Carcinoid heart disease is the heart damage resulting from substances causing the carcinoid syndrome, especially serotonin. Serotonin released from Carcinoid Cancer cells stimulates scar-forming cells (fibroblasts) to proliferate when serotonin attaches to serotonin receptors. These receptors exist within the body and are stimulated by high concentrations of serotonin. Since the liver is a common site of metastases from abdominal primary carcinoid cancers, blood which has passed through the liver of patients with carcinoid syndrome typically has a high concentration of serotonin released by the carcinoid liver metastases. This serotonin-rich blood then travels a short distance to the right side of the heart which contains serotonin receptors on the surface of the inner lining of the heart as well as on the heart valves. The damage (scarring) caused by serotonin is usually most prominent on the two right heart valves, the tricuspid and pulmonic valves; the scarring also affects the right ventricular inner lining. Since serotonin is deactivated during the blood’s passage through the lungs, the left heart structures are usually spared the ravages of carcinoid heart disease.