Diseases of the circulatory system (as described in Chapter 22) are the most common cause of death in the United States. Although several cardiovascular conditions are progressive (worsening over time), they are not normal changes that occur with aging. Myocardial infarction or stroke occurring in the elderly mainly results from the once seemingly inevitable progression of atherosclerosis. The development of several classes of blood lipid-lowering drugs in addition to more effective reduction of systemic hypertension has helped prolong life span.
During embryologic development (Figure 21-1), the two atria are connected by an opening in the atrial septum (foramen ovale), which allows blood to enter the left side of the heart, bypassing the high resistance of the pulmonary circulation in the uninflated fetal lung. At birth, decreased pulmonary vascular resistance and pressures allow closure of this foramen ovale. Unexpected neurologic injuries following anesthesia have been explained as resulting from “paradoxical embolism” (emboli gaining access to the left side of heart through a patent foramen ovale and then entering the cerebral circulation). Better monitoring of tissue oxygenation seems to have greatly decreased the incidence of these devastating events (or have at least provided alternate explanations).
Illustration of the fetal circulation just before birth showing the 3 shunts, the foramen ovale, ductus arteriosus and ductus venosus. Also shown is an estimation of the extent of oxygen saturation of the blood in the fetus. Note blood in the umbilical veins and ductus venosus has the highest oxygen saturation, approximately 80%.
The fetal circulation differs from that of the adult because of the presence of multiple parallel pathways including the placenta, the ductus venosus, a vessel from the umbilical vein that bypasses the liver, the foramen ovale, an opening between the right and left atria, and the ductus arteriosus (DA), a vessel connecting the pulmonary artery and the aorta. The two ventricles of the fetus pump in parallel; in the adult the two ventricles pump in series. The cardiac output in the fetus is equal to the output of the LV + the output of the RV whereas in the adult, CO of the LV = CO of the RV (Figure 21-1).
Gas exchange between the fetus and mother occurs in the placenta, as does the exchange of nutrients from the mother’s blood and waste products from the fetus. Basically the placenta performs the function of the lungs, GI tract, and kidneys. The placenta weighs about one-half kilograms at birth.
After the fertilized ovum is implanted in the wall of the uterus, blood-filled sinuses develop between the endometrium of the uterus and the surface of the embryo. These sinuses are supplied with arterial blood from ...