Bile and the Gall Bladder

Bile

Lipids begin to be digested in the mouthThe opening of the digestive tract where food enters and mastication begins.. Just like the cheese in your sandwich, salivary lipases start the breakdown process. But lipidsOrganic molecules including fats, oils, and steroids. are more difficult to digest because they are not water-soluble. Most lipid digestion occurs in the small intestines, where bileA digestive fluid produced by the liver and stored in the gallbladder; it helps emulsify fats for di from the liverA large organ that produces bile, detoxifies blood, and stores nutrients. and pancreatic lipases break the fats down.

Bile contains things that are necessary for lipid digestion.  Bile is mostly made of cholesterolA lipid molecule that is a key component of cell membranes and a precursor for bile acids and steroi because like dissolves like.  If we want to “dissolve” these fats we are eating, we will need a similar substance to do the dissolving.  Bilirubin is a pigment that gives mostly a yellow, sometimes a brown color.  You may associate diseases of the liver with the skinThe body’s largest organ, providing protection and regulation. condition jaundiceA yellowish skin discoloration caused by excess bilirubin, often due to liver dysfunction..  Jaundice results from inappropriate recycling of your bile.  Instead of excess bile going into fecesSolid waste material formed in the large intestine and expelled through the anus. and leaving the body, it is retained in the liver.  When the liver surpasses its storage space for bilirubinA yellow pigment produced from the breakdown of hemoglobin, excess amounts cause jaundice., it will start to accumulate in other parts of the body.

Bile contains salts that emulsify fats. Emulsify means to combine two or more liquids together that don’t easily mix, like oil and waterThe universal solvent essential for life.. In factA statement based on direct observation that is repeatedly confirmed., emulsifying is what you do to make a good salad dressing.  You have to use the blender to “cut up” the oil as you add it to the vinegar.  Also, Dawn dish soap has a claim on its label calling it a powerful emulsifier.  It is.  Dawn dish soap is used on wildlife that is covered in oil and other nonpolar substances from human activity.  The bile salts act like dishwashing detergent. They break up the fat into very small lipid balls. These get trapped in little phospholipid bubbles called micelles. This is only mechanical digestion because all we are doing with bile is making the lipid smaller. 

All this mechanical digestion simplifies the task. It helps the pancreatic lipases to perform chemical digestion of the triglycerides in the lipid.   That is where the next accessory organ, the pancreasA gland that produces digestive enzymes and hormones like insulin and glucagon., gets involved. The pancreas releases pancreatic lipases, which facilitate the breaking of the fatty acids from the triglyceride headRounded proximal end that fits into the acetabulum of the hip bone..

Lipid Digestion

Lipid digestion occurs very differently than carbohydrateOrganic molecules composed of carbon, hydrogen, and oxygen, primarily used for energy. or protein digestion.  With carbs and proteinsLarge molecules made of amino acids with various functions in the body., the breakdown products can be transported across the epithelium. They enter the venulesSmall veins that collect blood from capillaries and transport it to larger veins. present in a villus, such as one pictured here.  Small, polar breakdown products enter this venule where they will be transported to the liver via the hepatic portal veinThe main vein that transports blood from the digestive tract to the liver for processing..  Lipids are transported through these epithelial cellsThe basic structural and functional units of life. of the small intestine’s ileumThe last section of the small intestine, responsible for absorbing vitamin B12 and bile salts. but lipids enter this lactealA lymphatic vessel within a villus that absorbs dietary fats..  Lacteals are present in small intestine villiFinger-like projections in the small intestine that increase surface area for absorption. and are able to absorb large, nonpolar substances, like lipids.  This lacteal and all others in the small intestine are lymphatic vessels. They all converge and dump their contents into the subclavian vein.

The bile salts have emulsified the lipids. Smaller lipid droplets are now sequestered in micelles. These micelles contain lipid-soluble vitamins and maybe some excess cholesterol used to emulsify the lipid.  Micelles are pretty cool.  They are arranged like a lipid uni-layer.  They form just one row of phospholipid. Their polar heads face the outside aqueous environment. The nonpolar tails face the lipid and cholesterol on the inside.  I have a make-up remover that calls itself a “micellar” make-up remover.  Mascara is a lipid that coats your eyelashes and many eye shadows are also a lipid-based product.  Micelles are organized such that the water-soluble portions face the outside and the lipid-soluble portions face towards the center. This allows the micelles to readily interact with the cell surface.

Micelles and cell membranes are made of phospholipids. When a micelle contacts an epithelial cell of the small intestine, the contents inside it diffuse into the intestinal epithelial cell. This forms a water-soluble lipoprotein called a chylomicron, which is transferred to the lacteal. Chylomicrons are packaged with proteins, giving them the name lipoproteins.  At first, a low density lipoprotein (LDL) is formed.  Chylomicrons give the lymph fluid in lacteal a milky appearance and so we call the lymph contained in lacteals chyle.  Chyle, like all lymph, is dumped into general circulation via the subclavian vein.  Chylomicrons then travel to cells where the fatty acids within can be used for generating ATPThe energy currency of cells used for muscle contraction..  As chylomicrons lose contents, they become more dense, forming high density lipoproteins (HDLs).

Most of this process occurs in the ileum, the last portion of the small intestine. Any unabsorbed fat is shunted out with the feces, which results in the brownish color of feces from the bilirubin.

Ducts

Common hepatic duct drains bile from all of the central veinsBlood vessels that return deoxygenated blood to the heart (except pulmonary veins, which carry oxyge of the lobules in the liver. Most bile is shunted through the cystic duct which leads to the gallbladderA small organ beneath the liver that stores and releases bile into the small intestine. where the bile is stored until needed. When needed, the gallbladder can contract using its smooth muscle. This action pushes the bile through the cystic duct and into the bile duct. The bile duct then joins with the pancreatic duct. At this point, fluids from the pancreas contain tons of digestive enzymesProteins that speed up chemical reactions in the body.. These enzymes and the bile from the liver help with processing. They gather together at the hepatopancreatic sphincter. The hepatopancreatic sphincter is the valve that controlsVariables that remain constant to ensure a fair test. the hepatopancreatic ampulla. This ampulla is the opening connecting the plumbing from these accessory organs and the small intestine.

Something you may not have noticed is that bile ducting in the liver is not a circulatory system. Bile is drained from the liver and injected into the small intestine.

Gall Bladder

After being stored, bile is injected into the small intestine. If a person’s gallbladder is removed, the cystic duct can enlarge and function as a new gallbladder. The gall bladderA muscular organ that stores urine before excretion. not just stores bile, but it also concentrates bile.  This is how gall stones can form.  Gall stones are basically little rocks of bile salts.  These little rocks don’t flow out of the gall bladder and can accumulate in there, causing pain.  Yes, you can remove the gall bladder, as we mentioned.