51 Segment P1 Segment

52 Segment P2 Segment

53 Segment P3 Segment

Proximal Convoluted Tubule = Proximal Convolution = Pars Convoluta of Proximal Tubule = Convoluted Part of Proximal Tubule

Proximal Straight Tubule = Pars Recta (PR) of Proximal Tubule = Straight Part of Proximal Tubule

Descending Thin Limb (DTL)

Pars Descendens of Intermediate Tubule

Descending Part of Intermediate Tubule

Ascending Thin Limb (ATL)

Pars Ascendens of Intermediate Tubule

Ascending Part of Intermediate Tubule

Medullary Thick Ascending Limb (MTAL) Medullary Thick Ascending Limb of Henle (mTALH) Medullary Ascending Limb (MAL) Medullary Thick Limb

Medullary Straight Part of Distal Straight Tubule Medullary Straight Part of Thick Ascending Limb

Cortical Thick Ascending Limb (CTAL) Cortical Ascending Limb (CAL) Cortical Thick Ascending Limb of Henle (cTALH) Cortical Thick Limb

Cortical Straight Part of Distal Straight Tubule Cortical Straight Part of Thick Ascending Limb

Postmacular Segment of Distal Straight Tubule Postmacular Segment of Thick Ascending Limb

Distal Convoluted Tubule (DCT) Pars Convoluta of Distal Tubule Convoluted Part of Distal Tubule

Connecting Tubule (CNT) Connecting Segment

Initial Collecting Tubule

Cortical Collecting Tubule (CCT)

Outer Medullary Collecting Duct (OMCD) Outer Medullary Collecting Tubule (OMCT)

Inner Medullary Collecting Duct (IMCD) Inner Medullary Collecting Tubule (IMCT) Papillary Collecting Duct (PCD) Ducts of Bellini

FIGURE 28-1 Anatomy and nomenclatures of the nephron.

multiplier hypothesis" proposes that active transport in the TAL concentrates NaCl in the inter-stitium of the outer medulla, thus generating its hypertonicity. Since this segment of the nephron is impermeable to water, active transport in the ascending limb dilutes the tubular fluid. As the dilute fluid passes into the collecting-duct system, water is extracted if, and only if, vasopressin is present. Since the cortical and outer medullary collecting ducts have a low permeability to urea, urea is concentrated in the tubular fluid. The inner medullary collecting duct, however, is permeable to urea, so urea diffuses into the inner medulla, where it is trapped by countercurrent exchange in the vasa recta. Since the DTL is impermeable to salt and urea, the high urea concentration in the inner medulla extracts water from the DTL and concentrates NaCl in the tubular fluid of the DTL. As the tubular fluid enters the ATL, NaCl diffuses out of the salt-permeable ATL, contributing to the hypertonicity of the medullary interstitium.

In the glomerular capillaries, a portion of the plasma water is forced through a filter that has three basic components: fenestrated capillary endothelial cells, a basement membrane lying just beneath the endothelial cells, and the filtration slit diaphragms formed by the epithelial cells that cover the basement membrane on its urinary space side. Solutes of small size flow with filtered water (solvent drag) into the urinary (Bowman's) space, whereas formed elements and macro-molecules are retained by the filtration barrier.

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