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MCAT PHGY > GI system > Flashcards

GI system Flashcards

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MCAT flashcards
1
Q

What is enzymatic hydrolysis?

A

the primary catabolic reaction to break down food thats been ingested. It is the use of an enzyme to catalyze breakdown with water

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2
Q

what are the catalytic enzymes required for the catabolic break down of

  1. triglycerides
  2. polysaccharide
  3. disaccharides
  4. polypeptides
  5. small peptides
A
  1. fats –> pancreatic lipase
  2. polysaccharides –> pancreatic amylase
  3. di ““brush border disaccharidases (sucrase, maltase, lactase)
  4. pepsin and pancreatic proteases such as trypsin
  5. peptidases
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3
Q

what is special about the epithelial cells of the intestine?

A

they contain microvilli and crypts responsible for increased absorption area and (crypt) secretion of digestive enzymes.

the epithelial cells of the GI tract are connected by tight junctions

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4
Q

from inner most to outermost what is the layers of the GI tract?

A

inner most

  1. mucosa (epithelial tissue)
  2. submucosa
  3. circular muscularis followed by longitudinal muscualris
  4. serosa (connective tissue)
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5
Q

true or false, skeletal muscle and cardiac muscle are striated while smooth muscle is not?

A

true, smooth muscle is not

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6
Q

what are the 5 controls over the motility of the GI bolus? (2 internal 3 external)

A
  1. similar to cardiac muscle, GI smooth muscle has autorythmicity allowing for spontaneous depolarization
  2. similar to cardiac muscle, GI smooth muscle is connected by gap junctions allowing it to act as a functional syncytium.
  3. the enteric nervous system which is a independent branch of the ANS
  4. the actual ANS where the PNS stimulates motility and SNS inhibits it
  5. hormonal regulation
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7
Q

what are the two divisions of the ENS?

A
  1. myenteric plexus which is found in the circular and longitudinal muscle. Its responsible for gut motility
  2. submucosal plexus found in the submucosa. Responsible for blood flow and secretions.
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8
Q

explain the general structure of an exocrine gland within the GI tract.

A

the exocrine gland will be connected to the Gi lumen via a duct.

convoluted structures called acini are made up of many cells called Acinar cells. These cells secrete their products into a lumen which is collecting and drained at the duct where it is expelled into the GI tract.

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9
Q

what are goblet cells?

A

These are exocrine cells that line the entire GI tract. they secrete mucus to protect the body surfaces against harmful things.

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10
Q

What is the gastric gland?

A

Within the stomach, many cells arrange to form “gastric glands” which have an exocrine function to secrete HCl and pepsinogen

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11
Q

In the pancreas, what are pancreatic acinar cells and what are islets of langerhan cells?

A

pancreatic acinar cells are exocrine cells which secrete pancreatic enzymes such as pancreatic lipase, amylase, and more.

islets of Langerhans cells are endocrine cells which secrete
B - insulin
A - glucagon

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12
Q

what are 4 major component of saliva?

A

1) bicarbonate
2) salivary amylase which starts carbohydrate breakdown
3) lingual lipase which starts fat breakdown
4) lysozyme’s which attack bacterial cell walls.

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13
Q

what is ptyalin?

A

another name for salivary amylase

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14
Q

explain the pathway of food vs air upon entering the mouth

A

The first thing both pass is the pharynx. The pharynx splits into the trachea and the esophagus. A flap known as the epiglottis closes the trachea upon the swallow reflex.

food: passes thru the pharynx, over the epiglottis, and into the esophagus
air: passes the pharynx thru the open epiglottis, into the trachea

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15
Q

what is the uvula?

A

the ball hanging in back of throat

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16
Q

explain the purpose of the upper and lower esophageal sphincters

A

UES –> opens upon swallowing to permit food into the esophagus

LES –> Opens simultaneously with the UES. Its purpose is to protect the esophagus from harmful acid of the stomach

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17
Q

What are parietal cells?

A

these cells secrete H+ and Cl- (separately) into the stomach creating a ph of 2

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18
Q

what are chief cells?

A

chief cells secrete pepsinogen into the stomach

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19
Q

explain pepsin and pepsinogen conversion

A

the low ph caused by parietal cells HCl secretions converts inactive zymogen pepsinogen to pepsin which is responsible for protein catabolism.

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20
Q

what is proteolysis?

A

the most common why for a zymogen to become activated. in the case of pepsinogen, low pH encourages self proteolysis (cutting off of some peptide)

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21
Q

what is the pyloric sphincter? what controls its opening/closing?

A

The pyloric sphincter separates the stomach from the duodenum (first part of SI).

The pyloric sphincter will remain shut if
1. the duodenum is acidic
2. the duodenum is stretched
both indicate the small intestine is too full for more chyme.

22
Q

What is cholecystokinin (CCK)?

A

this is a hormone secreted by duodenum epithelial cells which inhibits stomach emptying and enzymatic secretions into the small intestine

23
Q

what are G cells of the stomach?

A

G cells secrete gastrin which is a hormone that acts on chief cells and parietal cells to release pepsinogen and acid.

24
Q

what is an enterchromaffin like (ECL) cell?

A

this cell secretes histamine which acts on parietal cells to stimulate acid release.

25
Q

explain the neural activity of the stomach

A

upon stomach distention: both a vagus nerve reflex and and enteric system mechanism are activated.

the ENS uses acetylcholine to synapse with all stomach cells.

note: histamine is paracrine, gastrin is endocrine

therefore there are 3 factors , n, e, and p working on parietal and chief cells

26
Q

what are the small intestine compartments, where does absorption/digestion occur?

A
  1. duodenum
  2. jejunum
  3. ileum

1 and 2 is where final absorption and digestion occur (for the most part)

27
Q

what is the brush border of the SI?

A

the microvilli on the villi in the SI form the brush-border responsible for exocrine enzymatic function

28
Q

what are three important features of SI villi

A
  1. villi contain capillaries which absorb AA’s and monosaccharides and take them to the hepatic portal vein
  2. they also contain small lymph vessels called lacteals which collects fats (chylomicrons) and brings them to the thoracic duct (and then a vein)
  3. Payers patches which are regions of lymphocytes
29
Q

What Two ducts empty into the duodenum? What sphincter do they both pass thru?

A

Pancreatic duct delivers exocrine secretions
Bile duct from liver delivers Bile

They both pass thru the Sphincter of Oddi

30
Q

duodenal enzymes: What is enterokinase (enteropeptidase)

A

This is an enzyme released by duodenal epithelial cells which activates trypsinogen from the pancreas into trypsin (active trypsin then activates the other pancreatic zymogens)

31
Q

Duodenal enzymes: brush border enzymes ?

A

along the microvilli there are brush border enzymes. these are typically Disacchardiases and dipeptidases

32
Q

duodenal hormones. Explain what CCK, Secretin, and Enterogastrone do?

A

CCK –> released in response to food entering the duodenum (distention). This increases pancreatic duct secretions, increases gall-bladder contraction (bile) and decreases gastric motility

secretin –> released when pH of the duo is to low. Secretin acts on the pancreas to release HCO3-

Enterogastrone –> inhibits gastric motility

33
Q

What substance is absorbed in the ileum which must be absorbed with intrinsic factor produced by parietal cells?

A

vitamin B12

34
Q

what is the junction between the ileum and the first part of the large intestine?

A

ileocecal valve (first part of the LI it the cecum)

35
Q

What vitamin do we get from gut bacteria? what does it do?

A

colonic bacteria provide us with Vitamin K which is essential for coagulation (blood clotting)

36
Q

explain pancreatic trypsinogen?

A

duodenal cells will release CCK which promotes the exocrine function of the pancreas. Upon secretion, trypsinogen meets with the enzyme enterokinase (aka enteropeptidase) which turns trypsinogen to trypsin.

Now active trypsin activates the rest of the pancreatic proteases (procarboxypeptidase, procollagenase, etc)

37
Q

true or false SNS increases pancreatic secretion

A

false, PNS does

38
Q

what are the three islets of langerhans cells? what they secrete

A
  1. alpha cells –> glucagon –> increases blood sugar by mobilizing fat and glycogen (liver)
  2. Beta cells –> insulin –> decreases blood sugar
  3. delta cells —> somatostatin –> decreases GI processes
39
Q

what affects stomach emptying?

A

CCK - inhibits it
somatostatin - inhibits it
gastrin - promotes it

40
Q

what is the hepatic portal system?

A

the lover receives blood from the hepatic arteries. It also receives blood carrying peptides and sugars directly from the small intestine (fats enter lacteals). i.e. the capillary bed from the gut goes to a bed in the liver.

the liver can monitor what is being absorbed

41
Q

what cells can store glucose?

A

Only liver and skeletal muscle cells can store glucose. And only hepatocytes can release glucose into the blood stream!!

42
Q

what is glucose-6-phosphate?

A

this is the product of glycogen breakdown which is not quite soluble glucose. In order for it to be released into the blood stream it gets dephosphorylated by glucose-6-phosphatase. (which only the liver has)

43
Q

explain how lipids get digested then absorbed by the intestine and then end up at the liver to be converted into lipoproteins?

A

salivary lipase starts the breakdown of fats. in the small intestine Bile salts emulsify them into micelles! these micelles are then easily broken by pancreatic lipase. The final product is a chylomicron which gets absorbed into lacteals which goes to the thoracic duct of the lymphatic system and drained into the blood.

the chylomicrons circulate to the liver where they get further degraded into fatty acids, glycerol, and high cholesterol chylomicron remnants such as (HDL, LDL). These get combined with proteins to form lipoproteins which become the source of cholesterol for other tissues

44
Q

what three hormones regulate satiety (feeling of being full) ? Where are they secreted from?

A
  1. Gastric cells secrete ghrelin which goes to the hypothalamus (arcuate nucleus) and stimulates hunger
  2. Peptide YY secreted from the jejunum goes to the hypothalamus (arcuate nucleus) to stimulate satiety
  3. Leptin is produced by adipocytes in response to high levels of triglycerides which also decreases hunger
45
Q

Once a carb has been fully digested how does its monosaccharide from become absorbed (what transporters)

A

Pancreatic amylase converts the carb into di/tri monosaccharides. Often brush border enzymes perform the final cleavage.

on the apical side, the bulky, hydrophilic monosaccharide must be pumped via secondary active transport into the epithelial cell. this is performed by a sugar / Na+ symporter.

on the basolateral side, the sugar leaves via facilitated diffusion and goes to the liver via hepatic portal

46
Q

explain how amino acids are absorbed

A

symporter specific for each AA carries in that amino acid + Na+ in secondary active transport. On the basolateral side, facilitated diffusion occurs and the amino acid makes its way to the liver (hepatic portal)

47
Q

why cant pancreatic lipase directly break down triglycerides in the duodenum?

A

the fats are so hydrophobic something else needs to emulsify them. Therefore, fat presence (distention) stimulates CCK which stimulates bile release. This turns fats to micelles which can now be degraded by pancreatic lipase.

48
Q

explain how fats are absorbed into the blood stream? from the duodenum

A

the final digestion of fats from pancreatic lipase converts them to monoglyceride’s and free fatty acids. Since these are so small and hydrophobic they directly diffuse into the epithelial cell of the jejunum

once inside the cell, they recombine to form triglyceride. They are then packaged into chylomicrons and released via exocytosis into lacteals which combine into the thoracic duct which empty into the blood stream. (lipemia)

now the chylomicrons are slowly taken up primarily by the liver and fat for storage.

49
Q

what enzyme does the liver and adipocyte cells have which permit hydrolysis of the chylomicron and therefore diffusion of fatty acids into them?

A

lipoprotein lipase hydrolyses chylomicrons. The fatty acids diffuse into the cell and then reform triglycerides for storage

50
Q

What are vitamins?

A

essential nutrients that the body cannot synthesis on its own

51
Q

where are fat soluble and water soluble vitamins stored?

A

water soluble vitamins aren’t stored, they are secreted by the kidneys
fat soluble ones are stored in adipocytes

52
Q

what are some fat soluble and water soluble vitamins?

A

Fat soluble: DEAK / ADEK
A - retinol, needed for vision
D - stimulates calcium absorption in gut
E - prevents oxidation of unsaturated fats
K - needed for hemostasis / coagulation

water soluble: BC
B - many functions
C - needed for collagen formation

MCAT PHGY (8 decks)

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