A lowering of pressure in the carotid sinus reflexly increases the force of atrial systole by (a) increasing sympathetic activity to the heart (carotido-sympatho-atrial reflex) and(b) decreasing efferent vagal activity to the heart (carotido-vago-atrial reflex); an elevation of carotid pressure has the reverse effect. The carotid sinus can thereby vary ventricular end-diastolic pressure and fiber length. A change of pressure in the carotid sinus reflexly modifies the ventricle's contractility such that from a given end-diastolic pressure or fiber length, with a low carotid pressure the ventricle's contraction will be substantially augmented and from a high carotid pressure it will be diminished. The role of the carotid sinus in circulatory regulation has been likened to a voltage regulating element in an electronic system; i.e., it causes an appropriate variation of input into the system so as to maintain a constant voltage when the current requirements of the system it is supplying are changed.