The Ability to Concentrate Urine Osmotically Is an Important Adaptation to Life on Land

When the kidneys form osmotically concentrated urine, they save water for the body. The kidneys have the task of getting rid of excess solutes (e.g., urea, various salts), which requires the excretion of solvent (water). Suppose, for example, we excrete 600 mOsm of solutes per day. If we were only capable of excreting urine that is isosmotic to plasma (approximately 300 mOsm/kg H2O), we would need to excrete 2.0 L H2O/day. If we can excrete the solutes in urine that is 4 times more concentrated than plasma (1,200 mOsm/kg H2O), only 0.5 L H2O/day would be required. By excreting solutes in osmotically concentrated urine, the kidneys, in effect, saved 2.0 — 0.5 = 1.5 L H2O for the

Tubular urine

Aquaporin-2

Collecting duct epithelium

./Vesicle with Çj aquaporin-2

Nucleus (f gene transcription)

t aquaporin-2 synthesis

./Vesicle with Çj aquaporin-2

Skeletal Muscle Cell Model

V2 receptor j¡— Adenylyl cyclase

Blood

V2 receptor j¡— Adenylyl cyclase

A model for the action of AVP on the epithelium of the collecting duct. The second messenger for AVP is cyclic AMP (cAMP). AVP has both prompt effects on luminal membrane water permeability (the movement of aquaporin-2-containing vesicles to the luminal cell membrane) and delayed effects (increased aquaporin-2 synthesis).

2 genes and produces an increase in the total number of aquaporin-2 molecules per cell.

The Loops of Henle Are Countercurrent Multipliers, and the Vasa Recta Are Countercurrent Exchangers

It has been known for longer than 50 years that there is a gradient of osmolality in the kidney medulla, with the highest osmolality present at the tips of the renal papillae. This gradient is explained by the countercurrent hypothesis. Two countercurrent processes occur in the kidney medulla—countercurrent multiplication and countercurrent exchange. The term countercurrent indicates a flow of fluid in opposite directions in adjacent structures (Fig. 23.23). The loops of Henle are countercurrent multipliers. Fluid flows toward the tip of the papilla along the descending limb of the loop and toward the cortex along the ascending limb of the loop. The loops of Henle set up the osmotic gradient in the medulla. Establishing a gradient requires work; the energy source is metabolism, which powers the active transport of Na+ out of the thick ascending limb. The vasa recta are countercurrent exchangers. Blood flows in opposite directions along juxtaposed descending (arterial) and ascending (venous) vasa recta, and solutes and water are exchanged passively between these capillary blood vessels. The vasa

Vasa recta

Loop of Henle

Collecting duct

Collecting duct

Inner medulla

Elements of the urinary concentrating mech-

'anism. The vasa recta are countercurrent exchangers, the loops of Henle are countercurrent multipliers, and the collecting ducts are osmotic equilibrating devices. Note that most loops of Henle and vasa recta do not reach the tip of the papilla, but turn at higher levels in the outer and inner medulla. Also, there are no thick ascending limbs in the inner medulla.

recta help maintain the gradient in the medulla. The collecting ducts act as osmotic equilibrating devices; depending on the plasma level of AVP, the collecting duct urine is allowed to equilibrate more or less with the hyperosmotic medullary interstitial fluid.

Countercurrent multiplication is the process in which a small gradient established at any level of the loop of Henle is increased (multiplied) into a much larger gradient along the axis of the loop. The osmotic gradient established at any level is called the single effect. The single effect involves movement of solute out of the water-impermeable ascending limb, solute deposition in the medullary interstitial fluid, and withdrawal of water from the descending limb. Because the fluid entering the next, deeper level of the loop is now more concentrated, repetition of the same process leads to an axial gradient of osmolality along the loop. The extent to which coun-tercurrent multiplication can establish a large gradient along the axis of the loop depends on several factors, including the magnitude of the single effect, the rate of fluid flow, and the length of the loop of Henle. The larger the single effect, the larger the axial gradient. Impaired solute removal, as from the inhibition of active transport by thick ascending limb cells, leads to a reduced axial gradient. If flow rate through the loop is too high, not enough time is allowed for establishing a significant single effect, and consequently, the axial gradient is reduced. Finally, if the loops are long, there is more opportunity for multiplication and a larger axial gradient can be established.

Countercurrent exchange is a common process in the vascular system. In many vascular beds, arterial and venous vessels lie close to each other, and exchanges of heat or materials can occur between these vessels. For example, because of the countercurrent exchange of heat between blood flowing toward and away from its feet, a penguin can stand on ice and yet maintain a warm body (core) temperature. Countercurrent exchange between descending and ascending vasa recta in the kidney reduces dissipation of the solute gradient in the medulla. The descending vasa recta tend to give up water to the more concentrated interstitial fluid; this water is taken up by the ascending vasa recta, which come from more concentrated regions of the medulla. In effect, much of the water in the blood short-circuits across the tops of the vasa recta and does not flow deep into the medulla, where it would tend to dilute the accumulated solute. The ascending vasa recta tend to give up solute as the blood moves toward the cortex. Solute enters the descending vasa recta and, therefore, tends to be trapped in the medulla. Countercurrent exchange is a purely passive process; it helps maintain a gradient established by some other means.

Operation of the Urinary Concentrating Mechanism Requires an Integrated Functioning of the Loops of Henle, Vasa Recta, and Collecting Ducts

Figure 23.24 summarizes the mechanisms involved in producing osmotically concentrated urine. Maximally concentrated urine, with an osmolality of 1,200 mOsm/kg H2O and a low urine volume (1% of the original filtered water), is being excreted.

Water Conservation Kidney Diagram

Osmotically concentrated urine. This diagram summarizes movements of ions, urea, and water in the kidney during production of maximally concentrated urine (1,200 mOsm/kg H2O). Numbers in ovals represent osmolality in mOsm/kg H2O. Numbers in boxes represent relative amounts of water present at each level of the nephron. Solid arrows indicate active transport; dashed arrows indicate passive transport. The heavy outlining along the ascending limb of the loop of Henle indicates relative water-impermeability.

Osmotically concentrated urine. This diagram summarizes movements of ions, urea, and water in the kidney during production of maximally concentrated urine (1,200 mOsm/kg H2O). Numbers in ovals represent osmolality in mOsm/kg H2O. Numbers in boxes represent relative amounts of water present at each level of the nephron. Solid arrows indicate active transport; dashed arrows indicate passive transport. The heavy outlining along the ascending limb of the loop of Henle indicates relative water-impermeability.

About 70% of filtered water is reabsorbed along the proximal convoluted tubule, so 30% of the original filtered volume enters the loop of Henle. As discussed earlier, proximal reabsorption of water is essentially an isosmotic process, so fluid entering the loop is isosmotic. As the fluid moves along the descending limb of the loop Henle in the medulla, it becomes increasingly concentrated. This rise in osmolality, in principle, could be due to one of two processes:

1) The movement of water out of the descending limb because of the hyperosmolality of the medullary interstitial fluid.

2) The entry of solute from the medullary interstitial fluid.

The relative importance of these processes may depend on the species of animal. For most efficient operation of the concentrating mechanism, water removal should be predominant, so only this process is depicted in Figure 23.24. The removal of water along the descending limb leads to a rise in [NaCl] in the loop fluid to a value higher than in the interstitial fluid.

When the fluid enters the ascending limb, it enters water-impermeable segments. NaCl is transported out of the ascending limb and deposited in the medullary interstitial fluid. In the thick ascending limb, Na+ transport is active and is powered by a vigorous Na+/K+ -ATPase. In the thin ascending limb, NaCl reabsorption appears to be mainly passive. It occurs because the [NaCl] in the tubular fluid is higher than in the interstitial fluid and because the passive permeability of the thin ascending limb to Na+ is high. There is also some evidence for a weak active Na+ pump in the thin ascending limb. The net addition of solute to the medulla by the loops is essential for the osmotic concentration of urine in the collecting ducts.

Fluid entering the distal convoluted tubule is hypoos-motic compared to plasma (see Fig. 23.24) because of the removal of solute along the ascending limb. In the presence of AVP, the cortical collecting ducts become water-permeable and water is passively reabsorbed into the cortical interstitial fluid. The high blood flow to the cortex rapidly carries away this water, so there is no detectable dilution of cortical tissue osmolality. Before the tubular fluid reenters the medulla, it is isosmotic and reduced to about 5% of the original filtered volume. The reabsorption of water in the cortical collecting ducts is important for the overall operation of the urinary concentrating mechanism. If this water were not reabsorbed in the cortex, an excessive amount would enter the medulla. It would tend to wash out the gra-

Urine Concentrating Mechanism

Mass balance considerations for the medulla as a whole. In the steady state, the inputs of water and solutes must equal their respective outputs. Water input into the medulla from the cortex (100 + 36 + 6 = 142 mL/min) equals water output from the medulla (117 + 24 + 1 = 142 mL/min). Solute input (28.5 + 10.3 + 1.7 = 40.5 mOsm/min) is likewise equal to solute output (36.9 + 2.4 + 1.2 = 40.5 mOsm/min).

dient in the medulla, leading to an impaired ability to concentrate the urine maximally.

All nephrons drain into collecting ducts that pass through the medulla. In the presence of AVP, the medullary collecting ducts are permeable to water. Water moves out of the collecting ducts into the more concentrated interstitial fluid. In high levels of AVP, the fluid equilibrates with the interstitial fluid, and the final urine becomes as concentrated as the tissue fluid at the tip of the papilla.

Many different models for the countercurrent mechanism have been proposed; each must take into account the principle of conservation of matter (mass balance). In the steady state, the inputs of water and every nonmetabolized solute must equal their respective outputs. This principle must be obeyed at every level of the medulla. Figure 23.25 presents a simplified scheme that applies the mass balance principle to the medulla as a whole. It provides some additional insight into the countercurrent mechanism. Notice that fluids entering the medulla (from the proximal tubule, descending vasa recta, and cortical collecting ducts) are isosmotic; they all have an osmolality of about 285 mOsm/kg H2O. Fluid leaving the medulla in the urine is hyperosmotic. It follows from mass balance considerations that somewhere a hypoosmotic fluid has to leave the medulla; this occurs in the ascending limb of the loop of Henle.

The input of water into the medulla must equal its output. Because water is added to the medulla along the descending limbs of the loops of Henle and the collecting ducts, this water must be removed at an equal rate. The ascending limbs of the loops of Henle cannot remove the added water, since they are water-impermeable. The water is removed by the vasa recta; this is why ascending exceeds descending vasa recta blood flow (see Fig. 23.25). The blood leaving the medulla is hyperosmotic because it drains a region of high osmolality and does not instantaneously equilibrate with the medullary interstitial fluid.

Urea Movement Along Nephron

i Movements of urea along the nephron. The

"numbers indicate relative amounts (100 = filtered urea), not concentrations. The heavy outline from the thick ascending limb to the outer medullary collecting duct indicates relatively urea-impermeable segments. Urea is added to the inner medulla by its collecting ducts; most of this urea reenters the loop of Henle, and some is removed by the vasa recta.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • kelsie
    How kidney concentrates urine?
    7 years ago
  • gabriel
    Which structure secretes urea helping produce high osmotic conditions deep in the medulla in nephron?
    7 years ago
  • asphodel sackville-baggins
    How the kidney produces urine?
    7 years ago
  • Annikki
    Is some salt removed by the vasa recta?
    7 years ago
  • aila
    How does the loop of henle and the collecting duct concentrate the urea in urine?
    7 years ago
  • lavinia
    Are humans capable of producing urine that is 4 time more concentrated than blood plasma?
    7 years ago
  • brendan
    Are humans capable of producing urine that is a 4 times more concentrated than blood plasma?
    7 years ago
  • Nora
    Why is it important to concentrate urine?
    7 years ago
  • ren
    Where does water reabsorption occur in the nephron?
    7 years ago
  • jessica custodio
    Can human urine be four times more concentrated than blood plasma?
    7 years ago
  • sam
    Where does urea leave in nephron?
    7 years ago
  • burtuka
    Is the ability to excrete an hyperosmotic urine a passive or active process?
    6 years ago
  • baldo
    Why it is necessary excrete concentrated urine?
    8 months ago

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