A&P of the liver, structure and functions, hepatic lobules (sinusoids, blood flow, hepatocytes, flow of bile, other cells of hepatic tissue). Live functions: blood filter, metabolism of carbohydrates, lipids and proteins, detoxification.
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The liver is an essential blood filter and the largest exocrine and endocrine gland.
The liver has 4 lobes: right, left, quadrate and caudate.
The liver has a dual blood supply:
– 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, and immediately drains into the inferior vena cava.
A liver contains 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, separated by blood-filled channels called sinusoids. Sinusoids are basically capillaries with fenestrated endothelium and no basement membrane. This 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.
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. Liver is responsible for production of many plasma protein including albumin and clotting factors.
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.