Final answer:
Stretch receptors in the atria detect volume changes in the blood, affecting vasodilation and blood flow to the kidneys.
Step-by-step explanation:
The correct answer is atria. Atria are the upper chambers of the heart that receive blood from the veins and pump it to the ventricles. Stretch receptors in the atria detect volume changes in the blood, affecting vasodilation and blood flow to the kidneys. These receptors, known as baroreceptors, are specialized stretch receptors located within thin areas of the heart chambers that respond to the degree of stretch caused by the presence of blood.