It is still not scientifically proven why some animals don’t have a gallbladder. Some proposed hypotheses have strong evidences 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 –
1. Gall bladder collects. stores, and concentrates bile secreted by the liver.
2. Bile salts help in fat emulsification in the intestinal lumen.
3. Bile is released on food intake, mainly fatty food acts as a stimulus for bile secretion.
4. The storage action reduces the number of cycles of enterohepatic circulation of bile.
5. Fat helps in absorption of fat-soluble vitamins. So this regulation of entero-hepatic circulation also regulates vitamin level in the body.
6. 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 takes 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 habit lack a gall bladder, this is explained by their herbivorous ancestors who lost their gall bladder.
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 tends to lack gall bladder, because of the chance of accumulation of bile
- Body size – some very large animals lack gall bladder to reduce weight that would have resulted from 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 liver in later phases of the life cycle – gall bladder disappears with it. This occurs in 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 case of rats compared to mice. So the requirement of concentration of bile is less in case of rats.
- Unexplained cases – the presence of gall bladder in teleost fishes is highly variable. A proper hypothesis is lacking.
N.B. We always keep the articles short and concept oriented to help build long-term memory in a short time 😜.
Replied by Narayan.
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