Book - Physiology of the Fetus 3
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Windle WF. Physiology of the Fetus. (1940) Saunders, Philadelphia.
1940 Physiology of the Fetus: 1 Introduction | 2 Heart | 3 Circulation | 4 Blood | 5 Respiration | 6 Respiratory Movements | 7 Digestive | 8 Renal - Skin | 9 Muscles | 10 Neural Genesis | 11 Neural Activity | 12 Motor Reactions and Reflexes | 13 Senses | 14 Endocrine | 15 Nutrition and Metabolism | Figures
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Chapter III The Fetal Circulation
Volume of Blood and Rate of Circulation
Blood volume is usually expressed as a function of body surface or body weight in the adult. The fetus has no surface from which heat is lost and therefore it is illogical to use surface relationships in indicating its blood volume. If we are to express it in terms of the weight of fetal tissues through which the blood passes we must consider not only the weight of the fetus itself but also that of the placenta. The relationship between the two changes greatly as gestation proceeds.
The blood volume of five huxnan infants 2.5 to 8 hours after birth was found to vary between 144 and 173 cc. per kilogram body weight, averaging 156.4 cc. per kilogram,1 but no data are available for human fetuses. The amount of fetal blood in the placenta is not known; but ioo cc. or more can be recovered from the placenta when the umbilical cord is clamped immediately after birth. Much of this blood will return to the fetus if clamping is delayed.
Determinations of blood volume have been made by Cohnstein and Zuntz2 in rabbits and by Elliott, Hall and Huggetts in goats. The latter investigators used the method of injecting a known amount of dye into thefetal yessels and estimating its dilution in the blood stream. Their results are summarized in Table 7.9 More recently Barcroft and Icennedy4 have employed an improved colorimetric dye injection method (Evan’s Blue, T 1,824) to study blood volume in sheep fetuses from the standpoint of development. Their results are illustrated in- Figs. 11 and I2.
Total Blood volume m Gou Pnrusns
Total Blood volume Total volume Fels« AS« htood Votum,- iu kskus ok htodd w. X IW w. X IW day« cc. ··W. in qmms wt in qmm
68 ................ .. 1·6.3 21.8 8.0
71 ............ 4p.4 « 40-4 8.8
77 ................ . St» e» IT
85 ................ .. 640 es« . 101 ................ .. 112.0 l 17.s 9.s us ................ . 145«..0 11.6 6.8 126 ................ .. 2o4,.0 14.2 9.9 136 ................ .. ist«-O« 14.2. », tu; 144 ............ 428.0 I 14.7 11.95
The placental cotyledons of the Template:Sheep reach maximal weight at about the middle of gestation when the fetus weighs only about 200 grams. It is evident that the blood forms a. much larger part of the fetal body weight at this time than it does toward full term. The absolute amount of blood increases greatly in the fetal body but that in the placenta remains nearly constant throughout the last third of gestation. At ido days of gestation the fetal blood is divided about equally between fetal body and placenta; at approximately full term only about one-fourth of it is in the p1acenta. Twin fetuses weigh a little less than single fetuses of comparable age, and their volume of blood is even less than encountered in single fetuses of comparable weight.
Fig. 11. The relation of blood volume in the ketus 4- the placenta to age in single ketuses (O) and twin ketuses (o) . (Barcrokt and Kennedy: Jour. Physiol. Vol. 95, i939.)
Although the amount of blood in the uterus at any one time during the last quarter of gestation remains about the same, the quantity traversing the uterus per minute more than doubles be— tween i io and i4o days. This may be related to the increase in fetal blood pressure (see chapter II) . The rate of blood How to and from the placenta is surprisingly great in the last quarter of fetal life.5 At i i i days of gestation 111 cc. of blood traversed the umbilical vessels per minute, and at 137 days this had increased to 568 cc. per minute. similarly on the maternal side in the same animals the rates of How were 1o6 cc. per minute and 475 cc. per minute. Lower values were obtained at ful1 term, and the Significance of this is not c1ear; contraction of the uterus upon the placenta may have impeded the flow. Tab1e ·8 contains the data which have been reported.
Fig. 12. The blood volume in relation to (a) fetal weight and (d) fetal age in single fetuses. (Barcroft and lcennedy: Jour. Physiol» Vol. 95. i939.)
The time taken for blood to circulate from the umbilical vein through the body of the human fetus and back to the umbilical artery has been estimated The mean circulation time before birth was found to be 30 seconds, and after birth, 60 seconds.
The cardiac output of the heart has been estimated in the sheep fetus,7-9 but in view of the surprisingly rapid How through the umbilical circuit it must be considerably greater than was reported in the earlier studies. More information is needed on this question.
TABLE 8 RATE or Bhoon PLow Tktnoucin THE: PLACENTA m THE SIIDDP
jk - H» sj jskjj H— j
I Rate ok blood How Age of ketus VVL of ketus Blosxs so! d « ( IN) Wams) Of kstus Umhi1ic21 cis-d Uterus cc.,Jmin. cc.,Jmin. 111 . . . . . . . . . . . . . . . . — 1,200 1;5 I 111 106
126 . . . . . . . . . . . . . . . . 3,000 270 218 161
127 . . . . . . . . . . . . . . . . 2850 265 162 179
129 . . . . . . . . . . . . . . . . 2,750 270 600 . . .
137 . « . . . . . . · . . . . . . . . 3,850 350 568 475
138 . . . . . . . . . . . . . «. . . ZEIT) I 860 284 347
152 . . . . . . . . . . . . . . 2800 412 300 I 268
The Course of the Fetal Blood
The course of the blood through the fetal heart has been the subject of controversy for more than a century. Most of the arguments need not be considered in this place, for readers who are interested in the historical aspects of this subject can consult the excellent review of Pohlman.10 Three principal theories are supported by experimental observations.
The one which has been favored by most writers until a few years ago holds that oxygenated blood from the placenta is brought by way of the umbilical vein, ductus venosus (part going through Iiver sinuses) and inferior vena cava to the right atrium of the fetal heart. A fold of endocardium known as the valve of the inferior vena cava directs this stream of more richly oxygenated blood across the blood ftlled right atrium and through the foramen ovale into the left atrium. Pulmonary venous blood, said to be negligible in quantity before birth, is added to it there. This still relatively well oxygenated mixture of blood enters the left ventricle, is pumped into the ascending aorta and the arch from which spring the great vessels supplying the heart, head and upper extremities A small quantity passes on through the aortic isthmus into the descending aorta. Venous blood returning from the rostral parts of the body by way of the superior vena cava enters the right atrium and is said to cross the stream from the inferior vena cava without mixing with it. This reduced blood passes into the right ventricle which forces it into the pulmonary arterial trunkz a little goes to the lungs but the greater part is shunted by the ductus arteriosus connecting the pulmonary artery with that part of the aorta distal to its great branches. There a smaller volume of the more highly oxygenated blood from the left side of the heart joins it and passes to the lower extremities as well as baclc to the placentas by way of the hypogastric and umbilical arteries. This is the sabatier (1791) doctrine, supported originally only by anatomical observations.
Opposed to it is the theory developed by PohlmanU and lcelloggIY 12 which holds that two streams of Iluid entering a common chamber from opposite directions must mix. Accotck ingly, the well oxygenated umbilical vein blood, diluted once when joined by venous blood of the abdominal inferior vena cava, enters the right atrium and is diluted again by that arriving from the upper part of the body in the superior vena cava. The re— sulting complete mixture goes two ways. Part passes into the right ventricle through the right atriowentricular oriücez the re— mainder traverses the foramen ovale and left atrium to enter the left ventricle. According to this view the aortic and the pulmonary ductus arteriosus streams of blood are approximately alilce in respect to oxygen content, and the upper parts of the body, including the heart which is the only organ working as hard in fetal life as it does in the newborn, receive no better blood than do the trunl(, inferior extremities and placenta. It is as— sumed that no signiiicant quantity of blood enters the fetal heart by channels other than the two venae cavae.
Let us defer consideration of the third hypothesis for the moment and examine the experimental evidence in favor of the sabatier and Pohlman-I(ellogg theories. The first study was made more than a century agoU szby injecting pastes of contrasting color into the superior and infexior venae cavae of three dead human fetuses under equal pressures. 1n one fetus the injection of the superior vena cava failed to reach the heart, in one the two colored masses underwent some mixing, and in the third, a fetus of seven months, the two crossed without mixing. The experiment was accepted as a demonstration of the validity»of the sabatier theory.
Direct observation of the beating heart of guinea pig fetuses whose placental circulation was intact has shown that the two sides difker in color, the left being bright and the right being darlc.9 similar1y in cat fetuses we have observed that when the umbilical vein blood is brilliantly red the carotid artery sometimes appears bright and the umbilical artery dull. There are certain objec— tions to accepting such observations. as these favoring the sabatier doctrine, however. The left ventric1e of the feta1 heart has a thinner wall than the right and may appear brighter even though the oxygen content of the blood is the same in both chambers. The bright red color of the umbilical vein of the cat fetus often results from contraction of the uterus which sends blood to the heart under higher pressures than are normally encountered. When the intra-uterine pressure is unevenly distributed in a fetus which has been removed from the Uterus, oxygenated blood may surge through the foramen ovale and into the left side of the heart in unusual proportions.
Experiments lending support to the second view, that the two caval blood streams mix completely in the right atrium, were initiated by Poh1manI0-I4 who was the first investigator to use living fetuses in a study of the course of blood before birth. He determined that both ventricles of the fetal pig heart have nearly equal capacities and that the ventricular pressures are practically identical. A saline suspension of corn starch was injected into the umbilical vein or into the fetal superior vena cava. Thereafter equal quantities of starch were recovered from samples of blood withdrawn from tlie two ventricles.
Kellogg 12 repeated and extended these experiments using a much larger series of pig fetuses. He demonstrated that material injected into either the umbilical vein or the fetal superior vena cava appeared at the same instant in both ventricles. After simuItaneously withdrawing equal samples under the same pressure from the two ventricles equal numbers of starch grains per cc. were counted in each sample. sedimentation of starch from larger volumes of blood talcen from the two ventricles confirmed these observations. Similar results have been obtained in chick embryos.I5
It was thought that injection of a foreign substance may have bloclced capillary beds besyond the heart, resulted in stasis and consequent churning of the blood within the heart. Therefore, KelloggU resorted to direct manometric gas analyses of small ventricular blood samples. For this investigation dog fetuses were delivered under 1ocal anesthesia, care being talscen to maintain the placenta1 circulation intact. It proved impossible to obtain adequate samples from the two ventricles simultaneous1y in the dog without collapsing the heart. Consequently blood was iirst withdrawn from one ventricle, then after a few minutes, from the other. The order of talcing was alternated. sixteen samples from each ventricle were analyzed. The average values obtained from right and left ventricular blood were 2.38 and 2.43 volumes per cent of oxygen and 44.33 and 42.69 volumes per cent of carbon dioxide. These data seemed to indicate that the two caval streams of the dog fetus undergo rather thorough mixing in the right atrium of the heart, assuming that the fetal pu1monary veins added a negligible quantity of blood to the left atrium. It will be shown presently however that the pulmonary return is much greater than was previously supposed. Therefore the studies under discussion do not prov complete mixture in the right atrium.
0ther states in goat and sheep fetuses by Huggett 16 and Barcroft9 see to indicate that the blood takes a figure of eight course through the fetal body, first through an "upper" circulation and then through a "lower," but without complete mixture in the atrium. The evidence is based on a comparison of values obtained by determining oxygen content of the blood drawn from upper and lower Circulations, e.g., from the carotid and umbilical arteries of the fetuses. The data in question are reproduced in Table 9. More recently additional blood—gas analyses have become available in sheep ketusesU and these are included in Fig. 13. The data lend considerable support to the classical Sabatier theory but do not prove that a complete crossing ok the streams occurs in the right atrium. All data based upon fetal blood-gas analysis are open to criticism. No one can doubt that physiologic conditions are up— set when fetuses are removed from the Uterus. Even though the p1acentas were left intact the uterineplacental relationships were altered and various degrees of anoxemia were set up in the fetuses.
TAZLIJ 9 OXYSEN copy-kniest· or· Byoov n: Tag III-km! AND Izowsa Fig-kar- Cmomwistons
I llJmbilical carotid Umbilical
Umbilical I carotid « Umbilical
Gent vem artery arte Säsep vein artery I artery no« (vol. W) I (vol. Z) H (vol. X) « I (vol. W) (vol. Z) (vol. NO, .....—....-......-....---..-..-..-—..l-..—.B..... 7.o I 6.0 I c.0 11..... 11.3-12.3 10.3 l 6.9—9.2— 6.5 «: 025 14..... 15.7 1o.4 1o.4 D..... 5.o 4.5 0.9 16..... 17.4 9.1 - «: M» 12.o H 9.0 I 4.0 19..... 6.8 337 1.7
9.7 «« s.0 27..... 10.5 6.9 5.9 o..... 7.5 l 7.o , as I Ave 7.96 5.9 2.94 Ave....l1.2.3—12.4 8.s 6.0-6.s I
Fig. 13. Comparison of the oxygen content of blood from the umbilical vein (o) , carotid artery (o) and umbilical artery (x) at different fetal ages (sheep). (Barcroft: "The Brain and Its Environment" Yale Univ Press.)
It is difficult to believe that the blood ok the ketal heart contains as little oxygen as was encountered in the dog at Caesarean section, and it is even questionable ik a proportional reduction could have talcen place in the two ventricular samples. The withdrawal of blood from only one ventricle at a time may have disturbed pressures and have led to serious errors.
The blood-gas analyses in goats and sheep are not quite as convincing when all the data are scrutinized as they appear on superficial inspection. In one of the experimental animals (sheep No. 11) the thoracic inferior vena cava blood appeared to contain less oxygen than that of the carotid artery, which of course is quite impossible; two different values were given for the blood of the umbilical artery, one of which was not a great deal less than that for the carotid artery. In other animals (sheep No. 14 and goat B) the carotid and umbilical artery blood contained identical amounts of oxygen.
Fig. 14. Roentgenogram showing injection of thorotrast into the superior vena cava of a living sheep fetus. The passage of blood through the right side of the heart, into the pulmonary arteries, ductus arteriosus and descending aorta may be seen. Gar-day, et al.: Brit. J. Raclio1., Vol. te, 1939.)
Apparently no one has been able to obtain blood samples simultaneously from the upper and lower Circulations. When this has been done the results will have more meaning than those available today. It should be pointed out that the oxygen contentof the umbilical vein blood of cat fetuses near birth fluctuates widely from moment to moment and this appears to be related to rhythmical uterine contractions. The inconsistencies in existing data may be related to this phenomenon. That species differences may exist must not be overloolced Perhaps less mixing takes place in the sheep and goat than in the pig and dog fetuses, as Patten has suggested.9 One of the most significant studies of the fetal circulation is that of Barc1ay and his colleagues.I9 By means of ac-ray cinephos tography during injections of radio-opaque substances into the jugular or umbilical veins of sheep fetuses they were able to follow the course of the s two vena caval streams very clearly. The superior vena cava blood appeared to pass directly through the right atrium into the right ventricle (Fig. 14) as it should according to the sabatier theory. The blood from the inferior vena cava took two courses. Most of it traversed the right atrium and foramen ovale to the left atrium andleft ventriclez a small part of it passed directly into the right ventricle (Fig. 15) . Barring the possibility that the force of injection added susiicient impetus to the blood to overcome a normal tendency to mix in the right atrium, these experiments prove that a very significant proportion of the more highly oxygenated blood from the placenta goes di— rectly to the left side of the heart of the sheep fetus to be delivered, after mixing with reduced pulmonary blood, to the heart, upper extremities and head. This is precisely the arrangement proposed by Wolff 20 more than 160 years ago and supported by experiments in dead fetuses by Ziegenspeck. 21
Fig. 15. Roentgenogram showing injection of thorotrast into the inferior vena cava of a living sheep fetus. The passage of blood through the right atrium, foramen ovale and left side of the heart may be seen. Its course through the hrachiocephalic artery a11d aorta is showtr A small amount of blood enters the right ventricle and may be seen in the pulmonary arteries. (Barclay, et a1.: Brit. J. Radiol.. Vol. te, 1939.)
Patten and Toulmin22 liave approached the question of blood flow through the heart from an entirely different standpoint. After determining that the average weights of the left and right ventricles are 7.11 and 8.o5 grarns, they measured the kunctional areas ok all the heart apertures in 2o normal human still—born ketuses at term. Their results expressed in average diameters and areas are illustrated diagrammatically in Fig. 16. Although they point out that it would be unwise to draw too specific conclusions about the circulation merely from the sizes of vessels, they are inclined to believe that their results demonstrate a mechanism un favorable to the classical sabatier theory. Since left and right ventricular pressures are equal in the fetal heart the should equal left ventricular weight are-I. ok aortic outlet
Fig. 16. Dimensions of the human heart and its orifices at term. (Patten & Toulmim White House conference Reports, D. Appleton-Century Co.)
8.05 85.0 FH should equal II. The rat1os are actually in the proportion of o.938 to i. Ik the volume ok blood entering the atria is proportional to the size ok the inlets the following proposition should be true:
From their data,
Ente-ist III- ais-sum Baker-Inn left ais-cumsupekiok veaa easy-o» s5.6 sq. mm. Iaferior vekka easy-a. .. 69.7 sq. mm. Coronaksy Sinne. . . . . IX? sq. mm. Tote-l . . . . . . . . 111.0 sq. mm. Total pulmonary veias . . . . . . . . . . 86.9 sq. mm. — 82.2 sq. mm.- Funetional okiäee ok foramen ovale. 82.2 sq. kam. Net . . . . . . . . . 78.8·sq. mm. Tot-a! . . . . . . . . . . . . . . . . . . . . 69.1 sq. man, Ente-sing seh sent-Hol- Entmsng leis reitst-fel
If these figures are functionally significant the ratio of blood received by the two ventricles should equal both the ratio of right ventricular to left ventricular weights and the ratio of pulmonary to aortic outlet areas. Substituting Hgures obtained from heart measurements:
THE-§ should equal T or 1.14o4 should equal 1.13o7, which
» »69.1 · ·7-.11 1s in agreement 1n ratio of o.989 to 1.
78.8 ss o —- h l l-— s oud equa A»
69.1 in agreement in ratio of o.941 to i.
or 1.14o4 should equal 1.21 1 i, which is
"Tentatively then," they conclude, "a ratio of about 8 to 7 may be accepted as expressing the relations between right and left di— visions of the fetal heart."
It is generally assumed that the prenatal lungs, requiring but a very small amount of blood for their own metabolism, are supplied with a negligible quantity until the moment of bikth when they expand and take over respiratory function. At that— time a radical rerouting of the blood is supposed to take place with dramatic suddenness. This conception has been questioned by Patten and his colleagues.22 In normal still—born human hearts they found the average combined cross-sectional area of the pulmonary arteries to be 22.8 sq. mm. and that of the pulmonary veins, 36.9 sq. mm. The functional orilice of the foramen ovale was only 32.2 sq. mm. Comparison of the pulmonary vessel measurements with others from newborns which had died after breathing showed them to have virtually the same capacity. The total cross-sectional area of the pulmonary veins was found to be about the same as that of the umbilical vein. The combined area of the left and right pulmonary arteries about equals that of the umbilical arteries.
The orifice of the foramen ovale is so restkicted by attachments of its flap—like valve in late fetal life that it alone can not be as— sumed to deliver enough blood to the left atrium to bring about an equalization of pressure there with that in the right atrium. And yet it has been proved that the two ventricular pressures are practically equal in the fetus before breathing startsPs 23 Although it is reasonable to assume from these facts that pulmonary venous return makes up the difference the evidence from the starch experiments casts doubt upon the theory,I0- U for if there is complete mixing in the right atrium and if there is a signiiicant volume of blood returning from the fetal lungs to dilute the starch laden blood from the foramen ovale less starch should have been recovered from the left ventricle than from the right.
An attempt to settle the question of fetal pu1monary circu1ation has been made by comparing the total iron content of apneic fetal lungs with that of the lungs of 1itter-mate lcittens which had been allowed to breathe air for an hour or more after clamping the umbilical cord.27 The average data from nine experiments are presented in Table to. If we can assume that the surgical procedures involved in the experiinents did not destroy a real difference between the apneic fetuses and the air breathing lcittens it is quite evident from Table Io that a circu1ation must be present in the lungs during late prenatal life which is wholly capable of caring for oxygenation pending the assumption of respiration, and that there is no sudden increase in its volume.
coupanrsodc or« Tun Tor-a« Pumonxnr InoN Am) Bsskruuixv Putz-rotem! Btoov m c« Fmusns AND Tun-In Am Bank-Inn«- Lwrtm Maskns
« bllcöscllzimtpjted « No. ok Tot-il F« Totol III« o W Um« speci- ikon P St« « Hbck per gut.
mens (mgs.) HEXE? (81n·s.) ashed Tom] in Pei- gut. IUUS lun s ashed lung COOZ (Oc·)
No air. . . 12 0 .822 5 .695 00959 I .698 0.9086 15.546 Breatheck 15 0 .s195 Z . 988 0 . 0951 l .782 0.8690 15 .527
«· Assuming all iron to be in hemoglobin ok blood; amount of tissue iron is unknown but would be equal in the two Krieg.
Direct observation of the fetal lungs before and after breathing lends support to this viewz the apneic fetal lungs present the appearance of highly vascular Organs. It has been estimated that more than five per cent of the fetal blood is in the human lungs before breathing beginsks Furthermore the recent studies of Barclay and his colleaguesW in the sheep fetus have demonstrated radiographically that the pulmonary arteries and veins carry a very appreciable volume of blood. Perhaps the crucial evidence favoring the concept of a significant pulmonary blood volume before birth is that afkorded by the exceedingly rare condition, congenital stenosis of the pulmonary veins, in which the left ventricle develops only about half its normal capacity and muscular powerks It seems probable that Patten’s original anatomical study portrays the true state of the pulmonary circulation before birth.
Correlation of the various studies just discussed leaves us with rather a different conception of the fetal circulation than is expressed by either of the two prevalent theories, although it must be granted that there are points for disputation on both sides. In the first place, one is justified in assuming some mixture of the two caval streams in the right atrium, at least in respect to the blood which goes to the right ventricle.. secondly, there can be little doubt that a greater quantity of the highly oxygenated blood from the placenta enters the left atrium than finds its way into the right ventricle. Icnowing that the fetal pulmonary veins re— turn a stream of blood to the left side of the heart approximately as large as that of the umbilical vein, we fail to see how complete mixing would have talcen place10-U in the right atrium and as much starch have been recovered from the left ventricle as from the right. More than half of the inferior caval blood must have traversed the foramen ovale. As a matter of fact KelloggU did recover more starch from the left ventricle in each of five dog fetuses of one series in which all the injectionshad been made into the umbilical veins. If there is a complete mixture of the two caval streams in the right atrium and if there is any loss of oxygen in the pulmonary circuit the lower half of the fetal body must receive more highly oxygenated blood than the heart and brain.
Perhaps after all the long discarded concept of Wolff and Ziegenspeclg which held that the superior vena cava blood goes entirely to the right ventricle but that the inferior caval stream passes to both left and right sides, is mpre nearly correct than either of the more prevalent theories of fetal circulation even though the methods originally used to arrive at this conclusion were not exactly physiologic It is this theory that the observations of Barcroft, Barclay and their colleagues support. The con cept is illustrated in Fig. 17. Again, a word ofcaution. There is no reason to believe that the same conditions are met in all species. It is quite possible that a greater proportion of the blood from the placenta reaches tl1e left side of the heart wisthout dilution in the sheep fetus than in man.
Fig. 17. The fetal circulation and probable course of the blood through the fetal hearr. The most highly oxygenated blood is contained in the vessels least shaded, the most completely reduced b1ood, in the darkest vessels, and four intermediate degrees of reduction due to mixing are indicated by four intermediate shades.
- Some recently completed injection experiments in living cat and guinewpig fetuses provide a strilcing demonstmtion of the crossing of the mval streut-s. They appear to support the sabatier doctrine even more completely than do Barclays studies.
Changes in the Circulation at Birth
Occlusion of the placental circulation is the most immediate event at birth. The umbilical arteries continue to pulsate for a few moments but very soon they constrict, alIowing no more blood to leave the body of the newborn. Much of the blood which is in the placenta drains back into the newborn if the umbilical cord is not clamped immediately.29 It has been estimated on the basis of 120 averaged cases that about 50 grams of blood returns to the child in the first minute and 98 grams by the thirtieth minute after birth.30 ·
It may be suggested that there is a reduction in the amount of blood entering the right atrium when the umbilical vessels are occluded. It is unsound to assume that development in fetal life has given the child an abdominal vena cava larger than necessary for the prenatal blood flow. This vessel probably does not accommodate immediately to the increased load placed upon it by obliteration of the umbilical arteries, but only gradually increases in diameter as postnatal growth takes place. What happens to the extra blood? We may surmise that the capillary bed of the newborn opens to receive more blood. This seems reasonable in view of the increased muscular work and initiation of new activities, including tonus.
Less blood enters»the right atrium and its iilling pressure consequently decreases. Hamilton and his colleagues23 found no appreciable diikerence between intraventricular pressures before and after clamping the umbilical cord, but they suggested that the umbilical circuit may have been shut off before their observations were made. If thefilling pressures of the two sides of the heart are about equal after the umbilical cord has been occluded any fall on the right side or any rise on the left will cause the valve of the foramen ovale to close. In the dog and the rabbit another flaplike valve guards the opening of the ductus arteriosus into the aorta; this too will close. With inspiration at birth in the dog and rabbit a negative intrathoracic pressure brings about a direct fall in pressure on both sides of the heart followed by a rise to the original 1evel with expiration. As the negative intrathoracic pressure becomes permanent1y established it is thought to lead to a decrease in the peripheral resistance within the blood channels of the lungs, but of course not in the systemic capil1aries; consequently the right intraventricular systolic pressure declines more than does the left which increases with development of systemic vasomotor tonus. The diastolic pressures remain about equal during the early hours after birth, but by two days in the rabbit diastolic pressures are considerably higher on the left side than on the right. As soon as the peripheral resistance is lowered, we may assume an increased blood iiow through the lungs with increased filling pressure of the left atrium which renders the valve of the foramen ovale permanently closed from a functional standpoinn
There is reason to believe that the events occurring at birth vary in the different species. In the sheep we lcnow nothing about intraventricular pressures but other information is at band. When the umbilical cord is clamped there is a brief transient rise in blood pressure followed by the establishment of respiration. When breathing becomes regular the mean systemic blood pressure reaches a higher 1evel than it had before respiration began. Deve1opment of sympathetic vaso-pressor tonus has been suggested to take place concomitant with establishment of respiration. It is known that the splenic smooth muscle begins to contract at that time and, more important, the ductus arteriosus closes by sphincter action.17-31 These events seem to indicate that provision is made for the right side of the heart to function at lower pressure levels than the left in the newborn lamb.
- Barclay and his colleaguesP have found recently that they were mistalcen about the identity of the ductus arteriosus in their iirst study. Nevertheless I am convinced, from direct observations in Professor Barcrofcs laboratoryg that the ductus does occlude within a few minutes after breathing starts.
What are the conditions in man? Here we are faced with inadequate data. It has been held that the systemic blood pressure does not change significantly with the advent of respiration at birth.32 The ductus arteriosus possesses no valve like that of the rabbit and dogz nor has it been shown to occlude immediately by sphincter action like that of the lamb. 1ts closure is said to be accomplished very gradually over a period of several weeks or months by a process resembling that encountered in endarteritis obliteransre ZHH Intimal pads are present in the vessel during late fetal life. If the ductus arteriosus remains open after birth it must be assumed that pressures on the two sides of the heart will tend to equalize and that blood can flow from the aorta into the pulmonary artery. 0ne should expect to hear the characteristic murmur of a patent ductus in every infant, but this is not the case. It would seem that there must be some mechanism as yet un— discovered in the human to bring about functional closure of the ductus arteriosus at birth. It is common to find an anatomically patent ductus at autopsy in early life, but it is questionable if postmortem ductus arteriosus patency soon after birth can be talcen as proof of a true physiologic patency existing before death. continued patency of the ductus arteriosus several years after birth is a relatively cornmon .occurrence. But the condition predisposes toward bacterial endocarditis and is in itself a danger because ok the additional load placed upon the lekt ventricle which may lead to cardiac decompensation. successful surgical ligation ok a patent ductus arteriosus in a 7 year old chi1d has been re— ported recently.3 The ekkects ok this operation upon the diastolic blood pressure are shown in Fig. i8.
Fig. 18. Dai1y blood pressures (upper: systolicx lower: diastolic) before and after ligating a patent ductus arteriosus in a 7 year old chi1d. Day of operation indicated by the arrow. (Gross and Habt-card: J.A.M.A., Vol. us. 1939.)
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Cite this page: Hill, M.A. (2020, July 9) Embryology Book - Physiology of the Fetus 3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Physiology_of_the_Fetus_3
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