A Review of the Liver for Nursing Students
Anatomy and Physiology of the Liver
The liver is primarily located on the right upper quadrant, from the right hypochondrium extending to the epigastric region, beneath the diaphragm and sits on top on the stomach, right kidney, and intestines.
The Liver is the largest solid organ, weighing at 1.2kg to 1.5 kg and carries around 13% of the body’s blood supply at any given time. Its shape is dependent on the surrounding organs but generally takes form of a blunt cone. A healthy liver will appear reddish-brown.
Anatomy of the Liver
There are four unequal sized lobes in the liver. However, it is divided into two major lobes; each are anteriorly divided into two lobes by the falciform ligament. The other two are identified inferiorly, the inferior quadrate lobe and posterior caudate lobe. In addition to this, these lobes are further divided into eight segments, these segments consist of smaller lobes or “lobules”.
The liver is supported by two major blood vessels, the portal vein and hepatic artery which enters though the porta hepatis or “gate of the liver”. Oxygenated blood flows through the proper hepatic artery from the celiac artery and is divided into the left and right artery supplying each of its respective lobe.
While “nutrient-rich” blood flows through the hepatic portal vein as it from the joining of other major veins from the abdomen (spleen, pancreas, and intestines). Through the liver, the vein is divided into three, right, middle (Cantlie line), then left hepatic vein ascends to the inferior vena cava. In total, only 25% of the blood is supplied through the hepatic artery, and 75% is through the portal vein.
The liver is surrounded by splanchnic and vagal nerves, which is supported by sympathetic and parasympathetic nerves, which regulates several a wide range of hepatic functions. The afferent fibers are responsible for sending glucose and lipid levels as well as osmolality to the brain. Wherein the efferent fibers regulates metabolism, bile secretion and blood flow.
One important accessory organ located under the right lobe of the liver is the gallbladder. The liver is both an endocrine and exocrine gland, although it is largely the latter as it excretes around 400ml to 800ml of bile every day. Bile breaks down fats and is a mixture of cholesterol, bilirubin, and bile salts.
Bilirubin is a metabolite of heme, which is the non-protein part of hemoglobin that carries iron. The gallbladder stores and releases excess bile through the cystic duct where it meets the hepatic duct (which is a combination of the left and right bile duct), making the common bile duct. This common duct then meets the pancreatic duct where it will also produce its own secretion and will enter the duodenum together at the ampulla of Vater.
These series of branches are also commonly known as the “biliary tree”. The hormone cholecystokinin controls the release of the bile and is secreted by the duodenum.
Physiology of the Liver
The liver is essential in many biochemical processes in the body. Known as the “chemical factory” of the body, the liver is said to perform around 500 functions. Besides its function to produce the bile, it also breaks down toxins, drugs to make the safer and easier for the body.
It also plays a big role in metabolizing carbohydrates, this includes glucogenesis, glycogenolysis and glycogenesis. Glucogenesis, or gluconeogenesis to be more accurate, is the production of new sugars (glucose) from the metabolic precursors. A majority of this happens in the liver, although it occurs in smaller sites in the body like the kidney.
Glycogenolysis is the breakdown of glycogen, a primary carbohydrate stored in the liver, producing glucose. Lastly, glycogenesis is the storing excess glucose. In addition to its other metabolic functions, it also has secretory and endocrine functions.
The liver metabolizes several essential vitamins needed for body function, this includes vitamin A, D, E, K and several variants of B (B1, B2, B3, B6, B12, Biotin, and Pantothenic Acid). It is only in the liver where coagulant factors, II, VII, IX and X are produced and are derived from metabolizing vitamin K. Vitamin E is an antioxidant which a product of lipoprotein metabolism, but its uptake, distribution and storage is in the liver.
Vitamin D is commonly associated with bones and teeth promotion, but it also helps in regulating insulin and supports our immune and nervous system. Vitamin A or retinoid is essential for eye health and helps with our immunity.
The numerous functions of the liver can be attributed to its structure and much of its histological organization supports its main function, blood filtering. The liver is mainly composed of hepatocytes which are epithelial cells that are quite unique in nature.
Hepatocytes may possess several organelles instead of a concentration of one, it is common for hepatocytes to have two nucleus and be polypoids. It also has two basal surfaces, whereas an ordinary cell only has one. It is cuboidal in nature and forms linear cords. Hepatocytes builds the basic histological unit of the liver, lobules.
Lobules are hexagonal units that are made of hepatocytes and sinusoids with a central lobular vein. Outside these hexagonal units is known as the “portal triad”, this consists of the hepatic vein, hepatic artery and bile duct. Sinusoids are endothelium-lines spaces where blood flows through and are in between rows of two-cells thick hepatocytes, otherwise known as hepatic cellular plates.
These “capillaries” that draws blood from both the hepatic vein and artery, allows close contact between the blood and functional hepatocytes through its porous walls as blood goes towards central lobular vein. There is a small space in between the sinusoids and hepatic plates called the spaces of Disse or perisinusoidal space which contains blood plasma. Microvilli of hepatocytes extend into this space allowing absorption of other nutrients.
In addition to this, the sinusoids are lined with “Kupffer cells”, or macrophages, a type of phagocytic cells that plays an important role in our immunity. This provides an additional layer of protection as the main source of blood comes from the hepatic vein, which drains from the gastrointestinal tract, which may bring bacteria if it enters the systemic blood supply.
This central lobular vein will connect with other lobular vein and forms the three hepatic veins taking the blood from the liver to the inferior vena cava then back to circulation. They form similar to a bicycle wheel, the hub is the central lobular vein, the spokes are the hepatic cellular plates and the space in between are the sinusoids.
The production of the bile happens in the hepatocytes, specifically in between two hepatocytes, which is collected through the canaliculi and is pushed out into bile ducts. The flow of blood is centripetal whereas the bile flows centrifugal.
As mentioned previously, the liver provides a layer of protection; it also produces lymph. This is produced in the space of Disse. Although not much is known how the lymphatic circulation in the liver, it is estimated that 25% to 50% of the total lymph found in the thoracic duct are from the liver.
It falls into three categories depending on what area of the liver they are produced, although it is estimated that around 80% of produced lymph drains into the portal lymphatic vessel, while the rest is from the sub lobular and superficial lymphatic vessels.
The liver plays a vital role in ensuring the body’s functions, from the filtration and storage of blood, metabolism and detoxication, and bile formation, and these are mostly attributed by how the liver is structured.
Although the most interesting feature of the liver is its ability to regenerate. One may lose up to 70% of its liver mass and the liver will be able to regenerate it in a matter of seven days. Although, despite its ability to regenerate, one must still take care of this vital organ.
Diseases of the Liver
Intake of too much fatty food and alcohol will lead to the early stages of fatty liver disease, or simple steatosis. If this continues, this will lead to inflammation or steatohepatitis, where you will start to see fat accumulation around the liver as well as necrosis.
The end stage of this disease if left untreated is cirrhosis, leaving the liver completely damaged as healthy liver tissue has been replaced by scar tissue (fibrosis). At this stage, it is irreversible. Cirrhosis may only be managed, and if it further progresses, may cause chronic liver disease end-stage liver disease (end-stage liver disease).
Several other diseases may lead to end-stage liver disease, but cirrhosis is the most common cause. This is life-threatening and can only be remedied if you are eligible of liver transplant.
The liver is also a potent area for colorectal cancer metastasis as the portal vein, the main source of blood supply comes from the intestines. It is estimated that around 70% of those with colorectal cancer will have metastases in the liver. As its foremost function to filter blood, if the liver is compromised in any way during this process, this will leave the person at risk of sepsis, multi-organ failure and death.
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