Science · Regulation & defense
Urinary System and Fluid Balance
Estimated time includes reading and one quiz. Take the time you need.
Start here: key ideas
- Kidneys filter blood and adjust water, ions, wastes, and pH.
- Filtration enters the nephron; reabsorption returns useful material to blood.
- Secretion moves selected material from blood into the tubule.
- ADH conserves water; aldosterone increases sodium reabsorption.
What you’ll be able to do
- Trace blood, filtrate, and urine routes.
- Compare filtration, reabsorption, secretion, and excretion.
- Explain hormone, electrolyte, and pH control.
Follow fluid through a nephron and ask what moves into blood or toward urine.
Optional review
If you’d like to review the basics, visit Cardiovascular System. You can start this lesson without completing those first.
Blood route, filtrate route, urine route
Renal arteries bring blood to the kidneys. In each nephron, an afferent arteriole enters the glomerulus. Fluid filters into Bowman’s capsule. It then moves through the proximal tubule, loop of Henle, distal tubule, and collecting duct. Urine flows to the renal pelvis, ureter, bladder, and urethra. The ureter carries urine to the bladder; the urethra carries it out.
Blood leaves the glomerulus through the efferent arteriole, then enters capillaries around the tubule. This close blood supply receives reabsorbed material and provides substances for secretion. The glomerular route and tubular route run beside each other but carry different fluids.
Blood and filtrate follow different routes
Follow two routes. Blood enters and leaves the filtering capillaries, called the glomerulus. Filtered fluid enters the capsule and moves along the kidney tubule. Nearby blood vessels exchange substances with that fluid. Fluid in the tubule is not whole blood.
Blood and filtrate follow different routes — OpenStax College, Anatomy & Physiology (2014 CC BY archive). CC BY 3.0. Original source image reproduced unchanged; explanatory caption added.
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