The Glomerulus Filters the Blood
The first step in renal function is filtration of the blood. Filtration takes place in the glomerulus, which is a network of
FIGURE 41-1 Schematic illustration of ju?tamedullary and superficial nephrons, listing functions of the segments of nephron and collecting duct.The glomerulus of a juxtamedullary nephron is located deep in the cortex near the Corticomedullary junction.
The thin limb of Henle's loop extends deep into the inner medulla. The glomerulus of a superficial nephron is located in the outer cortex, and Henle's loop extends only into the outer medulla. Arrows indicate the direction of tubule fluid flow. Segments are numbered in sequential order of modification of the tubule fluid, beginning with the glomerulus. {Modified from Madsen KM: Anatomy of the kidney. In Tisher CCt Wilcox CSt editors: Nephroloqy for the house officer, Baltimore, 1989, Williams & Wilkins.)capillaries that retains cellular components and medium- to high-molecular-weight proteins within the vasculature while extruding a fluid nearly identical to plasma in its electrolyte and water composition. This fluid is {he glomerular filtrate; the process of its formation is glomerular filtration.
The rate of glomerular filtration is a renal function parameter that is frequently evaluated in clinical practice. The glomerular filtration rate (GFR) is expressed as milliliters of glomerular filtrate formed per minute per kilogram of body weight (mL/min/kg). To understand GFR, it may help to think of these numbers in more tangible terms. An average-size beagle of 10 kg body weight with a typical GFR of 3.7 mL/min/kg would produce approximately 37 ml. of glomerular filtrate per minute, or 53.3 L (-14 gallons) of glomerular filtrate per day, almost 27 times the beagle’s extracellular fluid volume.