The Liver

Definition:

The liver is a reddish-brown, wedge-shaped organ located in the upper right quadrant of the abdomen. The human liver weighs approximately 1.5 kilograms (3.3 pounds) and comprises ~2% of total the body weight. It’s the largest gland in the human body.

It performs numerous essential functions, including metabolism, detoxification, protein synthesis, and the production of biochemicals necessary for digestion.

Anatomy of the Liver

The liver has four main lobes: the right lobe (larger), left lobe, caudate lobe (posterior), and quadrate lobe (inferior). These lobes are further divided into smaller units called lobules, which are the functional units of the liver.

Parts of the Liver:

  1. Lobes:
    • The liver is divided into two primary lobes:
      • Right Lobe: The larger of the two lobes.
      • Left Lobe: The smaller lobe.
    • Additionally, the liver has two smaller lobes located on the underside:
      • Caudate Lobe
      • Quadrate Lobe
  2. Lobules:
    • The liver is made up of small functional units called lobules. Each lobule consists of hepatocytes (liver cells) arranged in plates radiating from a central vein.

Liver Functions:

  1. Metabolism:
    • Carbohydrate Metabolism: The liver helps maintain blood glucose levels by storing glucose as glycogen (glycogenesis) and breaking it down (glycogenolysis) when needed. It also performs gluconeogenesis, the synthesis of glucose from non-carbohydrate sources.
    • Lipid Metabolism: The liver synthesises cholesterol, triglycerides, and lipoproteins. It also converts excess carbohydrates and proteins into fatty acids and triglycerides for storage.
    • Protein Metabolism: The liver synthesises non-essential amino acids and proteins, including albumin and clotting factors. It also converts ammonia, a toxic byproduct of protein metabolism, into urea for excretion.
  2. Detoxification:
    • The liver detoxifies and excretes various endogenous and exogenous substances, including drugs, alcohol, and environmental toxins. It converts these substances into less harmful compounds that can be excreted in bile or urine.
  3. Bile Production:
    • The liver produces bile, a fluid that aids in the digestion and absorption of fats and fat-soluble vitamins in the small intestine. Bile is stored in the gallbladder and released into the duodenum during digestion.
  4. Storage:
    • The liver stores essential nutrients, including vitamins A, D, E, K, and B12, as well as minerals like iron and copper. Glycogen, a form of glucose storage, is also stored in the liver.
  5. Immune Function:
    • The liver contains Kupffer cells, a type of macrophage that helps remove pathogens and debris from the blood. It also plays a role in modulating immune responses.
  6. Bilirubin Metabolism:
    • Production: Bilirubin is a byproduct of the breakdown of haemoglobin from red blood cells. The liver processes this bilirubin by converting it from its unconjugated (indirect) form to its conjugated (direct) form.
    • Excretion: Conjugated bilirubin is excreted into bile and eventually into the intestines, where it helps digest fats. It is ultimately excreted from the body in stool, which gives stool its characteristic brown colour.

Liver Circulation:

Dual Blood Supply: The liver has a unique dual blood supply:

  1. Hepatic Artery:
    • Supplies oxygenated blood from the systemic circulation. This accounts for about 25% of the liver’s blood supply.
  2. Portal Vein:
    • Supplies nutrient-rich blood from the gastrointestinal tract, spleen, and pancreas. This accounts for about 75% of the liver’s blood supply.

Blood Flow Pathway:

  • Blood enters the liver through the hepatic artery and portal vein.
  • The blood from both sources mixes in the liver sinusoids, which are capillary-like vessels within the liver.
  • Hepatocytes, the liver cells, process the blood, absorbing nutrients, and detoxifying substances.
  • Blood exits the liver through the hepatic veins, which drain into the inferior vena cava and return blood to the heart.

Sinusoids:

  • Liver sinusoids are highly permeable capillaries that allow for extensive exchange between the blood and hepatocytes.
  • Kupffer cells, located in the sinusoids, help filter out pathogens and debris from the blood.

Bile Flow Pathway:

  • Hepatocytes produce bile, which is secreted into bile canaliculi, small channels between liver cells.
  • Bile flows through the canaliculi into bile ducts, eventually draining into the common bile duct.
  • Bile is stored in the gallbladder or released into the duodenum to aid in digestion.