Page 49 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

38 APPLIED PHYSIOLOGY

basis for countercurrent multiplication and allows a single given level. This is the countercurrent multiplier concept
osmotic effect to be multiplied over the length of the of urinary concentration.
loop. The vessels accompanying the loops of Henle into
the medulla are called vasa recta. They prevent dissipation ROLES OF THE COLLECTING DUCTS
of the medullary osmotic gradient by a process called coun- AND ANTIDIURETIC HORMONE
tercurrent exchange (see Role of the Vasa Recta section). The collecting duct is divided into three segments: the
The countercurrent multiplier concept was first applied cortical collecting duct, outer medullary collecting duct,
to urine concentration by W. Kuhn, a physical chemist, and inner medullary collecting duct. These segments dif-
in 1942. 2,25 As early as 1909, however, K. Peter had noted fer in their permeability to sodium and urea (see
a correlation between the length of Henle’s loop and Table 2-2). The main role of the cortical collecting duct
the ability of a given species to concentrate its urine. is delivery of fluid with a very high urea concentration to
the outer medullary collecting duct. This occurs because
ROLE OF THE ASCENDING LIMB OF sodium chloride and water are removed from this seg-
HENLE’S LOOP ment of the nephron, but urea is not. The main functions
The ascending limb of Henle’s loop is impermeable to of the inner medullary collecting duct are to add urea to
water. Sodium chloride is actively transported from the the inner medullary interstitium and to produce maxi-
thick portion of the ascending limb without mally concentrated urine by osmotic equilibration of
accompanying water so that an osmotic gradient of tubular fluid with the hyperosmotic interstitium under
approximately 200 mOsm/kg is generated. This active the influence of ADH. 9,29 This segment of the nephron
transport of sodium chloride is the primary energy- is permeable to urea, and its urea permeability is increased
requiring step of the urinary concentrating mechanism. by ADH (see previous section on urea).
Active sodium transport is accomplished by the Na , As just described, fluid entering the distal tubule is
þ
K -ATPase located in the basolateral membranes of the hypoosmotic to plasma (approximately 100 mOsm/kg).
þ
tubular cells. This enzyme maintains a low intracellular Without the collecting duct, the so-called countercurrent
concentration of sodium and promotes passive entry of multiplier would dilute tubular fluid. In the presence of
sodium at the luminal membrane down a concentration ADH, this hypoosmotic fluid equilibrates osmotically
-
gradient. The luminal Na ,K , 2Cl carrier (NKCC2) with the cortical interstitium (osmolality, 300 mOsm/
þ
þ
binds one sodium ion, one potassium ion, and two chlo- kg) as the tubular fluid flows through the cortical
ride ions. 37 Chloride delivery is the rate-limiting step in collecting duct. By this process, approximately two thirds
this transport process, and loop diuretics such as furose- of the tubular water is removed before delivery to the
mide impair distal sodium reabsorption by competing medullary collecting duct. For example, 100 mOsm of
with chloride for the luminal carrier. 37 solute in 1 L of tubular fluid is reduced to 100 mOsm
Fluid reaching the distal convoluted tubule is of solute in 0.33 L of tubular fluid (300 mOsm/kg) with
hypoosmotic (100 mOsm/kg) compared with the fluid 0.67 L of water reabsorbed. Even more water can be
entering the descending limb of Henle’s loop (300 reabsorbed, depending on how much active sodium reab-
mOsm/kg). If fluid in the loops were stationary, the sorption occurs in the cortical collecting duct in response
active transport of sodium chloride out of the thick to aldosterone stimulation. These effects markedly reduce
ascending limb without water would increase the intersti- fluid delivery to the medullary collecting duct. Tubular
tial osmolality to 400 mOsm/kg and decrease the osmo- fluid entering the medullary collecting duct is thus isos-
lality of the fluid within the ascending limb to 200 motic with plasma but much reduced in volume. It is in
mOsm/kg. The descending limb of Henle’s loop is the medullary collecting duct that the final concentration
highly permeable to water, and water would be extracted of urine occurs.
from this site, increasing the osmolality of the tubular The water permeability of the epithelium of the
fluid in this segment of the nephron to 400 mOsm/kg. collecting duct is dependent on the action of ADH. In
However, the fluid within Henle’s loops is not the presence of ADH, water is removed from the
stationary. New tubular fluid with an osmolality of 300 collecting duct as the fluid osmotically equilibrates with
mOsm/kg is constantly entering the descending limb a progressively hyperosmotic medullary interstitium,
of Henle’s loop from the proximal tubule. As fluid and the final osmolality of the urine may approximate that
continues to move through the loops and an osmotic gra- of the papillary interstitium. In humans, this maximal
dient of 200 mOsm/kg is generated, this single osmotic urine osmolality is 900 to 1400 mOsm/kg. 44 In dogs
effect is multiplied over the length of Henle’s loop and cats, however, urine osmolality can approach 2800
(Fig. 2-12). The magnitude of the gradient from the and 3000 mOsm/kg, respectively. 24,45 Water reabsorp-
beginning of the loop to its hairpin turn is a function tion in the distal convoluted tubule and connecting
of the length of the loop itself. Thus, the vertical osmotic tubule is minimal because of their relative impermeability
gradient greatly exceeds the horizontal gradient at any to water, regardless of the presence or absence of ADH.