Nephron and Urine Formation: Filtration, Reabsorption and Counter-Current Explained (NEET)

Your kidneys filter roughly 180 litres of fluid every day, yet you pass only about 1.5 litres of urine. The other 178-odd litres are quietly taken back, and understanding exactly where and how is one of the most reliable scoring areas in NEET Excretory Products.

• Urine formation has three steps: glomerular filtration, tubular reabsorption and tubular secretion.

• GFR is about 125 mL/min, roughly 180 L/day.

• The loop of Henle and vasa recta create the medullary osmotic gradient (300 to 1200 mOsm/L) through counter-current exchange.

• The RAAS, ADH and ANF regulate filtration and water balance.

What Is the Functional Unit and How Is It Built?

The nephron is the functional unit of the kidney, and each human kidney has nearly one million of them. A nephron has two parts: the glomerulus plus Bowman's capsule (together the Malpighian or renal corpuscle) and the renal tubule (PCT, loop of Henle, DCT).

Most nephrons are cortical and have a short loop of Henle. In juxtamedullary nephrons the loop runs deep into the medulla and is accompanied by a well-developed vasa recta, which is why they matter for concentrating urine.

Step 1: Glomerular Filtration

Blood in the glomerular capillaries is filtered under pressure through three layers: the endothelium of the glomerular capillaries, the basement membrane, and the epithelial cells (podocytes) of Bowman's capsule. The slits between podocyte foot processes are called filtration slits.

Nearly all plasma constituents except proteins pass into the capsule, so this is called ultrafiltration. The glomerular filtration rate (GFR) is about 125 mL per minute in a healthy adult.

Step 2: Tubular Reabsorption, Segment by Segment

About 99% of the filtrate is reabsorbed. Learn it region by region, because NEET asks exactly this way.

• PCT: reabsorbs nearly all essential nutrients, and 70-80% of electrolytes and water. The brush-border (cuboidal) epithelium increases surface area. Here HCO3- is also reabsorbed and H+, K+ and ammonia are secreted, helping maintain pH and ionic balance.

• Descending limb of the loop of Henle: permeable to water, almost impermeable to electrolytes, so the filtrate becomes concentrated as it goes down.

• Ascending limb: impermeable to water but allows transport of electrolytes, so the filtrate becomes dilute as it goes up.

• DCT: conditional reabsorption of Na+ and water; reabsorbs HCO3- and selectively secretes H+, K+ and NH3.

• Collecting duct: extends from cortex to inner medulla, reabsorbs large amounts of water to produce concentrated urine, and allows passage of some urea into the medullary interstitium.

Step 3: Tubular Secretion

Tubular cells actively secrete substances such as H+, K+ and ammonia into the filtrate. This step is crucial for maintaining the ionic and acid-base balance of body fluids.

The Counter-Current Mechanism

The two limbs of the loop of Henle and the two limbs of the vasa recta run in opposite directions, forming a counter-current system. Interstitial osmolarity rises from about 300 mOsm/L in the cortex to 1200 mOsm/L in the inner medulla, caused mainly by NaCl and urea.

NaCl is transported by the ascending limb of the loop of Henle and exchanged with the descending limb of the vasa recta. Urea enters the thin segment of the ascending limb and is transported back to the interstitium by the collecting duct. This gradient allows water to leave the collecting duct, so humans can produce urine nearly four times more concentrated than the initial filtrate.

Pro Tip: Draw one nephron from memory and label, for each segment, whether it is permeable to water, to salts, or to neither. Students who drill this until it is automatic, as we encourage at Sukriti NeetJee, rarely lose marks on the loop of Henle statements in NEET, because most of those options simply swap "water" and "electrolytes" between the two limbs.

Regulation: RAAS, ADH and ANF

The juxtaglomerular apparatus (JGA) is formed by modified cells of the afferent arteriole and the adjacent DCT (macula densa). When GFR or blood pressure falls, JG cells release renin, which converts angiotensinogen to angiotensin I and then angiotensin II.

Angiotensin II is a vasoconstrictor that raises GFR and stimulates the adrenal cortex to release aldosterone, causing Na+ and water reabsorption in the distal tubule. ADH from the neurohypophysis increases water reabsorption and prevents diuresis. Atrial natriuretic factor (ANF) from the atria causes vasodilation and opposes the RAAS.

Common NEET Traps

• Confusing the permeability of the descending and ascending limbs.

• Saying that glucose is secreted; it is filtered and then reabsorbed in the PCT.

• Forgetting that ADH acts on the distal parts, increasing water reabsorption, and that low ADH leads to diabetes insipidus.

• Mixing up the functions of renin (enzyme), aldosterone (mineralocorticoid) and ANF (opposes RAAS).

Frequently Asked Questions

What is the GFR in a healthy person?

Approximately 125 mL per minute, which works out to about 180 litres of filtrate per day. Only around 1.5 litres is excreted as urine.

Where does most reabsorption occur in the nephron?

In the proximal convoluted tubule, which reabsorbs nearly all essential nutrients and 70-80% of water and electrolytes. Its brush-border epithelium gives a large surface area for this.

Which part of the loop of Henle is permeable to water?

The descending limb is permeable to water but nearly impermeable to electrolytes. The ascending limb is the opposite: impermeable to water but permeable to electrolytes.

What does the juxtaglomerular apparatus do?

It senses fall in GFR or blood pressure and releases renin. Renin starts the RAAS cascade, which raises blood pressure and GFR through angiotensin II and aldosterone.

How does ADH concentrate urine?

ADH, released from the neurohypophysis, makes the distal tubule and collecting duct more permeable to water. Water then moves out into the hyperosmotic medulla, leaving concentrated urine.

Have a question about excretion or any other NEET chapter? Leave it in the comments, and share this post with a fellow aspirant who is revising Human Physiology.