The nephron is the microscopic filtering unit of the kidney, and each kidney contains about one million of them. Nephrons remove wastes from the blood while keeping needed water, ions, glucose, and other useful molecules in the body. This balance helps maintain blood volume, blood pressure, pH, and salt concentration.
Understanding urine formation shows how the body controls its internal environment every minute.
Urine formation begins when blood pressure forces fluid from glomerular capillaries into Bowman’s capsule. As filtrate moves through the proximal tubule, loop of Henle, distal tubule, and collecting duct, the nephron reabsorbs useful substances and secretes extra wastes or ions. The loop of Henle and the surrounding medulla create a salt gradient that helps pull water out of the filtrate.
Hormones such as ADH adjust water reabsorption, allowing the body to make dilute or concentrated urine.
Key Facts
- Filtration occurs at the glomerulus, where blood pressure pushes water and small solutes into Bowman’s capsule.
- Filtration equation: filtration = substances entering Bowman’s capsule from the blood.
- Reabsorption returns useful substances from the tubule to the blood, including most water, glucose, amino acids, and sodium ions.
- Secretion moves selected substances from the blood into the tubule, including H+, K+, NH4+, and many drugs.
- Urine formation summary: excretion = filtration - reabsorption + secretion.
- ADH increases water reabsorption in the collecting duct, producing lower-volume, more concentrated urine.
Vocabulary
- Nephron
- A nephron is the functional unit of the kidney that filters blood and forms urine.
- Glomerulus
- The glomerulus is a ball of capillaries where blood pressure drives filtration into Bowman’s capsule.
- Filtrate
- Filtrate is the fluid that enters the nephron tubule after small substances are filtered out of the blood.
- Reabsorption
- Reabsorption is the movement of useful substances from the nephron tubule back into the bloodstream.
- Collecting duct
- The collecting duct is the final tubule segment where water reabsorption is adjusted and urine becomes more concentrated.
Common Mistakes to Avoid
- Thinking the nephron filters whole blood cells, which is wrong because healthy glomerular filtration keeps blood cells and most large proteins in the bloodstream.
- Confusing reabsorption with secretion, which is wrong because reabsorption moves substances from tubule to blood, while secretion moves substances from blood to tubule.
- Assuming all filtered water becomes urine, which is wrong because most filtered water is reabsorbed before the filtrate reaches the collecting duct.
- Forgetting the role of the medulla salt gradient, which is wrong because concentrated urine depends on water leaving the collecting duct as it passes through salty medullary tissue.
Practice Questions
- 1 A nephron filters 125 mL of fluid per minute, and 124 mL per minute is reabsorbed. How much urine is formed per minute if secretion is ignored?
- 2 A substance is filtered at 80 mg/min, reabsorbed at 50 mg/min, and secreted at 10 mg/min. Using excretion = filtration - reabsorption + secretion, what is its excretion rate?
- 3 A dehydrated person releases more ADH than usual. Explain how this changes water movement in the collecting duct and how it affects urine volume and concentration.