Anatomy and Physiology 12 · Biology · Lesson 11.2
Urine formation
- Level
- Grade 12
- Reading time
- 15 min reading
- Review
- Last reviewed 2026-09-03
- Standard
- Accuracy standard
- Errata
Abstract
Filtration, reabsorption, secretion.
How is urine formed in the kidney?
Short answer
Urine forms in the nephron through three steps. Filtration forces water and small solutes from the blood into the nephron at the glomerulus. Reabsorption returns useful substances — glucose, ions, and most water — back to the blood. Secretion adds extra wastes from the blood into the tubule. What remains is urine, carrying wastes out of the body.
Concept preview
What you will be able to do
- 01Describe glomerular filtration and state what passes into the filtrate and what stays in the blood.
- 02Explain tubular reabsorption and identify the substances returned to the blood.
- 03Explain tubular secretion and why it is needed.
- 04Explain how the loop of Henle helps concentrate urine using the counter-current idea.
Making urine is not a single event but a sequence of three linked processes that happen along the nephron, and keeping them straight is the key to this whole topic. The first is glomerular filtration, in which pressure forces a watery fluid out of the blood and into the nephron. The second is tubular reabsorption, in which the body takes back from that fluid nearly everything it still wants; most of the water, normally almost all glucose below transport capacity, and the salts it needs. The third is tubular secretion, in which a few extra wastes are added directly from the blood into the tubule. What finally trickles out of the collecting duct as urine is simply what is left over: the wastes and surplus water the body has chosen not to keep. A useful way to remember the logic is that filtration is deliberately wasteful; it dumps out almost everything; and the tubule then carefully rescues what matters.
School Plan
Urine formation
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Related topics
Frequently asked questions
- What are the three steps of urine formation?
- Filtration at the glomerulus, reabsorption of useful substances, and secretion of extra wastes into the tubule.
- Why is glucose normally absent from urine?
- It is completely reabsorbed into the blood during reabsorption; glucose in urine can signal a problem such as diabetes.