Calcium Flashcards
What is the daily calcium intake
500 - 1500 mg/day
About 30% of dietary calcium is absorbed
Where is calcium stored
99% of total calcium is stored in the skeleton
Remainder distributed between intracellular and extracellular fluid
What is calcium required for
Physiological processes
- skeletal growth and maintenance
- Normal blood clotting
- Muscle contractility
- Nerve function
What is the plasma concentration off calcium
Controlled to narrow limits
2.2-2.6 mmol/L
approx. 45% bound to plasma proteins - biologically unavailable
45% circulating ionised form
10% bound to other proteins
- citrate
- phosphate
What % of plasma calcium is bound to plasma proteins
45%
What % of plasma calcium is circulating in its ionised form
45%
What % of plasma calcium is bound to compounds such as citrate and phosphate
10%
When is calcium metabolically active
In ionised state
What organ regulates the amount of calcium excreted in the body
Kidney
Approximately what % of serum calcium filtered by the kidneys is reabsorbed
98%
Where is calcium reabsorbed in the kidneys
Passively - proximal tubule and ascending loop of Henle
Actively - Distal tubes of the kidneys
What happens when plasma calcium falls
Bone mineral reservoir acts by increasing the rate of bone resorption
Leads to a decrease in bone mineral density
What acts as a readily accessible source of exchangeable calcium
Skeleton
What are the most important hormones in calcium homeostasis
PTH
Calcitriol (1,25 dihydroxy Vitamin D)
What is the role of PTH in calcium homeostasis
Release is increased in response to low serum ionised calcium
Parathyroid gland senses changes through calcium sensing receptors
Acts in three ways to increase serum calcium
1. Decreasing renal calcium excretion
2. Increasing bone reabsorption
3. Enhancing dietary calcium absorption by stimulating the production of calcitriol in the kidney
What is the role of calcitonin in calcium homeostasis
Secreted by cells in the thyroid glands
Reduces serum calcium concentration
How much calcium is in the skeleton
1200g
How much calcium is in extracellular space
1g
How much in mmol/L is the ionised serum calcium
1.1. mmol/L
What is the affect of alkalosis on calcium
Higher pH albumin binds strongly to calcium
Total serum calcium = unchanged (2.4 mmol/L)
Ionised serum calcium = decrease (0.9mmol/L)
Extracellular fluid space calcium is in equilibrium with ionised calcium
What is low ionised calcium associated with
Contraction of the small muscles of the feet and hands = tetany
Due to depolarisation of the long nerves of the upper limb
What are the three ways homeostasis is maintained
Calcium absorption (from intestines) / endogenous faecal Ca
Calcium reabsorption (from kidneys) / Ca excretion
Bone resorption / bone formation
What are the source of dietary calcium
Major source = dairy products 2/3
Minor sources
- vegetables e.g. broccoli
- cereals e.g. white bread
- oily fish e.g. sardines
What % of dietary calcium do we absorb
30%
When do we absorb a higher fraction of calcium
When put on a low calcium diet
Mediated by calcitriol - the active form of vitamin D
Most calcium is absorbed by active transport
When can calcium be released rapidly
Exchangeable calcium on the bone surface
When can calcium be released slowly
By osteoclasts during bone resorption
What does the amount of calcium filtered by the glomerulus depend on
Glomerular filtration rate
Ultra filterable calcium
- ionised
- complexed
What increases/decrease the amount of calcium reabsorbed
More - PTH level is high
Less - filtered sodium is high
High sodium diet decreases Claudin-2 expression, resulting in increased TRPV5 and calcium transport
Describe calcitonin
Hormone produced by C cells (parafollicular cells) in thyroid
Secretion is stimulated by an increase in serum calcium
Effect is to lower bone resorption
Importance in man is uncertain
What is the action of the parathyroid hormone
Acts on
- bone
- kidney
Uses second messengers such as cyclic AMP, calcium
Positive and negative signs relate to actions in osteoblasts
What is the response to a low calcium diet
Low dietary calcium =
Less calcium is absorbed =
Results in low serum ionised calcium =
Results in higher PTH =
Results in fast and slow responses
Returns serum ionised calcium to normal
What is low dietary calcium an example of
Negative feedback
