When a blood vessel is damaged, blood platelets stick to the damaged region and release chemicals that attract more platelets to the region. At the site of infection, mast cells produce histamine which attracts white blood cells to the region. These in turn secrete chemoattractant cytokines that recruit more white blood cells to the region.During childbirth, the head of the baby presses on the cervix stimulating the pituitary gland to release oxytocin. This in turn causes the uterus to contract pushing the baby's head harder against the cervix and stimulating the release of more oxytocin.