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Grade 12 · Homeostasis · Extra lesson

Homeostasis: carbon dioxide, salts and thyroxine

Negative feedback beyond temperature and glucose: how the medulla oblongata controls carbon dioxide, how aldosterone balances salts, and how TSH and thyroxine regulate metabolism.

Negative feedback: the pattern

Every homeostatic mechanism has the same steps: a change from the normal level is detected by receptors, a control centre sends a message to effectors, and the effectors reverse the change. This is negative feedback.

Exam tip: describe each mechanism in this order — stimulus, receptor, control centre, message (nerve or hormone), effector, response, and return to normal.

Carbon dioxide

  1. During exercise, cells respire faster and the carbon dioxide level in the blood rises.
  2. Receptors in the carotid arteries and aorta (and in the medulla itself) detect the increase.
  3. The medulla oblongata sends impulses to the breathing muscles (diaphragm and intercostal muscles) and the heart.
  4. Breathing becomes faster and deeper, and the heart rate increases.
  5. More carbon dioxide is exhaled, so the level returns to normal.

Salt balance: aldosterone

  1. When the sodium ion level in the blood falls, the adrenal cortex secretes more aldosterone.
  2. Aldosterone causes the kidney tubules to reabsorb more sodium ions into the blood.
  3. The sodium level returns to normal.
  4. When the sodium level is too high, less aldosterone is secreted, so less sodium is reabsorbed and more is excreted in urine.

Water balance is controlled separately by ADH from the pituitary gland, which makes the collecting ducts more permeable to water.

Thyroxine and TSH

  1. When the thyroxine level is low, the hypothalamus stimulates the pituitary gland to secrete TSH (thyroid-stimulating hormone).
  2. TSH stimulates the thyroid gland to secrete thyroxine, which increases the metabolic rate.
  3. When thyroxine rises above normal, it inhibits TSH secretion, so the thyroid secretes less thyroxine.
ConditionCauseEffects
HyperthyroidismToo much thyroxineHigh metabolic rate, weight loss, nervousness, fast heart rate
HypothyroidismToo little thyroxineLow metabolic rate, weight gain, tiredness
GoitreLack of iodine (needed to make thyroxine)Swollen thyroid gland, because TSH keeps stimulating it

Worked example: explaining a feedback loop

✏️ Why iodine deficiency causes goitre
1Without iodine, the thyroid cannot make enough thyroxine.
2Low thyroxine no longer inhibits the pituitary, so TSH secretion stays high.
3TSH keeps stimulating the thyroid, which grows larger: a goitre.
0/8
Review the explanations above.
Answer in your exercise book, then open each memo to mark your work.
Question 1 · (5 marks)
Describe how the body returns a high blood carbon dioxide level to normal.
Show memo
Receptors in the carotid arteries/aorta detect the increase ✓. Impulses are sent to the medulla oblongata ✓, which sends impulses to the breathing muscles and heart ✓. Breathing becomes faster and deeper and heart rate increases ✓. More carbon dioxide is exhaled, so the level returns to normal ✓.
Question 2 · (4 marks)
Explain how aldosterone helps to maintain the salt balance of the blood when the sodium level is low.
Show memo
The adrenal cortex secretes more aldosterone ✓. Aldosterone travels in the blood to the kidney ✓ and causes the tubules to reabsorb more sodium ions ✓, so the sodium level in the blood rises back to normal ✓.
Question 3 · (4 marks)
Explain how thyroxine secretion is controlled by negative feedback.
Show memo
Low thyroxine stimulates the pituitary gland (via the hypothalamus) to secrete TSH ✓. TSH stimulates the thyroid to secrete thyroxine ✓. When thyroxine rises above normal, it inhibits TSH secretion ✓, so less thyroxine is secreted and the level returns to normal ✓.
Question 4 · (3 marks)
Explain why a person who lacks iodine in their diet may develop a goitre.
Show memo
Iodine is needed to make thyroxine, so thyroxine levels are low ✓. Low thyroxine does not inhibit TSH, so TSH stays high ✓, constantly stimulating the thyroid, which enlarges (goitre) ✓.