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Lecture 18 The Digestive System

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1 Lecture 18 The Digestive System

2 Food for Energy and Growth
Food provides animals with energy and essential nutrients for growth A healthy diet contains more carbohydrates than fats It also contains a lot of proteins The pyramid of nutrition

3 Who Is Overweight? The body mass index is used to determine who’s overweight body weight in kg (height in m)2 (height in in)2 = (body weight in lbs) X 703 BMI = Obesity

4 Essential Substances for Growth
Many vertebrates are unable to manufacture one or more of the 20 amino acids needed to make proteins Humans are unable to synthesize 8 amino acids These essential amino acids must be obtained through food In addition, all vertebrates cannot synthesize certain polyunsaturated fats Trace elements Minerals required in small amounts Iodine, cobalt, zinc, molybdenum, manganese Vitamins Essential organic substances required in small amounts Humans require at least 13 different vitamins Vitamin C (ascorbic acid) if not in diet, the disease scurvy will develop

5 Evolution of the Digestive System

6 Digestive System Overview
Consists of tubular gastrointestinal tract and accessory digestive organs Mouth and pharynx Esophagus – Delivers food to the stomach Stomach – Some preliminary digestion Small intestine – Digestion and absorption Large intestine – Water and mineral absorption Rectum – Waste excretion

7 General Structure of Digestive Organs
The gastrointestinal tract has a characteristic layered structure Mucosa Epithelium Submucosa Connective tissue Muscularis Smooth muscles Serosa

8 Breaking Down Food in the Mouth
Many vertebrates have teeth that are used for chewing (mastication) Carnivores have pointed teeth adapted for cutting and shearing Herbivores have large flat teeth well suited for grinding plant cellulose Omnivores have carnivorous teeth in front and herbivorous teeth in the back

9 The Structure of a Tooth

10 Tooth and Gum Disease: Periodontitis
Dental caries – gradual demineralization of enamel and dentin by bacterial action Dental plaque, a film of sugar, bacteria, and mouth debris, adheres to teeth Acid produced by the bacteria in the plaque dissolves calcium salts Without these salts, organic matter is digested by proteolytic enzymes Daily flossing and brushing help prevent caries by removing forming plaque Gingivitis – as plaque accumulates, it calcifies and forms calculus, or tartar Accumulation of calculus: Disrupts the seal between the gingivae and the teeth Puts the gums at risk for infection Periodontitis – serious gum disease resulting from an immune response Immune system attacks intruders as well as body tissues, carving pockets around the teeth and dissolving bone

11 Tongue Superior surface bears three types of papillae
Filiform – give the tongue roughness and provide friction Fungiform – scattered widely over the tongue and give it a reddish hue Circumvallate – V-shaped row in back of tongue Sulcus terminalis – groove that separates the tongue into two areas: Anterior 2/3 residing in the oral cavity Posterior third residing in the oropharynx

12 In The Mouth The tongue mixes food with a solution called saliva
Saliva moistens and lubricates food and contains amylase which initiates breakdown of starch into maltose The secretions of the salivary glands are controlled by the nervous system A continuous secretion of about 0.5 milliliters per minute keeps the mouth moist The presence of food in the mouth triggers an increased rate of secretion

13 Swallowing Prior to swallowing, the tongue moves food to the back of the mouth The soft palate elevates, pushing against back wall of pharynx This stimulates neurons to send impulses to the swallowing center in the brain Muscles contract and raise the larynx The glottis is pushed against the epiglottis which keeps food out of the respiratory tract, and into the esophagus

14 The Esophagus and Stomach
The esophagus is a muscular tube that connects the pharynx to the stomach The swallowing center stimulates successive waves of contraction Peristalsis moves food along the esophagus to the stomach

15 The stomach and gastric glands
Movement of food from esophagus into stomach is controlled by a ring of smooth muscle, the sphincter Humans lack a true sphincter and thus, can regurgitate The stomach is a saclike portion of the tract with a convoluted surface enabling expansion It contains an extra layer of smooth muscles for mixing food with gastric juices Two kinds of secretory cells Parietal cells – Secrete hydrochloric acid Chief cells – Secrete pepsinogen The human stomach produces about 2 liters of HCl and other gastric secretions every day This gastric juice has a pH of ~ 2 It kills most bacteria ingested with food and also denatures food proteins The mixture of partially digested food and gastric juice is termed chyme Chyme leaves the stomach to the small intestine through the pyloric sphincter

16 Ulcers The hormone gastrin regulates the synthesis of HCl
Overproduction of gastric acid can occasionally eat a hole through the stomach wall These gastric ulcers are rare Susceptibility increases when mucosal barriers are weakened by Helicobacter pylori infection Over 90% of gastrointestinal ulcers are duodenal ulcers Caused by too much chyme in the small intestine

17 The Small Intestine The small intestine is the body’s true digestive vat It breaks down large molecules into smaller ones which are then absorbed into the bloodstream The small intestine is ~ 6 m long The first 25 cm (~ 4%) constitute the duodenum The duodenum is the actual site of digestion The pancreas secretes digestive enzymes into it The liver secretes bile salts into it, to make fats easier to digest The ileum is the rest of the small intestine (~ 96%) It is devoted to absorption The lining is covered with finger-like projections called villi Each cell covering a villus has cytoplasmic projections called microvilli which increase the absorptive surface

18 Anatomy of the Small Intestine

19 The Large Intestine The large intestine, or colon is only 1 meter long
But has a larger diameter than the small intestine No digestion and little absorption take place in the large intestine Its primary function is to act as a refuse dump by collecting and compacting solid wastes Feces pass through the rectum as a result of peristalsis and leave the body through the anus

20 Accessory Digestive Organs
Pancreas Functions as both an exocrine & endocrine gland Exocrine: Cell clusters called acini secrete Trypsin and chymotrypsin which digest proteins Amylase which digests starch Lipase which digests fats Bicarbonate which neutralizes HCl in chyme Endocrine: Cell clusters called Islets of Langerhans secrete Insulin and glucagon which regulate sugar levels in blood Liver Largest internal organ of the body Its main exocrine secretion is bile Aids in the digestion of fats in the duodenum Chemically modifies substances absorbed in the gastrointestinal tract Converts poisons into less toxic forms Produces most of the proteins found in plasma Gall bladder Stores and concentrates bile Delivers it to the duodenum via the bile duct

21 Microscopic Anatomy of the Liver
Hepatocytes’ functions include: Production of bile Processing bloodborne nutrients Storage of fat-soluble vitamins Detoxification Secreted bile flows between hepatocytes toward the bile ducts in the portal triads Hexagonal-shaped liver lobules are the structural and functional units of the liver Composed of hepatocyte (liver cell) plates radiating outward from a central vein Portal triads are found at each of the six corners of each liver lobule Portal triads consist of a bile duct and Hepatic artery – supplies oxygen-rich blood to the liver Hepatic portal vein – carries venous blood with nutrients from digestive viscera Liver sinusoids – enlarged, leaky capillaries located between hepatic plates Kupffer cells – hepatic macrophages found in liver sinusoids

22 Homeostasis Homeostasis is the dynamic constancy of the internal environment Conditions fluctuate continuously within narrow limits Homeostasis is essential for life Play Hormones & Gastric Secretion

23 Regulating Blood Glucose
Blood glucose levels are monitored by the Islets of Langerhans in the pancreas When levels are high, insulin is released When levels are low, glucagon is released


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