Nephron and Urine Formation: How Kidneys Filter Blood Explained
The nephron is the functional unit of the kidney responsible for filtering blood and forming urine. Through a process of filtration, reabsorption, and secr
TL;DR: The nephron is the functional unit of the kidney responsible for filtering blood and forming urine. Through a process of filtration, reabsorption, and…
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
Nephron and Urine Formation: How Kidneys Filter Blood: The nephron is the functional unit of the kidney responsible for filtering blood and forming urine. Through a process of filtration, reabsorption, and secr
Nephron and Urine Formation: How Kidneys Filter Blood Explained Simply
The nephron is the functional unit of the kidney responsible for filtering blood and forming urine. Through a process of filtration, reabsorption, and secr This is a key Biology concept for Class 10.
Structure of the Nephron
Each nephron begins with the Bowman's capsule, a cup-shaped structure surrounding the glomerulus capillary network. Blood enters the glomerulus through the afferent arteriole and leaves through the efferent arteriole. The filtrate then flows through the proximal convoluted tubule, descending limb of the loop of Henle, ascending limb of the loop of Henle, distal convoluted tubule, and finally the collecting duct. This complex pathway allows precise filtering and reabsorption.
Ultrafiltration in Bowman's Capsule
Blood pressure forces small molecules through the filtration barrier in the glomerulus. Glucose, amino acids, ions, urea, and water pass through into the Bowman's capsule. Large proteins and blood cells cannot pass through because the filtration membrane has tiny pores that block them. About 180 liters of filtrate is produced daily, but only 1 to 2 liters becomes urine.
Selective Reabsorption in the Proximal Convoluted Tubule
The proximal convoluted tubule reabsorbs useful substances back into the blood. All glucose and amino acids are reabsorbed through active transport. Some ions are reabsorbed. Water is reabsorbed through osmosis. This selective process ensures only waste is left in the filtrate. The walls of this tubule have many mitochondria providing energy for active transport.
Concentration in the Loop of Henle
The loop of Henle creates a concentration gradient. The descending limb is permeable to water but not salt, so water exits and the filtrate becomes concentrated. The ascending limb is permeable to salt but not water, so salt exits and the filtrate becomes dilute. This counter-current multiplier system allows efficient water reabsorption later. It is essential for producing concentrated urine.
Final Adjustments and Urine Collection
The distal convoluted tubule and collecting duct make final adjustments to urine composition. The hormone aldosterone controls salt reabsorption. The hormone antidiuretic hormone controls water reabsorption. These hormones respond to blood sodium levels and blood volume, ensuring precise water and salt balance. The final urine is collected and flows to the ureters and bladder.
Worked Examples
Example 1. Why is glucose normally absent from urine but present in diabetic patients?
Solution: In healthy kidneys, all filtered glucose is reabsorbed in the proximal convoluted tubule. In diabetes, blood glucose levels exceed the reabsorption capacity of the tubule. Excess glucose cannot be reabsorbed and is lost in urine.
Example 2. What happens if the loop of Henle is damaged?
Solution: The concentration gradient cannot be maintained. The kidney loses its ability to produce concentrated urine. The person must drink large amounts of water and urinate frequently. This condition is called nephrogenic diabetes insipidus.
Where You See This in Real Life
- Your kidneys produce about 180 liters of filtrate daily, but your body reabsorbs 99% of the water, leaving only 1 to 2 liters as urine.
- Drinking coffee and alcohol inhibits antidiuretic hormone, which is why you urinate more after consuming these beverages.
- Eating a salty meal causes your blood to become saltier, stimulating your kidneys to reabsorb less water and produce more dilute urine.
- In the desert, your body releases antidiuretic hormone to conserve water, producing less and more concentrated urine.
Common Mistakes Students Make
- Thinking the distal convoluted tubule is where most reabsorption happens: The proximal convoluted tubule performs most reabsorption.
- Believing the loop of Henle only reabsorbs water: It creates the concentration gradient that enables water reabsorption elsewhere.
- Assuming all filtered substances are waste: Glucose and amino acids are filtered but are reabsorbed because they are useful.
Frequently Asked Questions
Why is the proximal convoluted tubule so long and twisted?
The long length and large surface area allow time and space for selective reabsorption through active transport. The twisted path slows the flow of filtrate, giving more time for reabsorption to occur.
How does antidiuretic hormone control water reabsorption?
When blood becomes concentrated, the brain releases antidiuretic hormone. This hormone increases the water permeability of the collecting duct walls, allowing more water to be reabsorbed. This produces less, more concentrated urine.
What is the function of the juxtaglomerular apparatus?
The juxtaglomerular apparatus senses blood pressure and sodium levels. It releases renin, which regulates blood pressure and sodium balance through a complex hormonal cascade.