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Gastrointestinal 1 > Physiology of the Stomach, Absorption > Flashcards

Physiology of the Stomach, Absorption Flashcards

1
Q

What are the functions of the stomach?

A
  • storage of food

- mixing of food with gastric secretions (forming chyme)

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2
Q

What reflex occurs when food stretches the stomach?

A

vagovagal reflex from stomach to brain stem and back (reducing tone in stomach muscular wall so it bulges progressively outward.

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3
Q

How is food propelled through the stomach?

A
  • weak peristaltic CONSTRICTOR WAVES (mixing waves initiated by gut wall) begin in the mid to upper portions of the stomach wall and move toward the antrum every 15-20 sec (distal).
  • as waves progress into antrum, they become more intense.
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4
Q

How is food mixed in the stomach?

A

constrictor rings. As peristaltic wave approaches pylorus, pyloric muscle contracts (impeding emptying), squeezing most antral contents towards stomach body.

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5
Q

What are Hunger contractions?

A

rhythmical peristaltic contractions in the stomach body that occur when the stomach is empty for several hours (hunger pangs).
*increased by low levels of blood sugar.

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6
Q

What promotes stomach emptying of chyme?

A

intense peristaltic contractions in stomach antrum (pyloric pump)= very tight ring-like constrictions. This is opposed by varying degrees of resistance at the pylorus.

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7
Q

What is the pyloric sphincter?

A

circular wall muscle of pylorus, separating stomach from duodenum, which remains sightly tonically contracted most of the time.
*it is usually opened enough for water/fluids to empty from stomach into duodenum.

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8
Q

What controls the degree of pyloric constriction?

A

nervous and humoral reflex signals

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9
Q

What regulates stomach emptying?

A
  • signals from stomach and duodenum (duodenum more potent) along with feedback inhibitory mechanisms, which work together to slow the rate of emptying (when there is too much chyme in the duodenum (volume), or when the chyme is too acidic, hypotonic, hypertonic, or irritating). This allows pancreatic and other secretions to neutralize the chyme.
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10
Q

What gastric factors promote emptying?

A
  • increased food VOLUME causes stretching of stomach wall, eliciting local myenteric reflexes that greatly increases the pyloric pump.
  • GASTRIN (mild stimulatory effect)
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11
Q

What 2 powerful duodenal factors mediate the inhibition of stomach emptying?

A
  1. ENTEROGASTRIC nervous reflexes from the duodenal wall via: 1) direct enteric nervous system, 2) through extrinsic nerves (from prevertebral sympatheic ganglia back through inhibitory sympathetics), and 3) through vagus to brainstem (inhibiting normal excitatory signals to the stomach).
  2. HORMONAL feedback via fats (which are slower to digest) extracting hormones from duodenal and jejunal epithelial cells. CCK enters the blood and blocks stomach motility induced by gastrin. Other hormones include gastric inhibitory peptide (glucose dependent peptide) and secretin (secreted in response to gastric acid in chyme).
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12
Q

What is the role of mucus secreting cells of the stomach?

A

secretion of viscous alkaline mucous to protect stomach lining and lubricate food for transport.

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13
Q

What are the 2 tubular glands of the stomach?

A
  1. oxyntic (gastric)

2. pyloric

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14
Q

** What are the oxyntic (gastric) tubular glands of the stomach?

A

composed of 3 cell types found in the body and fundus of stomach:

  1. mucus neck cells= secrete mucus.
  2. peptic (chief) cells= secrete pepsinogen.
  3. parietal (oxyntic) cells= secrete HCl and intrinsic factor.
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15
Q

What 3 factors stimulate gastric secretions?

A
  1. ACH
  2. GASTRIN
  3. HISTAMINE
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16
Q

*** What does ACh excite in the stomach?

A

secretion of:

  • PEPSINOGEN by peptic (chief) cells.
  • HCl by parietal cells.
  • MUCUS by mucosal cells.
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17
Q

*** What do gastrin and histamine (via H2 receptor) excite in the stomach?

A

strong secretion of HCl by parietal cells.

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18
Q

*** What happens to the pepsinogen, secreted by peptic (chief) cells, and what then does it do?

A
  • it is first cleaved by HCl to become active PEPSIN, which then cleaves proteins into peptides.
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19
Q

*** What is the role of intrinsic factor (secreted by parietal cells)?

A

essential for absorption of Vit B12 in ileum.

*B12 helps RBCs mature and stimulates bone marrow.

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20
Q

What is the mechanism of parietal cell HCl secretion?

A

H2O is first broken into OH- and H+ within the parietal cells. OH- combines with CO2 to form HCO3-, which will exit the parietal cell on the basolateral side to enter the blood in exchange for Cl-. The parietal cells contain large branching intracellular canaliculi into which H+ is secreted by an H+/K+ antiporter. Cl- continues to diffuse down it’s electrical gradient into the lumen of the canaliculus to combine with H+ forming HCl.
*To maintain the gradient Na+ will enter diffuse between the lumen and parietal cell, while being actively pumped out on the basolateral side in exchange for K+. K+ will then leak into the lumen passively due to electrochemical gradient.
Water will also diffuse passively form the extracellular space to the lumen of the stomach to create a watery consistency of HCl, NaCl, and KCl.

21
Q

What are enterochromaffin-like cells (ECL cells)?

A

lie deep in recesses of oxyntic glands and secrete HISTAMINE in response to gastrin (which is released from G cells when proteins enter the stomach) and other hormones secreted by stomach wall enteric nervous system.

22
Q

** What are the pyloric tubular glands of the stomach?

A
  • structurally similar to oxyntic glands, but few peptic (chief) cells and NO parietal cells.
  • instead they contain mostly MUCUS cells and G cells (secrete gastrin).
23
Q

What is the role of gastrin?

A

when protein-containg foods enter the antral end of the stomach, G cells release gastrin into the BLOOD to be transported to ECL cells in the oxyntic (gastric) glands, causing their release of histamine (which binds to H2 receptors on parietal cells), and subsequent parietal cell HCl secretion.

24
Q

What are some disorders of gastric H+ secretion?

A

gastric ulcer, duodenal ulcer, or Zollinger-Ellison syndrome

25
Q

What are the 3 phases of gastric secretion?

A
  1. cephalic phase
  2. gastric phase
  3. intestinal phase
26
Q

What is the cephalic phase of gastric secretion?

A
  • neurogenic signals that originate in the cerebral cortex and appetite centers of the amygdala and hypothalamus are activated by sight, smell, touch or taste that occurs before food enters the stomach. These signals are transmitted through dorsal motor nuclei of the VAGUS nerve to the stomach causing 30% of gastric secretion associated with a meal.
27
Q

What is the gastric phase of gastric secretion?

A
  • once food enters the stomach, it excites vagovagal reflexes, local enteric reflexes, and gastrin (60%) secretion.
28
Q

What is the intestinal phase of gastric secretion?

A
  • presence of food in upper duodenum will continue to cause stomach secretion of small amounts of gastric juice.
29
Q

What inhibits gastric secretion by other post-stomach intestinal factors?

A

distended small intestine, acid, protein degradation products, or irritation initiates REVERSE ENTEROGASTRIC reflex. This is transmitted through the myenteric, sympathetic, and parasympathetic (vagus) nervous system. This reflex also causes release of SECRETIN from the small intestine, which controls pancreatic secretion.
*GIP (increases insulin secretion from pancreas), vasoactive intestinal polypeptide, and somatostatin also have slight inhibitory effects.

30
Q

What are Brunner’s glands?

A

compound mucus glands of the duodenum (between pyloric sphincter and papilla of Vater), that secrete large amounts of alkaline mucus in response to tactile stimuli, vagal stimulation, or hormones (secretin).
*Note in individuals with heightened sympathetics, there will be increased inhibition of Brunner’s glands leading to enhanced susceptibility to peptic ulcers.

31
Q

What are the crypts of Lieberkuhn?

A

small pits between small intestinal villi that are covered by GOBLET cells and ENTEROCYTES (secrete water and electrolytes into crypts to be reabsorbed with chyme in adjacent cells).

32
Q

How is watery fluid secreted by the small intestinal enterocytes?

A

active secretion of Cl- ions and active reabsorption of bicarb ions causes electrical drag of Na+ ions through the membrane and into secreted fluid. Together they allow osmotic movement of water.

33
Q

What important enzymes do the enterocytes of the small intestine contain?

A
  • peptidases for splitting peptides into amino acids.
  • sucrase, maltase, isomaltase, and lactase for splitting disaccharides into monosaccharides.
  • lipase for splitting fats into glycerol and fatty acids.
34
Q

How much total fluid is absorbed by the GI tract daily?

A

8-9 liters!

85% by the small intestine

35
Q

What increases the surface area of the small intestine by nearly 1000 fold?

A

valvulae conniventes (folds of Kerckring), villi, and microvilli

36
Q

What is found at the tips of the microvilli?

A

actin filaments which extend from the epithelial cell body, causing rhythmical contraction of microvilli for continual movement of chyme.

37
Q

What is the vascular system of a microvilli?

A

has countercurrent mechanism for absorption along with a central lacteal for absorption into lymph.

38
Q

How is water transported in the small intestine?

A

via osmosis (transcellular and paracellular pathways)

39
Q

Why is sodium important for absorption of sugars and amino acids?

A

Na+ moves down its steep electrochemical gradient through Na+-glucose co-transporters, Na+-amino acid co-transporters, and Na+-H+ exchanger on the intestinal lumen side of the cells. Na+ is then actively transported through the basolateral walls in exchange for K+. Cl- also follows passively by electrochemical gradient.

40
Q

What does aldosterone do to sodium absorption (when dehydrated)?

A

greatly enhances it

41
Q

Since there is a ton of bicarb in the intestine (which came from the pancreas secretions), how is that reabsorbed?

A
  • H+ ions exchanged for Na+ on the gut lumen, will combine with bicarb to form carbonic acid. This dissociates to H2O and CO2. H2O remains with chyme and CO2 is absorbed into the blood.
42
Q

How are calcium, iron, potassium, magnesium, and phosphate absorbed?

A
  • as parathyroid hormone activates vitamin D, this causes Ca2+ ions to be ACTIVELY absorbed from the duodenum.
  • all other ions are ACTIVELY absorbed
43
Q

What is the most abundant monosaccharide absorbed?

A

80% glucose
(remaining 20% is galactose and fructose)
*ACTIVE transport

44
Q

How is glucose and galactose absorbed?

A

by active transport of Na+ through basolateral membrane causing decreased Na+ inside cells, allowing Na+ and glucose co-transport through from lumen into epithelial cell.

45
Q

How is fructose absorbed?

A

by facilitated diffusion all the way through the intestinal epithelium. Once in the cell it is phosphorylated and converted to glucose.

46
Q

How are peptides and amino acids absorbed?

A

most use Na+ co-transport mechanism and others use mechanism similar to fructose.
*at least 5 types of transport proteins.

47
Q

How are fats absorbed?

A
  • digested to monoglycrides + free fatty acids and then packaged into bile MICELLES (soluble in chyme). Once absorbed into epithelial cells, the fats diffuse out and the fatty acids go into the smooth ER where new triglycerides are released as chylomicrons to diffuse into the thoracic lymph duct and then into blood circulation.
  • short- and medium-chain fatty acids can also be directly absorbed into portal blood because they are more water-soluble and don’t need to be reconverted into triglycerides.
48
Q

What are some disorders of the stomach and absorption?

A
  • pyloric stenosis
  • gastritis (ulcers, autoimmune…)
  • peptic ulcers
  • gastric polyps
  • carcinoma

Gastrointestinal 1 (21 decks)

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