Medical brain notes 44

  Growth and Functional Development of the Fetus Initial development of the placenta and fetal membranes occurs far more rapidly than development of the fetus itself. In fact, during the first 2 to 3 weeks after implantation of the blastocyst, the fetus remains almost microscopic, but thereafter, as shown in Figure 83–1, the length of the fetus increases almost in proportion to age. At 12 weeks, the length is about 10 centimeters; at 20 weeks, 25 centimeters; and at term (40 weeks), 53 centimeters (about 21 inches). Because the weight of the fetus is approximately proportional to the cube of length, the weight increases almost in proportion to the cube of the age of the fetus.         Note in Figure 83–1 that the weight remains minuscule during the first 12 weeks and reaches 1 pound only at 23 weeks (5  1 / 2  months) of gestation. Then, during the last trimester of pregnancy, the fetus gains tremendously, so that 2 months before birth, ...

  Adjustments of the Infant to Extrauterine Life Onset of Breathing The most obvious effect of birth on the baby is loss of the placental connection with the mother and, there-fore, loss of this means of metabolic support. One of the most important immediate adjustments required of the infant is to begin breathing. Cause of Breathing at Birth.  After normal delivery from amother who has not been depressed by anesthetics, the child ordinarily begins to breathe within seconds and has a normal respiratory rhythm within less than 1 minute after birth. The promptness with which the fetus begins to breathe indicates that breathing is initiated by sudden exposure to the exterior world, probably result-ing from (1) a slightly asphyxiated state incident to the birth process, but also from (2) sensory impulses that originate in the suddenly cooled skin. In an infant who does not breathe immediately, the body becomes progressively more hypoxic and hypercapnic, which provides additional s...

  Onset of Breathing The most obvious effect of birth on the baby is loss of the placental connection with the mother and, there-fore, loss of this means of metabolic support. One of the most important immediate adjustments required of the infant is to begin breathing. Cause of Breathing at Birth.  After normal delivery from amother who has not been depressed by anesthetics, the child ordinarily begins to breathe within seconds and has a normal respiratory rhythm within less than 1 minute after birth. The promptness with which the fetus begins to breathe indicates that breathing is initiated by sudden exposure to the exterior world, probably result-ing from (1) a slightly asphyxiated state incident to the birth process, but also from (2) sensory impulses that originate in the suddenly cooled skin. In an infant who does not breathe immediately, the body becomes progressively more hypoxic and hypercapnic, which provides additional stimulus to the respiratory center and usually c...

  Circulatory Readjustments at Birth Equally as essential as the onset of breathing at birth are immediate circulatory adjustments that allow ade-quate blood flow through the lungs. Also, circulatory adjustments during the first few hours of life cause more and more blood flow through the baby’s liver, which up to this point has had very little blood flow. To describe these readjustments, we must first consider the anatom-ical structure of the fetal circulation. Specific Anatomical Structure of the Fetal Circulation.  Becausethe lungs are mainly nonfunctional during fetal life and because the liver is only partially functional, it is not necessary for the fetal heart to pump much blood through either the lungs or the liver. However, the fetal heart must pump large quantities of blood through the placenta. Therefore, special anatomical arrangements cause the fetal circulatory system to operate much differently from that of the newborn baby. First, as shown in Figure 83–4, blood...

  Nutrition of the Neonate Before birth, the fetus derives almost all its energy from glucose obtained from the mother’s blood. After birth, the amount of glucose stored in the infant’s body in the form of liver and muscle glycogen is sufficient to supply the infant’s needs for only a few hours. The liver of the neonate is still far from functionally adequate at birth, which prevents significant gluconeogenesis. Therefore, the infant’s blood glucose concentration frequently falls the first day to as low as 30 to 40 mg/dl of plasma, less than one half the normal value. Fortunately, however, appropriate mechanisms are available for the infant to use its stored fats and proteins for metabolism until mother’s milk can be provided 2 to 3 days later. Special problems are also frequently associated with getting an adequate fluid supply to the neonate because the infant’s rate of body fluid turnover averages seven times that of an adult, and the mother’s milk supply requires several days t...