Kidney secretion is a process in which the kidneys produce and secrete hormones, electrolytes, and other substances into the bloodstream. Renin is a hormone produced by the kidneys that helps regulate blood pressure and fluid balance. It is released when the kidneys detect a decrease in blood pressure or an increase in sodium levels. Renin is an important part of the renin-angiotensin-aldosterone system, which helps to regulate blood pressure and fluid balance.