Page 38 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Applied Renal Physiology 27

Connecting segment and Glomerulus Proximal tubule
cortical collecting duct
Filtration Isosmotic reabsorption of 65-70%
of filtered water and NaCl
Aldosterone-mediated
potassium secretion by
principal cells Reabsorption of 90%
−
of filtered HCO 3
+
H ion secretion by Major site of NH production
α-intercalated cells 3
Reabsorption of almost all
Potassium reabsorption
by α-intercalated cells filtered glucose and amino acids
Reabsorption of potassium,
ADH-mediated water phosphate, calcium, magnesium,
reabsorption
urea, and urate
Secretion of organic anions
and cations
Loop of Henle
Medullary collecting duct Countercurrent multiplier
Potassium reabsorption Reabsorption of 15-25% of
or secretion filtered NaCl
Final NaCl reabsorption Active regulation of
magnesium excretion
ADH-mediated water Distal tubule
and urea reabsorption
Small amount of
+
H ion and NH NaCl reabsorbed
3
secretion
Active regulation of
calcium excretion
Figure 2-1 Major functions of each portion of the nephron. (Drawing by Tim Vojt.)
the glomerular basement membrane and glomerular molecular mass of 5200 daltons and radius of 1.4 nm,
capillaries on the urinary side of the barrier with their pri- permeates freely, whereas serum albumin, with a molecu-
mary and interdigitating secondary foot processes. Filtra- lar mass of 69,000 daltons and radius of 3.6 nm,
tion slits, 10 to 30 nm in width, are located between the permeates minimally.
secondary foot processes. The podocytes are phagocytic The charge selectivity of the glomerulus resides in the
and may engulf macromolecules trapped by the filtration negatively charged sialoglycoproteins (e.g., laminin and
slits. They are invested with a negatively charged sialogly- fibronectin) and peptidoglycans (e.g., heparan sulfate)
coprotein coat that contributes to the charge selectivity of of the capillary endothelium, lamina rara interna, lamina
the filtration barrier. It is believed that the visceral epithe- rara externa, and visceral epithelium. At any given effec-
lial cells synthesize the glomerular basement membrane. tive molecular radius, negatively charged macromolecules
Themesangiumisnotapartofthefiltrationbarrierbuta experience greater restriction to filtration than neutral
stabilizingcoreoftissue, formingananchor for the glomer- ones. Positively charged macromolecules experience less
ulus at the vascular pole and along the axes of the capillary restriction to filtration than neutral ones of the same
lobules. The mesangial cells are in contact with the base- size (Fig. 2-3).
ment membrane in areas where there is no capillary endo-
thelium. The extraglomerular mesangium fills the space DETERMINANTS OF GLOMERULAR
between the macula densa and the glomerular arterioles FILTRATION
and constitutes part of the juxtaglomerular apparatus The term glomerular filtration rate refers to the total fil-
(JGA). The mesangial cells contain microfilaments and tration rate of both kidneys and represents the sum of the
can contract in response to specific hormones (e.g., angio- single-nephron glomerular filtration rates (SNGFRs)
tensin II), thus altering the surface area available for of all nephrons. The number of nephrons per kidney
filtration. They also synthesize prostaglandins that contrib- reflects the size of the animal. The feline kidney has
ute to renal vasodilatation. The mesangium also contains approximately 200,000 nephrons, the canine kidney
macrophages that can clear filtration residues from the approximately 400,000, and the human kidney approxi-
mesangial space by phagocytosis. mately 1,200,000 nephrons. SNGFR may differ among
The glomerular capillary wall is a size-selective and a some groups of nephrons under normal conditions,
charge-selective barrier to filtration. Its size selectivity and additional changes may occur in response to such
resides primarily in the lamina densa of the glomerular factors as water deprivation, increased water intake,
basement membrane. The glomerulus generally excludes increased salt intake, or increased protein intake. Superfi-
molecules with radii greater than 4 nm. Inulin, with a cial cortical nephrons have short loops of Henle with little