What is the principal cation of the ECF?

A. It stimulates angiotensin II secretion: Antidiuretic hormone (ADH) does not directly stimulate angiotensin II secretion; instead, it acts on the kidneys to promote water reabsorption.

B. It promotes water conservation: ADH promotes water conservation by increasing water reabsorption in the kidneys, reducing urine output, and thus helping to maintain fluid balance.

C. It inhibits salivation and thirst: ADH does not inhibit salivation or thirst; in fact, it can increase thirst to encourage fluid intake when dehydration is detected.

D. It stimulates hypothalamic osmoreceptors: While ADH affects hypothalamic osmoreceptors indirectly by promoting water retention, its primary function is to act on the kidneys to conserve water.

A. Tubuloglomerular feedback involves the macula densa sensing changes in sodium chloride concentration and adjusting the glomerular filtration rate (GFR) via the juxtaglomerular apparatus.

B. Renal autoregulation includes mechanisms like the myogenic mechanism and tubuloglomerular feedback that maintain stable GFR despite blood pressure changes. This option is too broad and not specific to the mechanism described.

C. The myogenic mechanism refers to the smooth muscle's tendency to contract when stretched, helping to stabilize GFR by adjusting the diameter of the afferent arterioles. This is the correct answer.

D. Sympathetic control influences renal blood flow and GFR through hormonal and nervous system regulation, but it is not the specific mechanism described in the question.