Blood calcium homeostasis

Major points covered here:  Importance of blood calcium levels, mechanisms used to control blood calcium levels, hormones involved in controlling blood calcium levels, and some things that happen of things go wrong with blood calcium levels (pathophysiology of blood calcium levels).

Proper blood calcium levels are very important to our health.  If calcium levels are not right it can effect our skeletal muscles, our heart’s rhythm, blood clotting, and more.  So, our bodies use three major mechanisms in order to maintain blood calcium homeostasis (keep blood calcium levels in a healthy range)…

  1. Calcium absorption —  basically we eat foods that contain calcium and our blood stream then absorbs calcium from our digestive tract.
  2. Calcium storage and release–  calcium can be moved from the blood stream and stored in bone tissue (bone mineralization), and calcium can be released from bone tissue and back into the blood when needed (resorption of bone).
  3. Calcium excretion —  calcium can be excreted from the blood stream by our kidneys and released from the body in urine.

Your body uses hormones to control each of these three major mechanisms.  Here are each of the major hormones involved and what they do…

  • Vitamin D (calcitriol):  This vitamin acts as a hormone, it increases blood calcium levels by stimulating more absorption of calcium from the digestive tract, decreasing calcium excretion into urine, and encouraging calcium release from bone tissue.  This vitamin/hormone is also an interesting molecule because our bodies can make their own calcitriol with some help from sunlight.  UV radiation from the sun hitting our skin allows the first chemistry to happen on the way to making this hormone, the molecule is then modified by the liver and finally by the kidneys before we finally have active calcitriol.
  • Calcitonin:  This hormone is released form the thyroid gland and it lowers blood calcium levels by stimulating the storage of calcium into bone tissue (“bone mineralization” or “bone deposition”), and by slightly increasing the excretion of calcium into urine by the kidneys.
  • Parathyroid hormone (PTH):  Parathyroid hormone is made by the parathyroid glands and it raises blood calcium levels by encouraging the release of calcium from the bone tissue and into the blood (resorption of bone), and by decreasing excretion of calcium into the urine by the kidneys (encourages the kidneys to reabsorb calcium, keeping more calcium in the blood).

So, what can happen if things go wrong with blood calcium levels?

Rickets:  Rickets is a weakening of bone tissue that usually happens if a child does not ingest or produce enough vitamin D.  Remember that calcitriol/vitamin D is important for increasing blood calcium levels, especially encouraging the absorption of calcium from the gut.  If the child is not absorbing enough calcium they will not have enough calcium available to build strong bones.  The child’s bones well simply be weaker as a result, this can lead to fractures or simply to the bending of bones that should be strong enough to handle normal stress without bending (having bowed legs as the femurs bend is somewhat common in rickets).   This can also happen in adults by the way, but in adults we call it osteomalacia.

Hypocalcemia:  Low blood calcium levels.

Some possible causes:  too little vitamin D, diarrhea, thyroid tumors, pregnancy and/or lactation, thyroid tumors, and more…

Signs and symptoms:

  • Oral and facial paresthesias (pins and needles feeling– kind of like the feeling you get after your arm or leg has “fallen asleep” as the feeling comes back you can have uncomfortable paresthesias).
  • Hyperactive skeletal muscles–  hyperactive reflexes, tetany, and other muscle spasms.  One form of tentany that is somewhat common in hypocalcemia is carpopedal spasms in which the hands and feet are effected with muscle spasms that are classical in appearance (do a google image search for carpopedal spasm and you will see that classic appearance).  These spasms can be painful.
  • Cardiac arrhythmia:  Usually include a decrease in the heart’s rate and strength of contraction and can be as severe as a deadly ventricular tachycardia known as torsade de pointes.
  • Petechia (tiny bruises):  bruising can occur because calcium is needed for blood clotting.  If calcium levels are too low the blood does not clot and any tiny break in a capillary can lead to patechia forming.  Larger bruises and excessive bleeding are also possible.

Hypercalcemia:  High blood calcium levels

Some possible causes:  Kidney failure, parathyroid tumor, high bone tissue turnover rate (usually due to another condition such as hyperthyroidism or due to being immobile for a long period), and more…

Signs and symptoms:  This depends on how quickly the calcium levels go up.  If they go up relatively slowly the body seams to compensate pretty well for the change and the person may not have any signs or symptoms.  However, if the increase in calcium is quick they may have…

  • Cardiac arrhythmia including an increase in heart rate and strength of contraction and other possible changes (like a shortened Q-T interval).
  • Depression of skeletal muscle activity–  hypoactive reflexes for example.
  • Formation of calcium stones:  These could be renal stones (in kidneys or bladder) or biliary stones (in gallbladder).
  • Gastric symptoms —  abdominal pain and possible vomiting
  • Increased urination

So, what is the take home here?  Blood calcium levels are important and they have to stay within certain limits for us to stay healthy.  Therefore, our bodies have important hormone controlled mechanisms to help maintain blood calcium homeostasis, and if these systems don’t work right blood calcium levels can move outside of homeostatic range causing us health problems that can be serious.

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