Digestive System

Digestive System Gastrointestinal Tract Alimentary Canal

Function of the digestive tract Physically and chemically prepares the food for absorption and use by the body cells

Processes called • ingestion • digestion • secretion • absorption • elimination

The end product to be absorbed is: • monosaccharides • amino acids • some lipids • vitamins and minerals • limited absorption in the stomach and in the small intestine

Organs of the Digestive System major organs and accessory organs The digestive is considered to be outside the body

Digestion involves Hydrolysis • Chemical process – a compound unties with water then splits into simpler compounds • Digestive enzymes • Specific to their action – only act on a specific substrate • Function at a specific pH • Different digestive enzymes require different pH • Often secreted as a proenzyme that has to be activated by a kinase to expose the active site • Ex: chief cells secrete pepsinogen. When it comes in contact with hydrochloric acid that is released by the parietal cells, it is converted to pepsin. Pepsin is a protein digesting enzyme.

Wall of the GI Tract 4 layers although in the different organs they may be a little different Mucosa innermost layer facing the lumen epithelial layer loose connective tissue thin layer of smooth muscle

Submucosa a thicker connective tissue that contains glands, blood vessels and nerves Muscularis thick layer of smooth muscle tissue inner layer – circular outer layer – longitudinal nerves and blood vessels

Serosa connective tissue that is a layer of the peritoneum. this epithelial tissue wraps around the organ, covering it the serosa is actually the called the visceral peritoneum connects the visceral peritoneum to the parietal peritoneum called the MESSENTARY

Mouth the beginning of the digestive tube beginning of mechanical and chemical digestion the parts of the mouth are: chewing called mastication swallowing called deglutition food being swallowed is called a bolus

after one swallows the process becomes involuntary Esophagus tough tube moves the food toward the stomach

Peristalsis wave-like ripple of the smooth muscle layer of a hollow organ – stretched wall triggers a reflex contraction of the circular muscle that pushes the bolus forward Segmentation a forward and backward movement that mixes and helps break down food these two movements can occur in alternating sequence that progresses food forward and helps to digest it

Phases of digestion Cephalic phase Triggered by smell and taste or maybe even thinking about food Prepares the stomach to receive food Stimulates the production of gastric juices in the stomach Gastric phase Begins when food arrives in the stomach Stretching of the stomach triggers mixing waves and stimulates secretion of the parietal and chief cells. Gastrin is stimulated to be released into the blood and starts the process of muscle contractions for mixing

Phases cont’d Intestinal phase Begins when chyme enters the intestine Controls the emptying of the stomach Neutralizes the pH Stimulates the stretch receptors in the intestine wall Alcohol and caffeine will stimulate this phase to happen faster

Stomach lies to the left under the diaphragm can be as small as a sausage hold as much as 1 – 1 ½ liters of food

Functions of the stomach • food reservoir • churns the food and moves chyme into the duodenum • limited absorption – cert • aindrugs, some water, alcohol and short chain fatty acids found in butter and milk • secrete gastric juices – acids and enzymes that are specific to what they digest and pH levels • destroys pathogens in food and mucus

Gastric wall of the stomach Stomach is lined by simple columnar epithelium that secretes a mucus that protects the stomach from the digestive juices. This lining contains gastric pits open to the surface and are actively dividing to produce new cells. What would the friction of digestion do to these cells? Parietal and chief cells deep in the lamina of the fundus produce about 1500 to 2000 ml of gastric juices a day. Depending on how one eats it could more or less. Cont’d on next slide

Parietal cells secrete intrinsic factor and hydrochloric acid Intrinsic factor helps with the absorption of vitamin B12 which will happen later in the intestine. The pH of the stomach is around a 2.0 or can be lower possibly 1.5 It kills microorganisms and can break down the cellulose of plants cells and the connective tissue in meats to release the nutrients from them

Secretion cont’d gastrinwhich regulates digestion ghrelinwhich regulates appetite. Chief cells secrete pepsinogen. When pepsinogen comes in contact with hydrochloric acid it is converted to pepsin a protein digesting enzyme.

Function of the small intestine is to absorb nutrients villi and microvilli – modifications of the mucosal layer that give it a velvety appearance – increases surface area hundreds of times The area also secretes an enzyme thought to inhibit bacterial growth in the small intestine

Chyme is released a little bit at a time – about every 20 seconds • As chyme enters the duodenum the pancreas adds a bicarboniate to change the pH of the chyme • The gall bladder will add bile to emulsify fats

Movement into the • Small Intestine • Page 530 in the text • The stomach slowly release it contents into the small intestine through the pyloric sphincter. • More gastrin is released to continue to break down proteins • Secretin is released to lower the pH and to increase the secretion of bile and buffers by the liver and pancreas • Cholecystokininslows down gastric activity to give the small intestine time to adjust and signals the gall bladder to release bile. • Gastric inhibitory peptide signal the pancreas to release insulin from its islets

Divisions of the small intestine • The small intestine is about 1 inch in diameter, about 20 or so feet long • Duodenum – about 10 inches to 2 feet long • Jejunum –where the tube turns downward – about 8 feet long • Ileum – no clear line of change • 12 feet long

Different areas of the small intestine absorb different substances

Liver The liver is responsible for regulating the composition of blood and receives about 25% of the heart’s output of blood The blood from the small intestines flows through the liver before it is circulated to the rest of the body Some of the functions of the liver are: Detoxifies substances from blood Produces bile salts Monitors the circulation levels of organic nutrients Synthesizes and breaks down glycogen Fat soluble vitamins A,D,K and E are absorbed and stored

Large Intestine About 5 feet long 3 inches wide Cecumfirst 2 – 3 inches, blind pouch, Ileocecal valve Ascending colon Transverse colon 2-90 degree angles Descending colon Sigmoid colon - “s” shaped Rectum – last 7 or 8 inches Anus

Function of large intestine Prepare residues of digestion for elimination called defecation Which is mostly cellulose, connective tissue from meat, undigested fats that have combined with dietary minerals--calcium and magnesium-- bacteria, pigments, water and mucus Reabsorbs bile Removes water for elimination Secretes mucus Intestinal bacteria generates 3 vitamins K – needed by the liver to synthesize clotting factors Biotin – water soluble vitamin important in glucose metabolism Vitamin B5 – a water soluble vitamin required in the manufacture of steroid hormones and some neurotransmitters

Cecum Connection of the small and large intestine Ileocecal valve Verivormappendex

Rectum Has two sphincters Internal anal sphincter - involuntary External anal sphincter – voluntary Stretching of the rectal walls stimulate the need to release feces

The effects of aging on the digestive system. • Muscle tone of the smooth muscle decreases and the sphincters can become weaker • constipation and heartburn • Cells do not reproduce as efficiently • Friction can cause damage so the digestive tube becomes more fragile • Other system damage • Examples: tooth loss, gum disease, liver damage from toxins, pancreatic disease, loss of sense of smell and/or taste • Greater cancer risk as one ages • Damage to other body systems • Check out the system interactions on page 550

Question to consider Apply what you have learned Why is diarrhea a potentially life threatening situation but constipation is not? If most of the stomach had to be removed would there be loss of absorption? If so what? What about vitamin absorption? What is the main function of the large intestine? 4. Some people have had apart or all their large bowl removed. If the large intestine were removed would it affect the body’s ability to synthize and/or absorb any nutrients? Why? If the exocrine pancreas were to be damaged what part of the digestive system would be the most impaired? If the esophageal sphincter were not functioning properly what might be the outcome? The stomach has more muscle layers that the rest of the alimentary canal. What is the advantage of that?

Some people suffer from what is called “dry mouth.” How might that affect digestion? A person is given a drug for another condition but the side effect of this drug may be that it reduces parasympathic—what you cannot control-- stimulation of the digestive tract. How might that affect digestion? A person has gum disease that is causing them to lose their teeth. Will there be an affect on the digestive system? What role does the soft palate play in the process of swallowing? After a stroke some people have problems swallowing. What role does the epiglottis play in swallowing and what might happen if one can not swallow correctly? A patient has cancer of the small intestine. Five feet of the organ has to be removed. Describe what might be a consequence of having to have this surgery. There are medications—some are now over the counter—that reduce stomach acids. Describe what might be the condition that would cause a person to need these stomach acid reducing meds.

15. Stomach acid is considered one of the strongest acids that comes in contact with living material. How is the stomach itself protected from the gastric juices its own cells secrete?

Digestive System