Human organs, such as the spleen, stomach and kidneys, consist of different types of tissue and may be grouped into structural and functional units that perform distinct functions. Organs are parts of major body systems that do not develop at the same rate.
Twelve organ systems make up a normally functioning human body: circulatory, digestive, endocrine, immune, integumentary, lymphatic, muscular, nervous, reproductive, skeletal, urinary/osmoregulatory, and respiratory. Even though all the organ systems work cooperatively, each system and each organ performs distinct functions within the body. Certain human organs, including the heart, lungs and liver, are called "vital," meaning necessary for life. Although they continue to grow in size, most are fully developed in functionality at birth.
Embryonic Organ Development
During early embryonic development, organ systems develop on an "as needed" basis. The brain and spinal cord, for example, which are required to control and coordinate the functions of other major organ systems of the body, are the first to form. At the end of the third week, the embryo is about two millimetres long and the blood vessels and alimentary canal have begun to develop. By the end of the fourth week, all major organs have begun to develop and the heart beats for the first time.
Fetal Organ Growth
Around the end of the eighth week of embryonic development, the fetal stage begins, when all the basic organ systems of the adult are established (though not all functioning) and recognisable. Around the 11th or 12th week, the external reproductive organs are visible. During the fetal stage, major organs grow significantly and body weight multiplies 600 times. Of the major vital organs, the lungs are the least developed at birth. Much postnatal lung development involves alveolar (air sac) formation, as alveoli and alveolar ducts increase in number.
Infancy and Childhood
During infancy and early childhood, lung development continues and rapid growth in the brain occurs as neural connections are formed and reorganised. Infancy is the time of fastest growth in a human after birth. Proportions change as the "top-heavy" body of the newborn develops into a more efficient stature conducive to standing with balance, as the other organ systems, arms, legs, and torso catch up with the brain and skull. Growth slows in childhood, but the brain continues to be the fastest-growing organ, increasing from 70 per cent to 90 per cent of its adult weight by age 6.
Adolescence to Adulthood
From adolescence to full adulthood, both genders exhibit improved fine motor skills as a result of increased further development of the central nervous system. Major internal organs increase in proportion to the growth of the skeletal and muscular system components, with boys normally displaying significant growth up to age 18. In adolescence, a human body exhibits the most relative organ growth in the reproductive system, as physical changes spurred by hormones result in obvious gender differences. With the conclusion of adolescence, organ growth is complete.
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