Digestive System

1. Digestion and absorption
2. Gastrointestinal tract (GI tract) 
• long tube that connects mouth with anus
• more than 9 meters long in adults
• upper and lower part
• upper includes mouth, esophagus, and stomach
• lower includes small and large intestines
• organs of gastrointestinal tract are covered by two layers of muscles that enable peristalsis
• peristalsis: rapid, involuntary, wave-like contraction of muscles
• pushes food through GI tract
• GI lined with mucous membranes
• mucous membranes moist tissues that secrete and absorb substances
• necessary for digestive system
3. Accessory organs of digestive system
• additional organs that play important roles in digestion
• food does not pass through them, but they make/store substances needed for digestion
• liver, gall bladder, and pancreas
4. Liver
• large organ next to stomach
• produces digestive substances that are carried by ducts, or tubes, to small intestine and gall bladder
• produces bile, which breaks down lipids
• maintains blood glucose levels
• gluconeogenesis: synthesis of glucose from certain amino acids, lactate or glycerol
• glycogenolysis: breakdown of glycogen into glucose
• glycogenesis: formation of glycogen from glucose
• liver is involved in getting rid of foreign substances and toxins, especially from gut
• toxins usually excreted in bile or urine
• breaking down toxins: drug metabolism
• usually done by using specialized enzymes produced in liver
• most blood being filtered comes from portal vein
• also carries blood to intestines
• liver can remove bacteria, fungi, viruses, and parasites from blood
• also does cholesterol synthesis and production of triglycerides
• cannot live without liver (of course) 
5. Pancreas
• exocrine gland: secretes pancreatic juice with digestive enzymes
• endocrine gland: produces several important hormones
• located below and behind the stomach
• endocrine cells grouped together in areas called islets of Langerhans
• produces insulin, glucagon, and somatostatin
• insulin and glucagon involved in controlling blood glucose levels
• insulin produced by beta cells and causes excess blood glucose to be taken up by liver and muscle cells
• stored as glycogen (polysaccharides) 
• glucagon produced by alpha cells and stimulates liver to break down glycogen into glucose, and released into the blood
6. Functions of digestive system
• 3 main functions
• digestion of food
• absorption of nutrients
• elimination of solid waste
• digestion: process of breaking down food into components body can absorb
• 2 types of digestion
• mechanical: physical breakdown of chunks of food into smaller pieces
• takes place mainly in the mouth and stomach
• chemical: breakdown of large, complex food molecules into smaller, simpler nutrient molecules that could be absorbed by blood
• mainly takes place in small intestine
• chemical digestion cannot take place without digestive enzymes
• digestive enzymes speed up reactions of chemical digestion
• secreted by glands in mucous membranes of mouth, stomach, small intestine, and pancreas
• digestive enzyme name typically ends with "-ase" (means enzyme) 
• after food is digested, nutrients are absorbed
7. Mouth
• the mouth begins digestion of starch
• enzyme: salivary amylase found in saliva
• mechanical digestion: tongue and teeth
• teeth shape reflects dietary habits
• humans are omnivores (both meat and plants) 
• incisors for cutting, canines for tearing, and molars for grinding
• saliva from salivary glands moistens food and makes it easier to chew
• muscular tongue mixes food with saliva and enzymes
• lump of chewed food (now called bolus) passes into pharynx
• pharynx connects mouth to rest of digestive tract
• connects mouth and nose to respiratory system
• when food is pushed back to back of the mouth by tongue, the pharynx is closed off from respiratory system
8. Esophagus
• bolus moves into esophagus
• narrow tube ~20 cm long
• begins at pharynx and ends at opening to stomach
• function: to pass food from mouth to stomach
• does not produce digestive enzymes and no digestive function
• sphincter: muscle at end of esophagus that controls entrance to stomach
• opens to let food in and closes again to prevent food from going back into esophagus
9. Stomach
• saclike organ located between end of esophagus and beginning of small intestine
• food digested both mechanically and chemically
• churning movement of stomach's muscular walls breaks down food mechanically
• also mixes food with fluids secreted by stomach
• hydrochloric acid (HCl) 
• gives stomach acidic environment
• helps destroy bacteria that entered stomach through food or drinks
• acidic environment needed for stomach's digestive enzymes to work
• digestive enzymes secreted in stomach helps break down proteins into peptides
• pepsin is main digestive enzyme in stomach
10. Small intestine
• narrow tube about 7 meters long
• site of most chemical digestion and most nutrient absorption
• small intestine much larger than large intestine
• called small because it is smaller in diameter
• pushes food along with peristalsis
• three parts: duodenum, jejunum, and ileum
11. Duodenum
• first part of small intestine
• 25 cm long
• most chemical digestion occurs there
• many enzymes active in duodenum
• only some produced by duodenum
• rest are produced by pancreas, and secreted in duodenum
• amylase: digests carbohydrates, made in pancreas
• trypsin: digests proteins, made in pancreas
• lipase, digests lipids, made in pancreas
• maltase: digests carbohydrates, made in small intestine
• peptidase: digests proteins, made in small intestine
• lipase: digests lipids, made in small intestine
• pancreas controlled by hormones
• gastrin (hormone) stimulates pancreas to secrete digestive enzymes
• bile reduces acidity of chyme (bolus that was churned around in stomach) entering from acidic stomach
• pancreas secretes biocarbonate, a basic substance that neutralizes acid
12. Jejunum 
• second part of small intestine
• 2.5 meters long
• most nutrients are absorbed into blood here
• mucous membrane of jejunum covered by villi
• villi have microvilli that increases surface area for absorption
• surface area adds up to the size of a tennis court
• greater absorption of nutrients
13. Ileum
• third part of small intestine
• 3.5 meters long
• few remaining nutrients are absorbed in ileum
• salts that form liver bile are absorbed
• also covered with villi and microvilli
• more absorption area
14. Large intestine
• connects small intestine with anus
• 1.5 meters long
• three parts: secum, colon, and rectum
• waste enters from small intestine (in liquid state) into the secum
• excess water is absorbed when waste passes through colon
• remaining solid waste called feces
• contains indigestible food substances, like fiber
• feces accumulates in rectum, and stored until eliminated from body
• sphincter controls anus and opens to let feces outside
15. Bacteria in large intestine
• large intestine provides home for intestinal bacteria and absorbs vitamin they produce
• most bacteria are helpful
• produce vitamins
• control growth of harmful bacteria
• break down toxin before they can poison the body
• break down indigestible food components
• produce substances that prevent colon cancer
16. Diseases of gastrointestinal tract
• inflammatory bowel disease
• inflammation of large intestine
• normal reaction of immune system
• causes swelling, redness, and pain
• 2 main forms
• Chrohn's disease
• abdominal pain, diarrhea, and weight loss
• caused by immune system reacting to body's own tissues
• no known cure, but treatment can control symptoms
• ulcerative colitis
• similar symptoms to Chrohn's disease
• unknown cause
• confined to colon and can be sometimes cured with surgery
• food allergies can affect GI tract
• disorders that occur when the immune system reacts to substances in food as if they were foreign invaders
• nuts, eggs, milk, fish, and shellfish
• symptoms: tingling in the mouth, vomiting, and diarrhea
• can also cause skin rashes and difficulty breathing
17. Diseases of stomach and esophagus
• layer of mucus normally protects lining of stomach from damage by stomach acid
• infection by Helicobacter plyori can weaken the mucus layer, causing gastritis or stomach ulcers
• gastritis is inflammation of lining of the stomach, and causes abdominal pain
• stomach ulcer is a sore in the lining of the stomach, and can cause severe abdominal pain and bleeding
• stomach acid can also damage lining of esophagus
• occurs when sphincter between stomach and esophagus do not close properly
• acid from stomach enters esophagus
• acid can cause esophagitis, or inflammation of the esophagus
• can be treated with medication and changes in diet
• if not treated, sometimes it will lead to cancer of esophagus
18. Diseases of the small intestine
• ulcers occur mostly in the duodenum because stomach acid enters duodenum during digestion
• symptoms and treatment of duodenal ulcers are similar to stomach ulcers
• salmonella and E. coli can cause infections in small intestine
• can enter body in contaminated foods or beverages
• symptoms: abdominal pain, cramping, vomiting, and diarrhea
• infections usually clear up without medical treatment
• celiac disease is immune reaction to gluten, found in grains
• tendency to have celiac disease is inherited
• symptoms: abdominal pain, diarrhea, and bloating
• gluten-free diet can prevent symptoms, but no cure for disease
19. Diseases of large intestine
• irritable bowel syndrome
• disorder in which large intestine is easily irritated
• common gastrointestinal disorders
• unknown cause, but may be due to excessive bacteria in intestine
• symptoms: abdominal pain, cramping, constipation, and diarrhea
• can be controlled with medication, stress management, and changes in diet
• no cure
• colitis
• inflammation of colon
• causes: bacterial infections to immune reactions against body's own tissues
• symptoms: pain and tenderness in abdomen
• treatment: medication, surgery, and changes in diet
• appendicitis
• inflammation of appendix
• most common in children and teens
• inflammation usually caused by bacterial infection
• symptoms: abdominal pain, loss of appetite, fever, and vomiting
• most often treated with surgery to remove infected appendix
• infected appendix can be fatal without treatment
20. Diseases of accessory organs
• cystic fibrosis
• affects the pancreas
• inherited disease where the body produces abnormally thick and sticky mucous
• mucus blocks the duct to duodenum, preventing pancreatic enzymes from reaching it
• proteins and lipids cannot be digested properly
• may take digestive enzymes by mouth to improve digestion
• no known cure
• hepatitis
• inflammation of liver
• caused by viral infection
• some viruses spread through contaminated foods or beverages, and others through sexual contact
• symptoms: fever, headache, vomiting, yellowing of skin and eyes, and abdominal pain
• disease may clear up without treatment if symptoms are mild
• however, if symptoms are severe, the disease may damage the liver so that it no longer produces bile
• medications are available to treat hepatitis
• some types can be prevented with vaccines
• gall stones
• crystals that form in the bile in the gall bladder
• caused by abnormal body chemistry or too much fat in diet
• may grow to size of golf ball
• may cause inflammation of gall bladder and severe abdominal pain
• only way to treat is to surgically remove gall stones or entire gall bladder