Digestion and Absorption in GI Tract Flashcards by Sarah Gillen

Majority of absorption occurs in the SI. What are the 2 major routes of movement of nutrients, H2O and electrolytes from intestinal lumen to blood?

Cellular:
Lumen -> apical membrane -> intestinal epithelial cell -> basolateral membrane -> blood

Paracellular (via tight junctions):
Intestinal epithelial cell -> lateral intercellular space -> intestinal epithelial cell

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2 general types of digestive activity

Cavital (or luminal):
Digestion resulting from actions of enzymes secreted by salivary glands, stomach, and pancreas

Membrane (contact):
Hydrolysis of enzymes synthesized by epithelial cells

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Structure of intestinal mucosa is ideal for absorption of large amounts of nutrients. The lumen surface of SI is arranged in longitudinal folds called folds of _______. Villi and microvilli increase surface area for absorption, and the longest villi are located in the _______

Kerckring; duodenum

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The microvillar surface, also known as the _____ border, is the site of activity of many digestive enzymes. It is a barrier that must be traversed by nutrients, water, and electrolytes. The enterocytes below the microvillar surface have junctional complexes near the surface, and the cells are connected via __________, leaving intercellular spaces between them

Brush; interdigitation

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Epithelial cells of intestines involved in digestion, absorption, and secretion with a turnover rate of 3-6 days

Enterocytes

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What effect might common cancer treatments have on intestinal enterocytes?

Enterocytes are susceptible to irradiation and chemotherapy

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Mucous secreting cells associated with intestinal epithelium, providing physical, chemical, and immunological protection

Goblet cells

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Intestinal epithelial cells that function as part of mucosal defenses against infection by secreting agents that destroy bacteria or produce inflammatory responses

Paneth cells

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What route of passage into enterocytes occurs at the base of microvilli and is the major mechanism for uptake of proteins?

Pinocytosis

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What route of passage into enterocytes occurs through pores in cell membranes and between cells?

Passive diffusion

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What are the remaining routes of passage into enterocytes besides pinocytosis and passive diffusion?

Membrane bound transporters via facilitated or active transport

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Transmural movement of digested material occurs during absorption. Solutes moving across the enterocyte will encounter what numerous barriers?

Unstirred layer of fluid (viscous)
Glycocalyx
Apical membrane
Cytoplasm of cell
Basolateral membrane
Basement membrane
Wall of blood capillary or lymphatic vessel

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Adaptations in digestion and absorption may occur in the body as needed to maintain homeostasis.

What are some clinical scenarios that require adaptation? Why is adaptation limited with terminal ileal resection?

Small bowel resection
Gastric bypass

Adaptation is limited with terminal ileum resections because this abolishes bile salts and vitamin B12 absorption

Genetic abnormalities can also limit and lead to loss of adaptability such as lactase deficiency

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Most meals are “mixed” in terms of nutrients provided. Sensing individual nutrients is done by what branch of the nervous system?

CNS

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Where does carbohydrate digestion/absorption begin? Where does most occur? Where do the absorbed products go?

In the mouth

Most in the duodenum

Absorbed products travel via portal circulation

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The goal of carbohydrate digestion is to get what 3 absorbable end products?

Glucose
Galactose
Fructose

[monosaccharides]

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Lactose is a disaccharide digested by ________ into _____ and ______

Lactase

Glucose
Galactose

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Sucrose is a disaccharide digested by ________ into _____ and ______

Sucrase

Glucose
Fructose

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What are the primary mechanisms for absorption of carbohydrates?

Co-transport mechanisms and facilitated diffusion

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What transporters are key to absorption of galactose?

SGLT1 and GLUT2

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What transporter, present on the apical membrane is required for the absorption of fructose?

GLUT5

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What transporter, on the basolateral membrane, is responsible for absorption of glucose, galactose, and fructose?

GLUT2

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What causes the symptoms of lactose intolerance?

Brush border enzyme lactase is deficient/absent so undigested lactose remains in lumen holding water with it, and osmotic diarrhea results

Undigested lactose also ferments into methane and gas

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Glucose tolerance test utilizes _______, a pentose sugar absorbed by ____cotransport and passive diffusion.

D-xylose; Na+

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Tests show that glucose is absorbed as expected by a patient if sucrose, lactose, or glucose is ingested; but not if complex starches are ingested. These data indicate a defect in which of the following? ``` A. Bile acid secretion B. Brush border enzyme levels C. Epithelial sodium-coupled glucose transport D. Pancreatic enzyme secretion E. Villus surface area ```

D. Pancreatic enzyme secretion

To what degree to proteins need to be digested in order to be readily absorbed in the small intestine?

Amino acids Dipeptides Tripeptides

What enzyme in the stomach operates at a low pH and participates in the beginning of protein digestion?

Pepsinogen (converted to active pepsin by low pH)

What enzyme participates in protein digestion in the small intestine and serves to activate all other proteases required? What enzyme activates it?

Trypsin (active form of trypsinogen) Activated by enterokinase in brush border

What other enzymes besides trypsin are required for protein digestion in the small intestine?

Chymotrypsin Elastase Carboxypeptidase A and B

What apical transporter is used for amino acids across the apical membrane?

Sodium - amino acid cotransporter

What apical membrane transporter is used for di and tripeptides? What are the next steps for their absorption into the blood?

Cotransported with H+ Di and tripeptides are further digested in the enterocyte by peptidases. After that there are separate facilitated diffusion mechanisms for each amino acid type (neutral, acidic, basic, imino, etc)

What protein assimilation disorder(s) are characterized by deficiency of pancreatic enzymes, specifically lack of proteases like trypsinogen?

Chronic pancreatitis and cystic fibrosis

What protein assimilation disorder is characterized by absence of trypsin, so all pancreatic enzymes are nonfunctional?

Congenital trypsin absence

What protein assimilation disorder is characterized by a defect in or absence of Na/amino acid cotransporters for di-basic amino acids? What amino acids are affected? What happens to these amino acids

Cystinuria Cysteine, lysine, arginine, ornithine They are secreted in feces

What protein assimilation disorder is characterized by inability to absorb neutral amino acids? What condition does this resemble and what are the symptoms?

Hartnup disease Similar to pellagra (niacin deficiency) Diarrhea, mood changes, neuro problems, red scaly rash, photosensitivity, short stature, urinary excretion of neutral amino acids like tryptophan and its byproducts: serotonin

What protein assimilation disorder is characterized by defect in regulated Cl- channel on apical membrane of pancreatic duct cells and can lead to acute and chronic pancreatitis due to loss of HCO3 secretion?

Cystic fibrosis

What key family of enzymes is responsible for lipid breakdown in digestion?

Lipases (lingual, gastric, pancreatic)

_____ and ______ lipases initiate digestion of lipids in the stomach. Lipids are broken down into small droplets and emulsified here by ________

Lingual; gastric; dietary proteins [no bile acids, so emulsification occurs by dietary proteins]

______ is secreted when lipids first appear in small intestine to slow the rate of gastric emptying and promote more mixing, this also activates gallbladder secretions

CCK

What emulsifies lipids in the small intestine?

Bile salts

What enzyme is inactivated by bile salts without colipase?

Pancreatic lipase

What enzyme displaces bile salts by binding to pancreatic lipase? Its proenzyme form is activated by what?

Colipase; trypsin

What enzyme catalyzes production of cholesterol and hydrolyzes TAGs into glycerol?

Cholesterol ester hydrolase

What enzyme is responsible for digestion of phospholipids? What activates it from its proenzyme form?

Phospholipase A2 Activated by trypsin

What molecular structure forms the solubilized products of lipid digestion?

Mixed micelles

What is the optimal pH for pancreatic lipase?

Steps for lipid absorption across enterocyte

1. Solubilization by micelles in the lumen 2. Diffusion of micellar content across apical membrane 3. Re-esterification inside the cell 4. Chylomicron formation inside the cell 5. Exocytosis of chylomicrons across basolateral membrane

Lack of what molecule leads to abetalipoproteinemia, aka no lipid absorption occurs?

ApoB - affects the ability to form chylomicrons inside the enterocyte

Key steps in lipid digestion/absorption are pancreatic enzyme secretions, bile acid secretion, emulsification, micelle formation, diffusion of lipids into intestinal epithelial cells, chylomicron formation, and transfer of chylomicrons into lymph. Problems with any of the above can result in what symptom?

Steatorrhea

What is pancreatic insufficiency?

Failure to secrete proper amounts of pancreatic enzymes

Pancreatitis occurs d/t impaired ____ and enzyme secretions

HCO3

What is the overlying theme in issues with pancreatic enzyme secretions?

Improper acidity of duodenum contents - lumen here must be neutralized by HCO3 containing pancreatic secretions

Deficiencies in bile salts interferes with formation of micelles in the small intestine What factors cause deficits in bile salts?

Ileal resection! - total bile salt pool is reduced due to loss of enterohepatic recirculation of bile salts Small intestinal bacterial overgrowth (SIBO) leading to deconjugation of bile salts, thus impairing micelle formation

What are the 2 main causes of SIBO?

Too little gastric secretion Small intestine dysmotility

What condition is characterized by a loss or decreased number of intestinal epithelial cells, reduced microvilli surface, steatorrhea, nutritional deficiencies in folate and Vit B12, diarrhea, and is treated with tetracycline and folate for 6 mos?

Tropical sprue

What is celiac sprue?

Autoimmune condition against gliadin (gluten component) - leads to destruction of villi in small intestine and hyperplasia of intestinal crypts Malabsorption related to deficiencies in folate, iron, calcium, vitamins A, B12, D

Fat soluble vitamins are absorbed via the same mechanism as lipids. Most water soluble vitamins are absorbed via __-dependent cotransport

Vitamin B12 (cobalamin) forms complexes with other proteins to be absorbed, what other proteins are they?

R proteins - secreted in salivary juices Intrinsic factor - IF Transcobalamin II

What effect does vitamin B12 deficiency have on the CNS?

Causes demyelination of the large nerve fibers of the spinal cord

What role do vitamin B12 and folate play in red blood cells? What condition results when there is a vitamin B12 deficiency?

Important for DNA synthesis in RBCs [vitamin B12 acts as coenzyme for reducing ribonucleotides to deoxyribonucleotides] Deficiency results in pernicious anemia - failure of RBCs to mature (other hematological disorders include macrocytic and megaloblastic anemias)

Vitamin B12 travels the GI tract bound to ______ until it gets to the ______ where it is absorbed across the mucosa and into the blood

IF; distal ileum

What 2 gastric surgical procedures affect the ability to absorb vitamin B12?

Gastrectomy - loss of parietal cells which are the source of IF Gastric bypass - exclusion of the stomach, duodenum, and proximal jejunum alters absorption of B12

Pernicious anemia may result if the stomach does not produce enough IF. What are common causes of this IF deficiency?

Atrophic gastritis - chronic inflammation of the stomach mucosa that leads to loss of parietal cells Autoimmune metaplastic atrophic gastritis - immune system attacks IF protein or gastric parietal cells

Problems with vitamin D absorption can lead to inadequate calcium absorption because its transporter in the small intestine is vitamin D dependent. How does this manifest in children vs. adults?

Children = rickets Adults = osteomalacia

Iron absorption occurs when the liver secretes ______ into bile, which then enters the duodenum. This compound binds with free iron and with hemoglobin, forming ________, which binds to receptors on membranes of enterocytes where it is absorbed via _______ and later released

Apotransferrin Transferrin; pinocytosis

Products and mechanism of carbohydrate digestion

Glucose Galactose Fructose Na-glucose cotransport, Na-galactose cotransport, facilitated diffusion

Products and mechanism of protein digestion

Amino acids Dipeptides Tripeptides Na-amino acid cotransport, H-dipeptide and H-tripeptide cotransport

Products and mechanism of lipid digestion

Fatty acids Monoglycerides Cholesterol Bile salts form micelles, diffusion of FAs, MAG, and cholesterol, reesterification in cell to TG and PLs, chylomicrons form in cell and transported to lymph

Bile salts are absorbed in the ileum via _____ cotransporter

Na-bile salt cotransporter

Fe3+ is reduced to Fe2+ in the _______ It binds to apoferritin in the ______, and it binds to ______ in the blood

Small intestine Intestinal cells; transferrin

Which of the following vitamin deficiencies is most likely in a person with steatorrhea? A. Riboflavin/B2 B. Vitamin E C. Vitamin C D. Niacin/B3

B. Vitamin E

Which of the following are key to absorption of galactose? ``` A. SGLT1 and GLUT2 B. SGLT1 and GLUT5 C. GLUT2 and GLUT5 D. GLUT4 and SGLT1 E. SGLT1 only ```

A. SGLT1 and GLUT2

Which of the following are key to absorption of fructose? ``` A. SGLT1 and GLUT2 B. SGLT1 and GLUT5 C. GLUT2 and GLUT5 D. GLUT4 and SGLT1 E. SGLT1 only ```

C. GLUT2 and GLUT5

A 55-year-old male patient comes to the office with chronic abdominal pain related to eating and diarrhea. He undergoes endoscopy, which demonstrates multiple small duodenal ulcers. Biopsies of the ulcers show benign histology. Gastrin levels are elevated. Secretin stimulation test is positive (ie, raises serum gastrin levels). Which of the following conditions would fit this symptomology? ``` A. Celiac disease B. H. Pylori gastritis C. Crohn disease D. Meckel diverticulum E. Zollinger-ellison syndrome ```

E. Zollinger-ellison syndrome

A 37-year-old female presents to office with diarrhea, abdominal pain, flatulence, and greasy, foul-smelling stools. She has a history of similar symptoms since adolescence, but they have worsened in the last 3 months. She has been eating a lot of desserts, especially ice cream, to help cope with the loss of her mother. Despite this, she has lost 11.3 kg (25 lb). Imaging reveals no abnormalities in the GI tract. A small bowel biopsy reveals a shortening and loss of intestinal villi.? ``` A. Celiac disease B. Hyperthyroidism C. Inflammatory bowel disease D. Irritable bowel syndrome E. Lactose intolerance ```

A. Celiac disease