A group of local regulators called prostaglandins (PGs) are modified fatty acids, often derived from lipids in the plasma membrane. They are so named because they were first discovered in prostate gland secretions that contribute to semen.

Released from most types of cells into interstitial fluid, prostaglandins regulate nearby cells in various ways, depending on the tissue. In semen that reaches the reproductive tract of a female, prostaglandins stimulate smooth muscles of the female‘s uterine wall to contract, helping sperm reach an egg. During childbirth, prostaglandins secreted by cells of the female‘s placenta cause the nearby muscles of the uterus to become more excitable, helping to induce labor.

In the immune system, various prostaglandins help induce fever and inflammation and also intensify the sensation of pain. These responses contribute to the body‘s defense by sounding an alarm that something harmful is occurring. The anti – inflammatory effects of aspirin and ibuprofen are due to the drugs‘ inhibition of prostaglandin synthesis. Prostaglandins also help regulate the aggregation of platelets, an early step in the formation of blood clots. This is why some physicians recommend that people who are at risk for a heart attack take aspirin on a regular basis.

In the respiratory system, two prostaglandins with very similar molecular structures have opposite effects on the smooth muscle cells in the walls of blood vessels serving the lungs. Prostaglandin E signals the muscle cells to relax, which dilates the blood vessels and promotes oxygenation of the blood. Prostaglandin F signals the muscle cells to contract, which constricts the vessels and reduces blood flow through the lungs. Shifts in the relative concentrations of these two antagonistic (opposing) signals help maintain homeostasis in changing circumstances.