The small intestine

Diffusion rate rises with surface area and concentration gradient and falls with the thickness of the barrier.

• A thicker barrier:
• Correct: A larger surface area:
• A smaller concentration gradient:

Microvilli are membrane projections with a core of actin filaments. They do not move; cilia beat, and gap junctions join cells.

• Hair-like projections that beat to move mucus:
• Correct: Tiny finger-like folds of the plasma membrane, supported by actin, that increase surface area:
• Channels that join neighboring cells:

Segmentation is alternating rings of contraction that chop and remix the contents; peristalsis is what moves them along.

• Correct: Mixes them with little forward movement:
• Pushes them rapidly toward the anus:
• Absorbs them into the blood:

The ileum is the only place where vitamin B12, bound to intrinsic factor, is absorbed. Without it, B12 from food cannot be taken up, so it must be given by injection.

• Tablets of lactase taken with every meal: Lactase sits in the brush border of the whole small intestine and is most active in the jejunum, which remains.
• Drugs that reduce stomach acid secretion: Stomach acid is made by the stomach, which is untouched.
• Correct: Injections of vitamin B12: Correct. Removing the ileum removes the site of B12 absorption.
• Injections of insulin before meals: Insulin comes from the pancreatic islets, not the intestine.

Chyme leaves the stomach at a pH near 2 and hits the first part of the duodenum before the pancreas's bicarbonate has mixed in. The alkaline mucus of the duodenal glands coats and protects that lining. That is also where most small-intestine ulcers form.

• It is where most fat is absorbed into the lacteals of the villi: Most fat is absorbed further along, mainly in the jejunum, and alkaline mucus does not absorb fat.
• It is where the migrating motor complex begins between meals: The migrating motor complex starts in the stomach and is driven by motilin; it has nothing to do with mucus secretion.
• Correct: It receives acidic chyme before pancreatic bicarbonate mixes in: Correct. The alkaline mucus meets the incoming acid.
• It is where vitamin B12 is absorbed with intrinsic factor: Vitamin B12 is absorbed at the far end of the small intestine, in the ileum.

Normal: 3 × 10 × 20 = 600 times a smooth tube. With flat villi: 3 × 1 × 20 = 60. That is one tenth of normal, so absorption falls roughly tenfold.

• To about half of normal: Losing a factor of 10 cuts the surface far more than in half.
• Correct: To about one tenth of normal: Correct. 60 is one tenth of 600.
• To about one twentieth of normal: A twentyfold loss would mean losing the microvilli, which remain here.
• It stays about the same: The microvilli stand on the villi, so flattening the villi removes most of the surface they sit on.

Lactase, built into the microvillus membrane, splits lactose into glucose and galactose right beside the transporters that absorb them.

• Pepsin carried from the stomach splits lactose in the lumen: Pepsin digests protein, not sugars.
• Intestinal juice from the glands splits lactose in the lumen: Intestinal juice is mostly water and mucus with very little enzyme; lactose is split at the brush border.
• Lactose is absorbed whole and split later inside the liver: Disaccharides such as lactose are not absorbed whole; they must be split into single sugars first.
• Correct: Brush border lactase splits it into glucose and galactose: Correct. Lactase is a brush border enzyme.

Absorptive cells package digested fat into droplets that are too large to cross into blood capillaries. They enter the lacteal in the core of each villus, and the fat-laden lymph looks milky.

• Milk proteins are absorbed whole from the lumen into the lymph: Proteins are split into amino acids and absorbed into the blood capillaries, not the lymph.
• White blood cells in the lymph multiply rapidly after every meal: The milky color comes from fat droplets, not from cells.
• Mucus from intestinal juice drains into the lymph vessels: Intestinal juice is secreted into the lumen, not into the lymph.
• Correct: Absorbed fat, in droplets too big for capillaries, enters lacteals: Correct. Fat droplets in the lymph give it the milky look that named the lacteals.

Stem cells at the base of the intestinal glands divide all the time, and their daughters live only 3 to 5 days before being shed from the villus tips. A drug that stops cell division halts the supply, so within days the lining thins and breaks down.

• Chemotherapy is concentrated in bile, which is secreted into the intestine: The effect comes from blocked cell division throughout the body, not from delivery in bile.
• Correct: Its stem cells divide constantly, renewing the lining every few days: Correct. Rapid renewal makes the lining one of the tissues most sensitive to drugs that kill dividing cells.
• The villi are made of smooth muscle, which chemotherapy targets directly: Villi are covered by epithelium over a core of connective tissue; only a few smooth muscle strands run in them, and smooth muscle is not what is damaged.
• The drug digests the brush border enzymes on the microvilli: Chemotherapy does not digest enzymes. The enzymes disappear because the cells that carry them are not replaced.

Brush border enzymes are part of the absorptive cells on the villi, so flattened villi carry far less of them. Lactase is the least abundant, so it is usually the first to fall short, and lactose goes undigested. Starch is still split to maltose by pancreatic enzymes, and the remaining maltase activity copes.

• Her pancreas has stopped secreting its digestive enzymes into the duodenum: Celiac disease damages the intestinal lining, not the pancreas, and a pancreatic problem would affect rice starch too.
• Her stomach no longer secretes pepsin because of the immune reaction: Pepsin digests protein, and the stomach is not affected by celiac disease.
• Correct: She has lost most lactase, the scarcest brush border enzyme: Correct. Lactase falls first when villi are damaged.
• She has lost her maltase, so she cannot digest the starch in rice: If she could not digest starch, rice would cause symptoms too. Maltase is more abundant than lactase and is not the first to run short.