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Difficulty: MediumHormonal Coordination in Plants and Animals

Arrange the following physiological events in the correct sequence to illustrate how parathyroid hormone (PTH) restores blood calcium homeostasis when plasma calcium concentration falls below normal.

  1. 1Detection of reduced extracellular calcium ions by calcium-sensing receptors on parathyroid chief cells
  2. 2Exocytosis of parathyroid hormone (PTH) from parathyroid glands into the bloodstream
  3. 3Binding of circulating PTH to specific membrane receptors on bone cells and renal tubule epithelia
  4. 4Activation of osteoclastic bone resorption and enhanced renal tubular reabsorption of calcium
  5. 5Elevation of blood calcium concentration back to normal, inhibiting further PTH release

Answer

The correct sequence begins with the detection of low calcium ions by parathyroid chief cell receptors, followed by PTH release into the bloodstream, binding of PTH to membrane receptors on bone and renal tubule cells, stimulation of bone resorption and renal calcium reabsorption, and finally the restoration of normal blood calcium levels, which inhibits further PTH secretion.
The correct order follows the canonical negative feedback pathway: sensor detection of low calcium by parathyroid chief cells -> endocrine hormone secretion (PTH release into blood) -> hormone-receptor binding at target tissues (bone and kidney) -> cellular effector actions (bone resorption and renal calcium reabsorption) -> homeostatic balance recovery and negative feedback shutdown of PTH secretion.

Step-by-Step Solution

1
Identify the initial physiological stimulus
Detection of reduced extracellular calcium ions by calcium-sensing receptors on parathyroid chief cells occurs first.
Homeostatic regulation begins with receptor detection of a deviation from the set point.
2
Determine the endocrine response
Exocytosis of parathyroid hormone (PTH) from parathyroid glands into the bloodstream occurs second.
Endocrine glands release hormones into circulation when stimulated by specific homeostatic changes.
3
Trace hormone transport and receptor interaction
Binding of circulating PTH to specific membrane receptors on bone cells and renal tubule epithelia occurs third.
Blood-borne peptide hormones must bind to target cell surface receptors to exert physiological effects.
4
Identify target tissue physiological activities
Activation of osteoclastic bone resorption and enhanced renal tubular reabsorption of calcium occurs fourth.
Target cells respond by releasing stored calcium into extracellular fluid and preventing urinary calcium excretion.
5
Identify the homeostatic outcome and feedback loop completion
Elevation of blood calcium concentration back to normal, inhibiting further PTH release occurs fifth.
Return of the variable to set point removes the stimulus, suppressing further hormone release via negative feedback.

Key Concept

Parathyroid hormone (PTH) negative feedback control mechanism in blood calcium osmoregulation/mineral homeostasis
Estimated Time:1m 30s
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Topics

Hormonal Coordination in Plants and Animals

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