Normal newborns: The period from birth, when the umbilical cord is tied, until 28 days before the whole day is defined as the neonatal period. The vast majority of newborns are delivered at full term, i.e. at 37 weeks (259 days) or more of gestational age, with a birth weight of more than 2500g and without any disease.
1.Breathing
The fetus has weak respiratory movements, but respiration is in a state of inhibition. At birth, due to the stimulation of proprioceptors and skin temperature receptors, which reflexively excite the respiratory center, most newborns start to breathe more regularly. It is thought that umbilical cord ligation causes sympathetic excitation, which accelerates the circulation of resentful flow through the carotid artery and increases the sensitivity of this receptor, which also acts on respiration. If there is a short period of asphyxia, the changes in blood and gas concentration and pH act as a powerful chemical stimulus, which stimulates the functional activity of the reticular formation and brings the excitability of the respiratory center to a new level.
Neonatal respiratory movements are more superficial, but the respiratory rate is fast (about 35-45 breaths), so the relative respiratory volume per minute is not lower than that of adults. Large fluctuations in respiratory rate during the first two weeks of life are normal for newborns. Short-term increases in respiratory rate >80 breaths/min are not clinically significant. When in the fast-acting eye sleep phase, breathing is often irregular and may be accompanied by a 3-5 s pause. In the non-REM phase, breathing is generally regular and superficial.
2.Circulation
The following important kinetic changes in the fetal circulation occur after birth, which are reciprocal with the anatomical changes
(1) Ligation of the umbilical vessels, which is artificially induced.
(2) The expansion and ventilation of the lungs resulting in lower resistance to pulmonary circulation.
(3) Functional closure of the foramen ovale, which is due to a decrease in right heart pressure and an increase in left heart pressure following a decrease in pulmonary vascular resistance. At this time, blood still passes through the arterial catheter from the left to the right shunt, playing a role in increasing the partial pressure of oxygen in the circumferential solid blood. Some newborns hear a heart murmur in the first few days, which may be related to the temporary non-closure of the ductus arteriosus.
The distribution of blood flow in normal newborns is mostly concentrated in the trunk and viscera, but less in the extremities, so the liver and spleen are easily palpable, while the extremities are easily cooled and the bundle tips are easily bruised. The distribution of blood flow in the brain is also unbalanced. The parsagittal area of the brain in full-term infants and the white matter around the ventricles in preterm infants are the areas with the least distribution of cerebral blood flow, which can easily cause ischemic injury in these areas when systemic hypotension is present.
The heart rate of normal term newborns is usually regular, 120-160 beats/min. Sometimes transient heart rate fluctuations can occur. Blood pressure ranges from 6.66/4kPa (50/30mmHg) to 10.66/6.66kPa (80/50mmHg).
3.Urinary
The fetal kidney at birth already has the same number of renal units as the adult, but is histologically immature, with insufficient filtration area and even less tubular volume, so the kidney’s function is only able to accommodate the normal metabolic burden in general, with limited potential.
The glomerular filtration rate, calculated from the body surface area, is only 1/4-1/2 of that of adults, and then gradually increases with the rise of blood pressure, the decrease of renal vascular resistance, the increase of filtration area and the improvement of basement membrane permeability.
Concentration function is relatively insufficient, with a maximum concentration capacity of 500-700 mOsm/L. Therefore, feeding newborns with a thicker milk formula can lead to an increase in blood urea nitrogen concentration. Renal dilution is still possible, with a minimum solute concentration of 50 mOsm/L in urine and limited acidification of urine under increased load discretion.
In neonates, blood chloride and lactate levels are higher due to inadequate renal function. Blood phosphorus and urine phosphorus are high in artificial feeders, which can easily cause imbalance of calcium and phosphorus balance and produce low blood calcium.
Most newborns urinate soon after birth, and if they are not fed enough, they may urinate only a small amount on the first day after birth.
4.Digestion
The digestive tract is relatively large and the muscle layer is thin, so it can adapt to the digestion and absorption of a large amount of liquid food. Swallowing function is perfect. Air is seen in the gastric sac soon after birth. The pharyngeal-esophageal sphincter does not close during swallowing and the esophagus does not peristaltic. The lower esophageal sphincter also does not close, so overflow is likely to occur. The entire gastrointestinal tract, especially the lower gastrointestinal tract, is more chivalrous in movement, and the air swallowed at birth reaches the rectum within 3-4 hours.
Neonatal saliva secretion less, often neutral or even acidic reaction, gastric acid in the postnatal temporary significantly higher, the first day after the gradual decline to the eighth day of free acid to zero, the newborn digestive tract can secrete sufficient digestive enzymes, but the pancreatic amylase to the postnatal 4 months to choose the adult level. The secretion of this enzyme may be promoted if starchy foods are fed earlier.
Newborns digest protein well, and rennet in their stomachs plays a larger role. The intestinal wall has a greater permeability, which facilitates the absorption of immunoglobulins in colostrum, so the concentration of IgA, IgG and IgM in the blood of breastfed infants is higher than that of cow’s milk-fed ones. However, other protein molecules can produce allergy through the intestinal wall, such as cow’s milk allergy and soy protein allergy.
Neonatal gastric lipolytic enzymes play a greater role in the digestion of fat. Human milk fat is absorbed at 85-90%, while cow’s milk fat is absorbed at a lower rate.
If the feces are not seen for 24 hours after birth, it is advisable to conduct an examination to exclude congenital anomalies such as anal atresia or megacolon.