The large intestine

Water follows solute: it moves by osmosis toward the side where the held-back solute is more concentrated.

• Correct: Toward the side with more solute:
• Toward the side with less solute:
• It does not move:

Visceral (autonomic) reflexes act on smooth muscle, cardiac muscle and glands. Somatic reflexes act on skeletal muscle.

• A somatic reflex:
• Correct: A visceral reflex:
• A stretch reflex of skeletal muscle:

Food passes through the stomach and colon, which are parts of the alimentary canal. The pancreas makes secretions and delivers them through a duct, so it is an accessory organ.

• The stomach:
• Correct: The pancreas:
• The colon:

The cecum is the blind pouch at the start of the large intestine, in the lower right, below the ileocecal valve. The appendix hangs from it.

• Ileum: The ileum is the last part of the small intestine, which opens into the large intestine beside the pinned pouch.
• Sigmoid colon: The sigmoid colon is the S-shaped loop in the pelvis, at the other end of the colon.
• Correct: Cecum: Correct. The blind-ended pouch is the cecum.
• Ascending colon: The ascending colon starts above the cecum and rises up the right side.

Filling stretches the rectum. Stretch-sensitive sensory receptors signal the sacral cord, which sends parasympathetic output that contracts the rectum and relaxes the internal sphincter. The last gate, the external sphincter, is voluntary, so defecation happens only when you relax it.

• Correct order: 1. A mass movement pushes feces into the rectum 2. The rectal wall stretches and its stretch-sensitive sensory receptors fire 3. Sensory neurons carry the signal to the sacral spinal cord 4. Parasympathetic neurons make the rectum contract and the internal anal sphincter relax 5. You choose to relax the external anal sphincter 6. Feces pass out through the anal canal

The small intestine absorbs about 7 to 8 L a day. The colon absorbs most of what is left, about 1.3 of the 1.4 to 1.5 L that reaches it. The colon's share is large compared with what arrives, but small compared with the total.

• In the stomach, before chyme leaves it: The stomach absorbs very little water; its lining is built to hold acid in, not to absorb.
• In the colon, where feces take shape: The colon absorbs about 90% of what reaches it, but only about 1.5 L arrives, a small fraction of the 9 L total.
• Correct: In the small intestine: Correct. Most absorption, of water as of nutrients, happens in the small intestine.
• In the rectum: The rectum stores feces briefly and absorbs little.

Slower movement gives the colon more time to absorb water, so stools become harder, drier and less frequent: opioid constipation. The cause is timing, not a change in the absorbing cells.

• Time feces spend in the colon: up. Opioids depress the enteric neurons that drive haustral contractions and mass movements, so contents move more slowly.
• Water absorbed from each day's contents: up. Contents stay longer in contact with the lining, so more sodium, and water with it, is absorbed.
• Water content of the stool: down. More water absorbed leaves less in the feces.
• Number of stools per week: down. Fewer mass movements deliver feces to the rectum less often, so the defecation reflex is triggered less often.

A settled gut microbiota crowds out invaders by taking up space and nutrients. Broad-spectrum antibiotics kill many of these bacteria. A resistant species can then overgrow, and its toxins inflame the colon, causing diarrhea.

• Correct: The antibiotic thinned her normal colon bacteria, and a resistant species overgrew and made toxins: Correct. Loss of the normal flora's crowding-out effect lets the resistant species take over.
• The antibiotic damaged her teniae coli and sped up her colon: Antibiotics act on bacteria, not on the smooth muscle of the teniae coli.
• The antibiotic blocked her colon's sodium channels and stopped water absorption: Antibiotics do not block the colon's sodium channels. The toxins cause the diarrhea.
• The antibiotic made her pancreas stop secreting enzymes: Pancreatic secretion is unaffected; missing enzymes would cause fatty stools, not a toxin-driven inflammatory diarrhea.

The defecation reflex runs through the sacral cord, which is intact, so stretch still makes the rectum contract and the internal sphincter relax. The injury cuts the pathways to and from the brain, so he loses the sensation of urge and voluntary control of the external sphincter.

• Nothing happens until signals arrive from the brain: The reflex arc runs through the sacral cord, not the brain, so it works without signals from the brain.
• Correct: The rectum contracts by reflex, but he feels no urge and has no voluntary control: Correct. The reflex survives; sensation and voluntary control are lost.
• He feels the urge normally, but his rectum cannot contract to push the feces out: Sensation travels up through the injured cord, so he does not feel the urge normally, while the reflex contraction of the rectum still works.
• His internal sphincter stays closed, but he can relax his external sphincter at will: The internal sphincter still relaxes by reflex, and voluntary control of the external sphincter needs signals from the brain, which the injury blocks.

Secreted chloride draws sodium and water into the lumen, so far more fluid than usual leaves the small intestine. The colon keeps absorbing, but it can take up only about 4 to 5 L a day. Everything above that leaves as diarrhea.

• The toxin destroys the colon's lining cells, so the colon can no longer absorb water: The colon is not destroyed. Cholera works mainly on the small intestine, and the colon keeps absorbing near its limit.
• The colon stops moving and fluid pools in it and leaks out: Cholera does not stop colon movement; the diarrhea comes from the volume of fluid.
• Chloride blocks the colon's sodium channels: Chloride does not block sodium channels; the colon's sodium absorption continues.
• Correct: The fluid arriving far exceeds the colon's maximum absorption of about 4 to 5 L a day: Correct. Secretory diarrhea overwhelms the colon's capacity.

Fiber is not digested by your enzymes. It holds water in the stool, feeds bacteria whose bodies add bulk, and makes a larger, softer mass. The stretch of a bigger mass triggers stronger contractions, so transit speeds up.

• Fiber is digested into glucose, which speeds colon movements: Your enzymes cannot digest most fiber; bacteria ferment some of it into short-chain fatty acids, not glucose for you.
• Correct: Fiber holds water in the stool and adds bulk that stretches the colon wall: Correct. Water held in bulky stool and stretch-driven contractions both help.
• Fiber blocks the sodium pumps of the colon's epithelial cells and stops water absorption: Fiber does not block sodium pumps; absorption continues normally.
• Fiber kills the gut bacteria that slow transit: Fiber feeds gut bacteria rather than killing them.