A&P digestion
8.Describe how and where lipids (fats) are broken down and absorbed. Be sure to include a description of the composition and function of bile.
... Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets). Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which "ferry" them to the intestinal mucosa. Micelles made up of fatty acids, monoglycerides, and bile salts Bile salts Fat droplets coated with bile salts Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other fatty substances and proteins to form chylomicrons. Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals and are carried away from the intestine in lymph.
7.Explain the steps involved in protein digestion and absorption which occur in the small intestine. Be sure you know where protein digestion first begins in the digestive system.
... Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxy- peptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase) of mucosal cells. The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side. The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts.
6.Explain the process of chemical digestion of carbohydrates. Be sure to state where chemical digestion begins, and the enzymes and monomers involved.
...Digestion -Digestion is a catabolic process that breaks down large food molecules to monomers to be absorbed by the GI tract lining; •Enzymes -Intrinsic and accessory gland secrete enzymes into the lumen of the GI tract that break down food •Hydrolysis -Water is added to break bonds
1.Distinguish between organs of the alimentary canal (gastrointestinal tract) and the accessory digestive organs.
1. Alimentary canal (gastrointestinal or GI tract) •Mouth to anus •Digests food and absorbs fragments •Mouth, pharynx, esophagus, stomach, small intestine, and large intestine 2. Accessory digestive organs •Teeth, tongue, gallbladder •Digestive glands -Salivary glands -Liver -Pancreas
4.List and describe the regions of the large intestine
1.Cecum: -first part of large intestine 2.Appendix: -masses of lymphoid tissue 3.Colon: -Ascending colon [right side; right colic (hepatic) flexure] -Transverse colon [left colic (splenic) flexure] -Descending colon (left side) -Sigmoid colon in pelvis opens to rectum 4.Rectum: -Three rectal valves stop feces from being passed with gas (flatus) 5.Anal canal: -Last segment of large intestine; Opens to body exterior at anus -Sphincters •Internal anal sphincter—smooth muscle •External anal sphincter—skeletal muscle
2.List the six processes of digestion and be able to describe the functions of each process and state where each process occurs.
1.Ingestion •Taking food into digestive tract, (mouth) 2.Propulsion •Move food through digestive tract. Includes swallowing (orapharynx) and peristalsis (stomach, esophogus, small and large intestines) 3.Mechanical breakdown •Include chewing, ( mouth )mixing food with saliva by tongue, churning food in (stomach) and segmentation (small intestines) 4.Digestion •Catabolic steps in which enzymes secreted into the lumen of GIT break down complex food molecules to their chemical building blocks. (small and large intestines) 5.Absorption •Passage of digested food from the lumen of GIT through mucosal cells by active or passive transport into blood or lymph. ( lymph and blood vessels) 6.Defecation •Elimination of indigestible substance from the body via anus in the form of feces. ( anus )
5.The large intestine is not required for life since it is not involved in chemical digestion. However, it does carry out important functions in the digestive system. List and describe these functions.
Bacterial Flora •Enter from small intestine or anus •Most bacteria is dead (killed by lysozyme, defensins, HCl and protein digesting enzymes), some are still alive -these constitute the bacterial flora of the large intestine •Colonize colon •Synthesize B complex vitamins and vitamin K •Ferment indigestible carbohydrates (such as cellulose) •Release of gases (~500ml of flatus per day) Digestive Processes in the Large Intestine •Residue remains in large intestine 12-24 hours •No food breakdown except by enteric bacteria •Vitamins (made by bacterial flora), water, and electrolytes (especially Na+ and Cl-) reclaimed •Major functions: propulsion of feces to anus; defecation Motility of the Large Intestine •Most contractions of colon -Haustral contractions •Slow segmenting movements •Haustra sequentially contract in response to distension •Gastrocolic reflex -Initiated by presence of food in stomach -Activates three to four slow powerful peristaltic waves per day in colon (mass movements) Defecation reflex (next slide)
5.Explain how the structure of the esophagus is best suited to its function of passing food down into the stomach via peristalsis.
Location: •Flat muscular tube from laryngopharynx to stomach •Pierces diaphragm at esophageal hiatus •Joins stomach at cardial orifice •Gastroesophageal (cardiac) sphincter •Surrounds cardial orifice Structure: •Esophageal mucosa contains stratified squamous epithelium -Changes to simple columnar at stomach •Esophageal glands in submucosa secrete mucus to aid in bolus movement •Muscularis externa - skeletal superiorly; mixed in middle; smooth inferiorly •Esophagus has fibrous adventitia (instead of serosa) composed entirely of connective tissue
7.Describe the regulation of gastric secretion including the processes that stimulate and those that inhibit the cephalic phase, gastric phase and intestinal phase (be sure you can provide a description of each phase).
Neural and hormonal mechanisms •Gastric mucosa secretes up to 3 L gastric juice/day -Vagus nerve (parasympathetic) stimulates stomach secretory activity -Sympathetic stimulation depresses stomach secretory activity -Hormonal control largely gastrin •Stimulate enzyme and HCl secretion Three phases of regulation of gastric secretion -Cephalic (reflex) phase -Gastric phase -Intestinal phase
4.Describe the two different types of cells found in salivary glands, and describe the composition of the substance released from each type of cell. Describe the general composition of saliva and explain how the composition changes based on parasympathetic versus sympathetic stimulation of the glands.
Two types of secretory cells -Serous cells •Watery, enzymes, ions, bit of mucin -Mucous cells •Mucus (stringy viscous solution) Composition of Saliva •97-99.5% water, slightly acidic (pH 6.75 - 7) •Electrolytes: Na+, K+, Cl-, PO4 2-, HCO3- •Digestive enzymes: Salivary amylase and lingual lipase •Mucin (protein) •Metabolic wastes: urea and uric acid •Lysozyme, IgA, defensins, protect against microorganisms Control of Salivation •Salivary glands are activated by parasympathetic nervous system •Ingested food stimulates chemoreceptors and mechanoreceptors in mouth send signals to brain stem therefore parasympathetic activity increases (via cranial nerve fibers) •Sympathetic stimulation inhibits salivation and results in dry mouth
3.List and describe the different types of innervation of the digestive system and compare and contrast between internal and external stimuli and control.
•Enteric Nervous System -Intrinsic nerve supply of alimentary canal consists of enteric neurons that communicate with one another to regulate digestive system activity. -Enteric neurons consists of 2 major intrinsic nerve plexuses: •Submucosal nerve plexus -Occupies the submucosa -Regulates glands and smooth muscle in the mucosa •Myenteric nerve plexus -Lies between the circular and longitudinal layers of the muscularis externa -Controls GI tract motility •Enteric Nervous System is linked to CNS via: 1.Afferent visceral fibers 2.Sympathetic and parasympathetic branches of the ANS •Sympathetic impulses inhibit digestive activities •Parasympathetic impulses stimulate digestive activities 1.Mechanoreceptors and chemoreceptors -Respond to stretch, changes in osmolarity and pH, and presence of substrate and end products of digestion -Initiate reflexes that •Activate or inhibit digestive glands •Stimulate smooth muscle to mix and move lumen contents 1.Intrinsic (nerve plexuses and hormone producing cells) and extrinsic controls (CNS centers and autonomic nerves) -Short reflexes - enteric nerve plexuses (gut brain) respond to stimuli in GI tract -Long reflexes respond to stimuli inside or outside GI tract; involve CNS centers and autonomic nerves -Hormones from cells in stomach and small intestine stimulate target cells in same or different organs to secrete or contract
6.Be able to describe the macroscopic and microscopic structure of the stomach. Explain the structure and function of the gastric pits, gastric glands, mucus neck cells, parietal cells, chief cells, enteroendocrine cells and mucosal barrier cells. Be sure you can explain the role each plays in chemical digestion in the stomach.
•Four tunics •Muscularis and mucosa modified -Muscularis externa •Three layers of smooth muscle •Inner oblique layer allows stomach to churn, mix, move, and physically break down food •Mucosa -Simple columnar epithelium composed of mucous cells •Secrete two-layer coat of alkaline mucus -Surface layer traps bicarbonate-rich fluid beneath it -Dotted with gastric pits which lead into tubular gastric glands •Gastric glands produce gastric juice -Mucous neck cells (secrete thin, acidic mucus of unknown function) -Parietal cells •Hydrochloric acid (HCl) -pH 1.5-3.5; denatures protein, activates pepsin, breaks down plant cell walls, kills many bacteria •Intrinsic factor -Glycoprotein required for absorption of vitamin B12 in small intestine -Chief cells •Pepsinogen - inactive form of enzyme pepsin -Activated to pepsin by HCl and by pepsin itself (a positive feedback mechanism) •Lipases -Digest lipids -Enteroendocrine cells •Secrete chemical messengers into lamina propria -Act as paracrines: Serotonin and histamine -Hormones: Somatostatin (also acts as paracrine) and gastrin
8.Be able to describe the anatomical junctions between the liver, gallbladder, pancreas and small intestines. Be sure you can explain the hepatopancreatic ampulla, hepatopancreatic sphincter, duodenal papilla, bile duct, and cystic duct.
•Hepatic artery and vein enter at porta hepatis •Bile ducts -Common hepatic duct leaves liver -Cystic duct connects to gallbladder -Bile duct formed by union of common hepatic and cystic ducts
1.Explain the structures of the small intestine which allow it to function primarily in nutrient absorption.
•Increase surface area of proximal part for nutrient absorption •Circular folds (plicae circulares) -Permanent folds (about 1 cm deep) that force chyme to slowly spiral through lumen, slowing its movement and allowing time for full nutrient absorption •Villi -Finger like projections of the mucosa (~1 mm high) of mucosa with capillary bed and lacteal for absorption. The epithelium includes simple columnar absorptive cells and mucus-secreting goblet cells. •Microvilli -Microvilli (brush border) of the absorptive cells of mucosa contain enzymes for carbohydrate and protein digestion
2.Be able to describe the general gross anatomy of the liver and know the primary functions of the hepatic artery, hepatic portal vein and common hepatic duct.
•Largest gland in body •Four lobes—right, left, caudate, and quadrate •Falciform ligament -Separates larger right and smaller left lobes -Suspends liver from diaphragm and anterior abdominal wall •Round ligament (ligamentum teres) -Remnant of fetal umbilical vein along free edge of falciform ligament •Hepatic artery and vein enter at porta hepatis •Bile ducts -Common hepatic duct leaves liver -Cystic duct connects to gallbladder -Bile duct formed by union of common hepatic and cystic ducts •Liver lobules -Hexagonal structural and functional units -Composed of plates of hepatocytes (liver cells) •Filter and process nutrient-rich blood -Central vein
3.Explain the general structure of the portal triad of a lobule and be able to describe the general functions of the liver.
•Portal triad at each corner of lobule -Branch of hepatic artery supplies oxygen -Branch of hepatic portal vein brings nutrient-rich blood -Bile duct receives bile from bile canaliculi •Liver sinusoids - leaky capillaries between hepatic plates •Kupffer cells (hepatic or stellate macrophages) in liver sinusoids remove debris & old RBCs •Hepatocytes - increased rough & smooth ER, Golgi, peroxisomes, mitochondria -Hepatocyte functions •Process bloodborne nutrients •Store fat-soluble vitamins •Perform detoxification •Produce ~900 ml bile per day
