In the muscular system, calcium plays a role in muscle contraction. It binds to calcium-modulated proteins that are necessary for the muscle contraction process to occur. In the nervous system, it is needed for neurotransmitter release from the neurons. In the skeletal system, calcium is stored in bone. When needed, calcium ions are released into the bloodstream via the action of osteoclasts. These are bone cells that remove bone tissue with the removal of the mineralized matrix and the bone is broken down. This is known as resorption. Hormones from the parathyroid glands, tiny glands on top of the thyroid, regulate the resorption of calcium from bone and reabsorption from the kidney back to the circulatory system. Vitamin D3 is converted to calcitriol which promotes absorption of calcium from the intestines and allows calcium to travel from the bones back to the circulation again. Calcium ions are stored in the mitochondria and in the endoplasmic reticulum inside of cells. Within cells, the intracellular calcium level is low compared to the extracellular fluid. This is known as a concentration gradient. The plasma membrane which is selectively permeable regulates this gradient by calcium pumps that use ATP for energy. Plasma levels of calcium are 1-2 percent. Half is unbound and the rest is found in albumin, a protein, as well as in phosphate, sulfate and bicarbonate, as well as citrate. Calcium is a very important mineral found in leafy greens and dairy products.