M6.2

• The cardiovascular system is one of the first systems to become functional in intrauterine life
• Development of the cardiovascular system


  1. Simple blood cells join yolk sac

  2. Form network of blood vessels

  3. Form single heart tube

  4. Heart beats by 24th day
• Heartbeat may be heard with a Doppler instrument

  10th to 12th week
• An electrocardiogram (ECG) may be recorded on a fetus

  11th week
• Early ECGs are not accurate until about the 20th week of pregnancy
• The heart rate of a fetus is affected by oxygen level, activity, and circulating blood volume
• After the 28th week of pregnancy, the heart rate stabilizes
• A consistent heart rate of 110 to 160 beats per minute is assessed after the 28th week of pregnancy
• Fetal circulation differs from extrauterine circulation
• Fetal circulation

  Fetus derives oxygen and excretes carbon dioxide from exchange in the placenta
• Blood arriving at the fetus from the placenta is highly oxygenated
• Umbilical vein


  Carries oxygenated blood to the fetal heart
• Blood flow in fetal circulation

  1. From umbilical vein to ductus venosus

  2. Discharges into fetal liver

  3. Connects to fetal inferior vena cava

  4. Directed to right side of heart
• The bulk of blood is shunted from the right atrium to the left atrium through the foramen ovale
• Path of oxygenated blood


  1. Left atrium to left ventricle

  2. Into aorta

  3. Out to body parts
• A small amount of blood leaves the right atrium by the adult circulatory route
• Blood flow from right atrium


  1. Through tricuspid valve

  2. Into right ventricle

  3. Into pulmonary artery

  4. To lungs
• The larger portion of blood is shunted away from the lungs through the ductus arteriosus
• Blood is transported from the descending aorta through the umbilical arteries back to the placental villi
• At birth, an infant's oxygen saturation level is 95% to 100%
• A newborn's pulse rate is 80 to 140 beats per minute
• The oxygen saturation level of fetal blood reaches only about 80%
• A normal fetal heart rate is 110 to 160 beats per minute
• Carbon dioxide does not accumulate in the fetal system
• Fetal hemoglobin


  Differs from adult hemoglobin in composition and efficiency
• Fetal hemoglobin has a different composition (two alpha and two gamma chains)
• A newborn's hemoglobin level is about 17.1 g per 100 mL
• A normal adult hemoglobin level is 11 g per 100 mL
• A newborn's hematocrit is about 53%
• A normal adult hematocrit level is 45%
• The change from fetal to adult hemoglobin levels begins before birth
• Major blood dyscrasias tend to be defects of the beta-hemoglobin chain
• Clinical symptoms of disorders do not become apparent until about 6 months of age
• The respiratory and digestive tracts exist as a single tube at the third week of intrauterine life
• Development of the respiratory system

  1. Septum divides esophagus from trachea

  2. Lung buds appear on trachea

  3. Diaphragm divides thoracic cavity from abdomen
• Spontaneous respiratory practice movements begin as early as 3 months gestation
• Specific lung fluid forms in alveoli to aid in expansion at birth
• Surfactant is formed and excreted by alveolar cells beginning at approximately the 24th week of pregnancy
• Surfactant decreases alveolar surface tension on expiration
• Surfactant has two components: lecithin and sphingomyelin