From birth to adulthood, infants are under constant growth and development. They are physically and functionally different from adults, for they have their own characteristics and laws. The younger they are, the more distinct these characteristics and laws will be. Therefore infants can never be regarded as the epitome of the adults. Doctors of past dynasties had made detailed discussions on the physiological characteristics. 

Their discussions can be summarized in the following two aspects: delicate zang-fu organs and underdeveloped physique and qi, vigorous vitality and rapid development. A full recognition of the physiological characteristics of the infants is helpful for understanding the developing rules of the infants, health care and diagnosis of diseases. 

 

The zang-fu organs refer to both the zang organs and fu organs. Delicacy means weakness, liability to attack and tenderness. The physique here refers to the configuration of the human body such as the limbs, skeleton, musculature, essence, blood and body fluid. Qi refers to physiological functional activities, such as lung qi, spleen qi and kidney qi. 

Delicacy of the Zang fu organs and underdevelopment of the physique and qi mean that, during the childhood, the morphological development and the physiological functions are immature and imperfect, and that the development and functions of the viscera are insufficient. Since the physical substances of the body pertain to yin and the functional activities of the body pertain to yang, doctors in the ancient times summarized the physiological characteristics of the infants as underdeveloped yin and yang.

The idea that the viscera of the infants are delicate and the body of the infants is underdeveloped is characterized by delicacy of the muscles and skin, loose striae, insufficient qi and blood, weak lung and spleen, insufficient kidney qi, timid will and fragile bones and muscles. 

For instance, from birth to the infancy, the infants are marked by big head, thin neck, longer upper portion and shorter lower portion. Within the period of one year and a half, the infants are prominent for metopism, thin and soft skin, loose muscular interstitial striae, yellowish and thin hair, liability to sweat during sleep, short voice in crying and instantaneous changes in emotions. The functions of both the five zang organs and six fu organs are underdeveloped, especially the lung, the spleen and the kidney.

(1) Constant insufficiency of lung  

The lung governs qi flowing all through the body. It is related to the skin, the hair and the striae externally. If the lung is delicate, defensive qi will be unconsolidated, making it easier to be attacked by the exogenous pathogenic factors. The lung depends on the spleen to provide nutrients. If the spleen and the stomach are vigorous, defensive qi will be consolidated. However, the spleen of the infants tends to be weak in functions, thus the lung qi of the infants is usually insufficient.

The spleen, the postnatal base of life, governs the transformation and transportation of the cereal nutrients and is the source for the generation of blood and qi. Since the spleen and stomach of the infants are underdeveloped, spleen qi is insufficient and the transforming and transporting functions are always weak. On the other hand, the infants are growing fast and need more nutrients than the adults. That is why the spleen qi often appears insufficient. 

The kidney is the congenital base of life. Kidney yin and kidney yang are the sources of life concerned with the constitution and growth of man. Yin of each organ depends on kidney yin to moisten and nourish it, while yang of each organ relies on kidney yang to warm and nourish it. Many aspects in the development of the infants are concerned with kidney qi, such as the growth, resistance of the body as well as the normal development and functions of the bones, brain, hair, ears and teeth. 

The kidney qi of the infants is underdeveloped congenitally and gradually becomes abundant with the increase of the age. During the growth and development of the body, the infants are in constant need of sufficient kidney qi to promote the growth. That is why kidney qi of the infants of ten appears insufficient.

Besides, the physiological functions of the infantile five zang organs are also marked by constant abundance of the liver and constant abundance of the heart. The former does not refer to hyperactivity of liver yang which means that during the development of the infants the liver governs smooth flow of qi and has the function of sending up and smoothing flow of qi in the whole body. The latter does not mean hyperactivity of the heart fire. In fact it refers to the phenomena in the infants marked by rapid development, vigorous heart qi and full vitality. 

Vitality refers to life-force. Full vitality and fast development mean that during the development and growth, the infants quickly become mature and perfect in both the morphological structure and physiological functions. The ancient doctors summarized such characteristics of infantile development as pure yang. The younger the infants are, the more vigorous their vitality and the faster their growth will be. 

Such a fast development and growth actually follows certain principles and typical changes, the manifestations of which include two aspects: physical development and mental development. Physically the weight of the infants of one year old is three times as much as, the length of their body is one and a half times longer and the head is half a time bigger than that when they were just born. This is the period marked by fast growth and development. As their ages increase, the speed of growth and development gradually slows down.