The Small Intestine. The Small Intestine

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1 3 ANATOMY.notebook Location: Abdominal Cavity Physical description: Your small intestine is approximately six metres long. (It the longest section of your digestive tract.) Although it is longer than your large intestine it has a smaller diameter. (This is why it's called the small intestine.) It has an average diameter of 2.5 cm. The absorptive surface area is roughly 250 square meters. (the size of a tennis court) Mechanical Digestion: After food is churned up in your stomach, a sphincter muscle at the end of your stomach opens to squirt small amounts of food into the top of your small intestine. This first section of the small intestine is called the duodenum. Mechanical digestion in the small intestine involves a coordinated set of contractions of smooth muscle which facilitates digestion and absorption in the small intestine. The primary functions of the small intestine are... the completion of chemical digestion of food the absorption of most nutrients 1

2 3 ANATOMY.notebook The small intestine is where most of the food you digest is absorbed into the blood stream. It contains many folds covered in tiny projections known as villi, which in turn are covered in even tinier projections known as microvilli. The microvilli contain blood vessels (capillaries) that absorb nutrients. Food is absorbed (taken in) to the body in the small intestine. The wall of the small intestine has small holes in it. Only small particles can pass through it: 2

3 3 ANATOMY.notebook Most digestion occurs in the first portion of the small intestine, while absorption occurs along the rest of the length. Duodenum: the first section of the small intestine is a short section that receives secretions from the pancreas and liver via the Duodenum: the first section of the small intestines which is a short section that receives secretions from the pancreas and liver via the pancreatic and common bile ducts. The food is a highly acidic mush and needs to be neutralised. pancreatic and common bile ducts. The food is a highly acidic mush and needs to be neutralized. Jejunum: considered to be roughly 40% of the small gut in man, but closer to 90% in animals. Ileum empties into the large intestine; considered to be about 60% of the intestine in man, but veterinary anatomists usually refer to it as being only the short terminal section of the small intestine. The Pancreas The primary functions of the pancreas is to manufacture chemicals (enzymes) for the chemical break down of food pancreatic fluid also contains bicarbonate which alters the chyme from very acidic (ph 1) to weakly basic (ph 8) these conditions make the enzymes of the pancreas work most efficiently the pancreas also manufactures the hormones insulin and glucagon, which regulate carbohydrate metabolism these hormones are made by cell clusters called "Islets of Langerhans" which are interspersed throughout the tissue chemical or mechanical digestion is NOT done in the pancreas 3

4 3 ANATOMY.notebook The Pancreas The pancreas manufactures the following enzymes... The Liver Location: Under your diaphragm, more to the right side of your body Physical description: * The liver is a wedge shaped, spongy organ. * The liver is the largest internal organ. * The average liver weighs about 1.6 kilograms. * Has 2 main lobes (large and small) * The average liver measures about 20 cm horizontally (across) and 17 cm vertically (down) and is 12 cm thick. 4

5 3 ANATOMY.notebook The Liver and Gall Bladder Mechanical and Chemical digestion is NOT done in the liver. The liver manufactures bile for use in helping in the digestion of fats. Bile salts, contained in bile, act like detergent, physically breaking up the fat droplets into smaller fat droplets. This creates a greater surface area to be exposed for the digestive enzymes. The Liver and Gall Bladder The gall bladder is a small sack located under the liver. The gall bladder is connected to the digestive tract through the bile duct. The gall bladder secretes the stored bile when fatty food is ingested. The gall bladder does NOT manufacture bile, it STORES bile for later use. 5

6 3 ANATOMY.notebook Jejunum: about 2.5 m long contains more folds than the duodenum breaks down the remaining proteins and carbohydrates so that they can be absorbed by the bloodstream Ileum about 3 m long contains fewer and smaller vili than either the duodenum or the jejunum absorbs nutrients, as well pushes the remaining undigested material into the large intestine Chemical Digestion: Chemical digestion in the small intestine involves various enzymes. Proteins: digested by protease Fats: digested by bile (physical) and lipase (chemical) Carbohydrates: digested by pancreatic amylase 6

7 3 ANATOMY.notebook Pancreatic amylase is released into the small intestine to digest carbohydrates into monosacharides (single sugars) which can be absorbed into the blood stream. Dietary lipids (fats and oils) do not dissolve in water, as a result they are not easily broken down by fat digesting enzymes called lipase in the watery content of the gastrointestinal tract. Fats must first be physically broken down into small droplets by bile salts so that they may be chemically digested by the lipase enzymes. Bile salts are produced in the liver but stored in the gallbladder. They enter the duodenum via the bile duct. Lipase enters the duodenum from the pancreas and chops up lipid molecules into fatty acid molecules and glycerol molecules. 7

8 3 ANATOMY.notebook The stomach begins protein digestion secreting HCl and pepsin. Then the other proteases are added to the small intestine to further digest the protein. Absorption in the Small Intestine Monosaccharides absorbed into the bloodstream are transported to the liver where monosaccharides (other than glucose) are converted to glucose glucose is carried to parts all over the body for energy. excess glucose is converted to glycogen. Amino Acids are absorbed into the bloodstream and carried: to the liver where they are converted to sugar for energy releasing reactions to other cells of the body where they are used to make enzymes and other protein such as keratin which forms skin and hair Glycerol and Fatty acids are absorbed by the lacteal they are reassembled to form triglycerides triglycerides are coated with protein to make them watersoluble so they can be transferred into the bloodstream provide energy to the cells 8

9 3 ANATOMY.notebook In a 24 hour period, the small intestine it is capable of absorbing... * several kilograms of carbohydrate * up to 1 kg of fat * 500 g protein * 20 litres of water The small intestine is... * the sole site for absorption of amino acids * the sole site for absorption of monosaccharides * the main site for absorption of lipids It normally takes about minutes for the first part of a meal we have eaten to reach the large intestine, and the last portion of the meal may not reach the large intestine for five hours. 9