Physiological Specifics of Babies

Physiological Specifics of Babies

Babies are not miniature adults, but rather have unique physiological specifics that require they be treated very differently. Due to their high metabolism and smaller mass in relation to organ systems, the normal physiological statistics of a baby are quite different from those of an adult.

A newborn baby typically weighs between 3 and 4 kilograms. Its resting heart rate is very rapid compared to an adult, between 100-160 beats per minute. Similarly, its rspiration rate is generally between 30 and 60 breaths per minute, which would seem like panting in an adult.

As the baby ages, its metabolism begins to slow a bit. At 6 months, with doubled weight or better, its pulse speeds up just a little to 110-160 beats per minute, but respiration slows dramatically to 24 to 38 breaths per minute. At a year, it will weigh around 10 kilograms, with pulse at 90 to 150 beats per minute and respiration of 22-30 breaths per minute.

A baby’s blood pressure tends to be lower than that of a child or adult. Systolic in a baby is generally around 70, increasing to 72 in a one-year-old child. Diastolic blood pressure can range from 34 to 66 in newborns, averaging around 50 to 55, and increases about 5 points overall in a one-year-old infant.

A baby’s temperature should be about the same as an adult’s, 98.6 F or 37 C. If temperature is taken axillary (under the arm), it will be a bit lower, 97.5 to 99.3 F or 36.5 to 37.4 C. Rectal temperature is a little higher, but should never exceed 100.2 F or 37.9 C. At 6 months, a baby’s temperature can be taken with an ear thermometer, and should range within one degree of the normal adult temperature.

A baby’s increased heart rate and respiratory rate, as well as its heightened metabolism, puts it at increased risk to a number of dangers adults would ignore. For instance, babies inhale more pollutants in relation to their masses. Similarly, medications taken by a baby will metabolize faster, making it difficult to properly gauge an efficacious dose.

Because babies are so small, it takes much less time for illnesses to take full effect in their bodies, and this may cause an outsized immune system response. Any parent who has observed a baby go from a normal temperature to a dangerously high one has seen this issue in action. For similar reasons, a baby will lose liquids (in volume compared to mass) much more rapidly than an adult. Parents should always watch sick infants closely for quick and sometimes dramatic physical changes.

Image: © Ruslan Olinchuk /

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