Atrial reflexes and renal function.

Distension of small balloons in the venous-atrial junctions results in an increase in heart rate, urinary flow and sodium excretion. Two types of atrial receptors are described: one type, histologically known, discharging into myelinated fibers, and a second type, discharging into nonmyelinated ("C") fibers. These responses are mediated by the myelinated fibers. Experiments have shown that simulation of receptors discharging into the large myelinated vagal fibers is responsible for a reflex increase in heart rate mediated only by sympathetic nerves and for an increase in urinary flow. The efferent pathway of the diuretic response is shown to be nervous and hormonal. Stimulation of atrial receptors causes (1) a reduction of activity in nerves to the kidney, causing an increase in both urinary volume and sodium excretion, and (2) the release of a blood-borne agent, possibly diuretic, that increases urinary volume but does not affect sodium excretion.