The human body works hard to maintain homeostasis, which means "steady state," with regard to different parameters. For instance, your body tries to maintain a constant temperature and acidity level. Similarly, you need stable levels of calcium in your blood to maintain normal function. Too much or too little blood calcium can result in a variety of medical problems, including cardiac arrhythmia. Calcitonin and parathyroid hormone help you to maintain normal calcium levels. While these two hormones have a relatively complex physiology, you can simplify their roles when describing how they work.
How They Work
Step 1
Before you can help someone understand how calcitonin and parathyroid hormone work, you'll need to explain that there are two major sources of calcium that enters the blood. You can get calcium from the food you eat, or you can pull it out of your bones, explains Dr. Lauralee Sherwood in her book "Human Physiology." While the bones are a source of large quantities of calcium, pulling calcium from the bones on a regular basis can make them less dense, resulting in a weakened skeletal system. Ideally, therefore, a healthy diet includes plenty of calcium so that pulling it from the bones isn't necessary.
Step 2
Calcium is far more than just a component of bone, even though most people think first of bone when considering the body's need for calcium. Calcium is also critical to muscular contraction -- including contraction of the heart muscle, which is why low blood calcium can lead to cardiac arrhythmia and inability to move the voluntary muscles. Your cells also use calcium to assist in their cell-to-cell communication, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry."
Step 3
The simplest way to describe calcitonin's role in the body is that it works to decrease blood calcium. As such, it causes the kidneys to excrete calcium in the urine, prevents the intestine from taking up as much calcium from food and increases the activity of specialized bone cells called osteoblasts. Osteoblasts build new bone matrix using calcium and other components from the blood; when osteoblasts are more active, your blood calcium levels fall. Calcitonin also decreases the activity of bone cells called osteoclasts, which break down bone matrix and release its components into the bloodstream. While the body's goal in releasing calcitonin is to decrease blood calcium, an indirect but important effect of the hormone is that it increases bone density. For this reason, physicians can use calcitonin in combination with vitamin D, which you need to help you absorb calcium from your food, to combat the bone loss associated with osteoporosis.
In a 1989 study published in the journal "Calcified Tissue International," Dr. Genaro M. A. Palmieri and colleagues noted that providing women who had osteoporosis with supplements of high levels of vitamin D in combination with calcitonin helped to prevent further bone loss.
Step 4
Parathyroid hormone counteracts calcitonin and works to increase blood calcium. It causes the kidneys to reabsorb calcium from filtered blood, meaning the calcium stays in the body instead of being excreted in the urine. It increases the activity of osteoclasts, while decreasing the activity of osteoblasts; this causes you to break down bone, and release the calcium -- and other components of bone matrix -- into the bloodstream. It also increases the absorption of calcium from the food you eat by increasing your production of vitamin D, which helps your intestines absorb calcium.
References
- “Human Physiology”; Lauralee Sherwood, Ph.D.; 2004
- “Biochemistry”; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Calcified Tissue International"; Effect of Calcitonin and Vitamin D in Osteoporosis"; Genaro M. A.. Palmieri et al; 1989
