A heart that fights high blood pressure
The heart is famous as a pump, but its muscle cells are also part-time endocrine cells. When the upper chambers stretch — a sign that blood volume or pressure is too high — the atria release atrial natriuretic peptide (ANP). ANP travels to the kidney and promotes natriuresis, the excretion of sodium and the water that follows it, which lowers blood volume and pressure. The ventricles release a close relative, B-type natriuretic peptide (BNP), when they are under strain.
These natriuretic peptides are an elegant counterweight to the renin-angiotensin-aldosterone system, which *raises* blood pressure and conserves salt. One arm of the body pushes pressure up; the heart's hormones push it back down. Because the strained heart pours out BNP, doctors measure it in the blood as a marker of heart failure — a beautiful case of an endocrine signal becoming a diagnostic tool.
Kidney, bone and a metabolic newcomer
Beyond renin, the kidney makes erythropoietin (EPO). Specialized kidney cells sense oxygen, and when it runs low — at high altitude, or with anemia — they release EPO, which travels to the bone marrow and stimulates red-blood-cell production. It is a clean example of a sensor and an effector linked by a hormone: the kidney measures oxygen, the marrow makes the cells, and EPO carries the message between them.
Bone, too, is endocrine. Bone cells secrete FGF23, a hormone that signals to the kidney to dump excess phosphate and to dial down active vitamin D — a key control point in phosphate balance. And the liver, muscle and fat release FGF21, a stress-and-fasting hormone that nudges the body to burn fat and adjust metabolism. FGF21 is being studied intensively because of its potential in metabolic disease.