The Liver

Figure 1: Diagram of the Liver

The liver is split into 2 main lobes as seen in the diagram which is made of 8 segmenets. These segments can be further split into around 1,000 lobules that all connect to the right and left hepatic duct and then the common hepatic duct. Lastly, the common hepatic duct joins with the cystic duct coming from the gallbladder to enter the duodenum along with the pancreatic duct.

The liver performs three main functions

a) Filtration + storage of blood

b) Metabolism + detox

c) Bile formation

Filtration + blood supply

There are two main blood vessels that are involved with this function, the portal vein that is responsible for draining the entire gastrointestinal tract and the hepatic artery that has just branched off from the aorta. Both the portal vein and hepatic artery's blood drains into the porous sinusoid surrounded by Kupffer cells. Kupffer cells are able to engulf pathogens or invading bacteria within the blood and transport that to the hepatocytes. Hepatocytes can then undergo any other process to make use of this blood such as bile synthesis, metabolism, detox, etc... All other blood that doesn't get engulfed by Kupffer cells enter the central vein to then enter the inferior vena cava.

The liver receives blood at low pressures and is close to the heart. Therefore, in cases of heart failure, blood has the route to back-flow into the liver as temporary storage.

Figure 2 - Diagram of a Sinusoid in the Liver

Metabolism + detox

Metabolism is when the body changes the monomers absorbed in the small and large intestines into energy. The liver has the ability to do many things with the monomers of carbohydrates, proteins, and lipids.

For carbohydrates, glucose is the primary final product to kickstart ATP. The liver has 3 main process it can undergo for glucose. Firstly, glycogenesis is turning glucose into glycogen, its natural form for storage to be used at a later stage. Conversely, glycogenolysis is turning glycogen to glucose so it's ready to be released into the blood and sent around th ebody. Lastly, gluconeogenesis is turning the glycerol parts of lipids into glucose.

Amino acids from proteins can be transaminated back into proteins, enzymes, and hormones for the body to use. Excess amino acids can be deaminated into ammonia, then urine to be excreted as a way of expelling large amounts of nitrogen found in the amino acids.

Fats have two routes for metabolism. One of them is the reformation into triglycerides as storage. Another is turning them into an ingredient for ATP and a byproduct of Ketone bodies - a back-up energy source for when glucose is scarce.

Figure 3 - Diagram of Hemoglobin in a Blood Vessel

The liver plays a role within the detoxification of hemoglobins. The heme part of hemoglobins, or the iron portion, cannot be recycled and must be expelled. It is turned into biliverdin, then unconjugated bilirubin. Unconjugated bilirubin is sent to the liver where an enzyme is attached that conjugates it and makes it soluble. Unconjugated bilirubin then gets turned into urobilinogen through reactions with the intestinal flora as it is headed through the small and large intestines to be excreted through defecation.

Bile formation

Bile is comprised of 95% H2O, bile acids and bile salts. The liver assists in the synthesis of bile acids and bile salts.

Cholesterol stored in the liver interacts with some enzymes to be turned into primary bile acids, more specifically, cholic acid and chenodeoxycholic acid. Half of them are stored in the gallbladder to be concentrated with other bile components while the other half go through the small intestines to the ileum. At the ileum, they interact with the inestinal flora to be changed into secondary bile acids. Cholic and chenodeoxycholic acid are transformed into deoxycholic acid and lithocholic acid respectively. Very little amounts of secondary bile acids continue down the gastrointestinal tract to be excreted, most are absorbed back into the liver through its own circulation called the enterohepatic circulation. Secondary bile acids are then paired with amino acids, either glycine or taumine, and are now bile salts because they are amphipathic, meaning they have both hydrophobic and hydrophilic ends. Bile salts lose their amino acids again at the ileum and return to the liver for the cycle to be repeated.

When chyme enters the duodenum, enteroendocrine cells sense it and release a hormone called CCK into the blood stream, stimulating the liver and pancreas to release bile and pancreatic juices. Biles mix with fat globules and breaks them down into emulsion droplets through emulsification. Emulsification works to cut up fat globules to have increased surface area for lipases and colipases to have an easier time to work on and turn into fatty acids.

Figure 4 - Diagram of How Bile Emulsifies Fats

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