The human power plant

Few organs are as deserving of writing about as the liver. It is the busiest organ in human abdomen, and the one with most functions, generally estimated to be over 500 of them! It is located in the right upper quadrant of the abdomen, and its complexity made it possible to transplant from a live donor only as of 1989. Things concerning the liver will often have the prefix hepat-, derived from the Greek word for liver (hepar).

Structure

The liver is a reddish-brown organ made out of two inequal parts, or lobes, weighing approximately 1.5 kg which establishes it both as the heaviest organ and the largest gland in the human body. The lobes are further divided into eight segments based on their vascularity. Each segment has its own vein, artery and bile duct, functioning separately from the rest of the liver. Important landmarks in the liver anatomy aris the biliary tract, collecting bile into the gallbladder, or draining it directly into the duodenum.

 Two large blood vessels siphon blood through the liver: the hepatic artery, which carries oxygen-rich blood from the heart, and the portal vein, carrying digested nutrients from the gastrointestinal tract, spleen and pancreas. These blood vessels branch into liver sinusoids, capillaries that form lobules.

Lobules (not to be mistaken for lobes!) are the functional units of a liver, made up of basic metabolic cells of the spleen – the hepatocytes. Their function is tied to their position in the liver, and can range from oxidation of fatty acids and cholesterol synthesis to sugar degradation, generation of fat and drug detoxification. Every lobule includes a portal vein, hepatic artery, lymphatic vessel and a bile duct. The merging and interplay between those four sources of both biological and chemical products gives rise to the complexity and importance of the liver.

Figure 1: Schematic view of lobules. Source: Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013.

Liver functions

The vast number of liver functions is too big to list and explain individually, but outlined here are the main ones.

The human body expresses about 20 000 protein coding genes, and 60% of them can be found in the liver. Out of those, 400 are found only in the liver, and consist of plasma proteins and hepatokines – hormone-like proteins secreted by hepatocytes into the blood stream. Other liver-specific proteins are liver enzymes, such as bile synthesis proteins, transporter proteins for drug metabolism, coagulation factors, and other.

Digestion

Bile is one of the most important excretes the liver produces. About 400 to 800 milliliters of bile is secreted every day and ultimately transported to duodenum, the first part of the small intestine. It is composed of water, bile salts, bilirubin, fats and inorganic salts. It acts as a surfactant, helping to emulsify and break down the food fats. Bile salts surround fat droplets (miceles) and provide a large surface area of the fat for the enzymes to digest. Without bile salts, more than 90% of food lipids would end up undigested.

The liver also helps digest a lot of biological material, so that it can be used as energy source and building block in our body, or to be excreted harmlessly from the organism. Breakdown of insulin and other hormones happens in the liver, and they get excreted via feces and urine. Ammonia, usually extremely toxic to the human body, is converted into urea and excreted as well. Bilirubin is another important waste product it helps to eliminate.

Other waste products of note include toxic substances and most medicinal products, which the liver digests in a process called drug metabolism.

Synthesis

Besides digestion, the liver also synthesizes and produces a repertoire biological material. Around 100 g of glycogen is produced and stored in the liver, binding glucose in a long and more stable chain, and serving as a long-term energy source. When blood sugar drops, this glycogen is broken down into glucose to supply the body with energy. In small doses, liver also produces both glucose and glycogen from their constituents.

All plasma protein, except Gamma-globulins are synthesized in the liver as well! Besides those, it also produces amino acids, blood clotting factors, red blood cells, cholesterol, triglycerides, fatty acids, and other factors, such as insulin-like growth factor!

Blood and lymph

Due to its sponge-like nature, large quantities of blood can be stored in the liver. Almost 10 percent of the circulating blood prefuses its blood vessels at all times! In case of high atrium heart pressure, the liver can expand to intake up to an additional liter of blood! Accordingly, if the blood pressure drops due to blood loss or other conditions, the liver can contract, expelling the stored blood and helping in blood pressure mainteinance.

As with the abundance of blood and blood vessels, the liver is also teeming with lymph vessels. Almost half of the overall lymph volume formed in the body arises from the liver, carrying precious proteins and regulatory molecules.

Other functions

The liver stores a multitude of substances, including vitamins A, D, B12, K and E, iron, copper, zinc, molybdenium and other trace elements crucial for function of certain enzymes. Coupled with the fact that 60% of all protein coding genes are active in the liver, it is easy to envision a vast storage unit that can quickly and efficiently assemble any building block a body is in need of.

As the abundance of blood flows through the liver, it is also a prime site to activate many immunological cells, who happen to come in contact with viruses and bacteria.

The liver also produces albumin to carry hormones and fatty acids, angiotensinogen, a prime hormone in blood pressure regulation, and the enzyme catalase, responsible for breaking down of hydrogen peroxide, a toxic oxidative agent.

Being the most active organ in the abdomen is not easy, and a lot can go wrong. Ongoing studies are researching the many diseases and how to battle them, from hepatitis to carcinoma. What exactly can go wrong, and what are the scientists doing to fix it, will be discussed in the net article!

References:

1.    Abdel-Misih, Sherif R.Z.; Bloomston, Mark (2010). "Liver Anatomy". Surgical Clinics of North America. 90 (4): 643–653.

2.    Elias, H.; Bengelsdorf, H. (1 July 1952). "The Structure of the Liver in Vertebrates". Cells Tissues Organs. 14 (4): 297–337.

3.    Starr, Cecie (2007-09-20). Biology: Concepts and Applications. Cengage Learning. p. 650. ISBN 978-0-495-11981-4.

4.    Tortora, Gerard J.; Derrickson, Bryan H. (2008). Principles of Anatomy and Physiology (12th ed.). John Wiley & Sons. p. 945. ISBN 978-0-470-08471-7.

5.    Zakim, David; Boyer, Thomas D. (2002). Hepatology: A Textbook of Liver Disease (4th ed.). ISBN 9780721690513.

6.    Anatomy of the Liver". Liver.co.uk. Archived from the original on 2015-06-27. Retrieved 2015-06-26.

7.    https://www.hopkinsmedicine.org/health/conditions-and-diseases/liver-anatomy-and-function

8.    https://columbiasurgery.org/liver/liver-and-its-functions

9.    https://www.webmd.com/digestive-disorders/picture-of-the-liver

10. https://www.stanfordchildrens.org/en/topic/default?id=anatomy-and-function-of-the-liver-90-P03069