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Urinary Concentration and Dilution: Physiology

Rex L. Jamison, James J. Gehrig

10.1002/cphy.cp080227

Source: Supplement 25: Handbook of Physiology, Renal Physiology

Originally published: 1992

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Control of Water Balance
1.1 General Principles
1.2 Regulation of Water Intake: Thirst
1.3 Regulation of Water Excretion: Antidiuretic Hormone
1.4 Hormones Other Than Arginine Vasopressin That Affect Urinary Water Excretion
1.5 Prostaglandins
2 Architecture of the Medulla
2.1 Nephrons and Collecting Ducts
2.2 Circulation
2.3 Topography of the Renal Medulla
3 Mass Balance and the Countercurrent Principle
3.1 Osmotic Gradient in the Medulla
3.2 Mass Balance for Water and Solute
3.3 Countercurrent Multiplication
4 Permeability and Transport Properties of the Renal Tubule Beyond the Pars Recta
4.1 Thin Descending Limb
4.2 Thin Ascending Limb
4.3 Thick Ascending Limb
4.4 Distal Tubule and Connecting Tubule
4.5 Collecting Tubule
5 Intracellular Osmoregulation
6 Microcirculation of the Renal Medulla
6.1 Countercurrent Exchange
6.2 Fluid Removal from the Medulla
6.3 Medullary Blood Flow
6.4 Regulation of Medullary Circulation
6.5 Summary of Role of Vasa Recta
7 Renal Pelvis
7.1 Structure
7.2 Mechanical Function: Peristalsis and Reflux
7.3 Water and Solute Exchange in the Renal Pelvis
Figure 1. Figure 1.

Plasma, urine, and renal tissue fluid osmolalities determined by vapor pressure osmometry in antidiuretic rabbit. Open triangles, data obtained when kidney tissue was placed directly into equilibration chamber; closed triangles, data obtained when filter paper wetted with tissue fluid was placed into equilibration chamber.

From Knepper 145
Figure 2. Figure 2.

Schematic of distribution of body water. Intracellular fluid makes up 55% of total body water. Remainder of body water is defined as comprising extracellular fluid. Plasma (7.5%) is separated from interstitial lymph fluid (20%) by capillary membrane; straight dashed line indicates that capillary membrane is highly permeable to water and solutes with the exception of plasma proteins. Three cavities enclosed by circular dashed lines are potential extracellular fluid spaces. Shaded portions of dense connective tissue and bone (15%) appear not to be readily accessible to interstitial lymph fluid. Transcellular fluid (2.5%) is contained in transcellular spaces such as gastrointestinal and urinary tracts.

From Maffly 173
Figure 3. Figure 3.

Anatomy of neurohypophysis and principal regulatory afferents. Shaded areas represent parts of central nervous system where blood‐brain barrier is deficient. pvn, paraventricular nuclei; or, osmoreceptors; son, supraoptic nuclei; oc, optic chiasm; ah, adenohypophysis; ds, diaphragm sella; nh, neurohypophysis; br, baroreceptors; ap, area postrema; nts, nuclei of tractus solitarius.

From Robertson and Berl 221
Figure 4. Figure 4.

A, B: relationship between plasma vasopressin and plasma and urine osmolality in healthy adults in varying states of water balance. Arrow indicates level of plasma osmolality at which thirst begins.

From Robertson and Berl 221
Figure 5. Figure 5.

Relationship of plasma arginine vasopressin (AVP) to percentage of increase in blood osmolality (open circles) or to percentage of decrease in blood volume (closed circles) in conscious rats. Each circle represents one animal.

From Dunn et al. 55
Figure 6. Figure 6.

Location of V2 receptors to arginine vasopressin along nephron and branched collecting duct in four mammalian species. Horizontal dashed lines indicate the corticomedullary junction. Segments outlined with dashed lines (thin ascending limbs) were not studied. Density of dotted areas is proportional to increase in adenylate cyclase activity induced by arginine vasopressin.

From Morel 187
Figure 7. Figure 7.

A: mean urine osmolality (vertical bars = ± S.D. of maximal value at each dose) during continuous intravenous infusion of antidiuretic hormone at 2.5 (•), 5 (○), 15 (▪), 30 (□, ▴), and 60 (Δ) μU min−1 (100 g body weight)−1. Value before antidiuretic hormone administration (star) represents mean of all groups. B: mean (± S.D.) urinary flow during continuous intravenous infusion of antidiuretic hormone at 2.5 (first graph), 5 (second), 15 (third), and 30 (fourth) μU min−1 (100 g body weight)−1. Value before antidiuretic hormone administration represents mean of all groups.

From Atherton et al. 9
Figure 8. Figure 8.

Effect of continuous infusion of AVP via osmotic minipump for 10 consecutive days on fluid balance, glomerular filtration rate, and renal blood flow in rats with hereditary lack of vasopressin (Brattleboro strain). Top: effect on body weight and plasma osmolality (Posm). Middle: effect on urinary flow (V) and urinary osmolality (Uosm). Bottom: effect on glomerular filtration rate (GFR) and effective renal blood flow (ERBF). Vertical bars represent 1 S.E. Asterisks indicate values significantly different from those for controls (before infusion) or for recovery after infusion (P < 0.05).

From Gellai et al. 73
Figure 9. Figure 9.

A: kidney of Brattleboro rat before dDAVP treatment. Semithin (1 μm) cross section through lower level of inner stripe of outer medulla. Descending thin limbs (D), thick ascending limbs (A), and collecting ducts (CD) are shown. B: kidney of Brattleboro rat after dDAVP treatment. Semithin cross section through lower level of inner stripe of outer medulla. Compared with untreated diabetes insipidus rat (in A), thick ascending limbs (A) are strikingly increased in diameter and cellular thickness. Many binuclear cells (asterisks) are found in thick ascending limb epithelium. D, descending thin limb; CD, collecting ducts.

From Bankir and Kriz 12
Figure 10. Figure 10.

Schematic of nephrons and collecting duct in superficial and juxtamedullary nephron of rabbit kidney. Throughout this series of illustrations, glomeruli and proximal tubules are in solid black; thin limbs, distal convoluted tubules, and collecting ducts are white; thick ascending limbs are gray; connecting tubules are black mottled with white flecks. C, cortex; OS, outer stripe; IS, inner stripe; IM, inner medulla.

Modified from Kaissling and Kriz 138
Figure 11. Figure 11.

Schematic of microvasculature, nephrons, and collecting ducts of mammalian kidney. C, cortex; OS, outer stripe; IS, inner stripe; IM, inner medulla. Left: arterial vessels and capillaries. Arcuate artery (arrow) gives rise to interlobular artery from which afferent arterioles of glomeruli originate. Efferent arterioles of juxtamedullary glomeruli descend into OS and divide into descending vasa recta, which together with ascending vasa recta establish vascular bundles of renal medulla. At intervals, descending vasa recta leave bundles to feed adjacent capillaries. Note different patterns of capillary plexuses in OS, IS, and IM. In C, capillary plexus of medullary rays (long‐meshed) and of cortical labyrinth are distinguished. Middle: venous vessels. Interlobular veins start in superficial C. Inner cortical parts (together with arcuate veins) accept medullary venous vessels. These are ascending vasa recta, which together with descending vasa recta establish vascular bundles. Most ascending vasa recta from IS ascend independently from bundles. All ascending vasa recta traverse OS as wide tortuous channels. Right: segmentation of short‐ and long‐looped nephrons and collecting duct. Glomeruli and proximal tubules are black; thin limbs, hatched; thick limbs, dotted; distal convoluted tubules, connecting tubules (arcade), and collecting ducts, white. Straight proximal tubule (pars recta) of juxtamedullary nephron takes tortuous course when descending through OS. Thin descending limb loops back within inner medulla. Thin ascending limb becomes thick ascending limb at transition from IM to outer medulla. It ascends through outer medulla to reach its corresponding glomerulus. Beyond macula densa, thick ascending limb becomes convoluted part of distal tubule. Connecting tubule follows (its separation from convoluted distal tubule not shown), which accepts further tributaries. Thereby an arcade is established ascending toward cortical surface before draining into cortical collecting duct. Straight proximal tubule of superficial nephron is long and descends within a medullary ray of C and OS. Thin descending limb turns back within lower part of IS. Thick ascending limb ascends through outer medulla and medullary ray of C. Beyond macula densa, it transforms into convoluted part of distal tubule. Via connecting tubule (not specifically marked), each nephron drains individually (i.e., without fusing with other nephrons) into cortical collecting duct. Collecting ducts are divided into cortical segment and outer and inner medullary segments. Cortical collecting ducts lie in medullary rays of C and have short side branches by which nephrons are accepted. Outer medullary collecting ducts do not fuse. Inner medullary collecting ducts coalesce by pairs in several steps and become large channels that empty into renal pelvis.

From Kriz 159
Figure 12. Figure 12.

Architectural organization of rat renal medulla in schematic longitudinal section (a) and cross sections through the outer stripe (b), inner stripe (c) of the outer medulla, and inner medulla (d). a: one long and two short Henle's loops, collecting duct, and vascular bundle. Individual vessels are not shown in bundle; rather, an attempt at its “three‐dimensional solid form” is shown. Vascular bundle contains ascending vasa recta coming from inner medulla (thick arrows), descending vasa recta, and thin descending limbs of short loop nephrons. Ascending vasa recta originating in inner stripe (thick wavy arrows) ascend directly within inner bundle region. b‐d, topographical relationships of four short and two long Henle's loops shown with collecting duct and vasa recta. In outer stripe (b), proximal straight tubule (PST) and medullary thick ascending limb (MAL) of long loops (l) among ascending vasa recta (AVR) are near vascular bundle. Displaced from bundle is collecting duct (CD) and PST and MAL of short loops (s), surrounded by AVR from inner stripe and capillaries forming plexus, which are smaller in diameter. In inner stripe (c), core of vascular bundle contains AVR and descending vasa recta (DVR), whereas now short descending thin limbs (DTL) are found among AVR coming from inner medulla in periphery. In inner bundle region, long DTL and CD run together with the thick ascending limb of short loop. Thick ascending limb of long loops border vascular bundle. In upper inner medulla (d), vascular bundle is still discernible, but AVR are already found uniformly throughout cross section. Note that CD is distant from vascular bundle, and between them are DTL and ascending thin limb (ATL) of long loops.

From Lemley and Kriz 168
Figure 13. Figure 13.

Renal tissue composition during sustained water diuresis (○) and during continuous intravenous infusion of vasopressin (•). Tissue slice numbers at top of each panel refer to level of transverse section: 1, papilla tip; 2, papilla base; 3, inner medulla; 4, outer medulla; 5, inner cortex; 6, outer cortex. Time (in hours) refers to time before or after the beginning of infusion of vasopressin or to corresponding time interval in water‐diuretic animals not infused with vasopressin. Values represent mean ± S.D. α: renal tissue osmolality, b: renal tissue content of water and total solute. UFDS, ureafree dry solid. c: renal tissue concentration of sodium and urea. d: renal tissue content of sodium and urea.

From Hai and Thomas 91
Figure 14. Figure 14.

Countercurrent multiplication (augmentation) by Henle's loop. According to countercurrent hypothesis, there are three requirements for loop to act as a multiplier: A, countercurrent flow; B, difference in epithelial permeability; and C, energy source. Thickened lining of ascending limb and distal tubule indicates water‐impermeable epithelium in contrast to water‐permeable descending limb. Source of energy is active reabsorption of sodium chloride by ascending limb. This increases sodium chloride concentration (indicated by size of type) in medulla, which osmotically extracts water from descending limb and collecting duct. This facilitates further salt reabsorption from ascending limb and simultaneously concentrates urine.

From Jamison and Maffly 132
Figure 15. Figure 15.

Kokko and Rector's passive inner medullary concentrating mechanism 140. Thin ascending limb, thick ascending limb, and first part of distal tubule are impermeable to water, as indicated by thickened lining (cf. Figs. 15 and 16). In thick ascending limb, active chloride reabsorption, accompanied by passive sodium movement , renders tubule fluid dilute and outer medullary interstitium hyperosmotic. In last part of distal tubule and in collecting tubule in cortex and outer medulla, water is reabsorbed along its osmotic gradient , increasing concentration of urea that remains behind. In inner medulla, both water and urea are reabsorbed from collecting duct . Some urea reenters Henle's loop (not shown). This medullary recycling of urea, in addition to trapping urea by countercurrent exchange in vasa recta (not shown), causes urea to accumulate in large quantities in medullary interstitium (indicated by the large type), where it osmotically extracts water from descending limb and thereby concentrates sodium chloride in descending limb fluid. When fluid rich in sodium chloride enters sodium chloride‐permeable (but water‐impermeable) thin ascending limb, sodium chloride moves passively down its concentration gradient , rendering tubule fluid relatively hypoosmotic to surrounding interstitium.

From Jamison and Maffly 132
Figure 16. Figure 16.

Simple three‐compartment model (left) and assignment of each compartment to structures of inner medulla (right inset, outlined by dashed line). C‐1, C‐2, and C‐3 represent three compartments whose initial solute compositions are indicated at bottom of respective compartments. As summarized below compartments, membrane I (M‐I) is impermeable to water, highly permeable to sodium chloride, and moderately permeable to urea. In contrast, membrane II (M‐II) is highly permeable to water and moderately permeable to urea but impermeable to sodium chloride. Reflection coefficient of M‐II membrane is higher to sodium chloride than to urea. tAL, thin ascending limb; CNW, capillary networks; PCD, papillary collecting duct; VB, vascular bundle; DLH, descending limb of Henle; P, permeability. As indicated by the abbreviations tAL, CNW, and PCD at the upper left part and toward the lower right part of the figure, the thin ascending limb (tAL) is incorporated as C‐1 (C1), the papillary collecting duct (PCD) as C‐3 (C3), and the capillary network (CNW) as C‐2 (C2).

From Imai et al. 117
Figure 17. Figure 17.

Schematic of tubulovascular counterflow systems. AVR, ascending vasa recta; SDL, short‐loop descending limb; MAL, medullary thick ascending limb; DVR, descending vasa recta; LDL, long‐loop descending limb; tAL, thin ascending limb; CNW, capillary networks (represent both ascending and descending vasa recta); MCD, medullary collecting duct; PCD, papillary collecting duct. Open arrows, water; closed arrows, sodium chloride; dotted arrows, urea. –, countercurrent pairs: , thick ascending limb and descending vasa recta; , descending limb of short loop and ascending vasa recta; , upper portion of descending limb of long loop and ascending vasa recta; , lower portion of long ascending limb and ascending vasa recta; , ascending vasa recta and collecting duct; , descending vasa recta and thin ascending limb.

From Imai et al. 107
Figure 18. Figure 18.

Generation of chloride and osmotic gradients in ascending thin limb in vitro by passive diffusion of solutes. Perfusate in each case is ultrafiltrate plus 300 mOsm/kg sodium chloride. Bath is either identical to perfusate (open circles) or isosmolar serum to which 300 mOsm/kg urea was added (closed circles). Left: plot of collected‐to‐perfusate (C/P) chloride concentration; right: collected‐to‐perfusate osmolality ratio. Lines connecting two points indicate two tubules studied at two different lengths.

From Imai and Kokko 113
Figure 19. Figure 19.

Sodium concentration in Henle's loop fluid (Na+LH) plotted against sodium concentration in vasa recta plasma (Na+VR) in rat. Open circles, paired samples in which descending vasa recta plasma was obtained; solid squares, paired samples in which ascending vasa recta plasma was collected. Line is line of identity.

From Johnston et al. 137
Figure 20. Figure 20.

Cellular model of mechanism of sodium chloride reabsorption in cortical thick ascending limb of Henle's loop and in early distal convoluted tubule. •, Na+K+ pump; ○, carrier systems; arrows, diffusive pathways. Bottom of figure gives estimates of voltages across cell membranes. With symmetrical Ringer's‐type solutions, transepithelial voltage in cortical thick ascending limb is +5 to +10 mV. Note that paracellular shunt pathway is sodium conductive.

From Greger and Velázquez 88
Figure 21. Figure 21.

A. The sum of the concentrations of two methylamines, betaine and glycerophosphorylcholine (GPC), and two polyols, sorbitol and myoinositol, in slices taken from renal cortex, and outer and inner renal medulla of the rabbit, plotted as function of sodium concentration in the corresponding section of the kidney. Open circles and dashed line depict results from animals in a diuretic state (mean urinary osmolality, 422 mOsm/kg water). Closed circles illustrate results from antidiuretic animals (mean osmolality, 1,285 mOsm/kg water). Vertical bars depict ± S.E. for total osmolyte determinations. Each point represents data for one kidney section. There is a strong correlation under both conditions. B. The concentration of GPC alone, from the same experiments, plotted as a function of the concentration of urea in the corresponding section of the kidney. Of all the osmolytes, only GPC showed a good correlation with urea and the same slopes of the regression lines (not shown) in both diuresis and antidiuresis.

From Yancey and Burg 313
Figure 22. Figure 22.

Principle of countercurrent exchanger. A source supplying heat (A, B) at 100 cal/min raises temperature of 10 ml/min of water flow 10°C in both A (straight flow) and B (countercurrent flow). However, because incoming water is heated by outgoing water in B, maximum temperature attained in countercurrent system is considerably higher than with straight flow. Graph in B compares temperature along flow tubes in each system. C: countercurrent flow as applied to vas rectum, capillary loop in medulla, showing that it is not necessary for limbs of loop to be in direct contact. In hypothetical illustration given, both limbs are in contact with same interstitial fluid, of progressively increasing concentration. Sodium salts (arrows) at first enter capillary blood and later partly return to interstitial fluid; final concentration of blood leaving area and interstitial fluid depends on various factors. Note analogy between B, in which heat is recirculated, and C, in which sodium salts are similarly retained in area.

From Berliner et al. 23
Figure 23. Figure 23.

Urine‐to‐plasma osmolality ratio (U/Posm) as a function of medullary blood flow.

From Thurau et al. 283
Figure 24. Figure 24.

Dual function of vasa recta. Medullary circulation consists of network of channels with main thoroughfares (vasa recta) and branch connections. Encircled Pr denotes plasma protein. Type size indicates relative concentration of each solute with respect to its location in medulla, but not necessarily with respect to concentrations of other solutes. Progressive rise in sodium chloride and urea concentrations in medullary interstitium is caused by reabsorption from Henle's loop and collecting tubule, respectively. Because capillaries are permeable to sodium chloride and urea, these solutes enter descending vasa recta and leave ascending vasa recta; such transcapillary exchange “traps” these solutes in medulla. Conversely, water is extracted from descending vasa recta, causing plasma protein concentration to increase. In ascending vasa recta, sum of oncotic (that caused by plasma protein) and osmotic (that caused by nonprotein solutes) transcapillary pressures results in capillary fluid uptake. In this way, water reabsorbed from collecting tubule and Henle's descending limb is removed from medulla and returned to general circulation. Vasa recta function in dual capacity, trapping solute and removing water, to preserve hyperosmolality of renal medulla.

From Jamison and Kriz 130
Figure 25. Figure 25.

Casts of pelvic cavities from rat (A) and sand rat (B, C). Microfil was injected into ureter in situ, after ligation of renal artery and vein. After Microfil solidified, kidney tissue was removed carefully and casts prepared for scanning electron microscopy. A, B: side view. Earliest part of ureter is cone shaped and shows gap (thin arrows) corresponding to location of papilla that was pressed against pelvic wall by Microfil. C: same cast as in B viewed from above as indicated by thick arrow in B. Papillary tip and ureter would be behind picture. Wide horizontal groove corresponds to penetration of inner medulla into pelvic space. Note well‐developed lateral pelvic fornices of Psammomys kidney. On these casts, borders of pelvic extensions in both species appear smooth and rounded, because cavities were distended by Microfil. They are sharper and thinner in unfilled kidney but possibly take this dilated shape in vivo during urine reflux caused by pelvic contraction.

From Bankir and de Rouffignac 130


Figure 1.

Plasma, urine, and renal tissue fluid osmolalities determined by vapor pressure osmometry in antidiuretic rabbit. Open triangles, data obtained when kidney tissue was placed directly into equilibration chamber; closed triangles, data obtained when filter paper wetted with tissue fluid was placed into equilibration chamber.

From Knepper 145


Figure 2.

Schematic of distribution of body water. Intracellular fluid makes up 55% of total body water. Remainder of body water is defined as comprising extracellular fluid. Plasma (7.5%) is separated from interstitial lymph fluid (20%) by capillary membrane; straight dashed line indicates that capillary membrane is highly permeable to water and solutes with the exception of plasma proteins. Three cavities enclosed by circular dashed lines are potential extracellular fluid spaces. Shaded portions of dense connective tissue and bone (15%) appear not to be readily accessible to interstitial lymph fluid. Transcellular fluid (2.5%) is contained in transcellular spaces such as gastrointestinal and urinary tracts.

From Maffly 173


Figure 3.

Anatomy of neurohypophysis and principal regulatory afferents. Shaded areas represent parts of central nervous system where blood‐brain barrier is deficient. pvn, paraventricular nuclei; or, osmoreceptors; son, supraoptic nuclei; oc, optic chiasm; ah, adenohypophysis; ds, diaphragm sella; nh, neurohypophysis; br, baroreceptors; ap, area postrema; nts, nuclei of tractus solitarius.

From Robertson and Berl 221


Figure 4.

A, B: relationship between plasma vasopressin and plasma and urine osmolality in healthy adults in varying states of water balance. Arrow indicates level of plasma osmolality at which thirst begins.

From Robertson and Berl 221


Figure 5.

Relationship of plasma arginine vasopressin (AVP) to percentage of increase in blood osmolality (open circles) or to percentage of decrease in blood volume (closed circles) in conscious rats. Each circle represents one animal.

From Dunn et al. 55


Figure 6.

Location of V2 receptors to arginine vasopressin along nephron and branched collecting duct in four mammalian species. Horizontal dashed lines indicate the corticomedullary junction. Segments outlined with dashed lines (thin ascending limbs) were not studied. Density of dotted areas is proportional to increase in adenylate cyclase activity induced by arginine vasopressin.

From Morel 187


Figure 7.

A: mean urine osmolality (vertical bars = ± S.D. of maximal value at each dose) during continuous intravenous infusion of antidiuretic hormone at 2.5 (•), 5 (○), 15 (▪), 30 (□, ▴), and 60 (Δ) μU min−1 (100 g body weight)−1. Value before antidiuretic hormone administration (star) represents mean of all groups. B: mean (± S.D.) urinary flow during continuous intravenous infusion of antidiuretic hormone at 2.5 (first graph), 5 (second), 15 (third), and 30 (fourth) μU min−1 (100 g body weight)−1. Value before antidiuretic hormone administration represents mean of all groups.

From Atherton et al. 9


Figure 8.

Effect of continuous infusion of AVP via osmotic minipump for 10 consecutive days on fluid balance, glomerular filtration rate, and renal blood flow in rats with hereditary lack of vasopressin (Brattleboro strain). Top: effect on body weight and plasma osmolality (Posm). Middle: effect on urinary flow (V) and urinary osmolality (Uosm). Bottom: effect on glomerular filtration rate (GFR) and effective renal blood flow (ERBF). Vertical bars represent 1 S.E. Asterisks indicate values significantly different from those for controls (before infusion) or for recovery after infusion (P < 0.05).

From Gellai et al. 73


Figure 9.

A: kidney of Brattleboro rat before dDAVP treatment. Semithin (1 μm) cross section through lower level of inner stripe of outer medulla. Descending thin limbs (D), thick ascending limbs (A), and collecting ducts (CD) are shown. B: kidney of Brattleboro rat after dDAVP treatment. Semithin cross section through lower level of inner stripe of outer medulla. Compared with untreated diabetes insipidus rat (in A), thick ascending limbs (A) are strikingly increased in diameter and cellular thickness. Many binuclear cells (asterisks) are found in thick ascending limb epithelium. D, descending thin limb; CD, collecting ducts.

From Bankir and Kriz 12


Figure 10.

Schematic of nephrons and collecting duct in superficial and juxtamedullary nephron of rabbit kidney. Throughout this series of illustrations, glomeruli and proximal tubules are in solid black; thin limbs, distal convoluted tubules, and collecting ducts are white; thick ascending limbs are gray; connecting tubules are black mottled with white flecks. C, cortex; OS, outer stripe; IS, inner stripe; IM, inner medulla.

Modified from Kaissling and Kriz 138


Figure 11.

Schematic of microvasculature, nephrons, and collecting ducts of mammalian kidney. C, cortex; OS, outer stripe; IS, inner stripe; IM, inner medulla. Left: arterial vessels and capillaries. Arcuate artery (arrow) gives rise to interlobular artery from which afferent arterioles of glomeruli originate. Efferent arterioles of juxtamedullary glomeruli descend into OS and divide into descending vasa recta, which together with ascending vasa recta establish vascular bundles of renal medulla. At intervals, descending vasa recta leave bundles to feed adjacent capillaries. Note different patterns of capillary plexuses in OS, IS, and IM. In C, capillary plexus of medullary rays (long‐meshed) and of cortical labyrinth are distinguished. Middle: venous vessels. Interlobular veins start in superficial C. Inner cortical parts (together with arcuate veins) accept medullary venous vessels. These are ascending vasa recta, which together with descending vasa recta establish vascular bundles. Most ascending vasa recta from IS ascend independently from bundles. All ascending vasa recta traverse OS as wide tortuous channels. Right: segmentation of short‐ and long‐looped nephrons and collecting duct. Glomeruli and proximal tubules are black; thin limbs, hatched; thick limbs, dotted; distal convoluted tubules, connecting tubules (arcade), and collecting ducts, white. Straight proximal tubule (pars recta) of juxtamedullary nephron takes tortuous course when descending through OS. Thin descending limb loops back within inner medulla. Thin ascending limb becomes thick ascending limb at transition from IM to outer medulla. It ascends through outer medulla to reach its corresponding glomerulus. Beyond macula densa, thick ascending limb becomes convoluted part of distal tubule. Connecting tubule follows (its separation from convoluted distal tubule not shown), which accepts further tributaries. Thereby an arcade is established ascending toward cortical surface before draining into cortical collecting duct. Straight proximal tubule of superficial nephron is long and descends within a medullary ray of C and OS. Thin descending limb turns back within lower part of IS. Thick ascending limb ascends through outer medulla and medullary ray of C. Beyond macula densa, it transforms into convoluted part of distal tubule. Via connecting tubule (not specifically marked), each nephron drains individually (i.e., without fusing with other nephrons) into cortical collecting duct. Collecting ducts are divided into cortical segment and outer and inner medullary segments. Cortical collecting ducts lie in medullary rays of C and have short side branches by which nephrons are accepted. Outer medullary collecting ducts do not fuse. Inner medullary collecting ducts coalesce by pairs in several steps and become large channels that empty into renal pelvis.

From Kriz 159


Figure 12.

Architectural organization of rat renal medulla in schematic longitudinal section (a) and cross sections through the outer stripe (b), inner stripe (c) of the outer medulla, and inner medulla (d). a: one long and two short Henle's loops, collecting duct, and vascular bundle. Individual vessels are not shown in bundle; rather, an attempt at its “three‐dimensional solid form” is shown. Vascular bundle contains ascending vasa recta coming from inner medulla (thick arrows), descending vasa recta, and thin descending limbs of short loop nephrons. Ascending vasa recta originating in inner stripe (thick wavy arrows) ascend directly within inner bundle region. b‐d, topographical relationships of four short and two long Henle's loops shown with collecting duct and vasa recta. In outer stripe (b), proximal straight tubule (PST) and medullary thick ascending limb (MAL) of long loops (l) among ascending vasa recta (AVR) are near vascular bundle. Displaced from bundle is collecting duct (CD) and PST and MAL of short loops (s), surrounded by AVR from inner stripe and capillaries forming plexus, which are smaller in diameter. In inner stripe (c), core of vascular bundle contains AVR and descending vasa recta (DVR), whereas now short descending thin limbs (DTL) are found among AVR coming from inner medulla in periphery. In inner bundle region, long DTL and CD run together with the thick ascending limb of short loop. Thick ascending limb of long loops border vascular bundle. In upper inner medulla (d), vascular bundle is still discernible, but AVR are already found uniformly throughout cross section. Note that CD is distant from vascular bundle, and between them are DTL and ascending thin limb (ATL) of long loops.

From Lemley and Kriz 168


Figure 13.

Renal tissue composition during sustained water diuresis (○) and during continuous intravenous infusion of vasopressin (•). Tissue slice numbers at top of each panel refer to level of transverse section: 1, papilla tip; 2, papilla base; 3, inner medulla; 4, outer medulla; 5, inner cortex; 6, outer cortex. Time (in hours) refers to time before or after the beginning of infusion of vasopressin or to corresponding time interval in water‐diuretic animals not infused with vasopressin. Values represent mean ± S.D. α: renal tissue osmolality, b: renal tissue content of water and total solute. UFDS, ureafree dry solid. c: renal tissue concentration of sodium and urea. d: renal tissue content of sodium and urea.

From Hai and Thomas 91


Figure 14.

Countercurrent multiplication (augmentation) by Henle's loop. According to countercurrent hypothesis, there are three requirements for loop to act as a multiplier: A, countercurrent flow; B, difference in epithelial permeability; and C, energy source. Thickened lining of ascending limb and distal tubule indicates water‐impermeable epithelium in contrast to water‐permeable descending limb. Source of energy is active reabsorption of sodium chloride by ascending limb. This increases sodium chloride concentration (indicated by size of type) in medulla, which osmotically extracts water from descending limb and collecting duct. This facilitates further salt reabsorption from ascending limb and simultaneously concentrates urine.

From Jamison and Maffly 132


Figure 15.

Kokko and Rector's passive inner medullary concentrating mechanism 140. Thin ascending limb, thick ascending limb, and first part of distal tubule are impermeable to water, as indicated by thickened lining (cf. Figs. 15 and 16). In thick ascending limb, active chloride reabsorption, accompanied by passive sodium movement , renders tubule fluid dilute and outer medullary interstitium hyperosmotic. In last part of distal tubule and in collecting tubule in cortex and outer medulla, water is reabsorbed along its osmotic gradient , increasing concentration of urea that remains behind. In inner medulla, both water and urea are reabsorbed from collecting duct . Some urea reenters Henle's loop (not shown). This medullary recycling of urea, in addition to trapping urea by countercurrent exchange in vasa recta (not shown), causes urea to accumulate in large quantities in medullary interstitium (indicated by the large type), where it osmotically extracts water from descending limb and thereby concentrates sodium chloride in descending limb fluid. When fluid rich in sodium chloride enters sodium chloride‐permeable (but water‐impermeable) thin ascending limb, sodium chloride moves passively down its concentration gradient , rendering tubule fluid relatively hypoosmotic to surrounding interstitium.

From Jamison and Maffly 132


Figure 16.

Simple three‐compartment model (left) and assignment of each compartment to structures of inner medulla (right inset, outlined by dashed line). C‐1, C‐2, and C‐3 represent three compartments whose initial solute compositions are indicated at bottom of respective compartments. As summarized below compartments, membrane I (M‐I) is impermeable to water, highly permeable to sodium chloride, and moderately permeable to urea. In contrast, membrane II (M‐II) is highly permeable to water and moderately permeable to urea but impermeable to sodium chloride. Reflection coefficient of M‐II membrane is higher to sodium chloride than to urea. tAL, thin ascending limb; CNW, capillary networks; PCD, papillary collecting duct; VB, vascular bundle; DLH, descending limb of Henle; P, permeability. As indicated by the abbreviations tAL, CNW, and PCD at the upper left part and toward the lower right part of the figure, the thin ascending limb (tAL) is incorporated as C‐1 (C1), the papillary collecting duct (PCD) as C‐3 (C3), and the capillary network (CNW) as C‐2 (C2).

From Imai et al. 117


Figure 17.

Schematic of tubulovascular counterflow systems. AVR, ascending vasa recta; SDL, short‐loop descending limb; MAL, medullary thick ascending limb; DVR, descending vasa recta; LDL, long‐loop descending limb; tAL, thin ascending limb; CNW, capillary networks (represent both ascending and descending vasa recta); MCD, medullary collecting duct; PCD, papillary collecting duct. Open arrows, water; closed arrows, sodium chloride; dotted arrows, urea. –, countercurrent pairs: , thick ascending limb and descending vasa recta; , descending limb of short loop and ascending vasa recta; , upper portion of descending limb of long loop and ascending vasa recta; , lower portion of long ascending limb and ascending vasa recta; , ascending vasa recta and collecting duct; , descending vasa recta and thin ascending limb.

From Imai et al. 107


Figure 18.

Generation of chloride and osmotic gradients in ascending thin limb in vitro by passive diffusion of solutes. Perfusate in each case is ultrafiltrate plus 300 mOsm/kg sodium chloride. Bath is either identical to perfusate (open circles) or isosmolar serum to which 300 mOsm/kg urea was added (closed circles). Left: plot of collected‐to‐perfusate (C/P) chloride concentration; right: collected‐to‐perfusate osmolality ratio. Lines connecting two points indicate two tubules studied at two different lengths.

From Imai and Kokko 113


Figure 19.

Sodium concentration in Henle's loop fluid (Na+LH) plotted against sodium concentration in vasa recta plasma (Na+VR) in rat. Open circles, paired samples in which descending vasa recta plasma was obtained; solid squares, paired samples in which ascending vasa recta plasma was collected. Line is line of identity.

From Johnston et al. 137


Figure 20.

Cellular model of mechanism of sodium chloride reabsorption in cortical thick ascending limb of Henle's loop and in early distal convoluted tubule. •, Na+K+ pump; ○, carrier systems; arrows, diffusive pathways. Bottom of figure gives estimates of voltages across cell membranes. With symmetrical Ringer's‐type solutions, transepithelial voltage in cortical thick ascending limb is +5 to +10 mV. Note that paracellular shunt pathway is sodium conductive.

From Greger and Velázquez 88


Figure 21.

A. The sum of the concentrations of two methylamines, betaine and glycerophosphorylcholine (GPC), and two polyols, sorbitol and myoinositol, in slices taken from renal cortex, and outer and inner renal medulla of the rabbit, plotted as function of sodium concentration in the corresponding section of the kidney. Open circles and dashed line depict results from animals in a diuretic state (mean urinary osmolality, 422 mOsm/kg water). Closed circles illustrate results from antidiuretic animals (mean osmolality, 1,285 mOsm/kg water). Vertical bars depict ± S.E. for total osmolyte determinations. Each point represents data for one kidney section. There is a strong correlation under both conditions. B. The concentration of GPC alone, from the same experiments, plotted as a function of the concentration of urea in the corresponding section of the kidney. Of all the osmolytes, only GPC showed a good correlation with urea and the same slopes of the regression lines (not shown) in both diuresis and antidiuresis.

From Yancey and Burg 313


Figure 22.

Principle of countercurrent exchanger. A source supplying heat (A, B) at 100 cal/min raises temperature of 10 ml/min of water flow 10°C in both A (straight flow) and B (countercurrent flow). However, because incoming water is heated by outgoing water in B, maximum temperature attained in countercurrent system is considerably higher than with straight flow. Graph in B compares temperature along flow tubes in each system. C: countercurrent flow as applied to vas rectum, capillary loop in medulla, showing that it is not necessary for limbs of loop to be in direct contact. In hypothetical illustration given, both limbs are in contact with same interstitial fluid, of progressively increasing concentration. Sodium salts (arrows) at first enter capillary blood and later partly return to interstitial fluid; final concentration of blood leaving area and interstitial fluid depends on various factors. Note analogy between B, in which heat is recirculated, and C, in which sodium salts are similarly retained in area.

From Berliner et al. 23


Figure 23.

Urine‐to‐plasma osmolality ratio (U/Posm) as a function of medullary blood flow.

From Thurau et al. 283


Figure 24.

Dual function of vasa recta. Medullary circulation consists of network of channels with main thoroughfares (vasa recta) and branch connections. Encircled Pr denotes plasma protein. Type size indicates relative concentration of each solute with respect to its location in medulla, but not necessarily with respect to concentrations of other solutes. Progressive rise in sodium chloride and urea concentrations in medullary interstitium is caused by reabsorption from Henle's loop and collecting tubule, respectively. Because capillaries are permeable to sodium chloride and urea, these solutes enter descending vasa recta and leave ascending vasa recta; such transcapillary exchange “traps” these solutes in medulla. Conversely, water is extracted from descending vasa recta, causing plasma protein concentration to increase. In ascending vasa recta, sum of oncotic (that caused by plasma protein) and osmotic (that caused by nonprotein solutes) transcapillary pressures results in capillary fluid uptake. In this way, water reabsorbed from collecting tubule and Henle's descending limb is removed from medulla and returned to general circulation. Vasa recta function in dual capacity, trapping solute and removing water, to preserve hyperosmolality of renal medulla.

From Jamison and Kriz 130


Figure 25.

Casts of pelvic cavities from rat (A) and sand rat (B, C). Microfil was injected into ureter in situ, after ligation of renal artery and vein. After Microfil solidified, kidney tissue was removed carefully and casts prepared for scanning electron microscopy. A, B: side view. Earliest part of ureter is cone shaped and shows gap (thin arrows) corresponding to location of papilla that was pressed against pelvic wall by Microfil. C: same cast as in B viewed from above as indicated by thick arrow in B. Papillary tip and ureter would be behind picture. Wide horizontal groove corresponds to penetration of inner medulla into pelvic space. Note well‐developed lateral pelvic fornices of Psammomys kidney. On these casts, borders of pelvic extensions in both species appear smooth and rounded, because cavities were distended by Microfil. They are sharper and thinner in unfilled kidney but possibly take this dilated shape in vivo during urine reflux caused by pelvic contraction.

From Bankir and de Rouffignac 130
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Article Title: Urinary Concentration and Dilution: Physiology
 

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Rex L. Jamison, James J. Gehrig. Urinary Concentration and Dilution: Physiology. Compr Physiol 2011, Supplement 25: Handbook of Physiology, Renal Physiology: 1219-1279. First published in print 1992. doi: 10.1002/cphy.cp080227