Atrial natriuretic peptide (ANP) or Atrial natriuretic factor (ANF) is a peptide hormone which reduces an expanded extracellular fluid (ECF) volume by increasing renal sodium excretion. ANP is synthesized, and secreted by cardiac muscle cells in the walls of the atria in the heart. These cells contain volume receptors which respond to increased stretching of the atrial wall due to increased atrial blood volume. ANP is one of a family of nine natriuretic peptides: seven are atrial in origin.

Based on this passage, secreted by cells in the walls of the hearts atria? Let's give stream of consciousness first:
The relevant information to answer the above question is: ANP is synthesized, and secreted by cardiac muscle cells in the walls of the atria in the heart.
So, the final answer is Atrial natriuretic peptide ( ANP ).