Where do most RBCs die?

Correct answer: C

A. Decreased osteoclast activity: osteoclasts resorb bone and are involved in calcium homeostasis, not the primary sensor/effector for correcting hypoxemia.
B. Increased osteoblast activity: osteoblasts build bone (also related to calcium metabolism), not the mechanism that increases RBC production in response to low O₂.
C. Secretion of erythropoietin: hypoxemia stimulates renal production (secretion) of erythropoietin (EPO), which promotes erythropoiesis in red marrow to raise RBC mass and correct low O₂.
D. Secretion of gastroferritin: gastroferritin (a gastric iron-binding molecule) helps iron handling in the gut but is not the feedback signal used to correct hypoxemia.

A. A chemical messenger that is triggered by the binding of a ligand on the cell surface and causes a change in DNA transcription: This describes how some hormones act inside a cell, but it is not the definition of a hormone.
B. A chemical messenger transported by the bloodstream that stimulates target cells in another organ often a good distance away: This is the textbook definition of a hormone: an endocrine signal transported in the blood to distant target organs.
C. Chemical messengers that travel across a synapse to stimulate another cell: This describes neurotransmitters, not hormones.
D. Chemical messengers that diffuse from their point of origin locally to affect other cells physiology: This describes paracrine or autocrine signals, not hormones.