The endocrine system controls the release of parathyroid hormone that triggers osteoclasts to breakdown (resorb) bone and release calcium into the blood to maintain homeostasis.
Proper calcium levels are important for the normal function of several systems. Calcium ions are used for blood clotting, the contraction of muscles, the activation of enzymes, and cellular communication. The parathyroid gland of the endocrine system is the main receptor and control center for blood calcium levels. When the parathyroid glands detect low blood calcium levels, they communicate with several organ systems and alter their function to restore blood calcium levels back to normal. The skeletal, urinary, and digestive systems all act as effectors to achieve this goal through negative feedback.
The release of parathyroid hormone from the endocrine system triggers osteoclasts of the skeletal system to breakdown (resorb) bone and release calcium into the blood. Similarly, this hormone causes the kidneys of the urinary system to reabsorb calcium and return it to the blood instead of excreting calcium into the urine. Through the altered function of the kidneys to form active vitamin D, the small intestine of the digestive system increases the absorption of calcium.
When blood calcium levels are elevated, the parathyroid gland senses that as well. But in this case, instead of increasing its secretion of parathyroid hormone, it decreases secretion of the hormone. This decreases bone reabsorption, increases calcium levels in the urine and decreases calcium absorption in the intestines.
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MCAT Official Prep (AAMC)
Biology Question Pack, Vol. 2 Passage 13 Question 87
Key Points
• The parathyroid glands of the endocrine system are the main receptor and control center for blood calcium levels; they communicate with skeletal, urinary, and digestive systems and alter their function to restore blood calcium levels back to normal.
• The release of parathyroid hormone from the endocrine system triggers osteoclasts of the skeletal system to breakdown (resorb) bone and release calcium into the blood.
Key Terms
homeostasis: the ability to maintain a constant internal environment in response to environmental changes
osteoclasts: a large bone cell which absorbs bone tissue during growth and healing
parathyroid glands: four tiny glands, located in the neck, that control the body’s calcium levels
endocrine system: a system of glands that produce hormones to control chemical processes in the body
negative feedback: causes the reduction of a hormone or chemical level in the body due to increased concentration of a substance
