Three Feedback Loops & Homeostasis Of Each

Calcium homeostasis refers to the regulation of the concentration of calcium ions in the extracellular fluid. The two most important hormones for maintaining calcium levels in the body are the parathyroid hormone and calcitonin. As shown in the diagram, when the levels of calcium are too high, the thyroid releases calcitonin, causing an increase of calcium deposition in the bones, a decrease of calcium uptake in intestines, and a decrease of calcium reabsorption from urine, causing the calcium levels to fall. When the levels of calcium are too low, the parathyroid releases PTH (parathyroid hormone), causing an increase of calcium release from the bones, an increase of calcium uptake in the intestines, and an increase of calcium reabsorption from the urine, causing the calcium levels to rise.

Circulating levels of glucose are controlled by two enzymes, insulin and glucagon. As shown in the diagram, when the blood glucose level is high (mainly occurs after eating), the pancreas releases insulin, causing the liver to take up glucose and store it as glycogen and the body cells to take up more glucose, which in the end should lower the blood glucose level. When the blood glucose level is low, the pancreas releases glucagon and the liver breaks down glycogen into glucose, which in the end should raise the blood glucose level.

Thermoregulation (body temperature homeostasis) refers to the process of keeping the internal body environment in a steady state, when the external environment is changed. As shown in the diagram, when the internal body temperature rises above normal, thermoregulation in the brain is activated, causing sweat glands to secrete sweat that evaporates, eventually cooling the body, or blood vessels in the skin to dilate, allowing heat to escape, which should allow the internal body temperature to decrease. When the internal body temperature falls below normal, thermoregulation in the brain is activated, causing skeletal muscles to rapidly contract, causing shivering, which eventually generates heat and blood vessels in the skin to constrict, minimizing heat loss, which should allow the internal body temperature to increase.