Digestion & Absorption Flashcards
Vitamin A, K and D are all absorbed in?
A The terminal ileum
B Ascending colon
C The stomach
D The proximal small bowel
D
Explanation
Most vitamins are absorbed in the upper small intestines. B12, however, is absorbed in the ileum.
Fat-soluble vitamins are highly INSOLUBLE in the gut, and their absorption is entirely dependent on their incorporation into micelles. Their absorption is depressed if fat absorption is depressed because of a lack of pancreatic enzymes or if bile is excluded form the intestines due to a biliary obstruction.
Regarding fat digestion and absorption, all the following statements are correct with the exception of?
A Free fatty acids (FFA) diffuse passively through the brush border of enterocytes
B Greatest absorption occurs mostly in the ileum
C Transport is in chylomicrons
D Pancreatic dysfunction may lead to steatorrhea
B
Explanation
Long chain FFA absorption is greatest in the upper parts of the small intestine, although appreciable amounts are also absorbed in the ileum. Cholesterol is readily absorbed from the small intestine. Steatorrhea that is caused due to pancreatic dysfunction is mostly due to lipase deficiency. Although fats enter the enterocytes by passive diffusion, there is some evdence to suggest that carriers are involved.
Extra: The correct answer is “Greatest absorption occurs mostly in the ileum”. Fat digestion and absorption primarily occur in the duodenum and jejunum, not the ileum. The ileum is more specialized for the absorption of bile salts and vitamin B12. Analysis of Each Option: “Pancreatic dysfunction may lead to steatorrhea” True. Pancreatic enzymes (especially lipase) are essential for the digestion of triglycerides into monoglycerides and free fatty acids. Pancreatic dysfunction reduces this digestion, leading to steatorrhea (fatty stools). Reference: Ganong’s Review of Medical Physiology, 26th Edition: Chapter on gastrointestinal physiology. Guyton and Hall, 14th Edition: Chapter on digestion and absorption. “Transport is in chylomicrons” True. Absorbed fatty acids and monoglycerides are re-esterified into triglycerides within enterocytes and packaged into chylomicrons, which are released into the lymphatic system. Reference: Ganong’s Review of Medical Physiology: “Chylomicrons are the primary vehicles for transporting dietary lipids.” Guyton and Hall: “Fat is absorbed as chylomicrons via lacteals.” “Free fatty acids (FFA) diffuse passively through the brush border of enterocytes” True. Fat digestion products (free fatty acids and monoglycerides) are lipid-soluble and passively diffuse across the enterocyte brush border. Reference: Ganong’s: “Free fatty acids and monoglycerides diffuse across enterocyte membranes due to their hydrophobic nature.” UpToDate: “Lipid absorption begins with passive diffusion across the apical membrane.” “Greatest absorption occurs mostly in the ileum” False. The ileum plays a minor role in fat absorption, mainly limited to bile salt reabsorption. Most fat absorption occurs in the duodenum and jejunum, where bile salts emulsify fats and lipase acts to digest them. Reference: Ganong’s: “Fat absorption occurs mainly in the proximal small intestine.” Guyton and Hall: “The jejunum is the principal site for fat absorption.” References: 1. Barrett, Kim E., et al. Ganong’s Review of Medical Physiology, 26th Edition, Chapters on gastrointestinal physiology. 2. Hall, John E., et al. Guyton and Hall Textbook of Medical Physiology, 14th Edition. 3. UpToDate: Fat digestion and absorption in the small intestine.
Regarding absorption of amino acids, which of the following statements is correct?
A Approximately 75% of digested protein comes form ingested food
B They are co-transported with ions
C The enzyme trypsin begins protein digestion in the stomach
D Absorption starts in the mouth
B
Explanation
Protein digestion begins in the stomach and absorption takes place rapidly in the duodenum and jejunum but slowly in the ileum. At least seven different transport systems are required for absorption. Three require NA, two both Na and Cl and two transport amino acids independent of ions. Only 2-6% of proteins escape digestion. Approximately 50% of the digested protein comes form ingested food, 25% from proteins in digestive juices and 25% of proteins from desquamated mucosal cells. Pepsin hydrolysis the bonds between tyrosine or phenylalanine and a second amino acid. So the products of pepsin digestion are polypeptides of very diverse sizes. Trypsin-in the small intestine begins to digest these polypeptides into free amino acids
Regarding fat digestion, which of the following statements is false?
A Lipase is found in the stomach, but has no real role there
B Mostly occurs in the jejunum
C Colipase is required for optimal activity of pancreatic lipase
D Fat is transported in chylomicrons
B
Explanation
Most fat digestion begins in the duodenum, pancreatic lipase being one of the most important enzymes. Lingual and stomach lipase are secreted but are of little quantitative significance for lipid digestion other than in the setting of pancreatic insufficiency.
Extra: Long chain free fatty acids (FFA) are absorbed in the greatest amounts in the upper parts of the small intestine but appreciable amounts are also absorbed in the ileum.
With regards to iron absorption, which of the following statements is correct?
A Occurs in the duodenum only
B Is increased by Vitamin C
C Is constant regardless of need
D Requires intrinsic factor
B
Explanation
Iron absorption is regulated by recent dietary intake, state of iron stores in the body and the state and level of erythropoeisis in the bone marrow. Almost all the absorption of iron occurs in the duodenum. Gastric secretions dissolve the iron and permit it to form soluble complexes with ascorbic acid and other substances that aid its reduction to the Fe2+ form (ferrous)-the absorbed form. Intestinal absorption of iron is regulated by three factors: recent dietary intake of iron, the state of iron stores in the body and the state of erythropoiesis in the bone marrow.
With regard to protein digestion, which of the following statements is correct?
A It is largely completed in the small intestine
B Protein digestion begins in the duodenum
C Pancreatic enzymes are the most important
D Commences on the action of saliva
A
Explanation
Protein digestion begins in the stomach and is not dependent on stimulation by saliva. Stomach and pancreatic enzymes both have an important role in protein digestion.
EXTRA: The final digestion to amino acids occurs in three locations: the intestinal lumen, the brush border, and the cytoplasm of the mucosal cells. Proteolytic enzymes from the stomach, pancreas and intestinal mucosa all play very important roles. Saliva has no digestive function on proteins. Protein digestion begins in the stomach.
Regarding absorption, which of the following statements is false?
A Fructose is absorbed by facilitated diffusion
B Glucose absorption is an example of secondary active transport
C Galactose is absorbed by the same mechanism as glucose
D Insulin regulates glucose absorption in the intestine
D
Explanation
Insulin has little to no effect on glucose absorption in the intestine
Which of the following statements is true regarding faeces?
A The solid portion contains 30% bacteria
B 50 ml is produced per day on average
C Solids form 75% of its composition
D The brown colour is due to melanin
A
Explanation
200-250mls of faeces is produced a day. Water forms 75% of the composition. The brown colour is due to bile pigments. Only 25% is of solid material
Which of the following is a nutritionally essential amino acid?
A Threonine
B Glutamine
C Glycine
D Cysteine
A
Explanation
Nutritional essential amino acids (aa) include: Valine, Leucine, Isoleucine, Threonine, Methionine, Phenylalanine, Lysine,
Note: Arginine and Histidine are considered conditionally essential AA-they are not necessary for the maintenance of nitrogen balance, but are needed for normal growth
Note, older versions of the textbook list of nutritional essential AA reports that alanine is an essential aa and tryptophan is not. However later in the chapter it reports that serotonin is synthesized from the essential aa tryptophan. A review on the internet points out that tryptophan is an essential aa and alanine is not.
The latest version of the textbook reports that tryptophan and threonine are essential AA.
Extra: PVT (Private) TIM HALL P - Phenylalanine V - Valine T - Theorinine T - Tryptophan I - Isoleucine M - Methionine H - Histidine A - Arginine L - Lysine L - Leucine
Concerning pancreatic secretions, which of the following statements is correct?
pancreas secretes trypsin inhibitor, not antitrypsin inhibitor
A pH is 6.0
B Pancreatic digestive enzymes are activated by trypsinogen
C The pancreas secretes gastrin
D Pancreatic alpha amylase also acts on ingested polysaccharides
D
Explanation
Pancreatic secretions are alkaline with a pH 8.0. The gastric mucosa secretes gastrin. Antitrypsin is formed in the liver. Pancreatic digestive enzymes are activated by trypsin in the intestinal lumen, a pancreatic protease that is released as an inactive precursor (trypsinogen). To avoid trypsin becoming active in the pancreas, the pancreas releases trypsin inhibitors (this prevents auto digestion by trypsin)
The majority of water ingested is re-absorbed in which of the following areas?
A Ileum
B Stomach
C Jejunum
D Colon
C
Explanation
Daily water turnover (ml) in the GIT tract
Ingested: Total 2000ml
Endogeneous secretion: Total 7000ml
Salivary glands 1500
Stomach 2500
Bile 500
Pancreas 1500
Intestine 1000
Total reabsorbed: 8800ml
Jejunum 5500
Ileum 2000
Colon 1300
Therefore balance in stool 200
Saliva contains all of the following EXCEPT?
A Peroxidases
B IgA
C Trypsinogen
D Mucin
C
Explanation
Saliva contains the following substances:
Water
Electrolytes: Na, K, Ca, Mg, Cl, HCO3, PO4, iodine
Mucins (to lubricate food)
Antibacterial: IgA, hydrogen peroxide
Epidermal growth factor
Enzymes: alpha-amylase, lingual lipase, kallikrein, antimicrobial enzymes (lysozyme, lactoperoxidase, lactoferrin, IgA), proline rich proteins, other minor enzymes
Cells:
Opiophin: a pain killing substance
Haptocorrin: protein that’s binds B12 to protect it against degradation in the stomach before it binds to intrinsic factor
Regarding digestive substrates and products- Which is INCORRECT?
A Sucrose= fructose and glucose
B Lactose= galactose and glucose
C Maltose= glucose and glucose
D Trehalose= galactose and fructose
D
Explanation
Lactose= galactose and glucose- main sugar in milk
Maltose= glucose and glucose- product of starch hydrolysis
Sucrose= fructose and glucose- common table sugar
Trehalose= glucose and glucose- found in fungi
Melibiose= glucose and galactose- found in legumes
Which is not an essential amino acid?
A Threonine
B Leucine
C Glutamine
D Valine
C
Explanation
Essential amino acids:
Valine, leucine, isoleucine, threonine, methionine, phenylalanine, arginine, lysine and histidine
Note, the current textbook reports that alanine is an essential aa and tryptophan is not. However, this may be wrong. A review on the internet points out that tryptophan is an essential aa and alanine is not.
Extra: in essential AA mnemonic PVT TIM HALL - there are no AA’s starting with the letter C or G (this rule can be used to answer both provided questions)
Which is correct regarding gastrointestinal absorption?
A Glucose absorption is directly energy independent
B Iron absorption is constant, regardless of need
C Most vitamin absorption occurs in the proximal small bowel
D Calcium absorption does not require a functioning kidney
C
Explanation
Vitamins ADEK are fat-soluble and must be digested by cholesterol esterase prior to absorption. These vitamins are highly insoluble in the gut and their absorption is dependent entirely on their incorporation into micelles. Most vitamins are absorbed in the upper small intestine. Calcium absorption form the intestine is dependent on 1,25 dihydroxycholecalciferol. This metabolite is formed in the proximal tubules of the kidney. Intestinal absorption of iron is regulated by three factors: recent dietary intake of iron, the state of iron stores in the body and the state of erythropoiesis in the bone marrow. The transport of glucose is dependent on sodium in the intestinal lumen. The energy for this glucose absorption is provided indirectly, by the active transport of sodium.
Which statement regarding protein digestion is TRUE?
A Pepsinogens are inactivated by hydrochloric acid
B Protein digestion begins in the mouth
C Pepsins have an optimal pH of 2.0-4.0
D Pepsinogen I is found only in the acid secreting regions of the stomach.
D
Explanation
Protein digestion begins in the stomach. Pepsins are responsible for protein digestion in the stomach. They are secreted in a inactive precursor form (proenzyme) and activated in the GIT tract. These pepsin precursors are called pepsinogens and are activated by HCL. Pepsinogen I is found only in the acid secreting regions of the stomach. Pepsinogen II is also found in the pyloric region. Maximal acid secretion correlates with pepsinogen I. Pepsins hydrolyse the bonds between aromatic amino acids. Pepsins have an optimum pH of 1.6-3.2. There action is terminated when they mix with the alkaline pancreatic juice in the duodenum and jejunum
28-year-old woman is found to have megaloblastic anaemia. She reports tingling of her hands bilaterally. She is likely deficient in a vitamin absorbed in which part of the gastrointestinal tract?
A Ileum
B Stomach
C Duodenum
D Colon
A
Explanation
Vitamin B12 deficiency causes megaloblastic pernicious anaemia and degeneration of posterolateral spinal cord tracts. Vitamin B12 binds to intrinsic factor and is absorbed in the terminal ileum. Vitamin B12 absorption and folate absorption are Na-independent, but all seven of the remaining water-soluble vitamins – thiamine, riboflavin, niacin, pyridoxine, pantothenate, biotin, and ascorbic acid – are absorbed by carriers that are Na cotransporters.
Which hormone increases gastric acid secretion, stimulates gastric mucosa proliferation, and stimulates gastric emptying?
A Cholecystokinin
B Gastrin
C Secretin
D Histamine
B
Explanation
Gastrin stimulates gastric acid secretion, increases gastric motility, and increases pancreatic and biliary secretions.
Secretin acts on pancreatic ducts to cause secretion of very alkaline pancreatic juice rich in HCO3 but poor in enzymes.
CCK augments the action of secretin in producing secretion of alkaline pancreatic juice.
Which statement regarding the colon is correct?
A The gastro-ileal reflex stimulates constriction of the caecum as food leaves the stomach
B The main function of the colon is to allow for the reabsorption of water, sodium, and other minerals
C Defecation is not a spinal reflex
D The first part of the test meal reaches the caecum in about 9hrs in most individuals.
B
Explanation
The colon serves as a reservoir for the residues of meals that cannot be digested or absorbed. Motility in this segment is likewise slowed to allow the colon to absorb water, sodium, and other minerals. 90% of the fluid is removed. It converts 1-2 litres of isotonic chyme that enters it form the ileum to about 250ml of semisolid faeces.
The first part of the test meal reaches the caecum in about 4hrs in most individuals.
When food leaves the stomach, the caecum relaxes and the passage of chyme through the ileo-caecal valve increases (gastro-ileal reflex). This is presumable a vagovagal reflex.
Distension of the rectum with faeces initiates reflex contraction of its musculature and the desire to defecate. In humans, the sympathetic nerve supply to the internal anal sphincter (involuntary) is excitatory, whereas the parasympathetic supply is inhibitory. The nerve supply to the external anal sphincter, a skeletal muscle, comes from the pudendal nerve. The urge to defecate occurs when the rectal pressure increases to about 18mmHg. When this pressure reaches 55mmHg, the external as well as the internal sphincter relax and there is expulsion of the contents of the rectum. This is why reflex evacuation of the rectum can occur in the setting of spinal injury.
Before the pressure that relaxes the external anal sphincter is reached, voluntary defecation can be initiated by straining. Normally, the angle between the anus and the rectum is approximately 90-100%, and this, added to contraction of the puborectalis muscle, inhibits defecation. With straining, the abdominal muscles contract, the pelvic floor is lowered 1-3cm, and the puborectalis muscle relaxes. The anorectal angle is straightened, and when this is combined with relaxation of the external sphincter, defecation occurs. Defecation is therefore a SPINAL reflex that can be voluntarily inhibited by keeping the external sphincter contracted or facilitated by relaxing the sphincter and contracting the abdominal muscles.
source: Ganong physiology 26 edition
