Hormones That Affect Blood Calcium Levels

Question 1

What does the skeleton have a vital function in?

Answer 1

• regulation of plasma calcium levels, it acts as a store for calcium

Question 2

What is most of the calcium ions deposited in the bones as?

Answer 2

• calcium phosphate crystals

Question 3

What does this bone mineral give it resistance to?

Answer 3

• compressive loads

Question 4

What is it in contact with?

Answer 4

• a pool of readily exchangeable bone calcium which can enter or leave the extracellular fluid calcium ion pool, rapidly

Question 5

Where else is calcium found?

Answer 5

• in solution inside cells = intracellular pool

Question 6

Where is some of the extracellular fluid calcium bound to?

Answer 6

• proteins, for example plasma proteins
• the remainder constitutes the free calcium

Question 7

The total calcium ion concentration in plasma is nearly…

Answer 7

• TWICE the free calcium ion concentration in plasma containing normal concentrations of plasma protein

Question 8

What is the only active form of calcium?

Answer 8

• FREE CALCIUM
• it only makes up a very small portion of total body calcium

Question 9

What does control of calcium levels act to regulate?

Answer 9

• the free calcium ion concentration in the short term
• the total body calcium ion content, in the medium-long term

Question 10

Regulation depends on two main endocrine hormones:

Answer 10

• parathormone
• vitamin D
• occasionally, calcitonin plays a part

Question 11

What is parathormone?

Answer 11

• a peptide hormone secreted by 4 parathyroid glands, located posterior to the thyroid gland in the neck

Question 12

What is parathormone responsible for?

Answer 12

• tight control of free calcium ion concentration in the extracellular fluid, that is essential for life

Question 13

What is parathormone secretion stimulated by?

Answer 13

• by a fall in calcium ion concentration
• it acts to elevate calcium ion concentration in many ways
• provides negative feedback control

Question 14

Where are the main sites of action of parathormone?

Answer 14

• kidney
• bone

Question 15

What does parathormone do in the bone?

Answer 15

• parathormone stimulates the rapid exchange of calcium ions from the readily exchangeable bone pool - to the extracellular fluid
• this raising extracellular fluid calcium ion concentration

Question 16

What does parathormone do in a long-term way for the bone?

Answer 16

• favours a continued but slower release of calcium ions
• due to osteoclastic breakdown of the bone itself

Question 17

What does parathormone do in the kidney?

Answer 17

• it effects the kidney in three different ways

Question 18

The first way:

Answer 18

• it increases the rate of calcium ion reabsorption from the renal tubules, so that less is lost in the urine than normal
• this helps conserve the extracellular fluid calcium ion content

Question 19

The second way:

Answer 19

• parathormone also has an important effect on the handling of (PO4)^3-
• it increases phosphate excretion
• this is important for calcium ion regulation, as the product of calcium ion x phosphate ion concentration in the extracellular fluid is constant
• so calcium and phosphate concentrations are reciprocally related
• because of this, reducing phosphate ion concentration actually increases calcium ion concentration

Question 20

The third role:

Answer 20

• parathormone increases the rate at which vitamin D is converted to its most biologically active form

Question 21

Where does vitamin D / cholecalciferol come from?

Answer 21

• the diet
• the skin

Question 22

What are the main dietary sources of vitamin D and where is it absorbed?

Answer 22

• fish, liver and UV irradiated milk
• it is absorbed along with the fat in the small intestine, it is a fat soluble vitamin

Question 23

What can ultraviolet radiation from sunlight do?

Answer 23

• convert a cholesterol derived precursor into vitamin D in the skin

Question 24

What happens vitamin D when it is in circulation:

Answer 24

• it undergoes a 2 stage activation
• it is hydroxylated twice
  1. First in the liver to form 25-hydroxyl-cholecalciferol
  2. Then in the kidney, to produce the mainly biologically active form = 1:25 dihydroxy-cholecalciferol

Question 25

What is the second hydroxylation stimulated by?

Answer 25

- parathormone

Question 26

What is the main purpose of vitamin D?

Answer 26

- to increase the rate of calcium ion uptake from the gut - this is the only way to replace calcium lost in the urine - it is the only mechanism for the maintenance of total body calcium

Question 27

Why does calcium uptake in childhood and adolescence need to exceed loss?

Answer 27

- since new bone is being formed rapidly, requiring a continuous increase in the total amount of body calcium stored as bone mineral

Question 28

What does bone mineralisation require?

Answer 28

- phosphate - calcium - vitamin D - these all stimulate phosphate absorption from the gut - it is the main hormone (vitamin D), responsible for maintaining enough phosphate in the plasma

Question 29

Where is calcitonin secreted from?

Answer 29

- by the parafollicular C cells of the thyroid gland

Question 30

What does calcitonin do?

Answer 30

- it acts on bone to reduce the release of calcium ions to the extracellular fluid - it tends to lower the extracellular fluid calcium ion concentration

Question 31

When is calcitonin secreted?

Answer 31

- during episodes of hypercalcaemia - it does not seem to play a significant role in normal calcium ion content control

Question 32

What can low calcium levels, hypocalcaemia result from?

Answer 32

- inadequate parathormone production = hypoparathyroidism - low vitamin D levels

Question 33

When is vitamin D deficiency going to occur?

Answer 33

- poor bone mineralisation - rickets in childhood - osteomalacia in adults

Question 34

What are the most dangerous effects of hypocalcaemia?

Answer 34

- result from increased neuro-muscular excitability - this can cause = fatal laryngeal spasm & respiratory arrest = due to spontaneous activity in motor nerves and muscle - can cause cardiac arrhythmias and abnormal sensations = due to spontaneous sensory nerve activity

Question 35

What do all these effects of hypocalcaemia result from?

Answer 35

- an increased permeability to sodium ions, in the presence of low calcium ion concentration - so the membrane potential depolarizes TOWARDS the threshold for action potentials

Question 36

What does high calcium ion concentration, hypercalcaemia result from?

Answer 36

- uncontrolled parathormone secretion = primary hyperparathyroidism due to adenoma of a parathyroid gland - prolonged excess of dietary calcium ions

Question 37

What does elevated calcium ion concentration do?

Answer 37

- decreases neuronal excitability, can cause psychiatric and gastrointestinal symptoms

Question 38

What else can happen?

Answer 38

- calcium phosphate may also be precipitated in renal tissues - this causes urinary stones & renal failure