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Urinalysis and Other Body FluidsLesson 1 of 13

Nephron Structure and Urine Formation

  • 3 min
  • 2 sections
  • 3 self-checks

Written and reviewed by Alexan Berberian, MS, MLS(ASCP)

Albumin, dysmorphic red cells, and red blood cell (RBC) casts in a urinalysis involve a different part of the nephron than urine that stays dilute when the body needs to conserve water. Both patterns relate to 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 gives 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 region Main function Urinalysis consequence of dysfunction
Glomerulus Filters water and small solutes while retaining cells and most plasma protein Albuminuria, hematuria, dysmorphic red blood cells (RBCs), and RBC casts support glomerular barrier injury
Proximal tubule Reabsorbs most filtered water and sodium, glucose, amino acids, bicarbonate, phosphate, and low-molecular-weight proteins; secretes organic acids, bases, and some drugs Normoglycemic glycosuria, aminoaciduria, phosphate or bicarbonate wasting, low-molecular-weight proteinuria, renal tubular epithelial cells, and tubular cell casts support proximal tubular dysfunction
Loop of Henle Establishes the corticomedullary osmotic gradient through countercurrent flow, NaCl transport, and urea recycling Loss of concentrating ability produces dilute urine despite a physiologic need to conserve water
Distal tubule and collecting duct Adjusts sodium, potassium, water, hydrogen-ion, and ammonium excretion under hormonal control Abnormal urine concentration or acidification can accompany vasopressin disorders, aldosterone disorders, and renal tubular acidosis
From filtration to urineA nephron crosses from cortex into medulla. The glomerulus is the capillary tuft enclosed by Bowman capsule; individual filtration-barrier layers are not shown. Blood enters and leaves the glomerulus through separate afferent and efferent arterioles. Filtrate enters Bowman capsule and follows the proximal tubule, descending limb, ascending limb, distal tubule, and collecting duct. White arrowheads show tubular flow. Water leaves the descending limb; the thick ascending limb reabsorbs sodium, potassium, and chloride. Vasopressin increases water uptake from the collecting duct. Tubule colors identify the segments; fluid composition is not color-coded.From filtration to urineTubular flowBlood flowCortexMedullaAfferentEfferentProximal tubuleBulk reabsorptionDistal tubuleFine adjustmentGlomerulusBowman capsuleDescending limbWater leavesThickascending limbNa, K, Cl leaveCollecting ductWater uptakewith vasopressinLoop of HenleUrine
A schematic nephron with segment-specific reabsorption, not to scale. The glomerulus is the capillary tuft inside Bowman capsule. Filtrate enters the capsular space before passing into the proximal tubule; individual filtration-barrier layers are not drawn. 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.

When you review a urinalysis, match each abnormal finding to the nephron segment that produces it. Findings that point to the same segment support each other, and a finding that fits no segment is a reason to recheck the specimen or the method.

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

Practice

Check yourself 1 of 3

Which nephron region forms the initial filtrate and normally keeps cells and most plasma protein out of it?

Correct. The glomerular barrier produces the filtrate and holds back cells and most plasma protein. Injury lets albumin and RBCs through, with dysmorphic RBCs and RBC casts in the urine.

Incorrect. The proximal tubule modifies the filtrate after it forms, reclaiming most filtered water, sodium, glucose, amino acids, and phosphate.

Incorrect. The collecting duct makes the final hormone-controlled adjustments to water, sodium, potassium, and acid excretion.

Review this section

Check yourself 2 of 3

Glycosuria with normal blood glucose, aminoaciduria, and phosphate wasting suggest impaired function in which segment?

Incorrect. Glomerular injury lets protein and RBCs into the filtrate. Loss of normally reclaimed small solutes with normal blood glucose indicates a defect in tubular reabsorption.

Correct. The proximal tubule normally reclaims filtered glucose, amino acids, phosphate, and bicarbonate, so its dysfunction lets them escape into urine.

Incorrect. The collecting duct adjusts water and electrolyte excretion. Glucose, amino acids, and phosphate are reclaimed earlier, in the proximal tubule.

Review this section

Check yourself 3 of 3

Which region builds the corticomedullary osmotic gradient through countercurrent flow and salt transport?

Incorrect. The proximal tubule reclaims most filtered water and sodium. Countercurrent flow in the loop of Henle builds the gradient.

Correct. Countercurrent flow in the loop, with NaCl transport and urea recycling, builds the gradient that concentrating ability depends on. Losing it produces dilute urine even when the body needs to conserve water.

Incorrect. The collecting duct uses the medullary gradient to concentrate urine under vasopressin. The loop of Henle builds it.

Review this section

References
  1. National Institute of Diabetes and Digestive and Kidney Diseases. Your Kidneys & How They Work. Accessed August 29, 2026. Back to text
  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. Back to text

Urinalysis and Other Body Fluids, lesson 1 of 13

End of Nephron Structure and Urine Formation

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https://latticemls.org/read/urinalysis-and-body-fluids/nephron-structure-and-urine-formation/ · Sources checked August 29, 2026 · © Berberian Labs LLC · CC BY-NC-SA 4.0