In a subject who loses approximately 20 percent of blood volume, plasma vasopressin concentration rises markedly even though plasma osmolality is unchanged. The dominant stimuli for this hormone release are:

• A Activation of right atrial stretch receptors by volume expansion
• B Osmoreceptor firing caused by hyperosmolality of the extracellular fluid
• C Unloading of arterial baroreceptors and increased angiotensin II acting on circumventricular organs ✓
• D Increased firing of carotid body chemoreceptors due to metabolic alkalosis

Explanation

Hemorrhage unloads carotid sinus and aortic arch baroreceptors, removing tonic inhibition of magnocellular neurons in the supraoptic and paraventricular nuclei, while angiotensin II generated by the activated renin system further stimulates ADH release via the subfornical organ. Vasopressin then acts on V1a receptors to constrict resistance vessels. Option B fails because osmolality is stated to be normal, and atrial stretch receptors inhibit, not stimulate, ADH secretion.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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