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🫀 Science · Human Anatomy and Physiology · Lesson 10 of 12 · 6 min

Urinary system: nephrons, urine formation, ADH and aldosterone

💧 Your kidneys filter about your whole blood plasma dozens of times a day and keep almost all of it. The TEAS wants to know how, and which hormones fine-tune it.

💡 Key idea

Filtration in the glomerulus, reabsorption mostly in the proximal tubule; ADH saves water, aldosterone saves sodium and dumps potassium.

🧠 The idea

Urine flows kidney → ureter → urinary bladder → urethra. The working unit of the kidney is the NEPHRON. Blood enters the glomerulus, a capillary knot inside Bowman's (glomerular) capsule, where pressure pushes water and small solutes out: this is FILTRATION. The filtrate then runs through the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule and the collecting duct. Along the way REABSORPTION returns water, glucose, amino acids and most sodium to the blood, and secretion adds wastes such as hydrogen ions and some drugs to the filtrate. Most reabsorption happens in the proximal tubule. Two hormones fine-tune the result. ADH (antidiuretic hormone, from the posterior pituitary) makes the collecting ducts more permeable to water, so less, more concentrated urine forms. ALDOSTERONE (from the adrenal cortex) makes the distal tubule reabsorb sodium and secrete potassium; water follows the sodium, raising blood volume and pressure.

✏️ Worked example

A hiker has had no water for a day. Predict ADH and urine. Step 1: dehydration raises blood concentration (osmolarity), which the hypothalamus senses. Step 2: the posterior pituitary releases more ADH. Step 3: collecting ducts reabsorb more water. Step 4: urine becomes small in volume and dark, concentrated. Alcohol does the opposite: it suppresses ADH, so urine output rises.

📌 Takeaways

  • Kidney → ureter → bladder → urethra; nephron = glomerulus, Bowman's capsule, PCT, loop of Henle, DCT, collecting duct.
  • Three steps: filtration, reabsorption, secretion.
  • More ADH = less, more concentrated urine; aldosterone = keep Na+, lose K+.

✅ Check yourself

Question 1 of 3

Where in the nephron does filtration of blood begin?

  1. Collecting duct
  2. Distal convoluted tubule
  3. Loop of Henle
  4. Glomerulus
Show the answer and rationale

Answer: D

Filtration happens at the glomerulus, where blood pressure forces fluid into Bowman's capsule. The loop of Henle, distal tubule and collecting duct come later and modify filtrate that has already formed.

Question 2 of 3

A rise in antidiuretic hormone (ADH) would most likely cause which change?

  1. Increased sodium loss in urine
  2. Larger volume of dilute urine
  3. Increased glucose in the urine
  4. Smaller volume of concentrated urine
Show the answer and rationale

Answer: D

ADH increases water reabsorption in the collecting ducts, so less water leaves and the urine is concentrated. A large dilute volume is what low ADH causes, and ADH does not directly change sodium or glucose handling.

Question 3 of 3

Aldosterone acts on the kidney to do which of the following?

  1. Reabsorb sodium and secrete potassium
  2. Reabsorb potassium and secrete sodium
  3. Secrete water into the collecting duct
  4. Block filtration at the glomerulus
Show the answer and rationale

Answer: A

Aldosterone, from the adrenal cortex, makes the distal tubule and collecting duct reabsorb sodium and secrete potassium, and water follows the sodium. The swapped version reverses the ions, and aldosterone does not secrete water or block filtration.

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