evka,
thats a very open ended question.
The heart and kidneys play important roles in maintaining the balance of fluid in the body. The salts (especially sodium) in the blood are one of the principle components which govern how much fluid the body retains.
2 of the more important hormonal pathways involved in the retention or excretion of sodium by the kidneys are the
renin-angiotensin system and the natriuretic peptides. Both of these hormones are secreted by cardiac tissues with the former also predominantly secreted by the kidneys.
Specialized sensors, or receptors, in the heart and kidneys and well as the blood vessels respond to changes in blood pressure, blood volume, and concentrations of blood salts to maintain an exquisite balance of fluid and salt in the blood.
Sometimes this system can go awry such as in heart failure. One of the mechanisms causing the spiral of fluid retention in heart failure occurs because the kidneys do not see adequate flow from the heart. They respond by secreteing hormones to try to increase the volume of fluid and hopefully perfusion pressure. Unfortunately, a sick heart cant handle the fluid and the spiral continues into what we know as congestive heart failure.
Hope this is a start.