Page 720 - Atlas of Histology with Functional Correlations

activates aquaporins, resulting in increased water absorption from the filtrate.

As a result, water freely leaves the filtrate in the collecting ducts and enters
the hypertonic interstitium established by the thin loops of Henle and the
surrounding capillary network, the vasa recta. Water in the interstitium is
then collected or absorbed and returned to the general circulation via the
peritubular capillaries and vasa recta, and the glomerular filtrate in the

collecting ducts becomes hypertonic (concentrated) urine.

In the absence of ADH, the cells of the collecting tubules and ducts
remain impermeable to water. Consequently, increased amount of water
remains in the glomerular filtrate of the collecting ducts, resulting in dilute

urine.

FIGURE 18.13 | Kidney: Ducts of Medullary

Region (Longitudinal Section)

The medullary region of the kidney consists of various sized tubules, larger
ducts, and blood vessels of the vasa recta. In this photomicrograph, different
kidney tubules and blood vessels have been sectioned in a longitudinal plane.

The tubules with large, light-staining cuboidal cells are the collecting tubules
(1). Adjacent to the collecting tubules (1) are tubules with darker-staining
cuboidal cells. These are the thick segments of the loop of Henle (2). Between
the tubules are blood vessels of the vasa recta (4) and the thin segments of the

loop of Henle (3). Blood vessels of the vasa recta (4) can be distinguished from
the thin segments of the loop of Henle (3) by the presence of blood cells in their
lumina.

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