Digestion And Absorption Flashcards
What is lactose intolerance?
Failure in ability to digest dairy carbs
Brush border lactase enzyme activity is deficient or absent resulting in undigested and unabsorbed lactose
Lactose in the SI is converted to SCFAs and H gas causing it to remain in the lumen holding H2O and leading to osmotic diarrhea
Ferments into methane and H gas
Describe carb digestion in the mouth
Begins breakdown of some starch
Contains salivary amylase which breaks starch down into maltose and 3-9 polymers of glucose
Describe carb digestion in the SI
Most starch breakdown occurs here
Pancreatic amylase is present which continues to breakdown starch
Many disaccharides reach SI intact
Describe the structure of intestinal mucosa
Ideal for absorption of large amounts of nutrients
Lumen surface of SI arranged in longitudinal folds (folds of Kerckring)
Villi and microvilli increase SA for absorption
Diseases that disrupt the mucosa will influence absorption of nutrients
Describe absorption of carbs
Co-transport mechanisms and facilitated diffusion
On the luminal side Na/K ATPase creates concentration gradient
Secondary active transport of glucose and galactose occurs through SLGT1
Facilitated diffusion of fructose through GLUT5
GLUT2 then is used for transport of all sugars into blood
What is one way to test for a CHO assimilation disorder?
After an overnight fast 25g D-xylose ingested and urine collected for next 5 hours
Key is that it will be absorbed not utilized
Absorbed by both active Na cotransport and by passive diffusion (at dose used will generally be able to absorb via passive diffusion)
Measures absorptive capacity of proximal SI specific to sugars
What are other tests that are used for CHO assimilation disorders?
Breath tests such as methane, isotopically labeled 13-CO2 from different sugars, lactose/sucrose breath test
What diseases are caused by a deficiency of pancreatic enzymes or defect in transporters of intestinal epithelial cells that lead to protein assimilation disorders?
Chronic pancreatitis, congenital trypsin absence, Cystinuria, hartnup disease, cystic fibrosis
What is chronic pancreatitis?
Deficiency of pancreatic enzymes
Lack of proteases (i.e. trypsinogen)
Inability to neutralize stomach acid
What is congenital trypsin absence?
Absence of trypsin
All pancreatic enzymes are gone
Cannot activate proteases
What is cystinuria?
Defect in transport or absence of di-basic amino acid transporter thus cysteine, arginine, lysine and ornithine are not reabsorbed at the proximal tubule
Due to intestinal deficiency - amino acids secreted in feces/urine
What is Hartnup disease?
Cannot absorb neutral amino acids
Sx resemble pellagra (niacin deficiency)
Autosomal recessive genetic disorder, Na dependent neutral amino acid transporter
Sx: diarrhea, mood changes, neuro problems (e.g. uncoordinated movements, abnormal muscle tone), red scaly skin rash, photosensitivity, short stature, urine sample displays high excretion of neutral amino acids (tryptophan) and by-products (serotonin)
What is cystic fibrosis?
CFTR regulated Cl channel on apical membrane of duct cells
Pancreas problems persist and present early
Some CFTR mutations associated with loss of HCO3 secretion
-cannot move enzymes from ducts
-can lead to acute and chronic pancreatitis
Cannot neutralize stomach acid
Describe protein digestion in the stomach
Pepsin secreted as pepsinogen and activated at low pH in the stomach (2-3)
Not essential for protein digestion (10-20%)
Describe the pancreas’s contribution to protein digestion
Enzymes: trypsin, chymotrypsin, carboxypeptidase, elastase
Mostly breaks proteins down into di and tripeptides, some amino acids
Trypsin and chymotrypsin -> small polypeptides
Carboxypeptidase -> cleaves aa at carbonyl ends
Describe protein digestion in the SI
Brush border enzymes: aminopolypeptidase and dipeptidases (some others)
Amino acids, di and trip peptides are absorbed into enterocytes
Pancreatic enzymes are secreted as what?
Zymogens and activated by either enterokinase (trypsinogen) or trypsin itself
Describe protein absorption in the SI
Na/K ATPase establishes Na gradient (blood side)
Separate cotransporters for each amino acid type (Na amino acid cotransporters on luminal side)
Separate facilitated diffusion mechanisms for each aa type on blood side
Diffusion of di and tri peptides into circulation
What is celiac sprue?
Autoimmune disorder with hereditary component
Abs develop against gluten components which leads to destruction of SI villi (atrophy) and hyperplasia of intestinal crypts
Malabsorption deficiencies in folate, iron, Ca and vitamins A, B12 and D
What are the GI sx of celiac sprue?
Abdominal pain, constipation, diarrhea, unexplained weight loss, vomiting nausea, steatorrhea
What are the non-GI sx of celiac sprue?
Related to malabsorption
Tingling or numbness of extremities (hands and feet), itchy skin with a rash, fatigue, seizures, easy bruising, bone fractures
What is tropical sprue?
Loss or decreased number of intestinal epithelial cells
Cause unknown
Reduced microvilli SA, lipid absorption impaired due to loss (steatorrhea)
Nutritional deficiencies occur (folate and vitamin B12)
Major sx: diarrhea (as well as cramps, nausea, weight loss, gas, indigestion)
Tx with tetracycline and folate for 6 months
Describe lipid digestion
Complicated by insolubility
Bile salts and lecithin emulsify lipid products into micelles
What enzyme is present in the mouth that is responsible for lipid digestion?
Lingual lipase
Describe lipid digestion in the stomach
Gastric lipase acts on TAGs
Dietary proteins perform emulsification here
CCK: time is required so CCK inhibits gastric emptying for more mixing along with activating gallbladder emptying
Describe lipid digestion in the SI
Most digestion of lipids occurs in the SI
Bile salts emulsify lipids
Pancreatic enzymes secreted into small intestine complete digestion (pancreatic lipase, cholesterol ester hydrolase, phospholipase A2)
What is pancreatic lipase?
Secreted as active enzyme
Inactivated by bile salts w/out colipase
What is colipase?
Activated by trypsin
Once activated it displaces bile salts by binding pancreatic lipase
What is cholesterol ester hydrolase?
Secreted as active enzyme
Catalyzes production of free cholesterol
Hydrolyzes triglycerides to glycerol
What is phospholipase A2?
Secretes as proenzyme activated by trypsin
What are the steps of lipid absorption?
Solubilization by micelles
Diffusion of micelles content across apical membrane
Re-esterification
Chylomicron formation
Exocytosis of chylomicrons into lymphatics
What are common problems that are associated with lipid digestion and absorption?
Pancreatitis, pancreatic insufficiency and Zollinger Ellison syndrome
What is pancreatitis?
Pancreatic enzymes (trypsin) become activated and self digest the pancreas Impaired HCO3 and enzyme secretions
What is pancreatic insufficiency?
Failure to secrete proper amounts of pancreatic enzymes
What is Zollinger-Ellison syndrome?
Gastrin secreting tumor of pancreas
Increased H secretion by gastric parietal cells
Overload of acid into duodenum
What factors cause deficits in bile salts?
Ileal restriction (total bile salt pool is reduced) due to loss fo enterohepatic recirculation of bile salts
Liver disease
Gallbladder disease
Interferes with formation of micelles in SI
What is small intestine bacterial overgrowth (SIBO)?
Two main causes: too little gastric secretion and SI dysmotility
Inappropriate growth of bacteria in SI causes pain, bloating, gas and diarrhea/constipation
Can result in malabsorption issues, steatorrhea, vitamin/mineral deficiency
Changes in pH result in improper activity of lipase enzymes in SI
Breath test detects methane and H
Bacteria deconjugate bile salts, impairing micelle formation
Severe bacterial overgrowth damages the intestinal mucosa
Describe the general principles of vitamin absorption
Vitamins must be acquired often from diet and absorbed along the GI tract
Fat soluble vitamins (ADEK) have same absorption mechanisms as lipids
Water soluble vitamins (B,C) are absorbed via Na/K dependent co-transport except for vitamin B12
Which proteins does vitamin B12 form complexes with to be absorbed?
R proteins (secreted in salivary juices) Intrinsic factor (IF) Transcobalamin II
What is pernicious anemia?
Caused by failure of RBC maturation when vitamin B12 is deficient
Other hematological disorders due to vitamin B12 deficiency are macrocytic and megaloblastic anemia’s
What are common causes for pernicious anemia?
Atrophic gastritis (chronic inflammation of the stomach mucosa that leads to los of parietal cells) Autoimmune metaplastic atrophic gastritis (immune system attaches IF protein or gastric parietal cells)
What are surgical implications of vitamin B12 absorption?
Gastrectomy (loss of parietal cells)
and gastric bypass (exclusion of the stomach, duodenum, and proximal jejunum alters absorption of vitamin B12)
Vitamin D deficiency results in what?
Inadequate Ca absorption
Rickets in children and osteomalacia in adults
Describe Ca absorption
Depends on presence of vitamin D
Absorbed across enterocytes but also reabsorbed in kidney
In SI, vitamin D dependent Ca binding protein (calbindin D-28K)
Calictrol and PTH also govern
Describe iron absorption
Liver secretes apotransferrin into bile and enters the duodenum
Apotransferrin binds with free iron and with hemoglobin forming transferrin
Transferrin binds to receptors on membranes of intestinal epithelial cells absorbed into cells via pinocytosis and later released
Describe electrolyte absorption
The SI and colon not only absorb large quantities of electrolytes (Na, Cl, HCO3 and K) and water but the epithelial cells lining the crypts of the SI also secrete fluid and electrolytes
The 1st step in this process is the absorption of solute followed by the absorption of water
Absorbate is always isosmotic meaning that solute and water absorption occur in proportion to each other
Describe electrolyte absorption in the SI
Jejunum absorbs lots of Na
Ileum absorbs Na and secretes HCO3
Describe electrolyte absorption in the colon
Absorbs Na and secretes K
Aldosterone modulates
Describe cholera
Cholera increases cAMP resulting in increased Cl secretion in the colon
The resulting Cl secretion is accompanied by secretion of Na and H2O
Volume of fluid secreted into the intestinal lumen overwhelms the absorptive mechanisms of the SI and colon leading to massive secretory diarrhea
