Endocrine Control of Calcium Metabolism Flashcards
What is the distribution of calcium in the body?
- 99% in bone
- 0.9% in ICF
- 0.1% in ECF
What is calcium tightly regulated with?
phosphorous obtained from the diet
What is phosphorus essential for?
- ATP
- cAMP second messenger systems (phosphorylation) - the backbone of DNA molecules
What are the 3 fractions of calcium in the ECF?
- free (ionised) - 50%; biologically active
- complexed (with PO43-) - 5%; not free to participate in chemical reactions
- bound (to protein) - 45%; restricted to the plasma
What is bone?
a living tissue that gets remodelled ~every 10 years
What are the 3 types of bone cell?
- osteocytes
- osteoclasts
- osteoblasts
What do osteocytes do?
transfer calcium to the ECF without destroying the bone structure (essentially retired osteoblasts)
What do osteoclasts do?
secrete hydrochloric acid that dissolves Ca3(PO4)2 crystals (to release calcium and phosphate back into the ECF) and enzymes that break down the organic matrix
What do osteoblasts do?
secrete organic matrix (osteoid) within which Ca3(PO4)2 crystals precipitate
What do osteoblasts act as?
a reservoir of calcium that can be released into the blood circulation when required
What are the 3 steps of resorption?
- osteoclasts anchor themselves to the surface of the bone which creates a microenvironment under the cell (sealed zone)
- within the zone, the osteoclasts create an acidic environment that dissolves the bone’s mineral content
- once the content is dissolved, enzymes released from osteoclasts remove the remaining collagenase bone matrix to compete resorption
What happens after resorption?
osteoblasts move into the resorption space and start to produce osteoid which then forms a scaffold in which calcium and phosphate crystallise
What are the 2 fates of osteoblasts?
- some are trapped in the matrix and become osteocytes
- some undergo apoptosis or revert back to lining cells which cover the surface of the bone
What is bone modelling?
when bone formation by osteoblasts occurs without prior bone resorption by osteoclasts which results in an increase in bone mass
What are early signs of hypocalcaemia?
- Trousseau’s sign
- Chvostek’s sign
How do you do test for Trousseau’s sign?
- a blood pressure cuff is placed around the arm and inflated to a pressure greater than the systolic blood pressure (>120mmHg) and held in place for 3 minutes to occlude the brachial artery
- in the absence of blood flow, the patient’s hypocalcaemia and subsequent neuromuscular irritability will induce spasm of the muscles of the hand and forearm
- the wrist and metacarpophalangeal joints flex, the DIP and PIP joints extend, and the fingers adduct
How do you do test for Chvostek’s sign?
- begin by telling the patient to relax his facial muscles
- stand directly in front of him and tap the facial nerve either just anterior to the earlobe and below the zygomatic arch or between the zygomatic arch and the corner of the mouth
- a positive response varies from twitching of the lip at the corner of the mouth to spasm of all facial muscles depending on the severity of the hypocalcaemia
What are the 7 main physiological functions of calcium?
- membrane excitation
- EC coupling
- haemostasis
- stimulus-secretion coupling
- second messenger and enzyme activity
- maintenance of tight junctions between cells
- bone formation
What are the effects of low calcium levels in skeletal muscle?
voltage gated sodium channels stay open and more sodium diffuses through the membrane which causes excess depolarisation and impulses are transmitted repeatedly which can lead to increased skeletal muscle contraction
What are the effects of low calcium levels in cardiac muscle?
decreased cardiac muscle contraction which leads to abnormal heart rhythms
What does the entry of calcium into secretory cells allow?
the release of the secretory product by exocytosis