It is still not scientifically proven why some animals don’t have a gallbladder. Some proposed hypotheses have strong evidence in support, while others are merely conjectures. We shall discuss them one by one. But to understand them properly, you need to remember the following points –
- Gall bladder collects. stores, and concentrates bile secreted by the liver.
- Bile salts help in fat emulsification in the intestinal lumen.
- Bile is released on food intake, mainly fatty food acts as a stimulus for bile secretion.
- The storage action reduces the number of cycles of enterohepatic circulation of bile.
- Fat helps in the absorption of fat-soluble vitamins. So this regulation of entero-hepatic circulation also regulates vitamin level in the body.
- Bile salts have harmful effects on intestinal mucosa when applied chronically. Cholecystectomy increases the risk of colon and pancreatic carcinomas.
Now the Hypotheses
The frequency of food intake – animals who take their food more frequently, requires a continuous supply of bile for proper digestion of that food. Reduced storage demand thus explains the loss of gall bladder in them.
Examples include – deer family (except Musk deer), horse, etc. these animals live on grass fields and takes their food continuously.
Food habit – fat is more available to carnivores than herbivores. So the lions, tigers, cats retained their gall bladder, while elephants, deer lost it
Ancestry – some animals in spite of having carnivorous food habits lack a gall bladder, this is explained by their herbivorous ancestors who lost their gall bladder. The whale is an example of this phenomena
Nature of bile salts – bile contains bile salts. Bile salts can be relatively water-soluble or insoluble depending on their biochemical structure. Hydrophobic salts tend to aggregate to form gallstones, they are also more harmful to the intestinal mucosa than hydrophilic salts
Those animals in whom proportionately more hydrophobic bile salts are produced tend to lack gall bladder, because of the chance of accumulation of bile
Body size – some very large animals lack gall bladder to reduce the weight that would have resulted from the storage of bile.
Whale, elephant lack gall bladder.
Absent liver – liver is absent in invertebrates. So gall bladder is unnecessary in them.
The disappearance of the liver in later phases of the life cycle – gall bladder disappears with it. This occurs in the lamprey.
Sphincter of Oddi – mice have more muscle tone in the sphincter of Oddi, compared to rats. Rats lack a gall bladder, while mice have one. Less sphincter of Oddi tone reduces the chance of bile retention.
Bile concentrating power of liver and biliary ducts – bile within the liver is more concentrated in the case of rats compared to mice. So the requirement of concentration of bile is less in the case of rats.
Unexplained cases – the presence of gall bladder in teleost fishes is highly variable. A proper hypothesis is lacking.
Dr. Arnab Mukherjee
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