Secretions and Absorption Flashcards
(39 cards)
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)
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
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
4
Q
Parietal cells
Location
A
Stomach body
5
Q
Chief cells
Location
A
Stomach body
6
Q
G cells
Location
A
Stomach antrum
7
Q
Mucus cells
Location
A
Stomach antrum
8
Q
D cells
Location
A
Stomach antrum and pancreas
9
Q
I cells
Location
A
Duodenum
10
Q
S cells
Location
A
Duodenum
11
Q
K cells
Location
A
Duodenum
12
Q
Parietal cells
Secretions
A
- HCl
- Intrinsic factor
13
Q
Chief cells
Secretions
A
Pepsinogin
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
15
Q
Mucus cells
Secretions
A
Mucus
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
Bile
| Functions
- Digestion and absorption of lipids and fat-soluble vitamins
- Cholesterol excretion (body's only mean of eliminating cholesterol)
- Antimicrobial activity (via membrane disruption)
26
Bilirubin
| Metabolism
- 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
Bile secretion
| Control
- 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
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Intestinal secretion
(Mechanism)
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- 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
Intestinal secretion
| Control
It is induced by VIP and ACh
30
Carbohydrates Absorption
| Luminal membrane
- 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
Carbohydrates Absorption
| Basal membrane
- 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
Protein Absorption
| Luminal membrane
- 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
Protein Absorption
| Basal membrane
All by simple diffusion
34
Lipids Absorption
| Luminal membrane
- 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
Lipids Absorption
| Basal membrane
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
Duodenum
| Water and Electrolytes Absorption
- 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
Jejunum
| Water and Electrolytes Absorption
Absorption of all water, Na+, K+, Cl- and HCO3- like the PCT of kidney
38
Ileum
| Water and Electrolytes Absorption
- 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
Colon
| Water and Electrolytes Absorption
- 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)
