**2. Physiology of the natriuretic peptides**

The natriuretic peptides are a family of peptides produced by the myocardium, vascular endothelium and the kidneys (Clerico et al, 2006). The "hormone" of the heart is involved in the regulation of intravascular homeostasis, myocardial function. On the other hand, natriuretic peptides counteract with the renin-angiotensin-aldosterone system, symphathetic nervous system, vasopressin release. Receptors of natriuretic peptides are mainly presented in the myocardium, kidneys, lungs and vascular endothelium. The action is mediated by cGMP, the cytoplasmatic calcium availability will be decreased in the smooth muscles and it will result in vasodilatation.(Silberbach & Roberts, 2001).

Atrial stretching triggers release of the preformed prohormone of atrial natriuretic peptide (NT-proANP) in the atrial wall. During secretion, the prohormone of atrial natriuretic peptide (ANP) is cleaved to atrial natriuretic peptide and N-terminal pro-atrial natriuretic peptide (Kangawa et al, 1984). Although brain natriuretic peptide chemically resembles atrial natriuretic peptide to some extent, it is synthetized both in the atrial and ventricular myocardium due to volume and pressure overload. The production of BNP requires time (6- 12 hours), since RNA transcription of the prohormone precedes the secretion. Then, the prohormone of brain natriuretic peptide is cleaved to brain natriuretic peptide (BNP) and Nterminal pro-brain natriuretic peptide (NT-proBNP). N-terminal peptides have several times higher concentrations and longer half life time in blood circulation than atrial natriuretic peptide and brain natriuretic peptide, making them reliable markers (Ham et al, 1995).
