 Dear students, in this topic we shall discuss the cardiac receptors. The mechanoreceptive and chemoreceptive afferent nerve endings innervate atria and ventricles of the heart. They collect information about the state of the heart and their signals are transmitted through this spinal cord to the cardiovascular center in the medulla which controls heart rate. Stimulation of some cardiac receptors also causes release of hormones. These hormones are released either directly from the atria or from other endocrine tissues. These hormones regulate the heart rate and blood pressure. First we shall discuss the receptors present in the walls of atria that is atrial receptors. The atrial walls have mechanoreceptive afferent fibres which include the myelinated A-type afferent fibres, myelinated P-type afferent fibres, unmyelinated C-type afferent fibres and stretch sensitive secretary cells. The myelinated A-type afferent fibres respond to changes in heart rate. Myelinated B-type afferent fibres respond to increase in the rate of filling and volume of atria. In response, the central cardiovascular centers generate two effects. One is increased heart rate and increase in urine excretion i.e. diurasis because when the fillings of atria are increasing and volume is increasing, it means that volume of blood has increased and therefore it is necessary to reduce the volume by increasing in urine production. The diurasis that is mediated by a decrease in the anti-diuratic hormone level in the blood which increases in urine production. The unmyelinated C-type afferent fibres innervate the junction of veins and atria. Fibre stimulation affects both heart rate and blood pressure. When the heart rate is low, due to the distension of this region, the effect that is increased in heart rate is created. But when the heart rate is high, due to their stimulation, the heart rate and blood pressure are reduced. The stretch sensitive secretory cells in the atrial wall produce atrial natriuretic peptide hormone. This hormone causes an increase in urine production and sodium excretion, which effectively reduces blood volume and blood pressure. Now we shall discuss the mode of action of atrial natriuretic peptide hormone. It acts in two ways by inhibiting the release of anti-diuretic hormone and by diminishing the renin angiotensin aldosterone system of the kidney. This system is involved in sodium ion reabsorption and increase in the volume of the blood. When it is inhibited, as a result, the blood is added to the urine volume. This way, the atrial natriuretic hormone is added to the urine volume. Dear students, now we shall discuss the ventricular receptors. Ventricle has both malenated and unmalenated sensory afferent nerve fiber endings, which are stimulated when blood flow to the coronary arteries is interrupted. Interruption level is low, stimulation level is low, and the effect produced is increased sympathetic outflow and decreased vagal outflow to the heart. Cardiac contractility, that is, cardiac output will increase and blood pressure will increase. However, when the ventricular receptors are stimulated at higher level, then these fibers help in the perception of pain in the heart.