There is a channel leading to the heart called Manovaha. It draws from every part of the human body the seminal fluid which is born of desire.
मध्ये च हृदयस्यैका शिरा तत्र मनोवहा। शुक्र संकल्पजं नृणां सर्वगात्रैर्विमुञ्चति॥