Absorption and Elimination

Q1, Sample Band 6 response (8 marks), annotated

The small intestine must absorb all major nutrients from a relatively narrow tube. The SA:V challenge (large volume of contents, small contact area) is overcome by three scales of structural folding that together amplify absorptive surface area approximately 600-fold. [1, SA:V principle applied]

At the macroscopic scale, plicae circulares are large circular folds of the entire intestinal wall that triple absorptive surface area and slow chyme transit, increasing nutrient–epithelium contact time. At the tissue scale, villi are finger-like projections (0.5–1.6 mm) extending into the lumen; each contains a capillary network and a lacteal, multiplying surface area a further 10-fold and positioning transport proteins directly in contact with lumenal contents. At the cellular scale, microvilli (the brush border, 1–2 μm) on each enterocyte surface amplify the membrane area a further 20-fold and are densely packed with transport proteins. Combined, these three levels produce ~250 m² of absorptive surface. [1, three scales with multipliers; 1, function of each described]

Glucose crosses the brush border via SGLT1a secondary active co-transporter that moves one glucose molecule in with two Na♠ ions. Active transport (powered indirectly by Na♠/K♠ ATPase on the basolateral membrane) is necessary because after a large meal, glucose may have already accumulated to a higher concentration inside the enterocyte than in the lumen, so simple diffusion would cease; the sodium gradient overcomes this and ensures continued glucose extraction. Glucose exits the basolateral membrane into the villus capillary via GLUT2 (facilitated diffusion, no ATP required because glucose moves down its gradient from cell to blood). [1, SGLT1/GLUT2 mechanism; 1, why active transport is necessary]

Glucose and amino acids enter the villus capillary network and travel via the hepatic portal vein directly to the liver for first-pass regulation (glucose: glycogen storage or release; amino acids: deamination, urea synthesis). Fatty acids, being lipid-soluble, enter the enterocyte by simple diffusion, are reassembled into triglycerides and packaged as chylomicrons in the Golgi apparatus, and are secreted into the lacteal. They travel via lymphatic vessels and the thoracic duct into the left subclavian vein, bypassing the liver entirely on first pass. [1, glucose/AA route correctly named; 1, fat route correctly named with chylomicron and lacteal; 1, contrast (portal vein vs thoracic duct, liver first vs bypass)]

Marking criteria (8 marks).

• 1 markStates the SA:V principle and explains that structural folding solves it for the small intestine.
• 1 markDescribes all three levels of folding (plicae, villi, microvilli) and gives the approximate multiplier or combined surface area (~250 m²) for at least two.
• 1 markExplains how each level specifically improves absorption efficiency (not just “increases SA”, must connect to function such as slowed transit, transport protein density, or capillary proximity).
• 1 markNames SGLT1 as the glucose brush-border transporter and explains it is a sodium co-transporter (secondary active transport).
• 1 markExplains why active transport is required (potential reverse gradient when enterocyte glucose is already high after previous absorption).
• 1 markNames GLUT2 and states it uses facilitated diffusion on the basolateral side; glucose/amino acids travel via hepatic portal vein to liver first.
• 1 markNames chylomicrons and lacteals as the fat absorption pathway and states fats enter the lymphatic system.
• 1 markStates fats reach the bloodstream via the thoracic duct/subclavian vein, bypassing the liver on first pass, and contrasts this with the glucose/AA hepatic portal route.

Q2, Sample Band 6 response (9 marks), annotated

Fatigue: Villous atrophy collapses absorptive surface area. Even though Jordan eats normally, glucose cannot be adequately absorbed across the damaged epithelium. Cells throughout the body receive insufficient glucose for cellular respiration, reducing ATP production. Low haemoglobin (from iron deficiency) further reduces oxygen delivery to tissues, compounding the energy deficit. [1]

Weight loss despite adequate intake: Digestion (enzymatic breakdown) is largely intact, starch, protein, and fat are hydrolysed to monomers in the lumen. However, without functional enterocytes, these monomers cannot be transported across the intestinal wall into the blood. Glucose, amino acids, and fatty acids remain in the lumen and pass through to the large intestine, where most are fermented or eliminated in faeces rather than absorbed. Jordan effectively starves at the cellular level despite eating. [1]

Iron-deficiency anaemia unresponsive to oral supplements: Iron is primarily absorbed in the duodenum and upper jejunum, the regions with the highest normal villus density. Villous atrophy is most severe in these proximal regions because they experience the greatest gluten exposure. Even supplemental iron placed into the lumen cannot be absorbed because the transport proteins that move iron across enterocytes require intact villous epithelium. Intravenous iron bypasses this problem because it is delivered directly into the bloodstream. [1, site of iron absorption; 1, why oral fails despite lumenal iron]

Abdominal bloating: Unabsorbed carbohydrates (glucose, maltose) reach the large intestine, where resident bacteria ferment them. Fermentation produces large volumes of gas (CO&sub2;, methane, hydrogen), causing distension and bloating. The unabsorbed carbohydrates also raise lumenal osmolarity, drawing water into the colon by osmosis, contributing to loose stools. [1]

Low calcium: Calcium is absorbed by specific transport proteins on villous enterocytes in the duodenum and jejunum, the same regions most affected by villous atrophy. Loss of the villous absorptive epithelium and its transport proteins means dietary calcium cannot cross the intestinal wall and passes through in faeces even when dietary intake is sufficient. [1]

Why oral iron fails: The iron is present in the intestinal lumen, digestion has not been impaired, but the transport machinery that moves iron from lumen to enterocyte to blood is destroyed along with the villi. No amount of lumenal iron can be absorbed without functioning epithelial transport proteins. [1]

After 12 months on a gluten-free diet: Removing gluten stops the immune-mediated attack on the intestinal epithelium. The intestinal epithelium has high regenerative capacity; over 6–24 months villi regrow and the brush border is restored. Absorptive surface area returns toward normal, transport proteins (SGLT1, GLUT2, calcium transporters, iron transporters) are re-expressed, and all nutrient absorptions recover. Haemoglobin, serum calcium, and albumin levels normalise; fatigue, weight loss, and bloating resolve. [1]

Marking criteria (9 marks).

• 1 markFatigue: links villous atrophy → reduced glucose absorption → reduced cellular ATP; anaemia (reduced O&sub2; delivery) named as compounding factor.
• 1 markWeight loss: correctly distinguishes digestion (intact) from absorption (impaired); states nutrients remain in lumen and are eliminated.
• 1 markIron anaemia: identifies duodenum/jejunum as the site of iron absorption and states villous atrophy is worst here.
• 1 markWhy oral iron fails: iron is present in lumen but cannot cross the damaged epithelium without functional transport proteins; IV iron bypasses this.
• 1 markBloating: links unabsorbed carbohydrates → bacterial fermentation → gas production in large intestine.
• 1 markLow calcium: identifies villous transport protein loss (calcium transporter) in duodenum/jejunum as the mechanism.
• 1 markLow albumin: liver synthesises albumin from absorbed amino acids; villous atrophy reduces amino acid absorption → liver has insufficient substrate → low albumin. (Must name the liver and amino acid absorption step.)
• 1 markGluten-free diet prediction: states epithelial regeneration restores villi and microvilli, restoring transport proteins and absorption capacity.
• 1 markNames at least two specific symptoms that would resolve and links each to the restored absorption mechanism.

Q3, Sample Band 6 response (6 marks)

The claim is partly correct but substantially wrong. [1, overall evaluative judgement]

What is approximately correct: By the time chyme leaves the small intestine, essentially all major macronutrients, glucose, amino acids, and fatty acids, have been absorbed. In this narrow sense, the large intestine performs no further macronutrient absorption. [1, concedes correct element]

What is wrong:

• “No biologically meaningful absorption.” The large intestine reabsorbs approximately 1.3–1.8 litres of water per day. Na♠ is actively pumped out of the colon lumen, and water follows by osmosis. Without this reabsorption, the body would lose this volume in faeces daily, clinically equivalent to ongoing severe diarrhoea, causing life-threatening dehydration and electrolyte loss. [1, refutes with water/electrolyte reabsorption detail]
• “No nutritional consequences.” Gut bacteria in the large intestine produce vitamins K and B12 through fermentation of dietary fibre. Vitamin K is essential for blood clotting; deficiency causes haemorrhage. Short-chain fatty acids (acetate, butyrate) produced by fermentation are also absorbed and serve as a significant energy source for colonic epithelial cells and the liver. These are genuine nutritional contributions the colon makes that would be lost after surgical removal. [1, refutes with bacterial vitamin synthesis]

Accurate account: The large intestine performs critical homeostatic absorption (water: ~1.5 L/day, electrolytes: Na♠, K♠, Cl²♠) and nutritional contributions via the microbiome (vitamins K and B12, short-chain fatty acids). Surgical removal (total colectomy) causes chronic dehydration risk, electrolyte imbalances, and vitamin K deficiency unless these are managed medically. The large intestine is not nutritionally trivial. [1, accurate overall statement; 1, uses specific quantities or named examples]

Marking criteria (6 marks).

• 1 markStates an overall evaluative judgement (e.g. “partly correct but substantially wrong”).
• 1 markCorrectly concedes that major macronutrient absorption is essentially complete before material enters the large intestine.
• 1 markCorrectly refutes “no meaningful absorption” with the water/electrolyte reabsorption mechanism (Na♠ pump → osmotic water movement, ~1.5 L/day).
• 1 markCorrectly refutes “no nutritional consequences” by naming bacterial production of at least one vitamin (K or B12) in the colon and its physiological importance.
• 1 markNames short-chain fatty acids or electrolyte reabsorption as a further contribution, or explains a specific consequence of total colectomy (dehydration, vitamin K deficiency, electrolyte imbalance).
• 1 markProvides a biologically accurate reformulation of the claim that acknowledges the large intestine’s genuine roles in water homeostasis and vitamin synthesis.