Final answer:
The slow exchange of Ca2+ between bone and plasma is part of maintaining calcium homeostasis, with less than 10% of bone calcium being exchangeable with plasma and the remainder exchanging slowly. Calcium homeostasis involves interplay between dietary intake, bone storage, and regulatory hormones such as PTH, vitamin D, and calcitonin. Plasma calcium levels are also influenced by plasma proteins and phosphate levels.
Step-by-step explanation:
The slow exchange of calcium ions (Ca2+) between bone and plasma is an essential aspect of calcium homeostasis, which maintains the blood calcium level around 9.5-10.5 mg/100 ml. In the blood, approximately 35-45% of calcium is bound to proteins, such as albumin, while the remainder is ionized (55-65%) or complexed with organic acids and phosphate.
The skeleton serves as a dynamic reservoir for calcium; a small portion (4-8 gm) of bone calcium is quickly exchangeable with plasma, residing on the surface of bone crystals as part the labile calcium storage pool. The vast majority of bone calcium is much more stable within the bone tissue and exchanges with plasma at a much slower rate.
Factors influencing calcium levels in plasma include the level of plasma proteins, with a decrease in proteins resulting in reduced total calcium levels, and plasma phosphate levels, which exhibit a reciprocal relationship with calcium (increased phosphate can lower serum calcium). Calcium ion exchange is critical for various physiological processes, including cardiac muscle function, where they play a pivotal role in the contraction mechanism through the troponin-tropomyosin complex.
Regulation of calcium homeostasis involves the interplay of bones, the endocrine system (PTH, vitamin D, and calcitonin), digestive system, and urinary system. Calcium cannot be synthesized biologically; it must be obtained through the diet. During states of low blood calcium levels, the body draws on the skeletal supply, and when levels are high, it deposits calcium back into the bones.