Calcium Homeostasis: Interactions of the Skeletal System and Other Organ Systems

______ is not only the most abundant mineral in bone, it is also the most abundant mineral in the human body. ______ ions are needed not only for bone mineralization but for tooth health, regulation of the heart rate and strength of contraction, blood coagulation, contraction of smooth and skeletal muscle cells, and regulation of nerve impulse conduction.

Calcium

_______, a condition characterized by abnormally low levels of calcium, can have an adverse effect on a number of different body systems including circulation, muscles, nerves, and bone. Without adequate calcium, blood has difficulty coagulating, the heart may skip beats or stop beating altogether, muscles may have difficulty contracting, nerves may have difficulty functioning, and bones may become brittle. The causes of hypocalcemia can range from hormonal imbalances to an improper diet. Treatments vary according to the cause, but prognoses are generally good.

Hypocalcemia

_______, a condition characterized by abnormally high levels of calcium, the nervous system is underactive, which results in lethargy, sluggish reflexes, constipation and loss of appetite, confusion, and in severe cases, coma.

Hypercalcemia

__________: The body deposits calcium in the bones when blood levels get too high, and it releases calcium when blood levels drop too low as negative feedback. This process is regulated by PTH, vitamin D, and calcitonin.

The bones act as a storage site for calcium

Hypocalcemia: Cells of the _______ gland have plasma membrane receptors for calcium. When calcium is not binding to these receptors, the cells release ________ hormone, or PTH.

parathyroid

PTH has three effects all leading to a drop-in blood calcium levels:

a. Bone response - stimulates osteoclasts so they accelerate the erosion of bone matrix which leads to the release of stored calcium ions into the blood.

b. Intestinal response - PTH increases the production of the hormone calcitriol, which is continuously secreted by the kidneys at low levels.  Calcitriol increases the calcium-absorbing effects of any digested food in the small intestine.

c. Kidney response - Calcitriol in turn stimulates calcium reabsorption at the kidney tubules.

Hypercalcemia: the C cells of the thyroid gland secrete ______.

calcitonin

Calcitonin has three effects all leading to a drop in blood calcium levels:

a. Bone response - calcitonin inhibits osteoclasts but does not affect osteoblasts so that they continue to deposit calcium ions into the matrix of bone.

b. Intestinal response - calcitonin decreases the rate of calcium absorption from foods in the digestive tract.

c. Kidney response - calcitonin inhibits the absorption of calcium in urine so that more calcium is excreted from the body.