What is the smallest organ in the human body?

The smallest organ in the human body is the pineal gland.

Explanation

• The pineal gland, also called the pineal body, is a tiny, cone-shaped endocrine organ located deep in the brain, between the two hemispheres, near the center where the two halves of the thalamus meet.
• It measures roughly 5–8 millimeters in length—about the size of a grain of rice—making it the body’s smallest functional organ.

Structure and Location

• Situated in the epithalamus, it lies behind the third ventricle of the brain.
• Composed mainly of pinealocytes (hormone-secreting cells) supported by glial tissue.

Function

• The pineal gland secretes the hormone melatonin, which:
  • Regulates sleep–wake cycles or circadian rhythms.
  • Helps control seasonal biological cycles (like reproduction in some species).
  • Influences female reproductive hormones, affecting menstrual cycles and fertility.