Other endocrine organs

Chondrocytes in the epiphyseal plate divide and are replaced by bone on the side toward the shaft, lengthening the bone. Adding bone at the surface makes it wider, not longer.

• Correct: At the epiphyseal plates, where cartilage cells divide and are replaced by bone:
• At the periosteum, by adding bone to the outer surface:
• In the medullary cavity, by filling it with new bone:

Growth hormone makes the liver and other tissues release insulin-like growth factor 1 (IGF-1), which drives chondrocyte division at the plates.

• It slows growth, through calcitonin:
• Correct: It drives growth at the epiphyseal plates, mainly through IGF-1:
• It closes the epiphyseal plates, through thyroid hormones:

The optic nerves meet at the optic chiasm, where fibers from the inner half of each retina cross to the other side.

• Correct: At the optic chiasm:
• At the lateral geniculate nucleus:
• In the primary visual cortex:

Light, especially blue-rich light, excites melanopsin-containing retinal ganglion cells, which signal the suprachiasmatic nucleus. The sympathetic drive to the pineal gland is shut off, so melatonin release falls. Evening light also shifts the body clock later.

• It rises with the eyes' effort: Visual effort does not drive the pineal gland. Light suppresses melatonin, whatever the eyes are looking at.
• Correct: It is suppressed: Correct. Light at the retina switches off melatonin release.
• It is unaffected by artificial light: Artificial light works through the same retinal cells as sunlight, though bright light has more effect than dim light.
• It rises as light passes through the skull: In humans, the pineal gland does not sense light itself. The signal comes from the retina through the SCN and a sympathetic pathway, and light lowers melatonin rather than raising it.

Stretched muscle cells in the heart's upper chambers release atrial natriuretic peptide. ANP makes the kidneys filter more and reabsorb less sodium, lowers renin, aldosterone and ADH, and dilates vessels, so the extra volume is excreted: negative feedback.

• More aldosterone, so his kidneys keep more of the sodium they filter: ANP lowers aldosterone. Aldosterone would keep more sodium, raising volume further.
• Less ANP, so his vessels constrict: Stretch raises ANP release, not lowers it.
• Correct: More ANP, so his kidneys excrete more sodium and water: Correct. Stretch releases ANP, which lowers blood volume.
• More renin, so angiotensin II rises: Higher volume and ANP both lower renin release.

At altitude the arterial blood carries less oxygen, and the kidneys respond within hours by releasing EPO. Red blood cells rise only over weeks. Once they have, the blood carries more oxygen, the kidney's oxygen supply improves, and EPO falls back.

• Oxygen in her arterial blood, first day: down. Each breath carries less oxygen at lower air pressure, so she is hypoxemic.
• EPO release from her kidneys, first day: up. Kidney cells sense less oxygen, and the oxygen-sensing protein survives and switches on the EPO gene within hours.
• Red blood cell count, first day: no change. EPO acts on the bone marrow, and new red blood cells take days to weeks to appear in large numbers.
• Red blood cell count, after six weeks: up. Weeks of raised EPO have increased red blood cell production.
• Oxygen carried per liter of her blood, after six weeks: up. More red blood cells in each liter carry more oxygen, partly offsetting the thin air.
• EPO release, after six weeks compared with the first day: down. The extra red blood cells deliver more oxygen to the kidney, so EPO release eases back toward normal: negative feedback.

After a large bleed, the blood that remains is normally loaded with oxygen, so there is no hypoxemia. But there are too few red blood cells and too little flow, so the tissues receive too little oxygen: hypoxia.

• A climber at 5,000 meters: At altitude, each breath brings in less oxygen, so the arterial blood itself is low in oxygen: hypoxemia, with hypoxia.
• A person with severe lung disease on room air: Lung disease impairs loading oxygen into the blood, so the arterial blood is low in oxygen: hypoxemia.
• A person breathing too slowly after an opioid overdose: Breathing too slowly brings too little fresh air into the lungs, so arterial oxygen falls: hypoxemia.
• Correct: A person who has just lost a lot of blood: Correct. The blood is fully loaded but there is not enough of it reaching the tissues.

Leptin rises in proportion to fat stores, but in most obesity the hypothalamus responds weakly to it: leptin resistance. That is why giving more leptin helps only the rare people born unable to make it.

• Correct: Their brains respond weakly to leptin: Correct. This is leptin resistance.
• Their fat cells release an inactive form of leptin: The leptin is normal and active; the weak link is the brain's response to it.
• Leptin stops acting on the brain after childhood: Leptin acts in adults too: adults born without it stay severely hungry until they are treated with it.
• High leptin increases hunger instead of lowering it: High leptin signals that energy stores are full and normally lowers food intake. It does not reverse its own effect.

The drug builds muscle, but it also switches off his own reproductive axis by negative feedback. LH and FSH fall, so his testes shrink and make less testosterone and fewer sperm. After he stops, recovery can take months.

• LH and FSH from his anterior pituitary: down. High androgen feeds back on the hypothalamus and pituitary, cutting GnRH, LH and FSH.
• Testosterone made by his own testes: down. Without LH, his testes make little testosterone of their own.
• Size of his testes: down. Without LH and FSH, the testes are no longer driven and shrink.
• Sperm production: down. Sperm production needs FSH and a high testosterone level inside the testes, both of which fall.
• Skeletal muscle mass: up. The drug binds androgen receptor proteins in muscle and increases protein synthesis, causing hypertrophy.

His testosterone is normal, yet his plates stay open. Without aromatase he makes almost no estrogen. So estrogen is the hormone that matures the plate cartilage and ends growth in length, in men as well as women.

• Testosterone by itself closes the epiphyseal plates in men: If testosterone closed the plates, his normal testosterone would have closed them by his late teens.
• Growth hormone, not sex hormones, closes the plates: Growth hormone drives growth at the plates; it does not close them.
• The plates close in women but stay open in men: Men's plates close too, normally by their early twenties, through estrogen made from testosterone. His stay open because he lacks that estrogen.
• Correct: Estrogen, not testosterone, closes the epiphyseal plates: Correct. Estrogen closes the plates in both sexes.

In adults, red blood cells are made in the red bone marrow, and that is where EPO acts. The liver makes red blood cells only before birth.

• Less oxygen reaches the kidney's tissue: This step is right. The kidney's tissue oxygen tracks what the blood can deliver.
• Oxygen-sensing cells in the kidney switch on the EPO gene: This step is right. An oxygen-sensing protein that survives when oxygen is low switches on the EPO gene.
• Correct: EPO travels in the blood to the liver, where red blood cells are made: This is the error. EPO acts on the red bone marrow.
• More red blood cells are produced and enter the blood: This step is right. More red cells enter the blood over days to weeks.
• Oxygen delivery rises, and EPO release falls back: This step is right. Better oxygen delivery turns EPO back down: negative feedback.