Urine formation is a complex process that involves several steps to filter blood, remove waste products, and maintain the body's fluid and electrolyte balance.
The primary site of urine formation is the nephron, which is the functional unit of the kidney.
There are three main steps in urine formation: glomerular filtration, tubular reabsorption, and tubular secretion.
Steps in Urine Formation
1) Glomerular filtration
Location: Glomerulus
Process: Blood is filtered, and water, electrolytes, and small molecules (e.g., glucose, urea) are forced into the Bowman’s capsule, forming the glomerular filtrate.
2) Tubular reabsorption
Location: Renal tubules (PCT, Loop of Henle, DCT)
Process: Essential substances and water are reabsorbed back into the bloodstream.
Details:
PCT: Reabsorbs most water, glucose, amino acids, and ions.
Loop of Henle: Water reabsorbed in descending limb, sodium, and chloride in ascending limb.
DCT: Reabsorbs additional ions and water, regulated by hormones like aldosterone and ADH.
3) Tubular secretion
Location: PCT and DCT
Process: Waste products and excess ions are actively transported from the blood into the renal tubule to become part of the urine.
4) Urine concentration and volume adjustment
Location: Collecting ducts
Process: Final concentration and volume of urine are adjusted under the influence of ADH, determining how much water is reabsorbed.
5) Urine transport and elimination
Pathway: Urine moves from the collecting ducts → renal calyces → renal pelvis → ureter → urinary bladder → urethra, where it is eliminated during urination.
