Model of short-term regulation of calcium-ion concentration

Abstract

This paper describes a digital simulation model of the short-term regulation of the concentration of calcium ions in the extracellular fluids of the body. The model includes four major classes of systems: terminal, intermediary, distribution, and coordinat ing. The terminal systems are the intestines (pri marily uptake) and the kidney (primarily excretion). The intermediary systems consist of two bone compart ments, viz., solid bone containing stable calcium salts and "fluid" bone in which recently formed unstable calcium salts are in equilibrium with the fluid that surrounds them. The distribution system is the extra-cellular fluid consisting of blood plasma and fluid between the cells in the tissues. The coordinating system functions via three hormones, viz., one secreted by the parathyroid glands (para thormone, PTH), one from the thyroid gland (calcitonin), and one derived from vitamin D through activation processes by the skin, liver, and kidney (1.25 dihydroxycholecalciferol). These four systems and their interrelationships are described in FORTRAN IV in conjunction with special simulation software. To illustrate the fundamental principles which govern calcium concentration, simple well-known experiments are simulated. These include calcium infusion, parathormone infusion under conditions of normal and abnormal high calcium concentration, and partial and total removal of the parathyroid glands. These simulations agree quite well with experimental findings. The model is designed to be expanded as new information becomes available.