The liver is one of the most important organs.
In addition to being an essential blood filter and the largest exocrine and endocrine gland,
it has hundreds of other functions concerning almost every body system.
The liver has 4 lobes: right, left, quadrate and caudate.
A unique feature of the liver is that it has a dual blood supply, namely:
- the proper hepatic artery carrying oxygenated blood from the aorta, and
- the hepatic portal vein carrying venous drainage from most of the gastrointestinal
(GI) tract, plus the spleen and pancreas.
This is how substances absorbed through the GI tract, including nutrients, toxins and
pathogens, are first screened and processed in the liver before they can reach the general
circulation.
These 2 vessels enter the liver via an opening in its inferior surface, which also serves
as the exit point for the common hepatic duct carrying bile.
Blood leaves the liver in hepatic veins, at its superior surface, and immediately drains
into the inferior vena cava.
A liver contains nearly a million of functional units called hepatic lobules.
A lobule is typically shaped like a 6-sided prism, appearing as a hexagon in cross-section.
At the corners of the hexagon are hepatic triads which are branches of the hepatic portal
vein, hepatic artery and bile duct.
In the center is a central vein - a branch of hepatic vein.
About 80% of the liver mass is made of specialized epithelial cells called hepatocytes, which
carry out most of liver functions.
In a lobule, hepatocytes are arranged in plates, or cords, mostly 1-cell thick, separated by
blood-filled channels called sinusoids.
Sinusoids are basically capillaries with fenestrated endothelium and no basement membrane.
This unusual tissue arrangement allows each hepatocyte to be fully submerged in blood
plasma, enabling free exchange of substances.
Blood from the portal vein and hepatic artery mixes together in sinusoids; after substance
exchange with hepatocytes, it pools into central veins, and exits the liver via hepatic veins.
Other cells of the hepatic tissue include liver-specific macrophages (Kupffer cells),
natural killer cells, and stellate cells with a variety of functions.
As an exocrine gland, the liver produces bile to aid with digestion of fats.
Bile is secreted into tiny canaliculi sandwiched between hepatocytes, then pooled into bile
ductules of the triads, and ultimately into left and right hepatic ducts, which converge
and exit the liver as the common hepatic duct.
Bile flows into the small intestine during digestion.
Outside mealtimes, it is diverted to the gallbladder for concentration and storage.
Metabolic wastes, such as bilirubin - a byproduct of red blood cell breakdown, can be secreted
in bile and excreted from the body.
As an endocrine gland, the liver directly synthesizes several hormones, and is involved
in the metabolism of many others.
The liver acts as a gatekeeper controlling substances that can enter the circulation,
both quantitatively and qualitatively.
It processes the nutrients and regulates the amount of nutrients that can pass into the
blood.
For example, after a meal, when glucose spikes from digestion of carbs, the liver converts
excess glucose into glycogen for storage.
When the body is fasting, glycogen is converted back to glucose to be released to the blood.
Likewise, the liver also serves as a storage for several vitamins and minerals.
The liver has a central role in lipid metabolism.
It synthesizes endogenous cholesterol and fats, and pack them, together with dietary
lipids, into particles of very-low-density lipoprotein to deliver them to other organs,
via the bloodstream.
Excess cholesterol is transported back to the liver, where it is secreted in bile and
excreted in feces.
The free amino acids resulting from protein digestion are also processed in the liver,
where they are synthesized into new proteins and pro-enzymes.
In fact, the liver is responsible for production of many plasma protein including albumin and
clotting factors.
Excess free amino acids, which can be harmful, are converted to other forms of energy storage,
or broken down to urea to be removed in waste.
The liver is a detoxification organ.
It screens the blood for potentially toxic substances and pathogens, and removes them
before they can reach the rest of the body.
Similarly, it also metabolizes, and may inactivate, orally administered medicines.
This is known as the first-pass effect.
For this reason, some medicines must be taken via other routes to bypass liver metabolism.
On the other hand, some drugs are specifically designed as pro-drugs, which only become active
after being metabolized by the liver.