The right answer is A: intercellular communication.
An example of intercellular communication is the one between the kidney, liver and lungs to preserve water-soluble homeostasis.
Angiotensinogen, an inactive protein produced by the liver, circulates in the blood. It is the precursor of the active peptides angiotensin I and II, and the only substrate of renin.
In the event of a drop in the pressure in the renal artery, renin (an enzyme sometimes considered as a hormone) is secreted in the kidney by differentiated myoepithelial cells of the arteriole afferent of the juxtaglomerular apparatus. There are also other stimuli promoting renin secretion: decreased natremia in the distal convoluted tubule, ß-agonists, hyperkalemia, PGI2 and stimulation of juxta-glomerular cells by the sympathetic nervous system)
The angiotensinogen is cleaved by the renin and forms a decapeptide called "angiotensin I", inactive.
Angiotensin I will then be converted primarily to angiotensin II by a carboxypeptidase, the angiotensin converting enzyme. This enzyme, secreted by the liver, acts at the pulmonary level.
