Unit 4—Maintenance of the Human Body Dr. Achilly 1

Part 1: The Digestive System “Concepts” chapter 26 2

Make sure you know the position/orientation of the organs of the GI tract 3

The digestive system--overview The food we eat contains lots of nutrients, but most of them are too large to be taken in by our cells. We must “digest” those nutrients, or make them smaller, so that they can be used. The digestive system performs 6 basic functions. 4

The digestive system -overview Ingestion—taking food into mouth. Secretion—cells that line digestive tract secrete chemicals that aid in the breakdown of nutrients. Mix & propel—myo contraction helps to mix and move the food thru the tract. 5

The digestive system -overview Digestion—a mechanical & chemical process. Absorption—when the products of digestion are brought into the cells that line the GI tract. Defecation—anything that’s not absorbed (cells & bacteria too!) leave the body as feces via the anus. 6

The digestive system -overview The digestive system is basically a tube formed from 4 layers of tissue. Mucosa is the inner lining. It contains cells that secrete enzymes & chemicals for digestion. It contains blood & lymphatic vessels. There are also lymphatic nodules that hold immune system cells. Submucosa is the next layer. It is mostly connective tissue as well as blood & lymph vessels. 7

The digestive system -overview Muscularis layer contains muscle tissue that is involuntary (not under your control). These myo’s help to propel and break up food. Serosa is the outermost layer. 8

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The digestive system -overview None of the digestive system is under voluntary control. The myo’s and cells of the GI tract respond to chemical & stretch receptors located thru out the digestive system. Outside influences (fear, stress, anger) can alter GI fxn. 10

The digestive system A large serous membrane called the peritoneum covers many of the digestive organs. It has many folds that bind the organs to each other & contain blood & lymphatic vessels. The peritoneum is outlined in pink 11

The digestive system -overview The largest fold of the peritoneum is called the greater omentum. It drapes over the small & large intestines like a “fatty apron.” Contains many fat cells and lymph nodes. These lymph nodes hold immune cells which help to combat infection.

The digestive system -overview 13

The digestive system--mouth Digestive system begins at the mouth or oral cavity. Mouth is formed by cheeks, & the hard & soft palates. 14

The digestive system--mouth Salivary glands There are 3 pairs. Parotid Submandibular Sublingual 15

The digestive system--mouth Saliva Fxns to begin chemical digestion & to lubricate food. Contains water, mucus, enzymes that destroy bacteria, & some that start chemical digestion. Salivary amylase is an enzyme that begins starch breakdown. Lingual lipase is released in mouth, but not activated until it hits the acid in the stomach. It begins fat breakdown. 16

The digestive system—mouth Tongue Forms the floor of the oral cavity. Has receptors for taste. It is considered an accessory digestive organ because it is needed for digestion, but food does not pass thru it. Composed of myo & mucous membrane. There are extrinsic myo’s of tongue which originate outside of tongue, but connect to it. They move tongue from side to side & in & out. Intrinsic myo’s originate & insert w/in tongue. They alter size & shape of tongue. 17

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The digestive system—mouth Teeth Accessory digestive organs located in sockets in the gingivae which covers the mandible & maxilla. 19

The digestive system—mouth As food enters mouth, mechanical digestion results from chewing or mastication. A soft, flexible mass results which is called a bolus. It can be easily swallowed. It passes from the mouth into the pharynx. 20

The digestive system Pharynx is made of myo & lined by a mucous membrane. Divided into 3 parts: Nasopharynx Oropharynx Laryngopharynx Contractions propel food into esophagus. 21

The digestive system—swallowing Voluntary stage—tongue pushes food backward against palate. Involuntary stage—once food goes into oropharynx, medulla oblongata is stimulated. Soft palate & uvula move up to prevent food entering nasopharynx Epiglottis closes, so that food doesn’t enter trachea. Esophageal sphincter relaxes. Food enters esophagus. 22

Swallowing 23

The digestive system--esophagus Esophagus is a muscular tube. Lies posterior to trachea. Travels thru the mediastinum, anterior to vertebral column. Pierces diaphragm & ends at the superior portion of the stomach. 24

The digestive system--esophagus A wave-like motion of myo contractions called peristalsis, forces food down esophagus. When lower esophageal sphincter relaxes, food enters stomach. Takes 4-8 seconds from mouth to stomach. 25

The digestive system--stomach Stomach performs many fxns: Mixing Storage Chemical digestion Secretes the hormone gastrin. 26

The digestive system—stomach Stomach has 4 main regions: Cardia Fundus Body Pylorus It also has many ridges called rugae. Pyloric sphincter open up to the duodenum. 27

Stomach 28

The digestive system--stomach The stomach contains the 4 basic cell layers discussed before. It has specialized secretory cells called gastric glands which line narrow channels called gastric pits. 29

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The digestive system—stomach All of these secretions form gastric juice (we make 2-3 qt/day). Gastric juice continues the chemical break down. Proteins are broken into smaller polypeptide chains, triglycerides into fatty acids and monoglycerides, etc. Food + gastric juices = chyme. 31

The digestive system—stomach While chemical digestion is occurring, mechanical digestion continues. Stomach has rhythmic contractions. Small amounts of chyme are passed into duodenum at regular intervals. Stomach takes 2-4 hours to empty. Very little absorption here—only water, small ions, certain drugs & alcohol. 32

The digestive system Food enters the small intestines next. The rest of the chemical digestion occurs here & is dependent upon enzymes secreted by many accessory organs: Pancreas Liver Gall bladder 33

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The digestive system—pancreas Has an exocrine portion: Secretions are put directly into a duct (pancreatic juices enter the pancreatic duct) This duct empties the secretions into the duodenum Endocrine portion: These cells secrete hormones into the blood stream which maintain homeostasis. Insulin Glucagon others 35

The digestive system—pancreas Pancreatic juice: Sodium bicarbonate—neutralizes chyme coming in from stomach. Pancreatic amylase—digests starch Trypsin, chymotrypsin, carboxypeptidase, elastase— digest proteins Pancreatic lipase—digest triglycerides Ribonuclease, deoxyribonuclease—digest DNA & RNA 36

The digestive system—pancreas Many enzymes are secreted in inactive forms. They are activated by other enzymes in the small intestines once they arrive there. 37

The digestive system—liver & gallbladder Next to the skin, liver is heaviest organ in body. Located inferior to diaphragm. Gallbladder is a sac located on the posterior surface of liver. 38

The digestive system—liver & gallbladder Liver has a large right lobe & smaller left lobe. Liver is suspended by the falciform ligament, ligamentum teres (round ligament) & coronary ligaments. 39

The digestive system—liver & gallbladder Fxn’l unit of liver is a lobule. Shaped like hexagon. Made of cells called hepatocytes. They are arranged around a central vein and there are many permeable capillaries called sinusoids. 40

The digestive system—liver & gallbladder Bile is secreted by hepatocytes. Some enters small intestines right away, some is stored in gallbladder. Brownish liquid = water, bile salts, cholesterol, lecithin, bile pigments. Bile is part excretory product & part digestive secretion. 41

The digestive system—liver & gallbladder Excretory The pigment is bilirubin which comes from the hemoglobin of broken down red blood cells. Bilirubin is further broken down in intestines to stercobilin which gives feces their brown color. 42

The digestive system—liver & gallbladder Digestive secretion Bile salts help to emulsify fats. Emulsification breaks down large lipid globules into smaller ones, so that pancreatic lipase can act on them. Salts also help with lipid absorption. 43

The digestive system—liver & gallbladder Blood supply of liver: Receives oxygenated blood from hepatic artery. Gets deoxygenated, but nutrient rich blood from hepatic portal vein. Blood goes to sinusoids. Oxygen, nutrients & toxins enter hepatocytes. Products of hepatocytes & unneeded nutrients secreted back into blood via central vein to hepatic vein. 44

The digestive system—liver & gallbladder Portal triad 45

The digestive system—liver & gallbladder The liver does a lot!! Regulates blood glucose levels Lipid metabolism Protein metabolism Processes drugs & hormones Excrete bilirubin Make bile salts Storage Phagocytosis of old cells & bacteria Activates vitamin D 46

The digestive system—small intestines Small intestines has 3 sections: Duodenum Jejunum Ileum It joins the large intestine at the ileocecal spinchter. 47

The digestive system—small intestines The epithelium of sm. intestine contains many cells which make up the intestinal glands: Absorptive—take in nutrients from chyme Goblet—secrete mucus Paneth—secrete bacteriocidal enzymes Hormone secreting cells (S, CCK & GIP) 48

The digestive system—small intestines The walls of the intestine are highly folded. In addition, the folds have folds or finger-like projections called villi. Lots of surface area for absorption. Each villi contains an arteriole, venule & lacteal (lymph vessel). 49

The digestive system—small intestines 50

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The digestive system—small intestines Each villi is covered with microvilli so that a fuzzy brush border is formed lining the intestines. 52

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The digestive system—small intestines Some physical digestion occurs as myo’s contract and move food along—called segmentation. This also forces nutrients against the wall of the intestine so that they are more easily absorbed. Chemical digestion takes place in the lumen of the small intestines & along the brush border. Digestion & absorption are aided by watery mix of intestinal & pancreatic juice, & chyme. The pH is slightly alkaline. 54

The digestive system—small intestines Chyme entering small intestines has partially digested carbs, proteins & lipids. Digestion finishes in small intestines. Carbohydrates (are broken down to)  monosaccharides Proteins  amino acids Lipids glycerol & fatty acid Nucleic acids nucleotides 55

The digestive system—small intestines Once all molecules are broken down, they must be absorbed. After they enter the cells of the villi they can be put into the circulatory system & transported to other parts of the body (liver first). 56

The digestive system—small intestines Nutrient Method of absorption into cells Monosaccharides Active or facilitated diffusion Amino acids Active transport Electrolytes Active Vitamins Simple diffusion Water (8.3 of the 9.3L) osmosis 57

The digestive system—small intestines Absorption of lipids is more complicated Bile salts surround fatty acids & monoglycerides into spheres called micelles. Micelle moves to brush border. Fatty acids & monoglycerides diffuse into border cells. Inside cell, they reform into triglycerides which clump together & get coated in proteins. Now they’re called chylomicrons. 58

The digestive system—small intestines Chylomicrons enter lacteal. These lymphatic vessels eventually dump their contents into the general circulation (left subclavian vein). They eventually make their way to the liver & adipose tissue. 59

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The digestive system—large intestines Large intestine is the end of the GI tract. Fxns: Complete absorption Vitamin production Form & expel feces 61

The digestive system—large intestines Ileocecal sphincter regulates passage of chyme from small intestine into large. Gastroileal reflex strengthens contractions of ileum & forces chyme into cecum. Hormone gastrin also relaxes sphincter. Strong muscular waves in large intestine propel chyme to rectum. 62

The digestive system—large intestines No enzymes are produced, but bacteria help. Ferment remaining carbs (release hydrogen & methane gas & CO2) Break down remaining proteins & amino acids Produce vit. K & some B vitamins 63

The digestive system—large intestines Chyme stays in large intestine 3-10 hours. Rest of water is absorbed. Feces formed: Water Salts Dead cells Bacteria Indigestible parts of food 64

The digestive system—large intestines Peristalsis pushes feces into rectum. As rectum stretches, defecation reflex is stimulated. Internal anal sphincter opens involuntarily. External anal sphincter is under voluntary control. 65

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The End