Figure 63

Scanning electron micrograph of the inner medulla, showing a prominent collecting duct, thin wall vessels, and abundant inter-stitium. A gradual increase in interstitial volume from the outer medullary stripe to the tip of the papilla occurs. In the outer stripe of the outer medulla, the relative volume of the interstitium is slightly less than is that of the cortex. This volume has been estimated to be approximately 5% in rats. It is in the inner stripe of the outer medulla that the interstitium begins to increase significantly in volume, in increments that gradually become larger toward the papillary tip.

The inner stripe of the outer medulla consists of the vascular bundles and the interbundle regions, which are occupied principally by tubules. Within the vascular bundles the interstitial spaces are meager, whereas in the interbundle region the interstitial spaces occupy some 10% to 20% of the volume. In the inner medulla the differentiation into vascular bundles and interbundle regions becomes gradually less obvious until the two regions merge. A gradual increase in the relative volume of the interstitial space from the base of the inner medulla to the tip of the papilla also occurs. In rats, the increment in interstitial space is from 10% to 15% at the base to about 30% at the tip. In rabbits, the increment is from 20% to 25% at the base to more than 40% at the tip.

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