An organ is a complex structure formed by 2 or more tissues. If we use this definition, then a corpuscle of touch (Meissner’s corpuscle) would be an organ and a very, very small one. Also, there are dozens or hundreds of lymph nodes around the body. They probably qualify as organs, and they are smaller than most other organs we think of.
In other words, it can be difficult to define what is an organ. The inner ear has a number of very small parts that I think are organs, including the ossicles (malleus, incus, and stapes), the organ of corti in the cochlea, and the very tiny organs of the vestibular/balance system (saccule, utricle and semi-circular canals).
Thus, a more appropriate question might be, “What is the smallest organ that is part of the gross anatomy of the human body?”
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid-base balance, and regulation of blood pressure. They serve the body as a natural filter of the blood, and remove wastes which are diverted to the urinary bladder. The kidney is approximately 11–14 cm in length, 6 cm wide and 4 cm thick.
The spleen is an organ with important roles in regard to red blood cells and the immune system. In healthy adult humans, it is approximately 11 centimetres (4.3 in) in length. It usually weighs between 150 grams (5.3 oz) and 200 grams (7.1 oz) and lies beneath the 9th to the 12th thoracic ribs.
The cerebellum ( little brain) is an organ located on the back of the (big) brain that plays an important role in motor control. It is also involved in some cognitive functions such as attention and language, and probably in some emotional functions such as regulating fear and pleasure responses. It has been estimated that its dimensions are 6 cm × 5 cm × 10 cm.
Hypothalamus is known as the ‘brain behind the endocrine system’. The automatic nervous system is governed by it. Located near the pituitary gland at the basal part of the skull, it also controls pituitary secretions. All the automatically adjusted factors such as body temperature, blood pressure, cardiovascular system and abdominal visceral regulation are controlled by the hypothalamus. As we grow older, hypothalamus requires support to maintain optimum level of performance. Dysfunction of the hypothalamus results in depression or abnormal responses to stress. It also leads to disturbances in the brain.
The gallbladder (cholecyst, gall bladder) is the second smallest organ that aids digestion and stores bile produced by the liver. In humans the loss of the gallbladder is usually easily tolerated. The gallbladder is a hollow system that sits just beneath the liver. In adults, the gallbladder measures approximately 8 cm in length and 4 cm in diameter when fully distended.
In healthy adult human males, average testicular volume is 18 cm³ per testis, with normal size ranging from 12 cm³ to 30 cm³. The average testicle size after puberty measures up to around 2 inches long, 0.8 inches in breadth, and 1.2 inches in height (5 x 2 x 3 cm).
The vertical measure of the eye is about 24 mm among adults, at birth about 16–17 mm (about 0.65 inch). The eyeball grows rapidly, increasing to 22.5–23 mm (approx. 0.89 in) by the age of three years. From then to age 13, the eye attains its full size. The volume is 6.5 ml (0.4 cu. in.) and the weight is 7.5 g (0.25 oz.)
3. Pituitary gland
Pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 g (0.02 oz.), in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity (sella turcica) covered by a dural fold (diaphragma sellae). The pituitary is functionally connected to the hypothalamus by the median eminence via a small tube called the infundibular stem (Pituitary Stalk). The pituitary fossa, in which the pituitary gland sits, is situated in the sphenoid bone in the middle cranial fossa at the base of the brain. The pituitary gland secretes nine hormones that regulate homeostasis.
2. Parathyroid gland
Parathyroid glands are small endocrine glands in the neck that produce parathyroid hormone. Humans usually have four parathyroid glands, which are usually located on the rear surface of the thyroid gland, or, in rare cases, within the thyroid gland itself or in the chest. Parathyroid glands control the amount of calcium in the blood and within the bones. They are about the size of a grain of rice and usually weigh between 25mg and 40mg in humans.
1. Pineal Gland
The pineal gland (also called the pineal body, epiphysis cerebri, epiphysis or the “third eye”) is the smallest endocrine gland in the human body. It produces the serotonin derivative melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions. Its shape resembles a tiny pine cone (hence its name), and it is located near the center of the brain, between the two hemispheres, tucked in a groove where the two rounded thalamic bodies join.
Also see which are the largest organs in the body.