats) into smaller parts:

   • free floating acids (FFA)

   • glycerol

3. bile salts cover the FFA, cholesterols, and fat soluble vitamins → micelles formed

4. Micelles carry lipids into enterocyte (intestinal cells)

   • diffused by incorporating into the membrane (collision), simple diffusion, carrier mediated transport

Exogenous lipoprotein pathway steps

1. smooth ER turns FFA back into triglycerides and cholesterols

2. triglycerides and cholesterols are backed into chylomicrons (transport protein for fats)

3. chylomicrons enter the lymphatic system at lacteals (tiny lymph vessels found in villi of small intestine)

Why is cholesterol necessary?

• biosynthesis of membranes, hormones, and bile

• energy metabolism

what is reverse cholesterol transport?

excess cholesterol is transported to the liver and eliminated

What is the size of lipoprotein?

size varies according to the lipid concentration and composition.

Exogenous lipoprotein?

from diet

• chylomicron

Endogenous liporotein?

lipids in tissues and bloodstream

• very low density lipoprotein (VLDL)

• low density lipoprotein (LDL)

• high density lipoprotein (HDL)

“good vs. bad” cholesterol…

does NOT reference cholesterol but the apolipoprotein responsible for transporting cholesterol

LDL (“bad”)

job: deliver cholesterol from liver → body cells

• stays in circulation longer due to low affinity for LDL receptors to target tissues

• can bind and penetrate arterial walls

• can trigger immune response due to higher susceptibility to oxidation

What happens if there is too much LDL in blood?

LDL deposits cholesterol in blood vessel walls → plaque buildup → atherosclerosis (narrowing of arteries) → increased risk of heart attacks and stroke

What is cholesterol?

type of lipid that serves as raw material for hormones

HDL (“good”)

job: deliver cholesterol from body cells → liver

• high affinity to cholesterol, it binds to plasma cholesterol and carries it to the liver

• reverse cholesterol transport

• can prevent damage to arteries through anti-inflammatory, anti- thrombotic, anti-apoptotic properties

• exercise and consumption of unsaturated fats promotes HDL formation /metabolism. Tabacco inhibits it

What happens after absorption?

• triglycerides stored in adipose tissue or used by cells.

• amino acids used by cells to make proteins needed

• glucose used by cells or stored as glycogen

• excess of any of these can be stored as fat