100 million of sperm cells is needed to help make a tiny opening on the wall of the egg cell this hole can be made by their secretion called as hyaluronic acid this will sort of melt a small part that allows only 1 sperm to enter and closes automatically as one sperm is in, if only few secretion is present it wont be able to make an opening so no sperm cell can fertilize the ovum.
2.in my opinion the needs of the body is the one regulating the hormone level like when blood sugar level is high there will be an increase secretions of insulin to lower blood sugar level, the level of Insulin will decrease when blood sugar is at normal level and when our blood sugar has decreased due to absence of intake of food the hormone glucagon is released.
3. endocrine and nervous system work together to maintain homeostasis or internal balance of the body; nervous detects stimulus from environment so the body can react to this stimulus appropriately and if it needs the help of endocrine glands nervous system will send message to endocrine system for a response like in emergency situations when you extra energy your NS will send message for the release of emergency hormone. And as for reproductive system it plays a part for the health of reproductive functions such normal menstrual cycle for female and healthy sperm cells for male.
4. Do you mean when the environment is cold? when its cold and you star running your cells will undergo much cellular respiration to release energy from the foods taken into the body, as respiration is taking place you are given the energy for you to be able to do the running and it also releases heat so you wont be able to feel the coldness anymore. Chloe I hope this isn't late yet and I hope I answered it right. Good luck young sis.