BIO 107 - Structure & Function of the Liver AND Structure & Function of the Gallbladder

What is the hepatic lobule?

The functional unit of the liver (hexagonal units)

• centered around a central vein

  • the central vein drains into the hepatic vein

List the major structures associated with the hepatic lobule:

• Interlobular vein (to hepatic vein)

• Central vein

• Plates of hepatocytes (liver cells)

• Sinusoids (sinusoidal capillaries - very leaky)

• Hepatic portal vein

• Hepatic artery

• Portal triad

• Bile duct

What structures make up the portal triad?

At each of the 6 outer corners of a lobule are portal triads made up of branches of the:

• bile duct

• hepatic portal vein

• hepatic portal artery

What are hepatocytes?

They are liver cells and about 70% of their surface faces the sinusoids

The liver has many metabolic and regulatory roles but its digestive function is to produce what?

Bile

What are the two cell types that are responsible for the production and release of the components of bile?

• the hepatocytes (liver cells)

• the epithelial cells lining the bile ducts

What is the major function of epithelial cells lining the bile ducts?

They release HCO3-

Where does the blood in the central vein eventually end up?

Into the hepatic vein

What are sinusoids?

They are large spaces lined with endothelium, through which blood passes

• very leaky capillaries

• big plasma proteins into blood

Which 2 blood sources supply the sinusoids?

The hepatic portal vein and the hepatic portal artery

What are bile canaliculi?

They are small channels into which bile is secreted

List the functions of the liver:

• Carbohydrate metabolism

  • important in maintaining normal blood glucose level

• Lipid metabolism

  • some triglyceride storage

  • cholesterol synthesis

• Protein metabolism

  • synthesis of most plasma proteins: (alpha and beta globulins, albumin, and fibrinogen)

• Detoxification

  • removal of drugs and hormones

• Excretion of bilirubin and cholesterol

• Synthesis of bile salts

• Storage (glycogen, vitamins and minerals such as iron and copper)

• Phagocytosis (Kupffer cells) “gobble up”

• Activation of vitamin D

• Synthesis of plasma proteins (albumin, fibrinogen)

In terms of digestion, what is the most important contribution of the liver?

The production/synthesis and secretion of bile

Which hepatic cells produce bile?

Hepatocytes

Is bile acidic, neutral or basic (alkaline)?

Basic (7.6-8.6)

Where is bile stored and concentrated?

By the gallbladder

Name the 4 major components of bile:

• Bile salts (made from steroid bile acids combined with amino acids) conjugation!

• Bile pigments

• Cholesterol

• HCO3-

Which of the major components of bile are produced by hepatocytes?

• Bile salts

• Bile pigments

• Cholesterol

Which of the major components of bile are produced by the bile duct epithelial cells?

HCO3-

What are two major functions of bile salts?

Emulsification: the breakdown of large lipid globules into smaller droplets

• the tiny lipid droplets present a very large surface area for the action of the pancreatic lipase (the major lipid digesting enzyme)

Lipid absorption: following their digestion

• via formation of micelles

Name one prominent bile pigment:

Bilirubin

• a build up of bilirubin causes jaundice

What role does bicarbonate play?

It neutralizes acid in the duodenum

What is some information about the cholesterol?

Excreted in feces

• bile is the major vehicle for cholesterol excretion from the body

• too much cholesterol or too few bile salts can lead to cholesterol crystallization (gallstones or biliary calculi)

What are the most important digestive components of bile?

The bile salts

What is the flow of bile?

Left & Right Hepatic Ducts → Common Hepatic Duct →
either Cystic Duct ↔ Gallbladder (for storage)
or → Common Bile Duct + Main Pancreatic Duct →
Hepatopancreatic Ampulla → Duodenum

When is bile secretion the greatest?

During a meal, but the liver is always producing and secreting some bile (ductile bile rich in bicarboante)

In between meals, is the sphincter closed or open? Explain.

It is closed and bile gets diverted to the gallbladder where it becomes concentrated

• it moves up via the cystic duct

• concentration: NaCl & H2O get absorbed into blood

Where is the sphincter of Oddi located?

It is located where the common bile duct and pancreatic duct open into the duodenum

When is the sphincter of Oddi open?

It opens during a meal (especially a fatty meal)

• the sphincter relaxes (opens)

• the gallbladder contracts

What role does CCK (released by duodenum) play in regulating the sphincter of Oddi?

It triggers dilation of the sphincter and contraction of the gallbladder

• This ejects bile into the duodenum through the duodenal ampulla

What are bile acids?

Amphipathic end products of cholesterol metabolism

• amphipathic means they both have a hydrophilic & hydrophobic regions and can form micelles

How does cholesterol form bile acids?

In the liver through 7α-hydroxylase enzymes

Where can primary bile acids get converted to secondary bile acids?

In the intestines

• intestinal bacteria can do this conversion

What does primary bile acids consist of?

• cholic acid

• chenodeoxycholic acid

What does secondary bile acids consist of?

• deoxycholic acid

• lithocholic acid

All bile acids are secreted by the hepatocytes in what form?

Conjugated form

What are bile salts?

Conjugated bile acids

• the amino acids glycine & taurine are used

• this increases water solubility

What is the advantage of conjugating bile acids into bile salts prior to secreting them into the bile canaliculi?

• They are charged and therefore, cannot cross cell membranes via passive means (they need active transporters)

• It increases their water solubility, making them more effective at emulsifying fats, functioning in the intestinal environment, and aiding lipid absorption

What does the enterohepatic circulation refer to?

This refers to bile salt recycling between the small intestinal lumen and the liver

• Normally about 95% of bile salts are reabsorbed by the terminal ileum

• bile salts that make it into the colon, can be converted back to bile acids by the bacteria there

• the bile salts can be returned to the liver via the hepatic portal vein

In which specific portion of the small intestine are bile salts reabsorbed?

The terminal ileum

Each hepatocyte is in contact with blood from what two sources?

The hepatic artery and hepatic portal system

What is the function of the hepatic artery?

It provides oxygenated blood to the liver

What is the function of the hepatic portal system?

It contains deoxygenated blood containing newly absorbed nutrients, drugs, microbes, and toxins

• The vascular connection between the digestive tract and liver

What is the order of the blood vessels in relation to the liver?

Hepatic artery & Hepatic portal vein → Liver sinusoids → Hepatic vein → Inferior vena cava

What is important about the hepatic portal vein?

They are rich in absorbed nutrients from GI tract

What is important about the Liver sinusoids?

Leaky capillaries

• receives blood from both the hepatic artery and the hepatic portal vein

What is important about the hepatic vein?

The blood leaves the liver via the hepatic vein which dumps its low oxygen blood into the inferior vena cava

Which blood vessel carries oxygenated blood to the liver sinusoids?

The hepatic artery

Which blood vessel brings low oxygen but high absorbed nutrient blood to the liver?

The hepatic portal system

Which blood vessel drains the liver sinusoids?

The central vein

Which vessels are part of the hepatic portal system?

• Hepatic portal vein

• Liver sinusoids

• Hepatic vein

What role does the gallbladder play in our bile story?

It stores and concentrates bile

The gallbladder is not essential to what?

Normal digestion & absorption of a meal

What are the major functions of the gall bladder?

• Storing and concentrating bile

• Musculature: contractions of the gallbladder release bile into the cystic duct!

  • has receptors for the:

    • vagus nerve (PNS)

    • Ach

    • CCK

Name the 3 major functions of the gallbladder that play a role in bile release from the gallbladder:

• parasympathetic stimulation

  • increases bile secretion via vagus nerve (Ach as the neurotransmitter)

• CCK

  • stimulates bile secretion by causing contractions of the wall of the gallbladder

• secretin

  • stimulates the secretion of bicarbonate (bile rich in bicarbonate) by epithelial cells lining the bile ducts

What 3 factors play a role in relaxing the sphincter of Oddi?

• CCK

  • opens the valve by decreasing sphincter tone (relaxing)

• VIP

  • locally relaxes the sphincter

• Nitric Oxide

  • locally relaxes the sphincter

What is the importance of this relaxation of the sphincter of Oddi?

Normally, the relaxation of this sphincter is coordinated with gallbladder contraction