Each kidney holds about a million nephrons, and the nephron is where students lose the thread — the tubule twists through several named segments, and it is easy to memorize the parts without understanding the job. The job is straightforward: a nephron filters blood using three processes in a fixed order — filtration, reabsorption, and secretion. Learn what each process moves and in which direction, and the nephron's anatomy becomes the route those processes follow.
The Nephron's Layout: Two Working Halves
A nephron has two parts that do different things.
The renal corpuscle is where blood is filtered. It contains the glomerulus, a tight ball of capillaries, wrapped in a cup called the glomerular (Bowman's) capsule. Blood arrives through the afferent arteriole and leaves through the efferent arteriole.
The renal tubule is the long pipe that processes the filtered fluid. Following it from the corpuscle outward: the proximal convoluted tubule (PCT), then the loop of Henle (a descending and an ascending limb), then the distal convoluted tubule (DCT), which empties into a shared collecting duct.
Wrapping the tubule are the peritubular capillaries — blood vessels that pick up and drop off whatever the tubule reabsorbs or secretes. Keep the tubule and these capillaries side by side in your mind: the whole job is moving substances between the two.
Step 1: Glomerular Filtration
Filtration happens in the renal corpuscle and is driven by blood pressure, not by any pump.
Blood enters the glomerulus through the afferent arteriole, which is wider than the efferent arteriole it leaves by. That mismatch — a wide inlet, a narrow outlet — builds high pressure inside the glomerular capillaries. The pressure forces fluid and small solutes out of the blood, through a filtration membrane, and into the glomerular capsule.
The membrane is size-selective. Water, glucose, amino acids, ions, and small wastes like urea pass through freely. Blood cells and large plasma proteins are too big and stay in the blood. The fluid that crosses into the capsule is called filtrate — essentially blood plasma minus its proteins and cells.
This is non-selective in content but selective by size: the nephron filters out a lot of useful material at this stage and will recover it later. About 180 liters of filtrate form per day — far more than the body could afford to lose.
Step 2: Tubular Reabsorption
If 180 liters were lost, you would be dead within the hour. Reabsorption is how the body gets nearly all of it back. It moves substances out of the tubule filtrate and into the peritubular capillary blood.
Most reabsorption happens in the proximal convoluted tubule. The PCT reclaims essentially 100% of the glucose and amino acids, the bulk of the sodium, and large amounts of water. Glucose moves by active transport coupled to sodium; water follows by osmosis.
The loop of Henle fine-tunes water and salt recovery: the descending limb is permeable to water (water leaves), and the ascending limb pumps out sodium and chloride but is impermeable to water. This sets up a salty environment in the kidney's medulla that lets the body concentrate urine.
The distal convoluted tubule and collecting duct handle the final, hormone-controlled adjustments. Antidiuretic hormone (ADH) makes the collecting duct more permeable to water, recovering more water when the body is dehydrated. Aldosterone increases sodium reabsorption. This is where the body tailors urine to its current needs. Of the 180 liters filtered, about 99% is reabsorbed — only roughly 1.5 liters becomes urine.
Step 3: Tubular Secretion
Secretion is reabsorption's mirror image: it moves substances out of the peritubular blood and into the tubule filtrate, to be excreted.
Filtration alone is too coarse — it cannot fully clear every unwanted substance, and some only need removing in fine amounts. Secretion catches them. The tubules secrete hydrogen ions to regulate blood pH, potassium ions to keep blood potassium in range, and drugs and toxins such as certain antibiotics that the body wants gone. Secretion happens along the tubule, especially in the PCT and DCT.
Secretion is what makes the kidney a precise regulator rather than a blunt sieve. It also gives the kidney a second route to clear substances that filtration missed.
Putting It Together: What Leaves as Urine
The three processes combine into one balance sheet. Urine = (what was filtered) − (what was reabsorbed) + (what was secreted). Filtration pushes a large, non-specific load of fluid into the tubule. Reabsorption rescues the water, glucose, and ions the body needs to keep. Secretion adds a last set of unwanted ions and toxins. Whatever remains in the collecting duct drains away as urine — concentrated metabolic waste, mostly urea, in just enough water to carry it.
Getting Help
The nephron is the urinary system's working unit, and its hormone-controlled final step connects straight to the endocrine system and its hormones. For more physiology walkthroughs, see the full set of Anatomy & Physiology study guides.
Conclusion
Nephrons filter blood in three ordered steps. Filtration uses blood pressure to push a protein-free fluid into the tubule. Reabsorption returns the roughly 99% of water and useful solutes the body needs. Secretion adds a final load of ions, acid, and toxins for disposal. Hold the direction of each step — out of blood, back into blood, out of blood again — and the nephron's twisting anatomy becomes the path that logic follows.
