Urinalysis and Other Body Fluids

Urine Formation

Nephron Structure and Urine Formation

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Every urine result reflects three nephron processes. The glomerulus filters plasma, the tubules return needed water and solutes to blood, and tubular cells add selected substances to the forming urine. Disease changes one or more of these processes. The resulting combination of protein, blood, cells, casts, crystals, concentration, and excretion provides a laboratory pattern that guides further testing.

From nephron to urine

Each kidney contains about one million nephrons. Blood enters a glomerulus through an afferent arteriole and leaves through an efferent arteriole. The efferent vessel supplies peritubular capillaries around cortical tubules and the vasa recta beside long loops of Henle. The kidneys normally receive about 20% to 25% of cardiac output, with renal blood flow near 1.0 to 1.2 L/min and renal plasma flow near 600 to 700 mL/min. These are physiologic approximations that vary with body size and hemodynamic state.1,2

Filtrate enters Bowman capsule, then travels through the proximal tubule, descending and ascending limbs of the loop of Henle, distal tubule, collecting duct, renal pelvis, and ureter.

Each nephron segment has a distinct role in urine formation:

Nephron regionMain functionUrinalysis consequence of dysfunction
GlomerulusFilters water and small solutes while retaining cells and most plasma proteinAlbuminuria, hematuria, dysmorphic red blood cells (RBCs), and RBC casts support glomerular barrier injury
Proximal tubuleReabsorbs most filtered water and sodium, glucose, amino acids, bicarbonate, phosphate, and low-molecular-weight proteins; secretes organic acids, bases, and some drugsNormoglycemic glycosuria, aminoaciduria, phosphate or bicarbonate wasting, low-molecular-weight proteinuria, renal tubular epithelial cells, and tubular cell casts support proximal tubular dysfunction
Loop of HenleEstablishes the corticomedullary osmotic gradient through countercurrent flow, NaCl transport, and urea recyclingLoss of concentrating ability produces dilute urine despite a physiologic need to conserve water
Distal tubule and collecting ductAdjust sodium, potassium, water, hydrogen-ion, and ammonium excretion under hormonal controlAbnormal urine concentration or acidification can accompany vasopressin disorders, aldosterone disorders, and renal tubular acidosis
Blood enters and leaves the glomerulus through separate arterioles. White arrows follow filtrate from Bowman capsule through the proximal tubule, down and up the loop of Henle, through the distal tubule, and into the collecting duct. The loop extends into the medulla. Water leaves its descending limb; sodium, potassium, and chloride leave its thick ascending limb.
A schematic nephron with segment-specific reabsorption. The efferent arteriole supplies capillaries around the tubules; that vascular network is omitted to keep the tubular path clear. The table above connects segment dysfunction with urine findings.

The later renal-function lesson develops filtration control, concentrating mechanisms, function tests, and disease patterns.

References
  1. National Institute of Diabetes and Digestive and Kidney Diseases. Your Kidneys & How They Work. Accessed August 29, 2026.
  2. Strasinger SK, Di Lorenzo MS. Urinalysis and Body Fluids. 7th ed. F.A. Davis; 2021. ISBN 978-0-8036-7582-7. F.A. Davis catalog record.