Answer Key

What are the four components of the digestive system….Motility, secretion, digestion, absorption

What is the difference between propulsive movements and mixing movements?

-propulsive movements push contents forwards through the digestive tract

-mixing movements mix food with digestive juices to promote digestion and absorption

What are the four major tissue layers of digestive tract?

-Mucosa, submucosa, muscularis externa, serosa

Describe the layers of the mucosa

1. Mucous membrane: highly folded. Contains exocrine gland cells that secrete digestive juices. Endocrine gland cells – hormones. And epithelial cells that are specialize for absorption.

2. Lamina propria: house gut-associate lymphoid tissue (GALT)

3. Muscularis mucosa: smooth muscle

Describe submucosa: thick layer of connective tissue (elastin), blood and lymph vessels. Nerve network (submucosal plexus)

Describe muscularis externa: smooth muscle consisting of circular inner layer and longitudinal outer layer

Describe serosa: secretes serous fluid; lubricates and prevents friction between digestive organs and surrounding viscera.

How are digestive motility and secretion regulated?

- intrinsic nerve plexuses, extrinsic nerves, gastrointestinal hormones stimulate smooth muscle (contraction for motility), exocrine gland cells to secrete digestive juices, endocrine gland cells to secrete gastrointestinal and pancreatic hormones

Describe digestive process:

Begins in the mouth. Teeth grind and break food into smaller pieces (making swallowing easier and increase food surface area). Salivary amylase begins digestion of carboyhdrates, moistens and lubricates, antibacterial lysozyme, solvent for molecules that stimulate taste buds.

Control of salivary secretion mechanism:

Pressure receptors and chemoreceptors in mouth and/or cerebral cortex stimulate salivary center in medulla stimulates autonomic nerves stimulate salivary glands increase salivary secretion

Swallowing reflex:

Swallowing center inhibits respiratory center in brain stem. Elevation of uvula prevents food from entering nasal passages. Position of tongue prevents food from reentering mouth. Epliglottis is pressed down over closed glottis as auxiliary mechanism to prevent food from entering airways. Tight apposition of vocal folds across glottis prevents food from entering respiratory airways.

Phayngoesophageal sphincter--------prevents air from entering esophagus

Gastroesophageal sphincter--------prevents reflux of gastric contents

Pyloric sphincter-------barrier between stomach and duodenum

Three main functions of the stomach:1. store food (takes place in body of stomach) 2. secrete HCl and enzymes that begin protein digestion3. mixing movement to convert pulverized food to chime (takes place in antrum of stomach)filling of stomach-----------triggered by act of eating, mediated by vagus nerve. Involves receptive relaxation.

Gastric emptying (largely controlled by factors in the duodenum) A peristaltic contraction originates in the upper fundus and sweeps down toward the pyloric sphincter. The

contraction becomes more vigorous as it reaches the thick-muscled antrum. The strong antral peristaltic contraction propels the chime forward. A small portion of chime is pushed through the partially open sphincter into the duodenum. The stronger the antral contraction, the more chime is emptied with each contractile wave. When the peristaltic contraction reaches the pyloric sphincter, the sphincter is tightly closed and no further emptying takes place. When chime that was being propelled forward hits the closed sphincter, it is tossed back into the antrum. Mixing of chime is accomplished as chime is propelled forward and tossed back into the antrum with each peristaltic contraction.

D cells--------secretes somatostatin

Parietal cells--------secretes hydrochloric acid and intrinsic factor

Chief cells-------secretes pepsinogen

Mucous cells-------secretes alkaline mucus

Enterochromaffin-like (ECL) cells--------secretes histamine

List five major function of hydrochloric acid in digestive process.

1. converts pepsinogen to pepsin

2. contributes to low pH (for optimal pepsin activity)

3. breakdown of connective tissue and muscle fibers

4. denatures protein

5. (along with salivary lysozyme) kills microorganisms ingested with food

Cephalic phase---------increased secretion of HCl and pepsinogen that occurs in response to stimuli acting in the head (nose, mouth, brain)

Gastric phase-----food in stomach, presence of protein increases gastric secretions

Intestinal phase------inhibitory phase, shut off form duodenum

Mechanism for cephalic phase of gastric secretion: stimuli in the head (seeing, chewing, tasting, swallowing food) stimulates vagus nerve stimulates intrinsic nerves and G cells. Intrinsic nerves increase secretion of Ach - stimulates chief and parietal cells to increase gastric secretion. G cells stimulate Gastrin and ECL cells to secrete histamine and increase gastric secretions

Mechanism for gastric phase of gastric secretion: stimuli in the stomach (protein, distension, caffeine, alcohol) stimulates vagus nerve stimulates intrinsic nerves and G cells. Intrinsic nerves increase secretion of Ach - stimulates chief and parietal cells to increase gastric secretion. G cells stimulate Gastrin and ECL cells to secrete histamine and increase gastric secretions

How do the components of the gastric mucosal barrier enable the stomach to contain acid without injuring itself? The luminal membranes of the gastric mucosal cells are impermeable to H+ so that HCl cannot penetrate into the cells. The cells are joined by tight junctions that prevent HCl from penetrating between them. A mucus coating over the gastric mucosa serves as a physical barrier to acid penetration. The HCO3- rich mucus also serves as a chemical barrier that neutralizes acid in the vicinity of the mucosa. Even when luminal pH is 2, the mucus pH is 7.

Islets of Langerhans (in pancreas)-------secrete insulin and glucagon

Proteolytic enzymes: Digest proteins

Trypsinogen - converted to active form trypsin

Chymotrypsinogen – converted to active form chymotrysin

Procarboxypeptidase – converted to active form carboxypeptidase

Pancreatic amylase: Breaks down polysaccharides

Pancreatic lipase:Only enzyme that can digest fat

Describe how the pancreas in controlled hormonally:

a. Acid in duodenal lumen stimulates duodenal mucosa to increase secretin secretin carried by blood and stimulates pancreatic duct cells increase secretion of aqueous NaHCO3 solution into duodenal lumen.

b. Fat and protein products in duodenal lumen stimulate duodenal mucosa to increase CCK release (CCK carried by blood) stimulates pancreatic acinar cells to increase secretion of pancreatic digestive enzymes into duodenal lumen.

Non-digestive functions of the liver:

1. metabolic processing of the major categories of nutrients

2.detoxifying hormones, drugs, and other foreign compounds

3. synthesizes plasma proteins

4. stores glycogen, fats, iron, copper, and many vitamins

5. activates Vitamin D

6. removes bacteria and worn-out red blood cells

7. excretes cholesterol and bilirubin

Describe blood flow in the liver: The liver receives blood from two sources:

1. Arterial blood (delivered by hepatic artery). Arterial blood provides the livers O2 supply and carries blood-borne metabolites for hepatic processing.

2. Venous blood (carried by the hepatic portal vein). Venous blood draining the digestive tract is carried to liver for processing and storage of newly absorbed nutrients.

**Blood leaves the liver via the hepatic vein

bile salts-----secreted by liver between meals. Stored in gallbladder, convert large fat globules into a liquid emulsion.

Describe circulation of bile salts: Secreted bile salts consist of 95% old, recycled bile salts and 5% newly synthesized bile salts. 95% of bile salts are reabsorbed by terminal ileum. Reabsorbed bile salts are recycled by enterohepatic circulation. 5% of bile salts are lost in feces.

Small intestine------majority of digestion and absorption occurs here, particularly in the duodenum and jejunum.

Intestinal secretions----------juice secreted by small intestine that does not contain any digestive enzymes

Describe mechanism by which carbohydrates are absorbed: The dietary polysaccharides starch and glycogen are converted into the disaccharide maltose through the action of salivary and pancreatic amylase. Maltose and the dietary disaccharides (lactose, sucrose) are converted to their respective monosaccharides by the disaccharides (maltase,

lactase, sucrase) located in the brush borders of the small-intestine epithelial cells. Glucose and galactose (monosacs) are absorbed into the epithelial cells by Na+ and energy-dependent secondary active transport (via the symporter SGLT) located at the luminal membrane. The monosaccharide fructose enters the cell by passive facilitated diffusion via GLUT-5. Glucose, galactose, and fructose exit the cell at the basal membrane by passive facilitated diffusion via GLUT-2. These monosaccharides enter the blood by simple diffusion.

Describe how protein is absorbed: Dietary and endogenous proteins are hydrolyzed into their constituent amino acids and a few small peptide fragments by gastric pepsin and the pancreatic proteolytic enzymes. Many small peptides are converted into their respective amino acids by the aminopeptidases located in the brush borders of the small-intestine epithelial cells. AAs are absorbed into the epithelial cells by means of Na+ and energy-dependent secondary active transport via a symporter. Various AAs are transported by carriers specific for them. Some small peptides are absorbed by a different type of symporter driven by H+, Na+, and energy-dependent tertiary active transport. Most absorbed small peptides are broken down into their AAs by intraceullular peptidases. AAs exit the cell at the basal membrane via various passive carriers. AAs enter the blood by simple diffusion (a small percentage of di and tripeptides also enter the blood intact).

Describe how dietary fat is absorbed: dietary fat (in form of large fat globules) is emulsified by detergent action of bile salts into a suspension of smaller fat droplets (increases SA available for attack by pancreatic lipase). Lipase hydrolyzes triglycerides into monglycerides and free fatty acids. These water-insoluble products are carried to the luminal surface of the small-intestine epithelial cells within water-soluble micelles, which are formed by bile salts and other bile constituents. When a micelle approaches the absorptive epithelial surface, the monoglycerides and faty acids leave the micelle and passively diffuse through the lipid bilayer of the luminal membranes. The monoglycerides and the free fatty acids are resynthesized into triglycerides inside the epithelial cells. These triglycerides aggregate and are coated with a layer of lipoprotein from the ER to form water-soluble chylomicrons. Chlyomicrons are extruded through the basal membrane of the cells by exocytosis. Chylomicrons are unable to cross the basement membrane of the capillaries, so instead they enter the lymphatic vessels, the central lacteals.

Describe how iron is absorbed: only a portion of ingested iron is in a form that can be absorbed (either heme iron or ferrous iron (Fe2+). Iron is absorbed across the luminal membrane of small-intestine epithelial cells by different energy-dependent carriers for heme and Fe2+. Dietary iron that is absorbed into the SI epithelial cells and is immediately needed for RBC production is transferred into the blood by ferroportin (membrane iron transporter). In the blood, the absorbed iron is carried to the bone marrow bound to transferrin (plasma protein carrier). Absorbed dietary iron that is not immediately needed is sotred in the epithelial cells as ferritin which cannot be transferred into the blood. This unused iron is lost in the feces as the ferritin-containing epithelial cells are sloughed. Dietary iron that was not absorbed is also lost in the feces.

Taeniae coli--------longitudinal bands of muscle in LI

Haustra ------------pouches or sacs; circular smooth muscle layer in LI

Haustral contractions ------------ main motility in LI

Large intestine---------------contains indigestible food residues, unabsorbed biliary components and remaining fluid

Colon -------extracts more water and salt from contents of LI.

Feces-------what remains to be eliminated after colon

Gastrocolic reflex -------------mediated from stomach to colon by gastrin and by autonomic nerves; most evident after first meal of the day, and is often followed by urge to defecate.

Defecation reflex ---------prescence of food in the stomach + presence of chime in the duodenum stimulates mass movement in large intestine/colon stimulates local defecation reflexes and parasympathetic controlled defecation reflexes causes internal anal sphincter to relax rectum and sigmoid colon contract - external anal sphincter (voluntary control)

Peptide hormones and catecholamines are hydrophilic hormones

Steroids and thyroid hormone are lipophilic hormones

List six major functions of the endocrine system:

1. metabolism + salt/water balance

2. adaptation to stress

3. growth and development

4. control reproduction

5. RBC production

6. Control circulation and digestion integration

Pineal gland --------produces melanin; secretes melatonin; affects reproductive development and daily physiologic cycles.

Melatonin ---------hormone of darkness; secretion falls to low levels during daylight; helps keep body’s circadian rhyms in synchrony w/ light-dark cycle, promotes sleep, influences reproductive activity (incl. onset of puberty), acts as antioxidant to remove free radicals, enhances immunity

Pituitary gland ------- produce HGH, ADH, and gonadotropins; controls growth of bones and muscles, increases reabsorption of H2O in kidneys, and controls development of ovaries and testes

Thyroid gland --------produces thyroxine; controls rate of metabolism and rate that glucose is used up in respiration; promotes growth. Follicular cells; lumen filled with colloid – serves as extracellular storage site for thyroid hormone

C cell----secrete peptide hormone calcitonin

Adrenal gland ----------produces adrenaline; prepares the body for emergencies; increases heart rate + depth of breathing, raises blood sugar level so more glucose is available for respiration, diverts blood from gut to limbs

Pancreas ------------- produces insulin and glucagon; converts excess glucose into glycogen in liver, convers glycogen back to glucose in liver

Ovaries -------produces estrogen and progesterone; controls ovulation and secondary sexual characteristics, prepares the uterus lining for receiving an embryo

Testes ----------produce testosterone; control sperm production and secondary sexual characteristics

Thymus -----------thymosin; promotes production and maturation of white blood cells

Tropic hormone----------regulates hormone secretion by another endocrine gland

Plasma concentration is determined by:

1. the hormone rate of secretion

2. rate of metabolic activation or conversion

3. transport

4. inactivation

5. excretion

posterior pituitary (neurohypophysis)-----------lobe of hypothalamus; consists of nervous tissue

anterior pituitary (adenohypophysis)------------lobe of hypothalamus; consists of glandular epithelial tissue

paraventricular nucleus axons neuronal terminals in posterior pituitary posterior pituitary

oxytocin -------stimulates uterine contractions and milk ejection during breast feeding

TSH thyroid gland thyroid hormone (T3, T4) increased metabolic rate

ACTH adrenal cortex release cortisol metabolic actions; stress response

LH sex hormone secretion (estrogen in progesterone in females, testosterone in males)

FSH gamete production (ova in females, sperm in males)

Growth hormone liver IGF-1 stimulates growth in bone and soft tissue

Growth hormone adipose tissue, muscle, liver metabolic actions

Prolactin mammary glands breast growth and milk secretion

Neurosecretory neurons in the hypothalamus produce hypophysiotropic hormones (releasing and inhibiting hormones) HPH enters the hypothalamic capillaries hypothalamic capillaries rejoin to form the hypothalamic-hypophyseal portal system portal system branches into the capillaries of the anterior pituitary HPH control the release of anterior pituitary hormones anterior pituitary secretes a given hormone into these capillaries anterior pituitary capillaries rejoin to form a vein, through which AP hormones leave for distribution throughout the body by the systemic circulation

6 peptide hormones produced by anterior pituitary:

1. FSH

2. Luteinizing Hormone

3. ACTH

4. Growth Hormone

5. TSH

6. Prolactin

Factors that effect growth

1. Normal levels of growth-influencing hormones

2. genetic determination of an individuals maximum growth capacity

3. adequate diet

4. freedom from chronic disease and stressful environmental conditions

growth hormone ------promotes growth by stimulating livers production of somatomedins

Insulin-like growth factor (IGF-1) ---------acts directly on bone and soft tissues; stimulates protein synthesis, cell division, and lengthening and thickening of bones

Exercise, stress, low blood glucose, diurnal rhythm hypothalamus growth-hormone releasing hormone (GHRH) anterior pituitary secrete growth hormone liver IGF-1 increase cell division, increase protein synthesis (decrease blood amino acids), increase bone growth. Growth hormone also increases fat break down (increasing fatty acid concentration in blood), decreases glucose uptake by muscles (increasing blood glucose), increases glucose output by liver (increases blood glucose)

Hormones besides HGH that are essential for normal growth:

1. Thyroid hormone: growth severely stunted in kids with hypothyroidism; hypersecretion does NOT cause excessive growth

2. Insulin: deficiency often blocks growth; hyperinsulinism often spurs excessive growth3. Androgens: play role in pubertal growth spurt; stimulate protein synthesis in many organs4. estrogens

Thyroid hormone negative feedback system:

feedback loop adrenal gland: stress + diurnal rhythm stimulate hypothalamus to release corticotropin-releasing hormone (CRH) stimulates anterior pituitary to release ACTH ACTH stimulates adrenal cortex to release cortisol which increases blood glucose (by stimulating gluconeogenesis and inhibiting glucose uptake), increase concentration of AA in blood (by stimulating protein degradation), increasing fatty acid concentration in blood (by stimulating lipolysis)

Integrated stress response feedback loop:

Stressor stimulates hypothalamus.

1. hypothalamus stimulates posterior pituitary to secrete vasopressin

2. hypothalamus releases CRH stim. Anterior pituitary to release ACTH stimulates adrenal cortex to secrete cortisol

3. stressor stimulates hypothalamus stimulates sympathetic nervous system stimulates adrenal medulaa to release epinephrine. Epi stimulates endocrine pancreas to release glucagon and inhibit insulin. Epi also stimulates arteriolar smooth muscle vasoconstriction decrease blood flow through kidneys activate RAAS

Glucose and Endocrine system: increase in blood glucose concentration (or increase in GI hormones or elevated blood AA concentration) stimulates islet B cells to secrete insulin decreases concentration of glucose, fatty acids, and AA in blood. Increases fuel storage and protein synthesis.List five functions of the skeleton:

1. support2. protection of vital internal organs3. Body movement (by giving attachment to muscles and providing leverage)4. Manufacture of blood cells (bone marrow)5. Storage for calcium and phosphate, which can be exchanged with the plasma to maintain plasma concentrations

of these electrolytes

Calcium regulation: (fast exchange) calcium is moved from the labile pool in the bone fluid into the plasma by PTH-activated calcium pumps located in the osteocytic-osteoblastic bone membrane. (slow exchange) calcium is moved from the stable pool in the mineralized bone into the plasma through PTH-induced dissolution of the bone by osteoclasts

Mechanism for calcium regulation: -decrease plasma calcium concentration stimulates parathyroid glands to release PTH increase [plasma calcium]

-elevated plasma calcium concentration stimulates thyroid C cells to release calcitonin decrease {plasma calcium}

Phosphate regulation: low phosphate concentration in plasma stimulates kidneys to release activated vitamin D increase phosphate absorption in intestine concentration of phosphate in plasma. Low phosphate concentration in plasma elevated plasma calcium concentration decreases release of PTH by parathyroid glands increases phosphate reabsorption by kidneys + decreases urinary secretion of phosphate. Simultaneously decreases calcium reabsorption by kidneys and increases urinary secretion of calcium.