U1vii / 17A05 / 16B01: Describe the control of plasma calcium

17A05: Exam Report

Describe the regulation of plasma calcium concentration.

51% of candidates passed this question.

High scoring answers discussed the three major hormones involved in calcium regulation – parathyroid hormone, vitamin D and calcitonin. For each of these it was expected that candidates include: site of production, stimulus for release, inhibitory factors and actions. In the case of renin it was expected that candidates also include the actions of angiotensin and aldosterone. Very few answers discussed inhibitory factors or negative feedback loops.

16B01: Exam Report

Outline the distribution of calcium in normal plasma (20% of marks). Describe the hormonal control of the calcium concentration in the plasma (80% of marks).

53% of candidates passed this question.

As stated in the question, the distribution in plasma (not body) was expected. Candidates are reminded to include units [mmol/l]. Many candidates spent considerable effort describing the roles of calcium and particularly its role in excitation contraction coupling – which was not asked and hence scored no marks.
It was expected that candidates were able to identify the roles of parathormone (PTH), 1,25 OH vitamin D and calcitonin as the major hormonal regulators. Better answers were able to describe the physiology and integration of these hormones at the gut, kidney and bone. Calcitonin and its transient and (probable) minor opposite role were also identified in these answers.
Better answers were also able to identify the permissive roles of growth hormone, cortisol and thyroxine.

U1vii / 17A05 / 16B01: Outline the distribution of calcium in normal plasma - Discuss the regulation of plasma calcium - Compare & Contrast Calcium Gluconate and Chloride

Definition

Ca²⁺ = a divalent cation

Distribution

99% → bone → not readily exchangeable
10% → PLASMA → readily exchangeable
- 45% free Ca²⁺
- 45% Ca²⁺ bound with Albumin
- 10% calcium complexes (citrate, carbonate, hydrogen phosphate)

Plasma Ca²⁺ Ref Range = 2.20 – 2.55 mmol/L

ICF Ca²⁺ is v. low = 100nM

Function

Membrane excitation → nerves + muscles
Haemostasis → activation of clotting factors
Muscle contraction → excitation – contraction coupling
Excitation – secretion processes → release NTs, hormones
Structural support → bone, membrane stabilisation + cell adhesion

Calcium Metabolism

Regulated by 3 hormones

PTH
1,25 hydroxycholecalciferol (1,25 Vit D3)
Calcitonin

Regulated by several organs

Parathyroid gland (PTH)
Thyroid gland (calcitonin)
Bone (Ca/PO₄ reservoir)
Skin (Vit D activation)
Liver (Vit D metabolism)
GI (Ca²⁺ absorption)
Kidney (Vit D metabolism)

PTH = ↑ plasma Ca2+

Polypeptide hormone
Released by Chief cells of Parathyroid Gland

Effects

Bone – ↑ clast activity → ↑free Ca²⁺
Kidney – ↑Ca²⁺ at distal tubule
↓PO₄ at proximal tubule
↑1α-hydroxylase activity (active Vit D3 production)
Gut – ↑Ca²⁺ (indirect via Calcitriol)

Regulation

Ionised Ca²⁺
- ↓Ca²⁺ → β-adr stimulation → ↑PTH release
- ↑Ca²⁺ → G_q → ↓PTH release
Calcitriol – ↓PTH synthesis
Requires Mg²⁺ for secretion

Vitamin D = ↑ Plasma Ca2+, ↑ Bone Formation

Steroid compound made from cholesterol

Synthesis

Effects

GUT – ↑Ca²⁺ reabsorption
Kidney – ↑Ca²⁺ reabsorption
Bone – ↑blast activity → formation & calcification of bone
PTH Gland – ↓PTH synthesis (neg feedback)

Regulation

PTH → ↑ Renal-1-hydroxylase
↑PO₄^2- → ↓ Renal-1-hydroxylase

Calcitonin = ↓ Plasma Ca2+

Polypeptide hormone
Secreted by PARAFOLLICULAR CELLS of thyroid gland

Effects

Bone – ↓blast activity = ↓ Ca²⁺ release
Kidney – ↑Ca²⁺ & PO4^2- excretion

Regulation

Plasma Ca²⁺ > 2.4mmol/L → ↑Calcitonin

Other Hormones

Glucocorticoids

Short T → inhibit bone reabsorption
Long T → ↓turnover ∴osteoporosis
Inhibit Ca²⁺ absorption gut

Growth Hormone

↑Ca²⁺ excretion urine
↑Ca²⁺ absorption gut
Overall +ve Ca²⁺ balance

Thyroid Hormone

Hypercalcaemia
Hypercalciuria
Osteoporosis

Oestrogen

Inhibits bone resorption