The decrease in pulmonary vascular resistance causes the pressure in the pulmonary artery to fall to approximately half its previous level (to ≈35 mm Hg). This change in pressure, coupled with a slight increase in aortic pressure, reverses the flow of blood through the ductus arteriosus. However, within several minutes, the large ductus arteriosus begins to constrict. This constriction produces turbulent flow, which is manifested as a murmur in newborn infants. Constriction of the ductus arteriosus is progressive and usually complete within 1 to 2 days after birth. Closure of the ductus arteriosus appears to be initiated by the high PO2 of the arterial blood passing through it; pulmonary ventilation with O2 closes the ductus, whereas ventilation with air low in O2 opens this shunt vessel. Whether O2 acts directly on the ductus or through the release of a vasoconstrictor substance is not known.
At birth, the walls of the two ventricles have approximately equal thickness. In addition, the muscle layer of the pulmonary arterioles is thick; this thickness is partly responsible for the high pulmonary vascular resistance of the fetus. After birth, the thickness of the walls of the right ventricle diminishes, as does the muscle layer of the pulmonary arterioles. In contrast, the left ventricular walls become thicker. These changes progress over a period of weeks after birth and reflect the effects of different hemodynamic forces (e.g., vascular resistance) on the two ventricles. Cardiac hypertrophy underlies the increase of heart weight during the normal growth period after birth. The physical demands imposed by the developing cardiovascular system, together with increased levels of soluble factors (e.g., growth hormone, insulin-like growth factor-1), account for physiological hypertrophy by which left ventricular mass more than doubles during the period from birth to early adulthood.
Theductusarteriosusoccasionallyfailstocloseafterbirth.Inthenewborn,thiscongenitalcardiovascularabnormality,calledpatent ductus arteriosus, cansometimesbecorrectedbytheadministrationofnonsteroidalanti-inflammatoryagentssuchasibuprofen.Ifthisdoesnotresultinclosureoftheductusorifthechildisolder,closuremustbeachievedsurgically.
1.
The vascular system is composed of two major subdivisions: the systemic circulation and the pulmonary circulation. These subdivisions are in series with each other and are composed of a number of vessel types (e.g., arteries, arterioles, capillaries) that are aligned in series with one another. In general, the vessels of a given type are arranged in parallel with each other.
2.
The mean velocity (v) of blood flow in a given type of vessel is directly proportional to the total blood flow being pumped by the heart, and it is inversely proportional to the cross-sectional area of all the parallel vessels of that type.
3.
Poiseuille’s law characterizes blood flow that is steady and laminar in vessels larger than arterioles. However, blood flow is nonnewtonian in very small blood vessels (i.e., Poiseuille’s law is not applicable).
4.
Flow tends to become turbulent when (1) flow velocity is high, (2) fluid viscosity is low, (3) fluid density is great, (4) vessel diameter is large, or (5) the wall of the vessel is irregular.
5.
Arteries not only conduct blood from the heart to the capillaries but also store some of the ejected blood during each cardiac systole. Hence, blood flow continues through the capillaries during cardiac diastole. Veins return blood to the heart from the capillaries and have a relatively low resistance and high capacitance that enables them to serve as reservoirs for blood.
6.
The aging process diminishes compliance of the arteries, as well as of the veins. The less compliant the arteries are, the more work the heart must do to achieve a given cardiac output. The less compliant the veins are, the poorer is their ability to store blood.
7.