Now, once they're inside the blood, they are water soluble and that means they do not need a carrier protein. And once they locate the target cells, these six proteins, these six hormones all bind to receptor proteins found on the plasma membrane of the target cells. So, because they can't actually pass across the cell membrane of the target cell, they have to bind to the surface of the plasma membrane of the target cells. So let's discuss each one of these hormones. Let's begin with the human growth hormone, also known as the growth hormone. So basically, the hormone that is released by the hypothalamus, that stimulates the release of the human growth hormone is known as the growth hormone releasing hormone or GHRH.