Bile, also known as gall, is a fluid produced by the liver in majority of the vertebrates. This fluid tastes bitter and its color ranges from deep green to yellowish brown. Bile facilitates the digestive process involving the lipids in the small intestine. In other words, bile can be described as a thick fluid that facilitates digestion of ingested foods. In several species, bile is accumulated in the gallbladder and when they eat something it is sent out into the duodenum. Bile facilitates in breaking down fats into simpler fatty acids that are soaked up by the body from the digestive tract. The chemical composition of bile is as follows: water (85 per cent), bile salts (10 per cent), pigments and mucus (three per cent), fats (one per cent), inorganic salts (0.7 per cent) and cholesterin (0.3 per cent).
According to medical suppositions widespread in the West from the period between the Classical Antiquity and the Middle Ages, the health of an individual's body depends on the balance or stability among four 'humors' or the essential fluids - blood, yellow bile (also known as 'ichor'), black bile and phlegm. Presence of too much of yellow bile and black bile is known to generate aggression as well as depression respectively. In effect, the Greek terms of the yellow bile and black bile led to the creation of the English words 'cholera' and 'melancholia'. These theories elucidate how the English term 'bilious' was derived from the word bile as well as the connotation of the term gall in English as 'impudence' or 'exasperation'.
Hepatocytes produce bile, which then passes through the numerous bile ducts that infiltrate into the liver. All through this process, a watery solution rich in bicarbonates is added to bile by the epithelial cells to dilute as well as enhance the alkalinity of the fluid. Subsequently, bile passes into the regular hepatic duct that connects with the cystic duct emerging from the gallbladder to form the common bile duct. In turn, the common bile duct connects with the pancreatic duct to open into the duodenum. When the sphincter of Oddi is shut down, it prevents bile from secreting into the intestine and, in its place, the liquid passes on into the gallbladder.
The gallbladder not only stores bile, but also increases its concentration to as many as five times its original strength or power between meals. The increase in its potency or concentration takes place by means of absorbing water as well as minute electrolytes, even as bile retains all its primary organic molecules. It may be noted that along with bile, cholesterol is also released, dissolved in the acids and fats present in the condensed solution. When the ingested food passes from the stomach to the duodenum in the shape of chyme (the semi-fluid mass into which food is changed by gastric discharges), the duodenum secretes cholecystokinin. In fact, it is cholecystokinin that induces the gallbladder to secrete the condensed bile to complete the digestive process.
Depending on the size of an individual's body, the liver of humans is able to produce nearly one litre of bile every day. Approximately, 95 per cent of the salts released in bile are again soaked up in the terminal ileum (the third and lowest part of the small intestine that extends from the jejunum to the cecum) and used again. In effect, blood from the ileum directly flows to the hepatic portal vein and goes back to the liver, wherein the hepatocytes once again absorb the salts and send them back to the bile ducts for reuse. This process occasionally occurs twice or thrice every time the individual takes a meal.
Up to a certain level, bile works as a surfactant (a surface-active agent) facilitating softening of the fats present in the ingested foods. The anions (negatively charged ions) present in bile salt possess a hydrophilic (affinity for water) aspect and, hence, they have a tendency to gather around fat droplets (triglycerides and phospholipids) to materialize into micelles (molecular aggregates of colloidal dimension) having the hydrophobic side in the direction of fats and hydrophilic towards the outer side. It may be noted that the hydrophilic surfaces are charged positively owing to the lecithin as well as other phospholipids that make up bile. In fact, the positive charge thwarts the fat droplets covered with bile from aggregating again to form large particles of fat.
In general, the micelles present in the duodenum measure approximately 14 μm to 33 μm in diameter.
The distribution of the fats in food into micelles actually mainly provides an augmented surface area for the enzyme pancreatic lipase to work on. In fact, this enzyme digests the triglycerides and has the aptitude to reach the fatty interior by means of the gaps present between the bile salts. The digestive process involves breaking down a triglyceride into two fatty acids as well as a monoglyceride, which are all soaked up by the villi (one of the tiny finger-like projections on specific membranes, like in the intestine) present on the walls of the intestine. Once the fatty acids are transferred through the intestinal membranes, they are restructured into triglycerides and subsequently absorbed into the lymphatic system (a network of blood vessels, tissues and organs in vertebrates that facilitates the body to control fluid equilibrium as well as combat infection) by means of the lacteals (any lymphatic vessel passing on chyle from the small intestine to the thoracic duct). In the absence of the bile salts, most of the lipids present in the ingested food would remain undigested and pass out in feces.
As bile helps to facilitates fat absorption by the body, it also forms a vital part of the absorption of substances that are soluble in fats - for instance, vitamins A, D, E and K. In addition to the digestive function of bile, it also functions as the way to excrete bilirubin - a spin-off of red blood cells (RBC) that are reprocessed by the liver. Bilirubin is derived from haemoglobin through a process called glucuronidation.
Besides, bile, which is alkaline by nature, also serves in counterbalancing excessive stomach acid before it is able to enter the ileum - the third and last division of the small intestine. Additionally, the bile salts also function as bactericides and help to eliminate several microorganisms that may be present in the food.
Any imbalance in the elements that make up bile - for instance, bilirubin, bile salts and cholesterol, results in the formation of gallstones. Generally, cholesterol is maintained in liquid form owing to the dissolving actions of the bile salts - an enhanced amount of cholesterol in the bile beats the capacity of the bile salts to dissolve and results in the development of cholesterol gallstones. Likewise, any shortage of bile salts also helps the formation of gallstone.
On the other hand, pigment or color gallstone is often related to chronic infection in the bile, particularly in some Asian nations wherein parasitic contagion of the bile duct is widespread. People who have been enduring ailments related to blood that result in unwarranted disintegration of the red blood cells may also possess augmented amounts of bilirubin (a product of red blood cell breakdown) in the bile. This, in turn, results in the formation of bilirubin gallstone.