S6) Chyme, Pancreas and Liver Secretions Flashcards

1
Q

Describe 3 properties of chyme

A
  • Hypertonic
  • Low pH
  • Partially digested
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2
Q

Explain how hypertonic chyme exits the stomach but isotonic chyme exits the duodenum

A
  • Initial digestion of food in stomach creates solutes that make the chyme hypertonic
  • Stomach wall largely impermeable to water so chyme cannot be diluted
  • Duodenum relatively permeable to water so hypertonic chyme draws in water from ECF/circulation
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3
Q

Duodenum secretes secretin and CCK in response to presence of chyme.

What is the action of secretin?

A

Secretin acts on pancreas to stimulate release an aqueous bicarbonate (HCO3-) component of the pancreatic secretion into the duodenum

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4
Q

Duodenum secretes secretin and CCK in response to presence of chyme.

What is the action of CCK?

A
  • Acts on pancreas to stimulate the release of enzyme component of pancreatic secretion
  • Contracts gallbladder
  • Relaxes sphincter of oddi
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5
Q

How are the exocrine secretions of the pancreas controlled?

A
  • Stimulated via gut hormones

I. Secretin

II. CCK

  • Also, autonomic control:

I. Sympathetic inhibits

II. Parasympathetic stimulates (vagus)

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6
Q

Describe the structure of the pancreatic acini

A
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7
Q

Which enzymes are released from the pancreas?

A

CCK & vagus nerve stimulate acini to produce enzymes:

  • Amylases and lipases (active)
  • Proteases (inactive)
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8
Q

Identify 4 activated proteases which are derived from the secretions of pancreatic acini

A
  • Trypsin
  • Chymotrypsin
  • Elastase
  • Carboxypeptidase
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9
Q

How are pancreatic proteases produced and stored?

A

Protease enzymes (inactive) produced within acinar cells are concentrated and stored in zymogen granules

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10
Q

What do zymogen granules contain?

A

Zymogen granules contain zymogen e.g. trypsinogen; which is converted to trypsin in intestinal lumen

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11
Q

What happens when pancreatic duct cells are stimulated?

A

Duct cells secrete aqueous component (isotonic) and bicarbonate which act to neutralise the acidic chyme

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12
Q

How do pancreatic secretions get to the duodenum?

A

Pancreatic secretions reach the duodenum via pancreatic duct and ampulla of vater

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13
Q

How does the liver respond to the presence of chyme?

A

Liver secretes bile (stored in gallbladder) into the duodenum

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14
Q

What does bile consist of?

A
  • Bile acid-dependent: bile acids and pigments
  • Bile-acid independent: alkaline solution
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15
Q

What is the role of bile in the duodenum?

A

Bile emulsifies fat (lipids) in duodenum so that they can be readily digested by pancreatic lipases

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16
Q

Describe the micro-anatomy of the liver

A
  • Lobules form the structural units of the liver
  • Hexagonal arrangement of cells, bile ducts and blood vessels
17
Q

Describe the arrangement of structures in the liver lobule

A
  • Triad of structures at each corner: portal vein (branch), hepatic artery (branch), bile duct
  • Central vein in middle
18
Q

Describe the flow of blood and bile in the liver lobule

A
  • Blood flows in towards central vein via sinusoids from the branches of the portal vein and hepatic artery
  • Bile flows out along canaliculi and bile duct into duodenum
19
Q

What is the name of the functional area of the liver lobule?

A

Functional area of the lobule is called the acinus

20
Q

With references to the liver, what is the significance of blood draining from the periphery of the lobule towards the centre?

A
  • Substances brought to the liver from gut therefore start at periphery and work towards middle (towards central vein)
  • Creates a series of zones (1,2,3) corresponding to distance from arterial blood supply
21
Q

Describe the production of bile in the liver

A
  • Bile is continuously produced by hepatocytes and duct cells in liver
  • It is only needed intermittently and, thus, is stored in the gallbladder
22
Q

Describe the secretion of bile into the duodenum

A
  • CCK stimulates the contraction of gallbladder
  • Bile is secreted via the common bile duct, then Ampulla of Vater into duodenum
23
Q

Which structure secretes bile acids and pigments?

A

Bile acids and pigments are secreted into canaliculi by hepatocytes

24
Q

Which structure secretes alkaline juices?

A

Alkaline juices are secreted by bile duct cells which are stimulated by secretin

25
Q

What are bile salts?

A

Bile salts are bile acids conjugated with amino acids (glycine, taurine)

26
Q

There are two primary bile acids.

Identify them

A
  • Cholic acid
  • Chenodeoxycolic acid
27
Q

Why are bile acids conjugated?

A
  • Bile acids are not always soluble at duodenal pHs
  • Bile salts have an amphipathic structure (hydrophilic and hydrophobic ends)
  • Bile salts allow emulsification of dietary lipids
28
Q

Why do bile salts need to emulsify fats?

A
  • Lipids form large globules once in duodenum, with a small surface area for lipases to act
  • Bile salts emulsify fat into disperse droplets, increasing surface area for lipases
29
Q

Bile salts then create micelles with products of lipid breakdown.

What do they do?

A

Micelles act as a vehicle for transporting hydrophobic molecules (products of lipid digestion) towards the luminal membrane of an enterocyte

30
Q

What happens after lipids diffuse into intestinal epithelium cells?

A
  • Inside the enterocyte, lipid molecules are re-esterified back to triglycerides, phospholipids and cholesterol
  • Then they are packed with apoproteins within enterocyte, forming chylomicrons
31
Q

What do chylomicrons do?

A

Chylomicrons carry digested fats via the lymphatic system to ultimately reach blood circulation

32
Q

Why are chylomicrons exocytosed from the basolateral membrane of the enterocyte?

A
  • Too large to enter capillaries
  • Enter lymphatic capillaries through thoracic duct
33
Q

The liver recycles bile acids.

Describe the entero-hepatic circulation of bile acids

A
  • Bile salts remain in gut (they do not enter enterocyte)
  • Reabsorbed in terminal ileum
  • Returned to liver in portal blood
34
Q

What is steatorrhea?

A

Steatorrhea is when undigested fat appears in faeces due to inadequate secretion of bile acids (salts) or pancreatic lipases

35
Q

How does steatorrhea present?

A
  • Pale
  • Floating
  • Foul smelling