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Urodynamic Events: Collecting Ducts |
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| We shall now see how the pelvic wall flow affects the flow in the collecting ducts. As the peristaltic wave moves down over the renal papilla, the urine is pushed through the papillary ducts as a bolus. Similar to the ureter, the collecting ducts are normally empty and closed behind the bolus. The bolus is short during low urine flow rates; the collecting ducts remain empty until the next bolus is pushed through. At low urine flow rates, they may be empty as much as 95% of the time. The fluid in the collecting duct is in contact with the collecting duct epithelium only 5% of the time. The linear velocity of the peristaltic wave is 1.6 mm/second, and is the velocity of the short bolus. |
Pulsatile Flow Through the Collecting Ducts |
| At higher urine flow rates, the bolus is longer and the contact time is increased. Under these conditions, the average linear velocity of the bolus is decreased. This is because the velocity of the leading edge of the urine, which is determined by the urine production, is lower than the velocity of the peristaltic wave pushing the trailing edge. As in the ureter, this velocity is independent of the urine flow rate. Thus, paradoxically, the average linear velocity of the urine in the collecting ducts is lower at higher flow rates compared to low urine flow rates. |
Higher Flow Rate |
| When the urine flow increases further, the bolus of urine is greatly increased in length and the collecting ducts are only empty when the peristaltic wave moves over the papilla. |
Highest Flow Rate (9.7 MB) |
| Light micrographs of the papilla fixed during contraction show the collecting ducts closed. |
Collecting Ducts at Contraction |
| They are wide open when the pelvic wall is not contracting. |
Collecting Ducts at Relaxation |
