BIOL 66- Chapter 18: The Digestive System

Functions

processes food into molecular forms that are transferred into the internal environment, which the circulatory system distributes to cells of the body

Motility

movement of food through the GI tract via ingestion, mastication, deglutition, and contraction of smooth muscle (peristalsis and segmentation)

Digestion

Chemical/mechanical breakdown of food from macromolecules (carbs, proteins, fats) to smaller molecules for absorption into blood stream for circulation in the body and secreted into cells

Secretion

Release of exocrine and endocrine secretions into the lumen of GI tract for digestion and regulation of digestion; bile salts, lipase, amylase, pepsin, HCl, NaOH

Absorption

Movement of digested end products into blood and lymph; the small molecules

Storage and Elimination

temporary storage followed by elimination of indigestible food and molecules (waste); feces

Immune Barrier

physcial barriers to pathological organisms and toxins due to tight junctions in epithelial lining of intestine; as GI mucosa is in contact with the external environment, almost 80% of immune system cells are here

Digestions via Hydrolysis Reactions

digestion of food occurs by hydrolysis reactions, the chemical breakdown of a substance involving a reaction with water

Structures- GI tract (Alimentary Canal)

mouth, pharynx, esophagus, stomach, small intestine, large intestine; appx 30 ft long

Structures- Accessory Organs and Tissues

teeth, tongue, salivary glands, liver, gallbladder, pancreas

Layers/Tunics of Gut Wall

mucosa, submucosa, muscularis, serosa

Mucosa layer

first layer, innermost; absorption and secretion, mucus secretion

Mucosa layer- Layers within

epithelium, lamina propria, and muscularis mucosae

Mucosa layer- Lamina propria

connective tissue with lymph nodules, hold immune function

Mucosa layer- Muscularis Mucosae

thin smooth muscle layer, numerous small folds to increase surface area for absorption; produces movement of villi and brush border of small intestine

Submucosa Layer

second layer; connective tissue, blood/lymph vessels, submucosal plexus

Submucosa Layer- Submucosal plexus

neuronal innervation for muscularis mucosae

Muscularis Externa

third layer; involved in segmental and peristaltic contractions, to move food through the tract, and pulverize and mix it with digestive enzymes; inner circular layer (around) and outer longitudinal layer (along) of smooth muscle; Myenteric plexus

Muscularis Externa- Myenteric plexus

neurons that control GI tract, like parasympathetic NS “rest and digest”

Serosa layer

fourth layer, outermost, connective tissue covered with epithelium

From mouth to stomach

mastication, deglutition, esophagus, stomach

Mastication

chewing of food, breaking down food with teeth and tongue; mixes food with saliva secreted by salivary glands

Where does digestion begin?

digestion starts with saliva, which contains salivary amylase, an enzyme that catalyzes the partial digestion of starch

Deglutition

swallowing; initiated when food or drink stimulate pressure receptors in the pharynx; the muscles of the mouth and tongue mix the food with saliva to create a bolus; tongue pushes the bolus to the back of the pharynx; upper esophageal sphincter relaxes; food descends into the esophagus

bolus

chewed up food (bocado); partially chewed up food starting to mix with secretion

Esophagus

motility only, no digestion; peristalsis

Esophagus- Anatomy

muscular tube connecting pharynx to stomach, posterior to trachea, lined with epithelium and skeletal and smooth muscle

Esophagus- Peristalsis

wavelike muscular contractions that push bolus to the stomach; circular smooth muscle contracts above the bolus and relaxes below it, then there’s shortening of the tube by longitudinal muscular contraction

Esophagus- food passage

Food passes through the lower esophageal sphincter to enter the stomach

Stomach

Digestion here results in chyme, partially digested food mixed with gastric juice; stores food; kills bacteria; starts digestion of proteins; Peristaltic waves mix and propel the chyme; moves chyme to SI, where most digestion and absorption occur

Stomach- acidity

kills bacteria with acidity of gastric juice, pH < 2

Chyme

bolus mixed with gastric juice in the stomach, makes it liquid

Most digestion and absorption occurs in the …

Small Intestine

Stomach- Specialized cells

specialized cells in the stomach synthesize and secrete mucus, enzyme precursors, hydrochloric acid (HCl), and hormones

Stomach-Gastric Rugae

inner surface of the stomach contains these folds

Stomach- abundant smooth muscle

abundant smooth muscle in the stomach is responsible for gastric motility, the movement of food

Gastric Glands Cell Types- Mucosa neck cells

secrete mucus, look like they have a neck

Gastric Glands Cell Types- Parietal cells

secrete hydrochloric acid (HCl)

Gastric Glands Cell Types- D Cells

secrete somatostatin, a hormone that inhibits parietal cells

Gastric Glands Cell Types- Chief (zygomatic) cells

secrete pepsinogen, an inactive form of pepsin (a protein-degesting enzyme), enzyme precursor

Gastric Glands Cell Types- Enterochromaffin-like (ECL) cells

secrete histamine and serotonin, for regulation of GI tract, control digestive activity

Gastric Glands Cell Types- G Cells

secrete gastrin, a hormone that stimulate parietal (to secrete HCl) and ECL cells (to secrete regularoty)

Gastric Glands Cell Types- Intrinsic factor

a polypeptide essential for intestinal absorption of vitamin B12, needed for production of RBCs and bone marrow

Gastric Glands Cell Types- Ghrelin

a hormone that regulates hunger

Gastric Glands Cell Types- Gastric Juice

secretions of gastric cell + water

Gastric Glands- Chief Cells

synthesize and secrete pepsinogen, the pepsin precursor

Gastric Glands- Parietal cells

synthesize and secrete the hydrochloric acid (HCl) responsible for the acidic pH in the gastric lumen

Activation of pepsin

the acidity in the gastric lumen (pH < 2) converts pepsinogen to pepsin. Pepsin partially digests proteins. Bicarbonate protects the stomach from acid damage

Small intestine

longest part of GI tract, 3 m long and small diameter; digestion and absorption

What does the SI digest?

carbohydrates, lipids, and proteins; include brush border enzymes and pancreatic enzymes

What does the SI absorb?

carbohydrates, lipids, AAs, vitamins, minerals, iron, water, electrolytes, and bile salts, into blood stream

SI- Villi

folds in mucosa, fingerlike projections

SI- Microvilli

fingerlike projections on villi; microvilli or brush border on the villi increase the surface area for absorption

SI- Brush Border Enzymes

They are embedded in the plasma membrane of microvilli; their active sites face the chyme in the lumen of the SI

SI- Motility

motility in the SI is slow, to ensure proper absorption of nutrients

SI- Contraction

main contraction is segmentation: muscular constrictions of lumen that occur simultaneously in different segments, mixes and moves chyme

Large intestine/Colon

large diameter; no villi; haustra are pouches on outer surface; no digestion; absorption and excretion

What does the LI absorb?

electrolytes, water, vitamins, and lots of fluid

What does the LI excrete?

waste products in feces, through rectum and anal canal; anything that can’t be digested or absorbed

GI tract- water intake

around 1.5L of water from food and drink enters GI tract per day

GI tract- fluid secretion

secretes around 8-10 L of fluid into its lumen

GI tract- places of fluid secretion

salivary glands, stomach, intestine, pancreas, liver, gallbladder

LI- absorption

absorption of most fluids so <200 ml of fluid is excreted in feces; active transport of Na+ into epithelial cells of LI → osmosis of water into ISF and then into bloodstream

Accessory organs

liver, gallbladder, pancreas

Accessory organs- Liver

many functions including synthesis of bile

Accessory organs- Gallbladder

storage of bile from liver and release into SI

Accessory organs- Pancreas

pancreatic juice for digestion in SI

Liver- functinos

detoxification of blood, carbohydrate metabolism, lipid metabolism, protein synthesis, secretion of bile, and storage of molecules

Secretion of Liver- hepatocytes

groups of liver cells are separated by hepatic sinusoids

Secretion of Liver- Blood enters liver lobule

blood enters liver lobule (functional unit) through portal triad, passes through hepatic sinusoids, and leave the lobule through central vein

Liver- Portal Triad

hepatic artery, portal vein, bile ductule

Secretion of Liver- Central vein

central vein converges to form hepatic veins that take venous blood away from the liver

Secretion of Liver- Bile

bile is synthesized by hepatocytes and secreted into bile canaliculi, which then drains into bile ductules into portal triad

Secretion of Liver- into gallbladder

bile is then funneled from the ductules into the gallbladder where it is stored, and then delivered into the small intestine

Hepa-

pertaining to the liver

Bile- Main Components

bile pigment- gives color, from breakdown of RBCs; bile acids or salts- amphipathic; lecithin- a lipid, in foods; bicarbonate- add bulk, increase volume; cholesterol- main way it’s recycle in the body; trace metals

Bile- Bile acids

they’re cholesterol based, and form micelles in aqueous solutions

Bile- Micelles

green block is nonpolar and black tail is polar; in SI, fats enter it, and the amphipathic property of micelles allows emulsification (breakdown) of fat

Bile- Emulsification

breakdown of large fat globules by bile acids into smaller globules for digestion by lipase

Circulation of Bile

bile enters the SI via the common bile duct, the bile in Si emulsifies fats to break them down before digestion by lipase

Pancreas- Endocrine functions

insulin, glucagon

Pancreas- Exocrine functions

pancreatic juice

Pancreatic Juice

synthesized by acinar cells and delivered to duodenum of SI through pancreatic ducts; a bicarbonate and around 20 enzymes

Pancreatic Juice- Enzymes

amylase, trypsin, lipase, and more; many enzymes are activated in SI; pancreatic enzymes and brush border enzymes accomplish complete digestion of food molecules

Why is it good that many Pancreatic Enzymes are activated in the SI?

if they were active before the SI, then they would digest the pancreas which is not good

Pancreatic Juice- Amylase

digest starch

Pancreatic Juice- Trypsin

digest protein; in the stomach it’s pepsin

Pancreatic Juice- Lipase

digest triglycerides

Pancreas- Endocrine portion

pancreatic islets of langerhans

Pancreas- Exocrine portion

Pancreatic Acini

Pancreatic Secretions- what would happen if digestive enzymes synthesized in active form?

if digestive enzymes secreted by the pancreas were synthesized in their active form, they would digest the very cells that make them. Hence, inactive precursors (zymogens) become activated in the SI

Pancreatic Secretions- Exocrine cells

the exocrine cells in the pancreas produce digestive enzymes that exit via the pancreatic duct to travel to the SI

Digestion and Absorption of Carbohydrates- Daily intake

250-300 g, mostly as starch- polysaccharide of glucose

Digestion and Absorption of Carbohydrates- Commonly ingested sugars

sucrose (table sugar) and lactose

Digestion and Absorption of Carbohydrates- Salivary Amylase

begins digestion in the mouth

Digestion and Absorption of Carbohydrates- Pancreatic Amylase

in SI, most of carb digestion, results in maltose, maltotriose, and oligosaccharides; breaks down larger sugar molecules to smaller molecules

Digestion and Absorption of Carbohydrates- Brush border amylases

in SI, hydrolyze these sugars into their component monosaccharides, which are then moved across the brush border membrane by active transport, to be absorbed into the bloodstream

Digestion and Absorption of Proteins- Daily intake

60-90 g, needed for AAs

Digestion and Absorption of Proteins- Pepsin in stomach

produces short-chain polypeptides

Digestion and Absorption of Proteins- in SI

pancreatic enzymes trypsin, chymotrypsin, and carboxypeptidase, and brush border enzyme aminopeptidase, digest polypeptides into free AAs, dipeptides, and tripeptides