The Animation

In late relaxation, which lasts about 2 seconds, the shape of the papilla remains normal. Collecting ducts open as urine flows into them from above during the last second of this period. Vasa recta remain open and blood flows through them. Loops of Henle remain open as fluid continues to flow through them.

Peristaltic contraction, about 0.5 seconds. In front of the contraction, the collecting duct fluid is pushed through the ducts at a velocity greater than the velocity with which the fluid is formed, thus creating an increment in the pressure on the wall of the ducts. About half the fluid in the terminal collecting duct is absorbed. Our findings indicate that the collecting duct cell volumes in the terminal collecting duct is increased by approximately the volume of the fluid absorbed from the collecting duct lumen during each peristaltic contraction. This suggests that the reabsorbed water entered the collecting duct cells. Collecting ducts close behind the urine bolus as the papilla is stretched, becoming longer and narrower. The vasa recta close and the blood stops. Some blood moves retrograde, some down towards the tip of the papilla. The loops of Henle close as the fluid is pushed both retrograde and towards the tip.

Rebound. The papilla becomes both shorter and broader. The collecting ducts are still closed, but water moves out of the cells into the interstitium due to negative hydrostatic pressure generated by the elastic properties of the interstitial matrix. Ascending vasa recta are tethered to other structures and are opened as the tissue expands during rebound, permitting water to enter the vasa recta. At this point in time, there is no blood in either the ascending or descending vasa recta. The loops of Henle, the descending as well as ascending, are still empty.

Early relaxation, 1 second. The papilla resumes its original shape. The collecting ducts are still empty and closed, because the urine has not reached them yet. Vasa recta, first the descending and then the ascending, fill with blood, pushing the column of water, which had entered the vasa recta, towards the cortex. Loops of Henle also fill with fluid.

Simulation: Longitudinal Section

Simulation: Cross Section