Pediatric Physiology – Basics and Clinical Relevance

What is Pediatric Physiology?

Pediatric physiology is the branch of physiology that examines the normal biological functions and processes of the human body during childhood and adolescence. It covers all organ systems – from the cardiovascular and respiratory systems to the nervous system, metabolism, and hormones – and investigates how these systems function and change during growth and maturation. A thorough understanding of pediatric physiology is fundamental to pediatric medicine, as many diagnostic and therapeutic measures must be adapted to the age and developmental stage of the child.

Key Characteristics of Childhood Physiology

Children are not simply small adults. Their organs, tissues, and regulatory mechanisms are in a continuous process of development and maturation, with far-reaching implications for diagnosis, treatment, and medication dosing.

Cardiovascular System

Newborns and infants have a significantly higher heart rate than adults. In newborns, heart rates of 120–160 beats per minute are normal; this decreases gradually with age. Blood pressure is also lower in young children and rises progressively into adulthood. Cardiac output relative to body weight is higher, reflecting the greater oxygen demand per kilogram in children.

Respiratory System

Children breathe faster than adults because their tidal volumes are smaller. Respiratory rates range from 30–60 breaths per minute in newborns, to 20–30 in toddlers, and 15–20 in school-age children. The airways are narrower, making children more susceptible to obstructive conditions. The relatively low functional residual capacity means children can deteriorate rapidly during respiratory distress.

Nervous System

The central nervous system undergoes intensive development during childhood. Myelination – the formation of the protective myelin sheath around nerve fibers – is incomplete at birth and is not fully achieved until early adulthood. This accounts for the progressive motor and cognitive development seen in the first years of life. Primitive reflexes such as the Moro reflex and grasp reflex are characteristic of the neonatal period and disappear as neurological maturation progresses.

Kidneys and Fluid Balance

Renal function is limited in newborns. The glomerular filtration rate (GFR) does not reach near-adult values until the second year of life. Children have a higher total body water content and a more rapid fluid turnover, making them more vulnerable to dehydration. The concentrating ability of the kidneys is also limited during the first months of life.

Metabolism and Thermoregulation

The basal metabolic rate relative to body surface area is considerably higher in children than in adults. Infants and toddlers have a larger surface-area-to-volume ratio, resulting in greater heat loss and increased susceptibility to hypothermia. Brown adipose tissue plays an important role in non-shivering thermogenesis in newborns.

Immune System

The immune system of a newborn is not yet fully mature. Neonates are protected from infection by maternal antibodies (IgG) transferred transplacentally. This passive immunity declines during the first months of life as the infant's own immune system gradually matures. Breastfeeding provides additional protection through secretory IgA and other immune factors.

Hormonal System and Growth

Growth hormone (somatotropin) and insulin-like growth factor IGF-1 play a central role in childhood growth. The growth plates (epiphyseal plates) in the long bones allow longitudinal growth and fuse during puberty under the influence of sex hormones. Puberty itself is a complex physiological process triggered by the activation of the hypothalamic-pituitary-gonadal axis.

Clinical Relevance of Pediatric Physiology

Knowledge of the physiological characteristics of childhood is essential in pediatric clinical practice. Reference ranges for laboratory values, vital signs, and imaging findings must always be interpreted in an age-appropriate context. Medication dosages in pediatrics are frequently calculated based on body weight or body surface area, as the absorption, distribution, metabolism, and excretion of drugs differ substantially between children and adults.

• Age-dependent normal values for heart rate, blood pressure, and respiratory rate
• Weight- or body surface area-based medication dosing
• Special protection against dehydration and hypothermia
• Early detection of developmental delays through knowledge of maturation processes
• Vaccination schedules aligned with the maturation of the childhood immune system

Developmental Stages at a Glance

Pediatric physiology is structured according to the following developmental phases:

• Neonatal period: 0–28 days of life
• Infancy: 1 month to end of the 1st year of life
• Toddler age: 1st–3rd year of life
• Preschool age: 3rd–6th year of life
• School age: 6th–12th year of life
• Puberty and adolescence: from approximately 10–12 years to approximately 18 years of age

References

1. Kliegman, R.M. et al.: Nelson Textbook of Pediatrics. 21st Edition. Elsevier, Philadelphia 2020.
2. World Health Organization (WHO): Child Growth Standards. URL: https://www.who.int/tools/child-growth-standards (accessed 2024).
3. Rudolph, A.M. et al.: Rudolph's Pediatrics. 22nd Edition. McGraw-Hill Education, New York 2011.