Question
Describe the structure of a nephron. Explain the three processes by which urine is formed.
Solution — Step by Step
A nephron is the structural and functional unit of the kidney. Each human kidney contains about one million nephrons. Each nephron consists of:
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Bowman’s Capsule: A double-walled cup that surrounds the glomerulus. The glomerulus is a knot of capillaries that brings blood to the nephron.
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Proximal Convoluted Tubule (PCT): Highly coiled tubule immediately after Bowman’s capsule; located in the cortex; lined with microvilli (brush border) for increased surface area.
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Loop of Henle: A U-shaped loop that descends into the medulla. Has a descending limb (water permeable, not solute permeable) and an ascending limb (impermeable to water, actively transports solutes out).
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Distal Convoluted Tubule (DCT): Coiled tubule in the cortex; site of regulated ion reabsorption controlled by hormones.
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Collecting Duct (CD): Not strictly part of the nephron but receives fluid from many nephrons; passes through the medulla and joins the renal pelvis.
Two types of nephrons:
- Cortical nephrons (85%): Short loop of Henle; mainly in cortex
- Juxtamedullary nephrons (15%): Long loop of Henle extending deep into medulla; important for concentrating urine
Blood enters the glomerulus through the afferent arteriole (wider) and leaves through the efferent arteriole (narrower). This difference in diameter creates high pressure inside the glomerulus (~60–70 mmHg), forcing water and small molecules out of the blood into Bowman’s capsule.
What is filtered (passes through):
- Water, glucose, amino acids, urea, uric acid, creatinine, ions (Na⁺, K⁺, Cl⁻, HCO₃⁻)
What stays in blood (does NOT filter):
- Large proteins (albumin), blood cells, fats
The fluid that enters Bowman’s capsule is called glomerular filtrate (or primary urine). It has a composition similar to blood plasma but without large proteins.
Filtration rate: About 180 litres per day (125 mL/min = Glomerular Filtration Rate, GFR). But we produce only 1.5 litres of urine per day — so 99% of the filtrate is reabsorbed.
As the filtrate moves through the tubules, the body selectively reabsorbs useful substances back into the peritubular capillaries.
PCT (Proximal Convoluted Tubule):
- Active transport: All glucose, amino acids, vitamins are completely reabsorbed (threshold reabsorption)
- Na⁺, K⁺, HCO₃⁻ actively reabsorbed; water follows by osmosis
- About 75–80% of water is reabsorbed here
Loop of Henle:
- Descending limb: Permeable to water; water moves out by osmosis into the hypertonic medullary interstitium; filtrate becomes concentrated
- Ascending limb: Impermeable to water; actively transports NaCl out; filtrate becomes dilute
This counter-current mechanism creates a concentration gradient in the medulla, essential for producing concentrated urine.
DCT (Distal Convoluted Tubule):
- Regulated reabsorption of Na⁺ and water (controlled by aldosterone and ADH)
- H⁺ and K⁺ may be secreted here to maintain blood pH
Collecting Duct:
- Further water reabsorption under the influence of ADH (antidiuretic hormone/vasopressin)
- When ADH levels are high, collecting duct becomes more permeable to water → concentrated urine
- When ADH is absent (e.g., after drinking lots of water), dilute urine is produced
This is the active transfer of waste products from the peritubular capillary blood INTO the tubular fluid. It is the reverse of reabsorption.
Substances secreted:
- Urea, uric acid, creatinine: Metabolic waste
- H⁺ ions: Maintain blood pH (if blood is too acidic, more H⁺ is secreted)
- K⁺ ions: Regulated by aldosterone
- Some drugs and toxins: Penicillin, certain metabolites
Tubular secretion ensures that even substances not fully filtered (because blood pressure didn’t push them through) are cleared from the blood.
Final urine contains:
- Water (about 95%)
- Urea (~2%), uric acid, creatinine
- Ions: Na⁺, K⁺, Cl⁻, phosphates, sulphates
- No glucose, no proteins, no amino acids in healthy urine
Key hormonal control:
- ADH (vasopressin): Increases water reabsorption in collecting duct → concentrated urine → secreted during dehydration
- Aldosterone (from adrenal cortex): Promotes Na⁺ and water reabsorption in DCT → increases blood volume → secreted when blood pressure is low
- ANF (Atrial Natriuretic Factor): Promotes Na⁺ and water excretion → reduces blood pressure → secreted when blood pressure is high
Why This Works
The kidney works by a combination of non-selective filtration (everything small exits the blood) followed by highly selective recovery (only what’s needed is taken back). The result is a remarkably efficient system: harmful wastes like urea and creatinine, which are in low concentration in blood, are concentrated up to 60× in urine.
The counter-current mechanism in the loop of Henle is what allows the kidney to produce urine far more concentrated than blood plasma — an essential adaptation for conserving water.
Alternative Method
For quick revision, remember the three steps as: Filter → Reabsorb → Secrete. The filtrate starts as 180 L/day, most is reabsorbed, a small amount is secreted, and 1.5 L/day leaves as urine.
Common Mistake
Students often say “glucose is found in urine.” In a healthy person, glucose is completely reabsorbed in the PCT. Glucose appears in urine (glucosuria) only when blood glucose exceeds the renal threshold (~180 mg/dL), as in diabetes mellitus. Never write “urine contains glucose” in your answer without specifying it refers to a diabetic condition.