Download HBDigPhys

LectureDigestion
Structures of Digestive System
1. alimentary canal (gastrointestinal [GI] tract): Four Layer of the Gut (Digestive Tract)
Mucosa , Submucosa, Muscularis, Serosa (called Peritoneum)
a. digestion - break down molecules is mechanical and chemical
b. absorption - move into circulatory system
c. mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus
2. accessory digestive organs teeth, tongue, gallbladder, salivary glands, liver, pancreas
B. Primary Functions of Digestive System
1. ingestion - getting food into the GI tract (eating)
2. propulsion - moving food along the tract
a. swallowing and peristalsis (wave-like motion)
3. mechanical digestion - the physical grinding and churning of foodstuffs to breakdown and
expose to enzymes and the surface of the GI tract
4. chemical digestion - breakdown of larger molecules into absorbable parts by enzymatic action
5. absorption - transport of digested molecules, vitamins, minerals, water, into blood
6. defecation - elimination of unused foodstuff (feces)
C. Control of Conditions in the GI Tract
mechanoreceptors and chemoreceptors respond to:
a. stretching of the lumen by foodstuffs
b. solute concentration and pH within the lumen
c. presence of digestible and digested molecules
Digestive Processes Occurring in the Mouth, Pharynx, Esophagus
A. Composition of Saliva & Control of Salivation
1. major components of saliva: water (97-99.5%), electrolytes: Na+, K+, Cl-, PO4- , mucin - protein that forms
thick, slimy mucus, IgA antibodies - immune defense, lysozyme - antibacterial enzyme,
2. Mouth is the first site of chemical digestion of STARCH: uses salivary amylase 1. Starch is Broken down to Maltose
. control of salivation: salivatory nuclei (pons & medulla of the brain) ----->
PARASYMPATHETIC nerve activation -----> increases salivation
SYMPATHETIC nerve activation ->decreased salivation
B. Mechanical Processes
1. mastication (chewing) - cheeks, tongue, and teeth involved in both voluntary and involuntary
grinding, ripping, and tearing of foodstuffs
2. Swallowing - moving "bolus" on its way
a. tongue compacts ground food into a "bolus", epiglottis blocks off trachea, peristaltic waves moves food through
the esophagus to stomach
STOMACH: A large muscular, STORAGE organ.
1.
2.
Functions to store food, Begins chemical digestion of proteins with pepsin.
Controls movement of chyme into the small intestine (duodenal end) with pyloric sphincter
3. There are 3 layers of muscle in the muscularis layer of the stomach wall aid in mechanical digestion
4. The mucosa layer has deep folds called rugae and gastric pits . Gastric glands in Pits secrete gastric
juice
5. Gastric juice contains pepsin, an enzyme that breaks down proteins, acid (HCl) and mucus
6. HCl gives the stomach a pH of 2 which activates pepsin and helps kill bacteria found in food
7. A bacterium, Helicobacter pylori, lives in the mucus and can cause gastric ulcers
8. The stomach empties chyme into the small intestine after 2-6 hrs
Regulation of Gastric Secretion, Motility, and Emptying
A. Regulation of Gastric Secretion ("Gastric Juice") has 3 stages: Cephalic , Gastric and Intestinal.
1. Cephalic: vagus nerve (parasympathetic) -----> (Hormone Gastrin) increased gastric secretion (HCl and
activation of pepsinogen to pepsin)
2. gastric phase. food reaches the stomach --------> stomach distends, more (GASTRIN) gastric secretion (HCl and
activation of pepsinogen to pepsin). Parietal cell in the mucosa.in Pits secrete HCl
3.
intestinal phase chyme enters the duodenum (small intestine); inhibition of gastric processes initiation of
sympathetics release of inhibitory hormones:(secretin, cholecystokinin CCK
4.
HORMONES INVOLVED IN DIGESTION
1.
2.
3.
Gastrin (paraSYMPATHETIC activation and stomach activation)
Secretin (activated by acid chyme in duodenum): stimulate
pancreas to secret Alkaline solution to neutralize acid chyme
Cholecystokinin (CCK, activated in duodenum): stimulates
pancreas to secrete enzymes and gallbladder to secrete bile
Sphincter muscle along the gut act as valves and control the rate of flow.
Three Accessory Gland: Liver, Gallbladder and Pancrease.
Liver
1.
Large metabolic organ that lies under the diaphragm and is made of ~100,000 lobules
2. Filters blood from the GI tract thus acting to remove poisons and detoxify the blood, Removes
iron, vitamins A, D, E, K and B12 from the blood and stores them
3. Stores glucose as glycogen and breaks it down to help retain blood glucose levels
4. Makes plasma proteins and helps regulate cholesterol levels by making bile salts
5. Makes bile that is then stored in the gallbladder to be secreted into the small intestine to
emulsify fats
6. Breaks down hemoglobin
A. Content of Bile (made in Liver, stored and released by Gall Bladder). Emulsify fats - separate fats into tiny
droplets for digestion & absorption
1. bile salts, bile pigments, cholesterol, neutral fats, phospholipids, electrolytes
2. bile salts - derivatives of cholesterol (cholic acid, chenodeoxycholic acid)
3. bile pigment (bilirubin) - waste product of heme from broken-down erythrocytes
(urobilinogen - breakdown product of bilirubin, causes darker coloration of feces)
B. Bile Release to Small Intestine is stimulated by parasympathetic nerves - stimulates gall bladder release
and cholecystokinin (CCK) - hormone released by cells of the mucosa of the duodenum
acidic, fatty chyme enters duodenum ----->duodenal mucosa secretes CCK ----->
a. gall bladder contracts to release bile and b. pancreas secretes pancreatic juices
Gallstones - crystallized formation of cholesterol and salts, causing obstruction of bile release
Liver Disorders
1.
2.
Hepatitis: Inflammation of the liver, Caused by Viruses, this can lead to liver damage, cancer and/or death
Cirrhosis: the liver becomes fatty and eventually this tissue is replaced by fibrous scar tissue; Seen in
alcoholics and obese people; can lead to liver failure in which the liver cannot regenerate as fast as it is
being damaged
Composition of Pancreatic Juice and Regulation of Secretion
1. Very basic (alkaline) bicarbonate solution
2. enzymes - precursors and active digestive forms
a. trypsin breaks down protein to polypeptides
b. amylase breaks down starch to maltose
c. lipases breaks down fats to fatty acids and glycerol,
d. nucleases breaks down nucleic acids to nucleotides
e. proteases break down polypeptides to amino acid
f. maltase breaks down maltose to glucose
B. Regulation of Pancreatic Secretion
1. parasympathetic - causes release during cephalic and gastric phases of gastric secretion
2. secretin - hormone that causes release of "bicarbonate-rich" pancreatic juices in response to
the presence of HCl
3. cholecystokinin - hormone that causes release of "enzyme-rich" pancreatic juice in response
to the presence of proteins and fats
Small Intestine Mucosa is modified for absorption with columnar epithelial cells called villi and
membrane extensions called microvilli.
1.
2.
3.
4.
6m (18 ft) in length, Enzymes secreted by the pancreas to digest carbohydrates, proteins and fats
Bile is secreted by the gallbladder into the small intestine to emulsify fats
Digested food is absorbed by numerous villi (finger-like projections) and microvilli
Amino acids and sugars enter the capillaries;fatty acids and glycerol enter the lacteals (small lymph
vessels)
Digestive Processes of the Small Intestine
A. Optimal Conditions for Digestion & Absorption
1. pancreatic juice & bile - enzymes, emulsifying fats, and adjust pH from acid to basic
2. small intestine is PRIMARY site for absorption of nutrients into the cardiovascular system
B. Movement in the Small Intestine using segmentation and peristalsis
Digestive Processes of the Large Intestine (Cecum (contains appendix), Colon (ascending, transverse and
descending), Rectum and Anus)
A. Bacterial Flora digest remaining carbohydrates, responsible for producing gas (flatus) and synthesize
complex B vitamins and vitamin K
B. Digestion and Absorption reclaim most of the water and reclaim some of the electrolytes (Na+ and Cl-)
C. Defecation
D. defecation reflex - when feces (stool) enters rectum, spinal cord reflex is triggered by pressure
Disorders of Large Intestine
1.
Diarrhea – increased peristalsis and water is not reabsorbed due to either an infection or nervous stimulation
2. Constipation – condition when feces are dry and hard that may be controlled with water and fiber
3. Hemorrhoids – enlarged and inflamed blood vessels of the anus due to chronic constipation,
pregnancy, aging and anal intercourse
4. Diverticulosis – occurrence of pouches of mucosa from weak spots in the muscularis layer that can
become infected often in the colon
5. Irritable bowel syndrome (IBS) – Inflammatory bowel disease/colitis (IBD) – disorders
characterized by chronic diarrhea and abdominal pain
6. Polyps and cancer – small growths found in the epithelial lining that can be benign or cancerous
7. Chemical Digestion
A. Enzymatic Hydrolysis ("water" "breaking")
1. hydrolysis - a water molecule is added between two "monomers" of a complex organic molecule
in order break it down into its component parts
B. Carbohydrate Digestion
1. monosaccharides - "monomers" such as glucose, fructose, and galactose
2. disaccharides - sucrose (table sugar), lactose (milk sugar), and maltose (grain sugar)
3. polysaccharides - starch (grains), glycogen (muscle)
4. carbohydrate hydrolyzing enzymes
a. salivary amylase - produces "oligosaccharides" starch to maltose (disaccharide)
b. pancreatic amylase - in small intestine : starch to maltose (disaccharide)
c. intestinal maltase or sucrase – complete the hydrolysis of carbohydrates to monosacchrides.
5. lactose intolerance - decreased ability to digest lactose in the diet (use "lactase" supplements)
C. Protein Digestion
1. amino acids - the "monomer" components of protein
2. Step 1: stomach - pepsinogen -----> pepsin (low pH) forms polypeptides
3. Step 2: small intestine
a. trypsin an enzymes that cleave throughout the protein
b. protease enzymes completely hydrolyze to amino acids.
D. Lipid (Fat) Digestion
1. lipid structure - glycerol + 3 triglycerides
2.Step 1: Emulsify the fat globule with bile salts - "emulsify" fats in 1 micron "micelles"
3.Step 2: Chemical breakdow with lipases - enzymes that break down lipids to fatty acids
E. Nucleic Acid Digestion small intestine: pancreatic nucleases - break down DNA and RNA
Nutrition: we must consume the folllowing. The amount you consume should reflect your energy output.
1.
4 Macromolecules (contained in food groups) Each must be hydrolysed to monomer for uptake (absorption)
2.
3.
4.
5.
into bloodstream.. 6-11 serving (3-5cups) of Grains, 2-3 servings of Meat or Beans (must combine with grain to
obtain 20 amino acids); Dairy (
Fiber: Indigestible cellulose plant matter makes the cell walls of plants. Obtained in whole foods like whole
grains instead of refined or processed; fresh fruits and vegetables
Minerals
Vitamins Organic compounds (not including proteins, fats or carbohydrates) used for metabolism but are not
produced in high enough quantity by the body; they are often enzyme helpers (coenzymes); There are a total of
13 vitamins in two groups: fat-soluble and water soluble
Antioxidants.
Absorption of Nutrients :
A active transport - most nutrients must be transported across membrane using ATP of the cells
B.Carbohydrate Absorption facilitated diffusion - glucose and galactose (coupled with active transport of Na+)
C. Protein (Amino Acid) Absorption
1. facilitated diffusion - amino acids and small peptides (coupled with Na+ active transport)
a. "carrier molecule" has binding sites for both amino acid and Na +; relies on Na+ gradient
2. food allergies - absorption of proteins in infant gut causes early immune reaction
D. Lipid Absorption USES LACTEALS OF THE LYMPHATIC SYSTEM
a. contain cholesterol and fat-soluble vitamins
b. diffuse through lipid bilayer of membrane
c. lacteals of the lymphatic system
E. Nucleic Acid Absorption pentoses, nitrogen bases, phosphates - absorbed by similar processes as sugars and
amino acids
Vitamin Absorption
1. fat soluble – Consume cautiously, excess leads to toxicity. Four are A, D, E, K are absorbed with lipid micelles
OLESTRA - will carry fat soluble vitamins out in feces with it
Vit D, 15-30 min of sunlight, milk ;becomes Calcitrol; a hormone that help absorption Ca.
Vitamin A, E, K from leafy greens and oils
2. water soluble - Vitamins B & C absorbed by diffusion
a. Vitamin B12 - large and electrically charged, must bind with "intrinsic factor" before being taken into the a
cell by endocytosis. Too Little leades to Pernicious anemia.
b. Vit B(Folic acid) Too little in pregnant mothers leads to poor neural tube development (Spina bifida)
Electrolyte (Minerals )Absorption: Major need greater than 5 mg a day (Ca, P, Na, .K, Su, Cl, Mg) Minor need less
than 5 mg a day (Fe, I, Co, Se, Mn). Know the Role of the following
1. Fe and Ca - primarily absorbed in small intestine; low intake leads to Fe-deficiency anemia
Fe. Obtained form meats, whole grains and prune juice; Needed to form hemoglobin
Ca: Needed to make bone, nerve impulse conduction and muscle contraction
2. Na - exchanged for sugars and amino acids; needed for water balance, nerve and muscle conduction
500mg /day needed.
3. Cl - absorbed into cells and exchanged for HCO3- needed for electrolyte balance.
4.K - absorbed into cells due to osmotic gradients; needed for , nerve and muscle conduction
5. I: obtained from salt and seafood; needed for thyroid hormone synthesis
Eating Disorders
Anorexia nervosa - psychological disorder due to fear of getting fat and usually results in self-induced starvation,
high physical activity and may include purging
Bulimia nervosa - disorder in which people eat large amounts of high-calorie food (binge-eating) followed by
purging to avoid weight gain often more than once a day