Cells use paracrine signaling to communicate with their neighboring cells.In paracrine signaling, a cell secretes signaling molecules that can diffuse over short distances.As a result, such molecules can only trigger a response in cells with specific receptors in the immediate vicinity.In fact, signaling molecules that are not bound to the receptors are quickly degraded by enzymes present in the extracellular matrix.For example, physical activities like exercise increase the requirement for oxygen in skeletal muscle tissues. To fulfill this need for more oxygen, the endothelial cells lining the inner circumference of the blood vessels in this region secrete nitric oxide gas, or NO, as a paracrine mediator.NO enters the extracellular matrix, and diffuses into the neighboring smooth muscle cells, causing them to relax.As a result, the blood vessel dilates, increasing the blood flow to the deoxygenated tissues.However, similar to other paracrine signaling molecules, NO degrades rapidly outside the endothelial cell and can only trigger a localized response.