The presence of specific receptors on the target cells allows them to respond to the signaling molecules.
Explanation:
Signaling molecules move throughout the body but exhibit their effect only on certain cells. This is due to the presence of signaling molecule specific receptors on the target cells. The target cells have cell surface or intracellular receptors to which the signaling molecules bind to generate their effect. The absence of these receptors in other cells does not allow the signaling molecules to exert any physiological response on them.
For example, liver and muscle cells mostly have insulin receptors and respond to the presence of insulin in blood by increased uptake of glucose from the blood.
higher altitudes have a lower level of oxygen which in turn, trains your lungs to hold and breathe more oxygen which also helps you cycle, run, etc. longer than usual
