Ganong's Review of Medical Physiology, 23rd Edition

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582 SECTION VI Cardiovascular Physiology O 2 saturation Relative units FIGURE 34–15 Changes in uterine blood flow and the amount of O 2 in uterine venous blood during pregnancy. (After Barcroft H. Modified and redrawn with permission from Keele CA, Neil E: Samson Wright’s Applied Physiology, 12th ed. Oxford University Press, 1971.) size of the conceptus increases much more, changing from a single cell to a fetus plus a placenta that weighs 4 to 5 kg at term in humans. Consequently, more O 2 is extracted from the uterine blood during the latter part of pregnancy, and the O 2 saturation of uterine blood falls. Corticotrophin-releasing hormone appears to play an important role in up-regulating uterine blood flow, as well as in the eventual timing of birth. PLACENTA Uterine blood flow The placenta is the “fetal lung” (Figures 34–16 and 34–17). Its maternal portion is in effect a large blood sinus. Into this “lake” project the villi of the fetal portion containing the small branches of the fetal umbilical arteries and vein (Figure 34–16). O 2 is taken up by the fetal blood and CO 2 is discharged into the maternal circulation across the walls of the villi in a fashion analogous to O 2 and CO 2 exchange in the lungs (see Chapter 36). However, the cellular layers covering the villi are thicker and less permeable than the alveolar membranes in the lungs, and exchange is much less efficient. The placenta is also the route by which all nutritive materials enter the fetus and by which fetal wastes are discharged to the maternal blood. FETAL CIRCULATION Parturition Uterine venous blood Parturition Time after conception Fetal weight Systemic venous blood The arrangement of the circulation in the fetus is shown diagrammatically in Figure 34–17. Fifty-five percent of the fetal cardiac output goes through the placenta. The blood in the umbilical vein in humans is believed to be about 80% saturated with O 2 , compared with 98% saturation in the arterial circulation of the adult. The ductus venosus (Figure 34–18) diverts Amnion Septum Umbilical arteries Umbilical vein Umbilical cord Chorion Villus Intervillous space Spiral arteriole Basal plate Chorionic plate Endometrium Myometrium FIGURE 34–16 Diagram of a section through the human placenta, showing the way the fetal villi project into the maternal sinuses. (Reproduced with permission from Benson RC: Handbook of Obstetrics and Gynecology, 8th ed. Originally published by Appleton & Lange. Copyright © 1983 McGraw-Hill.) some of this blood directly to the inferior vena cava, and the remainder mixes with the portal blood of the fetus. The portal and systemic venous blood of the fetus is only 26% saturated, and the saturation of the mixed blood in the inferior vena cava is approximately 67%. Most of the blood entering the heart through the inferior vena cava is diverted directly to the left atrium via the patent foramen ovale. Most of the blood from the superior vena cava enters the right ventricle and is expelled into the pulmonary artery. The resistance of the collapsed lungs is high, and the pressure in the pulmonary artery is several mm Hg higher than it is in the aorta, so that most of the blood in the pulmonary artery passes through the ductus arteriosus to the aorta. In this fashion, the relatively unsaturated blood from the right ventricle is diverted to the trunk and lower body of the fetus, while the head of the fetus receives the betteroxygenated blood from the left ventricle. From the aorta, some of the blood is pumped into the umbilical arteries and back to the placenta. The O 2 saturation of the blood in the lower aorta and umbilical arteries of the fetus is approximately 60%. FETAL RESPIRATION The tissues of fetal and newborn mammals have a remarkable but poorly understood resistance to hypoxia. However, the O 2 saturation of the maternal blood in the placenta is so low that the fetus might suffer hypoxic damage if fetal red cells did not
DA Placenta Body L heart Lungs R heart Superior vena cava Foramen ovale Right atrium Right ventricle Ductus venosus Portal vein Umbilical vein From placenta FO DA Body L heart Lungs R heart Body L heart Lungs R heart FETUS NEWBORN ADULT Umbilical arteries Left atrium Inferior vena cava To placenta Ductus arteriosus Pulmonary artery Left ventricle Aorta FIGURE 34–18 Circulation in the fetus. Most of the oxygenated blood reaching the heart via the umbilical vein and inferior vena cava is diverted through the foramen ovale and pumped out the aorta to the head, while the deoxygenated blood returned via the superior vena cava is mostly pumped through the pulmonary artery and ductus arteriosus to the feet and the umbilical arteries. CHAPTER 34 Circulation Through Special Regions 583 FIGURE 34–17 Diagram of the circulation in the fetus, the newborn infant, and the adult. DA, ductus arteriosus; FO, foramen ovale. (Redrawn and reproduced with permission from Born GVR et al: Changes in the heart and lungs at birth. Cold Spring Harbor Symp Quant Biol 1954;19:102.) have a greater O 2 affinity than adult red cells (Figure 34–19). The fetal red cells contain fetal hemoglobin (hemoglobin F), whereas the adult cells contain adult hemoglobin (hemoglobin A). The cause of the difference in O 2 affinity between the two is that hemoglobin F binds 2, 3-DPG less effectively than hemoglobin A does. The decrease in O 2 affinity due to the binding of 2, 3-DPG is discussed in Chapter 32). Some hemoglobin A is present in blood during fetal life (see Chapter 32). After birth, production of hemoglobin F normally ceases, and by the age of 4 mo 90% of the circulating hemoglobin is hemoglobin A. O 2 content (mL/dL) 22 20 18 16 14 12 10 8 6 4 2 0 Fetus Mother 10 20 30 40 50 60 70 80 90 100 PO2 (mm Hg) FIGURE 34–19 Dissociation curves of hemoglobin in human maternal and fetal blood.
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Copyright © 2010 by The McGraw-Hil
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iv Key Features of the 23rd Edition
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About the Authors KIM E. BARRETT Ki
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viii CONTENTS 27. Digestion, Absorp
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2 SECTION I Cellular & Molecular Ba
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10 SECTION I Cellular & Molecular B
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80 SECTION II Physiology of Nerve &
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100 SECTION II Physiology of Nerve
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168 SECTION III Central & Periphera
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302 SECTION IV Endocrine & Reproduc
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430 SECTION V Gastrointestinal Phys
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490 SECTION VI Cardiovascular Physi
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632 SECTION VII Respiratory Physiol
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640 SECTION VIII Renal Physiology T
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642 SECTION VIII Renal Physiology m
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644 SECTION VIII Renal Physiology F
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646 SECTION VIII Renal Physiology N
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650 SECTION VIII Renal Physiology G
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652 SECTION VIII Renal Physiology t
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654 SECTION VIII Renal Physiology I
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660 SECTION VIII Renal Physiology C
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662 SECTION VIII Renal Physiology d
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668 SECTION VIII Renal Physiology p
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670 SECTION VIII Renal Physiology l
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672 SECTION VIII Renal Physiology A
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676 SECTION VIII Renal Physiology E
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678 SECTION VIII Renal Physiology o
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680 SECTION VIII Renal Physiology I
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682 SECTION VIII Renal Physiology C
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688 Answers to Multiple Choice Ques
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690 INDEX Angiotensin III, 671 Angi
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692 INDEX Cannabinoids, 145, 179 Ca
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694 INDEX Climbing fibers, 255 Clin
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696 INDEX Endocrine functions of pa
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698 INDEX Flaccid paralysis, 244 Fl
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700 INDEX Hearing (continued) affer
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702 INDEX Intrinsic system, 532 Int
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704 INDEX Monoamines (continued) hi
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706 INDEX Pacemaker (continued) imp
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708 INDEX Proprioceptors, 632-633 P
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710 INDEX Respiratory compensation,
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712 INDEX Supratentorial lesions, 2
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714 INDEX Vestibular system, 213-21
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Ganong's Review of Medical Physiology, 23rd Edition

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