LECTURE 22 : RENAL PHYSIOLOGY PART 2 & DIGESTIVE SYSTEM

know the different components of the R.A.A.S. system?

• know the cells type responsible for secreting renin? what triggers the release of renin

• what do the macula densa cells detect?

• where is angiotensinogen produced? what enzyme is responsible for converting angiotensinogen to angiotensin 1

• what enzyme converts angiotensin 1 —→ angiotensin 2?

• know the different targets of angiotensin 2

RENIN ANGIOTENSIN-ALDOSTERONE SYSTEM (R.A.A.S)

-juxtaglomerular cells found surrounding the afferent arteriole produce and secrete the enzyme renin

• triggers:

  • low blood pressure

  • sympathetic innervation

  • input from the macula densa cells

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MACULA DENSA CELLS

-found within distal convoluted tubule

-detect Na+ concentration

• low Na+ —→ secretion of prostaglandins —→ bind to juxtaglomerular cells triggering the release of renin

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ANGIOTENSINOGEN CONVERSION

-angiotensinogen: hormone that is produced and released by the liver

• inactive

-renin converts angiotensinogen to angiotensin 1

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ANGIOTENSIN 2

-angiotensin converting enzyme converts angiotensin 1 —→ angiotensin 2

• active form of angiotensin

-angiotensin 2 targets:

• smooth muscle in blood vessels —→ vasoconstriction —→ increased resistance in vessel

• kidneys —→ increases water reabsorption —→ increases stroke volume

• pituitary gland —→ secretion of antidiuretic hormone

• adrenal glands —→ increased aldosterone secretion

how do physical and chemical digestion differ? what is involved in each process?

PHYSICAL/MECHANICAL DIGESTION

-mechanical break down of food into smaller particles

• increases the surface area of the food particles —→ allows for the chemical digestion of food

-occurs throughout the oral cavity, stomach, and small intestine

-involves the teeth, tongue, saliva, muscle, or bile

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CHEMICAL DIGESTION

-chemical break down of macromolecule polymers into monomers

• occurs via digestive enzymes

• ex. break down of starch into disaccharides and monosaccharides

-occurs within the oral cavity, stomach, small intestine, and large intestine

what enzyme is responsible for the chemical digestion of carbohydrates?

starch and glycogen (carbohydrates) require initial breakdown via amylase to maltose. maltose is then broken down via maltase to its glucose monomers

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-at the oral cavity chemical digestion: salivary amylase breaks down starch into the disaccharide maltose

how do absorption and secretion differ? know which way molecules are moving in each process.

ABSORPTION

-movement of molecules from the lumen of the GI tract, through the epithelial lining, and into the surrounding vessels or lacteals

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SECRETION

-occurs throughout the GI tract

-includes:

• acids

• enzymes —→ break macromolecules down

• buffers —→ neutralize

  • bicarbonate

• mucus

• saliva

what is deglutition?

• what mass is swallowed during deglutition?

• where does this mass travel to?

deglutition = swallowing

food forms a mass in the oral cavity that can easily be swallowed

• called the bolus

food enters the esophagus following swallowing —→ stomach

what paste like substances is formed in the stomach? what is a part of this paste?

-contains 3 layers of smooth muscle to churn and mix arriving bolus with acids and enzymes

• bolus is churned into a paste like substance called chyme

  • form of mechanical digestion

-cardiac sphincter prevents the backflow of acidic stomach contents into esophagus

what molecules are absorbed in the stomach?

-thin layer of epithelium allows for easy absorption of water and certain drugs

what is the pH of the stomach? why is this pH required for digestion?

HCl lowers the pH of the stomach to 1

• gastric pepsin works optimally at a pH of ~1

• allows for the digestion of proteins

goblet cells in simple columnar epithelial lining produce mucus —→ prevents digestion of stomach walls

what are gastric pits? know the 3 major cell types found in these pits.

• what do each of these cells produce? how do these secretions aid in digestion?

indentations/pits within the stomach —→ contains cells that aid in digestion

contain 3 major cell types:

• parietal cells

• enterochromaffin-like cells (ECL)

• chief cells

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Parietal cells —→ produce HCl acid that creates the acidic environment of the stomach

Enterochromaffin-like cells (ECL) —→ produce and release histamines —→ increases mucus production when HCl is being secreted

Chief cells —→ produce pepsinogen —→ converted to its active form of pepsin when HCl is present

how is HCl formed in the lumen of the stomach?

by parietal cells!!

carbonic anhydrase

H2CO3 dissociates into HCO3- and H+

• H+ ions are pumped into the lumen

HCO3- is pumped out of the cell via the HCO3-/Cl- cotransporter

• Cl- pumped into the cell

Cl- diffuses down its concentration gradient into the lumen

• becomes HCl due to presence of H+ ions

what stimulates the production of HCl in the stomach

HCl production is stimulated by:

• gastrin

  • hormone released by the stomach following “stretching” of the stomach

• histamines

• acetylcholine release form the parasympathetic nervous system

HCl production is inhibited by:

• somatostatin

  • hormone released by the hypothalamus

what is the inactive for of pepsin? how does this form become “active”?

pepsinogen is secreted by the chief cells

• inactive —→ protein digestion does not occur

HCl allows for cleavage of pepsinogen forming pepsin

• active —→ protein digestion occurs

what secretions and materials does the duodenum receive?

duodenum: proximal

• receives chyme from the stomach

receives secretions from the liver/gallbladder and the pancreas

• liver/gallbladder —→ bile

• pancreas —→ enzymes, messengers, and buffers

  • buffer neutralizes acid chyme arriving from the stomach

jejunum: site where the most absorption occurs

ileum: distal

• carries chyme to the large intestine

• same absorption

• reabsorption of bile and enzymes

know what macromolecules trypsin, chymotrypsin, carboxypeptidase, lipase, ribonuclease, and deoxyribonuclease, and amylase break down

TRYPAIN, CHYMOTRYPSIN, ELASTASE

• proteins

CARBOXYPEPTIDASE

• proteins

RIBONUCLEASE, DEOXYRIBONUCLEASE

• nucleic acids

AMYLASE

• polysaccharides

LIPASE

• triglycerides

what structures are responsible for increasing the surface area within the small intestine? why is this important?

small intestine contains circular folds called plicae with villi

• increase the surface area of the small intestine —→ increase absorption

lumen lined with layer of simple columnar epithelium

• epithelial cells contain microvilli —→ increases surface area of the cell

what two hormones are secreted by the enteroendocrine cells? what is the effect of each?

enteroendocrine cells secrete hormones that regulate absorption, reabsorption, and movement through the GI tract

includes:

• somatostatin —→ decrease motility and secretion of digestive enzymes

• ghrelin —→ signals to the brain that you are hungry

how are monosaccharides absorbed within the small intestine? (2 ways)

-pancreatic amylase breaks down polysaccharides to maltose (disaccharide)

-enzymes within the brush border break down maltose and other disaccharides to corresponding monosaccharides

-GLUT transporters pump fructose from the lumen into the epithelial cell

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-glucose and galactose are pumped into the cell via the Na+/Glucose cotransporter SGLT

• Na+ concentration is established by Na+/K+ ATPase in the basolateral

membrane

-GLUT transporters in the basolateral membrane pump glucose, fructose, and galactose into the interstitial fluid —→ moves into the blood stream

how are amino acids absorbed within the small intestine?

-pancreatic proteases and peptidases break down proteins into smaller peptide fragments or into amino acids

-smaller peptides are broken down by brush border enzymes into amino acids

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-amino acids are pumped into the cell via Na+/Amino acids cotrasnporters

• Na+ gradient is established by Na+/K+ ATPase

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-small peptides are moved into the cell via peptide/H+ cotransporters

• peptides must be broken down into amino acids inside the cell

!!!for all of these they end with —→ -amino acids transporters pump amino acids through basolateral membrane into the surrounding interstitial fluid —→ taken up into the blood stream

how are fats/lipds globules digested in the small intestine?

• know the enzymes and secretions involved.

• which form of digestion is this?

-triglycerides are hydrophobic —→ form large fat globules/aggregates in the small intestine

-bile produced by the liver emulsifies fat inside the small intestine (emulsification; mechanical digestion)

• forms smaller droplets

-increased surface area allows triglycerides to come in contact with lipase

-lipase breaks down triglycerides —→ 1 monoglyceride + 2 fatty acids (hydrolysis; chemical digestion)

• monoglycerides continue on through GI tract

• fatty acids are absorbed —→ lacteals

what is absorbed in the large intestine? what molecules are digested in the large intestine?

LARGE INTESTINE FUNCTIONS

reabsorption/absorption of water from the GI tract

absorption of water-soluble vitamins and minerals

breakdown of cellulose and chitin

absorption of ion

• Na+, K+, etc.

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DIGESTION IN LARGE INTESTINE

bacteria within the large intestine digest some undigested fiber

• converted to short-chain fatty acids for absorption

• results in slightly acidic environment that is neutralized by HCO_3-

-bacteria are able to digest and convert molecules to absorbable forms of vitamins B and K

what molecules are secreted by the large intestine?

-digestive enzymes are not secreted in the large intestine

-mucus is secreted forming a protective barrier

-HCO_3- and K+ are secreted into the lumen

how are Na+ and H2O absorbed in the large intestine?

Na+ and H2O are absorbed through the intestinal lumen

• Na+ is actively moved into the epithelial cells lining the lumen

  • creates a concentration gradient that allows for the absorption of water