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2014-‐07-‐17 Recall The Diges)ve System Learning About Our Guts! • Diges-on has four stages – Inges-on – Diges-on (mechanical and chemical) – Absorp-on – Elimina-on The Alimentary Canal Inges)on/Diges)on • Adult humans have a diges-ve tract that ranges from 6.5m to 9m • Although it is one con-nuous tube, it can be broken down into specialized segments • Inges-on occurs when food is put into the mouth • Mechanical and chemical diges-on both begin in the oral cavity (mouth) Mechanical – Food is mas)cated (ground down by teeth) and formed into bolus by tongue Chemical diges-on – Saliva aids the chemical breakdown of carbohydrates (by salvary amylase enzyme) Diges)on -‐Chemical-‐ Diges)on -‐Chemical-‐ Saliva – a watery secre-on in the mouth that begins the diges-ve process • Contains amylase enzymes – break down starches (complex carbs) • Lubricates food for swallowing • Dissolves par-cles & makes food tas-ng possible • Dissolved par-cles penetrate cells of taste buds on tongue and cheeks • Nerve cells for taste s-mulated when receptor sites filled by chemical compounds with complementary shape Salivary Glands • Sublingual glands release saliva into mouth through smaller ducts 1 2014-‐07-‐17 Diges)on -‐Mechanical-‐ Teeth Diges)on • Glands in the lining of the esophagus produce mucus which keeps the passage moist and aids in swallowing • The bolus then enters into the stomach through the esophageal sphincter (aka cardiac sphincter) Diges)on • Esophagus – muscular tube through which food passes from the mouth to the stomach • Bolus ac-vates peristalsis – Involuntary rhythmic muscle contrac-on moves the bolus down esophagus into the stomach Diges)on • Sphincters – regulate movement of food into and out of the stomach • Cardiac sphincter– relaxa-on allows food to enter the stomach • Pyloric sphincter – allows food to exit stomach (enter duodenum) Diges)on • Stomach – 1.5L capacity, “j” shaped, food storage • Located in the abdominal cavity • Rugae -‐ stomach walls are folded, allows for expansion when full • Stomach also undergoes peristalsis to con-nue mechanical diges-on and mixing of food with gastric juices • 3 layers of muscle fibres that contract and then relax to churn and mechanically break up pieces of food • Chyme – a thick liquid produced in the stomach and made of digested food combined with gastric juice 2 2014-‐07-‐17 Diges)on Chemical Diges-on • Gastric juices – secreted by glands in stomach • Mixture of HCl(aq), salts, enzymes, water, and mucus • Alkaline mucus protects stomach lining from being digested by gastric juices Absorp)on is made possible by enzymes Diges)on HCl • Kills harmful substances • Converts pepsinogen (protein diges-ng enzyme) to its ac-ve form pepsin • PH ranges from 2.0 – 3.0 Absorp-on • Diges)ve enzymes – help break down food substances into forms that can be absorbed and used by the body • Diges)ve enzymes are normally secreted… • In the mouth (part of saliva) • By the stomach • Released into small intes-nes from liver and pancreas Enzymes Involved in Diges)on Enzymes Involved in Diges)on • Amylase: • Protease: – breaks down of – breaks down protein starches. – Secreted by pancreas – (secreted in the saliva and the pancrea-c • Pepsin juices) – Breaks down protein • Mycozyme: – Released in stomach – Breaks down starches – (secreted in stomach) • Lipase: – breakdown fats (lipids) – (secreted by the pancreas) 3 2014-‐07-‐17 Absorp)on • Most diges-on occurs in the small intes-ne • Up to 7m in length, 2.5cm in diameter • Three dis-nct parts: – Duodenum – Jejunum – Ileum • Contains folds (villi) Absorp)on SMALL INTESTINE Duodenum • First 25 – 30 cm • Not mucus lined (cannot handle stomach acids) • Liver and pancrea-c secre-ons bring pH up to between 5-‐6 Absorp)on: Accessory Organs Liver • Largest internal organ (1.5kg) • Secretes bile, a greenish-‐ yellow fluid • Helps to digest fat • Green colour from destruc-on of old RBCs (pigment eliminated as waste) Absorp)on • Villi – finger like projec-ons lining the surface of the small intes-ne that increase the surface area to improve the absorp-on of nutrients Absorp)on: Accessory Organs Pancreas • Secretes 1L of pancrea-c fluid into duodenum daily • Pancrea-c fluid contains: – Enzymes to digest carbohydrates, lipids, proteins – Bicarbonate (helps to regulate enzyme func-on and pH in duodenum) Absorp)on: Accessory Organs Gall Bladder • Stores bile that is created in liver • Contains bile salts – Essen-al for diges-on of fats – Fats are insoluble, are suspended in chime as small droplets – Bile salts act like detergent and allow for emulsifica-on 4 2014-‐07-‐17 Absorp)on SMALL INTESTINE Jejunum • Next 2.5m • Contains lots of folds of the inner wall • Breaks down remaining proteins and carbohydrates • pH between 7 – 9 Absorp)on The Importance of pH on Enzyme Ac)vity • Enzyme func-on and rate of ac-vity is governed by: – Temperature – pH Absorp)on SMALL INTESTINE Ileum • Last 3m • Contains fewer and smaller villi than duodenum or jejunum • Func-on is to absorb nutrients, as well as to push remaining undigested material into large intes-ne • pH between 7-‐9 Absorp)on LARGE INTESTINE • Also known as colon • Absorb water from alimentary canal • About 90% of H2O is absorbed back into the blood • Volume of indiges-ble food mostly (indiges-ble plant maher) is reduced by about 2/3 Absorp)on LARGE INTESTINE • Three sec-ons: – Ascending – Transverse – Descending • Billions of anerobic bacteria • Some of these bacteria produce vitamins: folic acid, B vitamins, vitamin K ELIMINATION 5