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GIT > Secretions and Absorption > Flashcards

Secretions and Absorption Flashcards

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1
Q 
Salivary Glands
(General notes)
A
  • Parotid gland secretions are entirely serous
  • Submandibualr and sublingual glands secretions are mixed serous and mucus
  • All are almost entirely under parasympathetic control (VII and IX cranial nerves)
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2
Q 
Salivary Glands
(Formation mechanism)
A
  • Initiated by Cl- pumped in to the acinus which will pull Na+ and water into acinus too
  • In the duct Na+ and Cl- will be pumped out and K+ and HCO3- will be pumped in
  • The result will be a hypotonic solution rich in K+ and HCO3- that dilutes and neutralizes
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3
Q

Saliva

Other components

A
  • Alpha-amylase (ptyalin): secreted in active form and begins the digestion of carbohydrates
  • Mucus and glycoproteins
  • Immunoglobulins and lysozymes
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4
Q

Parietal cells

Location

A

Stomach body

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5
Q

Chief cells

Location

A

Stomach body

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6
Q

G cells

Location

A

Stomach antrum

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7
Q

Mucus cells

Location

A

Stomach antrum

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8
Q

D cells

Location

A

Stomach antrum and pancreas

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9
Q

I cells

Location

A

Duodenum

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10
Q

S cells

Location

A

Duodenum

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11
Q

K cells

Location

A

Duodenum

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12
Q

Parietal cells

Secretions

A
  • HCl

- Intrinsic factor

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13
Q

Chief cells

Secretions

A

Pepsinogin

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14
Q

G cells

Secretions

A

Gastrin (that will directly and indirectly [through stimulation of enterochromaffin-like (ECL) cells that will secrete histamine] stimulates parietal cells

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15
Q

Mucus cells

Secretions

A

Mucus

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16
Q

D cells

Secretions

A

Somatostatin

17
Q

I cells

Secretions

A

Cholecystokinin (CCK)

18
Q

S cells

Secretions

A

Secretin

19
Q

K cells

Secretions

A

Glucose-dependent insulinotropic peptide (GIP)

20
Q

Acid Secretion

Control

A
  • Stimulated by:
  • ACh that is stimulated by smell/sight of food and reflexly in response to stomach distension (vagovagal reflex)
  • Locally released histamine (stimulated by ACh and gastrin)
  • Gastrin (stimulated by GRP)
  • Inhibited by somatostatin that inhibits gastrin secretion (feedback regulation)
21
Q 
Acid Secretion
(Cellular mechanism)
A
  • H+ is generated inside parietal cells from CO2 (cellular and from arterial blood) by carbonic anhydrase
  • H+ is pumped into stomach by H+/K+ antiport (the target of PPIs)
  • HCO3- generated during H+ formation will be pumbed out through the basolateral membrane into venous blood by HCO3-/Cl- antiport (alkaline venous blood)
  • K+ and Cl- will diffuse through apical membrane into stomach
  • Stomach secretion will be rich in H+, K+ and Cl- but low in Na+ (vomiting will lead to metabolic alkalosis and hypokalemia [mainly due to alkalosis effect on the kidney])
22
Q

Pancreatic Secretions

Control

A
  • Vagovagal reflex and sympathetic control are of minor influence
  • Most control is via secretin (stimulates aqueous component by increasing cAMP [Gs] and the Cl- channel CFTR) and CCK (stimulates the enzymatic component)
23
Q

Pancreatic Secretions

Enzymes

A
  • Alpha-amylase (cannot break beta linkages of cellulose)
  • Pancreatic lipase (glycerol ester lipase) that needs a colipase (which displaces bile salt from surface of micelles allowing the lipase to work)
  • Cholesterol esterase (sterol lipase)
  • Pancreatic proteases (the only ones that secreted in inactive form and called zymogens) include trypsinogen, chymotrypsinogen and procarboxypeptidase
  • Trypsinogen is activated by enterokinase (that is secreted by lining of small intestine [not brush boarder]) to trypsin (endopeptidase)
  • Chymotrypsinogen is activated by trypsin to chymotrypsin (endopeptidase)
  • Procarboxypeptidase is activated by trypsin to carboxypeptidase (exopeptidase)
  • Trypsin also activates other trypsinogens (exerting a positive feedback loop)
24
Q

Bile

Components

A
  • Bile salts (conjugated to glycine or taurine to make them water soluble); they form the micelles:
  • Cholic acid (primary) —> deoxycholic acid (secondary)
  • Chenodeoxycholic acid (primary) —> lithocholic acid (secondary)
  • Phospholipids
  • Cholesterol
  • Bile pigments (particularly bilirubin)
  • Water and ions (especially HCO3-)
25
Q

Bile

Functions

A
  • Digestion and absorption of lipids and fat-soluble vitamins
  • Cholesterol excretion (body’s only mean of eliminating cholesterol)
  • Antimicrobial activity (via membrane disruption)
26
Q

Bilirubin

Metabolism

A
  • Heme is metabolized by heme oxygenase to biliverdin, which is subsequently reduced to bilirubin
  • Indirect (unconjugated) bilirubin is transferred to the liver by albumin
  • In liver, indirect bilirubin is conjugated with glucuronate by UDP-glucuronosyl-transferase and then excreted in bile
  • It is then converted by gut bacteria to urobilinogen
  • 80% of urobilinogen will be excreted in feces as stercobilin (brown color of stool)
  • 20% of urobilinogen will reabsorbed with 10% of that will be excreted by kidney as urobilin (yellow color of urine) and 90% will go back to liver via the enterohepatic circulation
27
Q

Bile secretion

Control

A
  • Secretin causes secretion of HCO3- and fluid into bile canalicular ducts
  • Bile salts secretion by hepatocytes is directly proportional to hepatic portal vein concentration of bile salts
  • CCK causes gallbladder contraction and sphincter of Oddi relaxation
28
Q 
Intestinal secretion
(Mechanism)
A
  • Basolateral membrane has Na+/K+/2Cl- (secondary active transport) that transport the Cl- (mainly) inside the cell
  • The elevated intracellular Cl- and -ve intracellular potential drive the diffusion of Cl- through channels on the apical membrane
  • The opening of apical Cl- channel is increased by increase in cytosolic Ca++ and/or cAMP (also increased by cholera toxin through Gs). cAMP-dependent Ca++ channels are CFTR channnels
29
Q

Intestinal secretion

Control

A

It is induced by VIP and ACh

30
Q

Carbohydrates Absorption

Luminal membrane

A
  • Glucose and galactose are absorbed via the same carrier (secondary active transport linked to sodium [SGLT1])
  • Fructose is absorbed independently by facilitated diffusion (GLUT-5)
31
Q

Carbohydrates Absorption

Basal membrane

A
  • All of them are absorbed passively mainly via facilitated diffusion (GLUT-2)
  • D-xylose absorption test is used to distinguish GI mucosal damage from other causes of malabsorption
32
Q

Protein Absorption

Luminal membrane

A
  • Amino acids by secondary active diffusion linked to sodium (the transporters are selective for the amino acids [transporter of tryptophan is deficient in Hartnup’s disease])
  • Small peptides (di- and tri-peptides) powered by Na+/H+ antiporter
33
Q

Protein Absorption

Basal membrane

A

All by simple diffusion

34
Q

Lipids Absorption

Luminal membrane

A
  • By micelles which are diffused to the brush boarder of intestine; the content of micelles are:
  • Fatty-acids (long chain)
  • 2-monoglyceride
  • Cholesterol
  • Lysolecithin
  • Vitamins A, D, E and K
  • Bile salts, which stabilize the micelles
  • Short chain fatty acids are directly absorbed into bloodstream by simple diffusion
  • Bile salts are also actively reabsorbed in distal ileum
35
Q

Lipids Absorption

Basal membrane

A

Triglyceride is resynthesized and form lipid droplets (chylomicrons with apolipoprotein B-48) which leave the intestine via the lymphatic circulation (lacteals). Then they enter the bloodstream via the thoracic duct

36
Q

Duodenum

Water and Electrolytes Absorption

A
  • Water is moving into lumen because the fluid entering duodenum is usually hypertonic
  • Then there will be absorption of water, Na+ and other electrolytes
  • Ca++ absorption is enhanced by calbindin (its synthesis is induced by calcitriol [active vitamin D])
  • Fe++ absorption is enhanced by ferritin
  • Most of water-soluble vitamins are absorbed by Na+ symporter
37
Q

Jejunum

Water and Electrolytes Absorption

A

Absorption of all water, Na+, K+, Cl- and HCO3- like the PCT of kidney

38
Q

Ileum

Water and Electrolytes Absorption

A
  • Absorption of water, Na+, K+ and Cl-
  • Excretion of HCO3- by HCO3-/Cl- antiporter
  • In distal ileum and just in distal ileum reabsorption of bile salts and absorption of intrinsic factor with vitamin B12 are done
39
Q

Colon

Water and Electrolytes Absorption

A
  • Limited absorption of water, Na+ and Cl- mainly in ascending and transverse colon
  • Excretion of HCO3- by HCO3-/Cl- antiporter
  • It can be a target for aldosterone in which case there will be increase in Na+ and water reabsorption and excretion of K+ (so diarrhea will lead to metabolic acidosis and hypokalemia)

GIT (3 decks)

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