Foetal Circulation PDF

Fetal circulation Fraser Chisholm [email protected] Learning Outcomes After this lecture, time spent in the dissecting room, and further private study you should be able to: 1. Explain in detail how blood circulates through the foetal heart 2. Explain in general terms the circulation of bloo...

Fetal circulation Fraser Chisholm [email protected] Learning Outcomes After this lecture, time spent in the dissecting room, and further private study you should be able to: 1. Explain in detail how blood circulates through the foetal heart 2. Explain in general terms the circulation of blood in the foetus and placenta 3. Explain the functions of the foramen ovale, ductus arteriosus and ductus venosus 4. Explain the changes that occur in the circulation of blood in the foetus at the time of birth 5. Explain briefly the consequences of non-closure of foramen ovale and ductus arteriosus 6. Review and describe the development of trilaminar disc, the 3 germ layers (ecto, endo and mesoderm), and folding 2 Adult Circulation • Involves separate pulmonary and systemic circulations • No mixing of oxygenated and deoxygenated blood • Oxygenation of the blood occurs in the lungs • There is extensive hepatic blood supply • Are the latter 2 going to occur in the foetus? 3 Foetal Circulation • A developing foetus has special needs RA – Lungs are not functioning as in adults – Digestive system is not functioning as in adults so has a restricted hepatic circulation • The foetal circulation is modified by three shunts or by-passes to avoid the lungs and the liver – By-pass of the hepatic circulation via ductus venousus – By-pass of the pulmonary circulation via the foramen ovale – By-pass of the pulmonary circulation via the ductus arteriosus RV IVC Foramen ovale Body PA Lung Ductus Arteriosus Aorta PV LV LA 4 Placenta • O2 and nutrients diffuse across the placental barrier from mother to foetus while CO2 and metabolic waste are removed via the placenta • Maternal and foetal blood never mix • Usually posterior-fundal 5 Placenta 6 Umbilical cord • The umbilical cord is surrounded by the foetal membrane, amnion, and contains Wharton’s jelly. • Embedded in this jelly are two umbilical arteries and single umbilical vein 7 Umbilical vein to Ductus Venosus • Umbilical vein, carries oxygen- and nutrientrich blood from the placenta • Divides into two branches: • Smaller branch to the liver • Larger branch, ductus venosus by-passes liver and drains into inferior vena cava • Oxygenated blood mixes with venous blood from the lower limbs. 8 Foramen ovale • Most of the blood coming through inferior vena cava passes from right atrium into left atrium through foramen ovale • Then onward to L. ventricle and aorta • Remember valve of the inferior vena cava? 9 Foramen ovale 10 Ductus Arteriosus • Blood carried by superior vena cava (deoxygenated) → RA → RV → pulmonary trunk → ductus arteriosus (90%) → aorta • Small amount • pulmonary trunk • pulmonary arteries (10%) • Lungs • pulmonary veins • LA → LV → aorta Note! Where does DA open? Why here? 11 Umbilical arteries • One third of the blood in the descending aorta is distributed in the abdomen, pelvis and lower limbs • Two thirds of the blood goes to the placenta via internal iliac artery → umbilical arteries → Placenta 12 Foetal Circulation 13 Post natal circulation • Shunts (by-passes) must be obliterated at birth when the pulmonary circulation comes on stream!! • The umbilical cord is tied and cut – Loss of the blood flow through the placenta – Increased systemic vascular resistance – Increased aortic → left ventricular → left atrial pressures • First breath – Expansion of the lungs (alveoli) – Decreased pulmonary vascular resistance – Reduced pulmonary arterial → Reduced right ventricular → Reduced right atrial pressures 14 Closure of Foramen ovale • Once pulmonary circulation is established, blood from the pulmonary circulation (lungs) is returned to the LA • Pressure rises in the LA • Valve of the foramen ovale is pushed rightward and closes the foramen • It becomes structurally closed by 4 months → fossa ovalis and falx septi 15 Patent foramen Ovale • Patent foramen ovale (PFO) is a consequence of non-closure – Usually asymptomatic – May cause paradoxical emboli 16 Obliteration of Ductus Arteriosus • Functional closure (first hour): • Constriction of the smooth muscle on the wall of ductus arteriosus – Increased oxygen – Fall in Prostaglandins • Structural closure (between 1 and 4 months): Anatomical closure by thickening of the tunica intima 17 Patent Ductus Arteriosus • Postnatally the aortic pressure is greater than in the pulmonary trunk • Blood will flow back into the pulmonary circulation – Left to right shunt – Increased pulmonary blood flow → pulmonary hypertension – Congestive cardiac failure 18 Obliteration of Ductus Venosus • Blood flow through the umbilical vein ceases. • The muscle wall of the ductus venosus contracts • Portal venous blood flows through the hepatic sinuses • Functionally closes within 1 to 3 hours of birth • Obliterates and becomes Ligamentum venosum 19 Obliteration of Umbilical Vessels • Umbilical vein obliterates and forms the round ligament of the liver (lig. teres hepatis) • Distal parts of the umbilical arteries atrophy around 2-3 months to become the medial umbilical ligaments (within medial umbilical folds) 20 Before and after 21 First three weeks • The morula travels down the Fallopian tube, enters the uterine cavity and implants into the endometrium • At about the time of implantation, fluid begins to accumulate in the intracellular spaces of the morula • Gradually the intercellular spaces merge to form a single cavity. • If we take a section from the blastocyst, that is how it looks (right • figure) This cavity is called blastocele (blastocyst cavity) • We have an “inner cell mass” at one pole and these cells start to • differentiate to form the primary germ layers (endoderm and ectoderm) • – This process is called gastrulation • At this stage the embryo is called a blastocyst 22 Morula to blastocyst • If we take a section from the blastocyst, that is how it looks This cavity is called blastocele (blastocyst cavity) • We have an “inner cell mass” at one pole and these cells start to differentiate to form the primary germ layers (endoderm and ectoderm) 23 Day 9 • Implanting in uterine mucosa • Forming bilaminar disc with – Hypoblast and yolk sac – Epiblast and amnion • 0.2mm across 24 End of week 2 • • • • • Bilaminar disc with connecting stalk Within chorionic sac Amniotic sac on “dorsal” side Yolk sac on “ventral” side Epibalst and Hypoblast are in contact • Day 14 Primitive streak appears – cells start to move • Gastrulation has begun • Next 3-4 days a lot happens 13/09/2023 25 Gastrulation • The process of turning the bilaminar disc into a tri-laminar disc • Epiblast cells migrate through the primitive streak displacing the hypoblast cells – New layer is Endoderm • Later more epiblast cells migrate and fill in between the two layers* – Mesoderm • Epiblast is now know as Ectoderm • 3 Germ layers • Sets up definitive body axis • Allows cells to move then induce differentiation 13/09/2023 26 Notochord • Notochordal process grows from primitive pit • Changes from tube to disc to solid rod (17 -20 days) • Induces changes in overlying ectoderm – Forms neural plate • Differentiation of mesoderm – – – – Paraxial (somites) Intermediate Lateral plate → somatic and splanchnic 13/09/2023 27 Mesoderm • • • • • • • • • • The mesoderm has 3 parts: paraxial mesoderm, intermediate mesoderm and lateral plate mesoderm – Paraxial mesoderm will differentiate into somites (sclerotome, myotome, dermatome) and head mesenchyme – Intermediate mesoderm will develop into the urogenital system – Lateral plate mesoderm will become the walls of the body cavities, serous membranes (parietal and visceral peritoneum) and differentiate into vascular system 28 Sources of figures and Images • • • • • • • • • DAFFNER: Daffner & Hartman. Clinical Radiology. The Essentials ,4th Ed, 2014 © Lippincott Williams & Wilkins. DRAKE: Drake et al. Gray’s Atlas of Anatomy 2nd Ed, 2018 © Elsevier. GILROY: Gilroy et al. Atlas of Anatomy 2nd Ed, 2012 © Thieme KELLEY: Kelly&Petersen. Sectional Anatomy for Imaging Professionals, 3rd Edition, 2013 © Mosby, Elsevier Inc MOORE: Moore et al. Clinically Oriented Anatomy 8th Ed, 2018 © Wolters Kluver. NETTER: Hansen. Netter's Clinical Anatomy, 3rd Edition, 2014 © Saunders, Elsevier Inc SOBOTTA: Putz&Pabst. Sobotta Atlas of Human Anatomy, 14th Edition, 2006 © Urban and Fischer Verlag, Elsevier Inc ONI: Soni et al. Point-of-care Ultrasound, 2015 © Saunders. Langmans Medical Embryology 14th ed. 2019. © Lippincott Williams & Wilkins 29