Mechanically calcium content

Rea.ctivity ofLea.d—Ca.lcium Alloys. Precise control of the calcium content is required to control the grain stmcture, corrosion resistance, and mechanical properties of lead—calcium alloys. Calcium reacts readily with air and other elements such as antimony, arsenic, and sulfur to produce oxides or intermetaUic compounds (see Calciumand calciumalloys). In these reactions, calcium is lost and suspended soHds reduce fluidity and castibiUty. The very thin grids that are required for automotive batteries are difficult to cast from lead—calcium alloys. 

Kamitakahara, M., Kawashita, M., Miyata, N., Kokubo, T. and Nakamura, T. (2002) Bioactivity and mechanical properties of polydimethylsiloxane (PDMS)-CaO-Si02 hybrids with different calcium contents. Journal of Materials Science-Materials in Medicine, 13, 1015-1020. 

Although bone is not considered a major calcium sensing organ in humans, the cells of bone tissue control over 99% of the human body s calcium content. The principal calcium sensors that regulate bone calcium uptake and release are in the parathyroid glands. Bone function is also modified by vitamin D and by calcium transport in the kidney and intestine. These indirect mechanisms of controlling bone calcium metabolism are beyond the scope of our considerations here. In spite of processing... 

The mechanism of action of cardiac glycosides has always been a subject of debate. The main site at which glycosides act is the NaVK -ATPase of the cell membrane, which constitutes the Na /K pump, and they bind to the K -binding site, thus inhibiting the enzyme (see ATPASE INHIBITORS). This inhibition, through a series of interrelated actions, eventually causes depolarization and affects cardiac rhythm. There is also an eventual increase in sarcoplasmic calcium content and an increase in the amount of calcium released by the action potential - and thus the force of contraction is increased. These are the principal beneficial actions. 

The mechanisms by which PTH might impair central nervous system function are only partially nnderstood. The increased calcium content in such diverse tissues as skin, cornea, blood vessels, brain, and heart in patients with hyperparathyroidism suggests that PTH may somehow facilitate the entry of Ca into such tissues. The finding of increased calcium in the brains of both dogs and humans with either acute or chronic renal disease and secondary hyperparathyroidism is consistent with the conception that part of the central nervous system dysfunction and EEG abnormalities found in acute renal failure or chronic renal failure may be due in part to a PTH-mediated increase in brain calcium. Calcium is essential for the function of neurotransmission in the central nervous system and for a large number of intracellular enzyme systems. Thus, increased brain calcium content could disrupt cerebral function by interfering with any of these processes (Rasmussen, 1986). It is also possible that PTH itself may have a detrimental effect on the central nervous system. 

The average daily intake of calcium is about 1250 mg, although there is a considerable variation between different parts of the world. Fifteen to forty percent is absorbed from the intestine. The intestine also secrets calcium, and the net calcium absorbed each day approximates 200 mg. At calcium balance, the kidneys therefore excrete slightly less than this amount. In case of low calcium intake a greater fraction is absorbed than at a higher calcium intake. This adaptation to the calcium content of the food is a slow process, which is connected to complex changes in the concentrations of the calcium-transporting mechanisms in the mucosa cells [2]. 

---