Parathyroid and Calcium Flashcards
what do calcium salts (with phosphate) do ?
provide structural integrity of bone
What are calcium ions necessary for ?
Both intracellular and extracellular ions are essential for
> neuromuscular excitation
> blood coagulation
> hormone secretion
> enzyme activity
> fertilization
how much calcium is in the body ?
70 kg and 1200g of Ca2+
> 99% of Ca2+ is in your bones (stably stored as Hydroxyapatite)
> Also found in your soft tissues; intracellular (11g) and extracellular (1g)
Calcium Background
(regulation of Ca2+)
Extracellular Ca2+ is very tightly regulated:
>ionized calcium (“free”; 50%)
> protein-bound calcium (40%)
> calcium complexed with phosphate and citrate (10%)
Cellular Ca2+ is also highly regulated:
> largely associated with membranes in mitochondira, endoplasmic reticulum and plasma membrane
how is total body calcium maintained ?
**Total body calcium = intake - output**
Intake: Diet
> approx. 1/3 absorbed in small intestine. absorption is hormone regulated
- body can’t make it so it has to be replaced (RECOMMENDED: approx 1000 mg/day)
Output: Kidneys
Total body Calcium
**Total body calcium = intracellular + extracellular (ECF/plasma + bone)**
small intestine brings in calcium into extracellular fluid (0.1%) and 99% of calcium is found in the bone
there’s 0.9% free Ca2+ in the blood
and your kidneys filters and reabsorbs Ca2+, some leaving in your urine
**READ TEXTBOOK**
How does the endocrine system control [Ca2+] ?
3 hormones regulate movement of calcium between BONE, KIDNEY & INTESTINE
- Parathyroid hormone (PTH)
- Calcitriol (Vit D3)
- Calcitonin
** first two are the most important in adults **
Outline the relationship between calcium and the parathyroid hormone
Parathyroids are seen in terrestrial amphibians and onwards
There are 2 cell types: Chief cells (produce PTH) and Oxyphils (function unknown)
When plasma Ca2+ begins to fall, PTH acts to raise blood Ca2+ back to normal levels
what are the 3 mechanisms that PTH raises blood Ca2+
- stimulates osteoClasts to resorb bone (Primary Mechanism)
- Stimulates kidneys to resorb Ca2+
- Stimulates kidneys to produce enzyme needed to activate Vitamin D, which in turn promotes better absorption of Ca2+ from food and drink across the intestinal epithelium
Calcium & PTH : Hypo vs Hyper
secreted continuously, not stored..
Hypocalcaemia (too low):
> PTH secretion increases (stimulates resorption to get more Ca2+ back into blood)
Hypercalcaemia (too high):
>PTH secretion decreases
How does PTH stimulate osteoclasts to resorb bone
Bone is constantly formed and resorbed. Calcium in bone is mainly crystals (Hydroxyapatite)
> a small fraction is ionized and readily exchangeable
Describe bone deposition and resorption
Bone deposition:
OsteoBLASTs secrete a matrix of collagen protein, which becomes hardened by deposits of hydroxyapatite
Bone resorption:
osteoCLASTs dissolve the hydroxyapatite and return the bone calcium (and phosphate) to the blood
PTH stimulates kidneys to resorb Ca2+
>resorption of Ca2+ results in increased blood Ca2+ which negatively feedbacks to decrease blood Ca2+ maintaining ideal conditions
Calcium and Vitamine D3:
stimulates kidneys to produce enzyme needed to activate Vit D3
Overall : Vit D3 acts on the intestines to absorb more Ca2+ from food and drink
- Vit D3 is produced from its precurcor molecule 7-dehydrocholesterol under the influence of ultraviolet B sunlight
- Vit D3 is secreted into the blood (from skin or intestine) and functions as a PREhormone (not active). Goes to the liver, chemically changed, hydroxyl group added to carbon #25
- Still not active, needs a hydroxyl added to carbon #1, This is done by enzyme in the kidneys that is stimulated by PTH
> result is 1,25-dihydroxyvitamin D3 (maintains calcium balance in the body)
Calcium and Calcitonin
Calcitonin is made in C cells of thyroid in response to high Ca2+, plays only a minor role in adult humans.
Calcitonin works on the kidneys to stimulate
