Hormonal Control of the Kidney: PECTAA Class 12 Biology, Punjab Board

After a salty meal or a hot afternoon, your urine is dark and small in volume. After drinking a lot of water, it is pale and plentiful. The kidney does not decide this alone. Two hormones give the orders, and this note explains both.

Chapter 14 of the PECTAA Biology 12 book is the Human Urinary System. After the nephron and urine formation, the book covers the hormonal control of kidney functions and then the two capillary beds of the kidney. This note is for FSc Part 2 (2nd year, Class 12) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.

What to know

Two kinds of urine. When the body must save water, the kidneys pass a hypertonic (concentrated) urine. When there is too much water, they pass a hypotonic (dilute) urine. Hormones coordinate this switch.

1. Antidiuretic hormone (ADH). The book calls it the body's main tool for conserving water.

• Made and released: produced by the hypothalamus, secreted by the posterior pituitary gland.
• Trigger: dehydration or salty food raises the salt concentration of the blood. Osmoreceptors in the hypothalamus detect this.
• Response: the hypothalamus signals the posterior pituitary. The person feels thirsty, and ADH is released.
• Action: ADH makes the walls of the collecting ducts more permeable to water. Water moves out of the filtrate into the surrounding hypertonic fluid and then into the blood. Concentrated urine is produced and water is saved.
• Switching off: when blood salt concentration falls, the hypothalamus stops ADH secretion. The collecting duct walls become less permeable, less water is reabsorbed, and dilute urine is passed. Blood volume and blood pressure then fall.
• Side effect noted in the book: ADH raises blood volume and so blood pressure. It also narrows blood vessels, which raises blood pressure further.

Diabetes insipidus. A person who cannot make enough ADH, for example because of pituitary damage, keeps passing large amounts of dilute urine. Such a person is at risk of severe dehydration.

2. Aldosterone. It controls sodium and potassium levels.

• Made by: the adrenal cortex, the outer layer of the adrenal gland that sits on top of the kidney.
• Trigger: low blood pressure or low blood sodium.
• Action: it acts on the distal convoluted tubule and the collecting duct, increasing the reabsorption of sodium ions from the filtrate into the blood. With more sodium in the blood, water follows from the filtrate into the blood.
• Potassium: aldosterone also promotes the excretion of potassium ions in urine. This helps prevent hyperkalaemia, a condition of high blood potassium that can cause dangerous heart rhythm problems.

Two capillary beds in the kidney. They work one after the other.

1. Glomerular capillaries lie inside the Bowman's capsule. They are built for filtration and work at high blood pressure, which pushes water and small solutes into the nephron.
2. Peritubular capillaries surround the renal tubules. They handle reabsorption and secretion at low pressure. They take back glucose, ions and water from the tubules, and they secrete certain toxins into the tubule.

Key points

• ADH targets water; aldosterone targets sodium (water follows the sodium).
• ADH acts on collecting ducts; aldosterone acts on the distal convoluted tubule and collecting duct.
• High pressure filters, low pressure reabsorbs: glomerular versus peritubular capillaries.

Worked examples

Example 1. A student eats a very salty meal. Trace the steps to concentrated urine.

1. Blood salt concentration rises.
2. Osmoreceptors in the hypothalamus detect it.
3. The posterior pituitary releases ADH, and the student feels thirsty.
4. Collecting ducts become more permeable, water returns to the blood, and urine becomes concentrated.

Example 2. Why does a person with diabetes insipidus feel very thirsty? Without enough ADH, the collecting ducts stay less permeable. Large volumes of dilute urine are lost, so the body is short of water.

Example 3. Blood pressure drops. Which hormone responds, and how does this help? Aldosterone. It increases sodium reabsorption. Water follows sodium into the blood, so blood volume and pressure rise.

Common mistakes in the exam

1. Saying the pituitary makes ADH. The hypothalamus produces it; the posterior pituitary secretes it.
2. Saying aldosterone comes from the adrenal medulla. The book says adrenal cortex.
3. Reversing ADH's effect. More ADH means less urine and more concentrated urine.
4. Confusing the capillary beds. Glomerular capillaries filter at high pressure; peritubular capillaries reabsorb and secrete at low pressure.
5. Leaving out potassium. Aldosterone also increases potassium excretion.

Exam technique

• Use a four-step chain for ADH questions: stimulus, detector, hormone release, effect on the collecting duct.
• In "compare" questions, set ADH and aldosterone side by side: source, trigger, target, effect.
• Name diabetes insipidus with its cause and its main sign.
• Check your board's notice for the topics in your paper.

Practice questions

1. Which cells detect high salt concentration in blood? Answer: Osmoreceptors in the hypothalamus.

2. On which part of the nephron does ADH act? Answer: The walls of the collecting ducts.

3. Name the condition caused by too little ADH. Answer: Diabetes insipidus.

4. What is hyperkalaemia, and which hormone helps prevent it? Answer: High blood potassium; aldosterone, by increasing potassium excretion.

5. Which capillaries work at low pressure, and what is their job? Answer: Peritubular capillaries; reabsorption and secretion.

6. A person drinks a large volume of water. What happens to ADH and to the urine? Answer: Blood salt concentration falls, ADH secretion stops, the collecting ducts become less permeable, and a large volume of dilute urine is passed.

Ready to practise this? Continue in 2nd Year Biology.