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Red blood count

Red blood count

Bilirubin

6 8 10 12 14 16 Age in weeks

Figure 83-6

Changes in the red blood cell count and in serum bilirubin concentration during the first 16 weeks of life, showing physiologic anemia at 6 to 12 weeks of life and physiologic hyperbilirubinemia during the first 2 weeks of life.

count to normal within another 2 to 3 months. Immediately after birth, the white blood cell count of the neonate is about 45,000 per cubic millimeter, which is about five times as great as that of the normal adult.

Neonatal Jaundice and Erythroblastosis Fetalis. Biliru-bin formed in the fetus can cross the placenta into the mother and be excreted through the liver of the mother, but immediately after birth, the only means for ridding the neonate of bilirubin is through the neonate's own liver, which for the first week or so of life functions poorly and is incapable of conjugating significant quantities of bilirubin with glucuronic acid for excretion into the bile. Consequently, the plasma biliru-bin concentration rises from a normal value of less than 1 mg/dl to an average of 5 mg/dl during the first 3 days of life and then gradually falls back to normal as the liver becomes functional. This effect, called physiologic hyperbilirubinemia, is shown in Figure 83-6, and it is associated with mild jaundice (yellowness) of the infant's skin and especially of the sclerae of its eyes for a week or two.

However, by far the most important abnormal cause of serious neonatal jaundice is erythroblastosis fetalis, which is discussed in detail in Chapter 32 in relation to Rh factor incompatibility between the fetus and mother. Briefly, the erythroblastotic baby inherits Rh-positive red cells from the father, while the mother is Rh negative. The mother then becomes immunized against the Rh-positive factor (a protein) in the fetus's blood cells, and her antibodies destroy fetal red cells, releasing extreme quantities of bilirubin into the fetus's plasma and often causing fetal death for lack of adequate red cells. Before the advent of modern obstetrical therapeutics, this condition occurred either mildly or seriously in 1 of every 50 to 100 neonates.

Fluid Balance, Acid-Base Balance, and Renal Function

The rate of fluid intake and fluid excretion in the newborn infant is seven times as great in relation to weight as in the adult, which means that even a slight percentage alteration of fluid intake or fluid output can cause rapidly developing abnormalities.

The rate of metabolism in the infant is also twice as great in relation to body mass as in the adult, which means that twice as much acid is normally formed, which gives a tendency toward acidosis in the infant. Functional development of the kidneys is not complete until the end of about the first month of life. For instance, the kidneys of the neonate can concentrate urine to only 1.5 times the osmolality of the plasma instead of the adult three to four times. Therefore, considering the immaturity of the kidneys, together with the marked fluid turnover in the infant and rapid formation of acid, one can readily understand that among the most important problems of infancy are acidosis, dehydration, and, more rarely, overhydration.

Liver Function

During the first few days of life, liver function in the neonate may be quite deficient, as evidenced by the following effects:

1. The liver of the neonate conjugates bilirubin with glucuronic acid poorly and therefore excretes bilirubin only slightly during the first few days of life.

2. The liver of the neonate is deficient in forming plasma proteins, so that the plasma protein concentration falls during the first weeks of life to 15 to 20 per cent less than that for older children. Occasionally the protein concentration falls so low that the infant develops hypoproteinemic edema.

3. The gluconeogenesis function of the liver is particularly deficient. As a result, the blood glucose level of the unfed neonate falls to about 30 to 40 mg/dl (about 40 per cent of normal), and the infant must depend mainly on its stored fats for energy until sufficient feeding can occur.

4. The liver of the neonate usually also forms too little of the blood factors needed for normal blood coagulation.

Digestion, Absorption, and Metabolism of Energy Foods; and Nutrition

In general, the ability of the neonate to digest, absorb, and metabolize foods is no different from that of the older child, with the following three exceptions.

First, secretion of pancreatic amylase in the neonate is deficient, so that the neonate uses starches less adequately than do older children.

Second, absorption of fats from the gastrointestinal tract is somewhat less than that in the older child. Consequently, milk with a high fat content, such as cow's milk, is frequently inadequately absorbed.

Third, because the liver functions imperfectly during at least the first week of life, the glucose concentration in the blood is unstable and low.

The neonate is especially capable of synthesizing and storing proteins. Indeed, with an adequate diet, as much

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