A patient takes an oral lipophilic (fat-soluble) medication. Compared to a water-soluble drug of similar dose, how does its absorption route and first-pass hepatic metabolism differ?
AThe lipophilic drug is absorbed more slowly and undergoes more extensive first-pass hepatic metabolism because the liver must process all absorbed substances
BThe lipophilic drug is packaged into chylomicrons and exits via the lymphatic system, bypassing the liver entirely; the water-soluble drug enters portal circulation and undergoes first-pass hepatic metabolism
CBoth drugs are absorbed via the portal vein but the lipophilic drug is sequestered in adipose tissue before reaching the liver
DThe lipophilic drug requires active transport into enterocytes, which is slower than the passive diffusion used by water-soluble drugs
Lipophilic substances (including dietary fats, fat-soluble vitamins, and fat-soluble drugs) are packaged into chylomicrons inside enterocytes and exported via lacteals into the lymphatic system. They enter the bloodstream at the left subclavian vein, bypassing the liver entirely on first pass. Water-soluble nutrients and drugs are absorbed into capillaries within the villi and travel via the portal vein directly to the liver, where first-pass metabolism can significantly reduce bioavailability. This is why some fat-soluble drugs have very high oral bioavailability — the liver never gets the first chance to metabolize them.
Question 2 Multiple Choice
A patient with pernicious anemia (autoimmune destruction of gastric parietal cells) develops vitamin B12 deficiency despite consuming adequate dietary B12. What is the most direct mechanism?
BWithout intrinsic factor (secreted by parietal cells), the terminal ileum cannot absorb B12 via receptor-mediated endocytosis, regardless of luminal B12 concentration
CThe autoimmune attack destroys B12-specific transporter proteins in the small intestinal epithelium
DParietal cell loss impairs bile production, reducing the micelle formation needed for B12 solubilization
Vitamin B12 absorption requires intrinsic factor, a glycoprotein secreted exclusively by gastric parietal cells. The B12-intrinsic factor complex binds to specific receptors (cubam receptors) in the terminal ileum and is absorbed by receptor-mediated endocytosis. Without intrinsic factor, there are no functional receptors to bind B12, and the vitamin passes through the ileum unabsorbed — even if luminal B12 concentrations are high from dietary sources. This is why pernicious anemia requires B12 supplementation by injection (bypassing the gut entirely) rather than oral supplementation.
Question 3 True / False
Glucose absorption across the apical membrane of enterocytes is a passive process — glucose simply diffuses down its concentration gradient without any energy input from the cell.
TTrue
FFalse
Answer: False
Glucose absorption across the apical membrane uses SGLT1 (sodium-glucose linked transporter 1), which is a secondary active transporter. SGLT1 co-transports one glucose molecule with two sodium ions; the sodium moves down its electrochemical gradient, providing the driving force to pull glucose into the cell even against a concentration gradient. The sodium gradient itself is maintained by Na⁺/K⁺-ATPase on the basolateral membrane, which actively pumps sodium out using ATP. So glucose absorption is ultimately driven by ATP hydrolysis, just indirectly. Fructose, by contrast, uses GLUT5 (passive facilitated diffusion) — explaining why fructose absorption has a lower capacity ceiling.
Question 4 True / False
Dietary fat is not subject to first-pass hepatic metabolism because chylomicrons are too large to enter blood capillaries and instead travel through the lymphatic system before reaching the bloodstream.
TTrue
FFalse
Answer: True
Chylomicrons assembled in enterocytes are large lipoprotein particles (~75–1200 nm diameter) — far too large to squeeze through the tight junctions and small fenestrae of blood capillaries. They are instead exocytosed into lacteals (lymphatic capillaries within each villus), travel through lymphatic vessels to the thoracic duct, and enter the venous bloodstream at the left subclavian vein. This route entirely bypasses the portal vein and hepatic first-pass metabolism. Dietary fat reaches the general circulation before the liver has any chance to process it — which is why triglycerides in chylomicrons are delivered first to peripheral tissues and adipose tissue, with the liver receiving chylomicron remnants only after lipolysis in peripheral capillaries.
Question 5 Short Answer
Explain why the three-level surface amplification (mucosal folds → villi → microvilli) is functionally necessary, rather than just an anatomical curiosity.
Think about your answer, then reveal below.
Model answer: The small intestine must absorb the entire day's nutritional intake — hundreds of grams of carbohydrates, proteins, and fats — within a few hours, across the wall of a tube roughly 6 meters long. Without surface amplification, the absorptive area of the intestinal tube would be approximately 0.33 m² (the area of a smooth cylinder). The three-level amplification increases this to ~200 m² — about 600-fold — bringing the effective absorptive surface to roughly the area of a tennis court. This massive surface area is necessary because absorption rate is proportional to surface area (Fick's law); without it, nutrients would pass through faster than they could be absorbed, leading to malabsorption. The clinical consequence of losing this surface — as in celiac disease (villus atrophy) — is profound malabsorption of all macronutrients and many micronutrients.
Each level of amplification serves the same purpose: maximizing the number of transporter-bearing membrane surface per unit length of intestine. The brush border microvilli also carry digestive enzymes (brush border enzymes like maltase, sucrase, lactase) on their surface, so digestion and absorption occur simultaneously at the same membrane — a further efficiency gain.