THE FETAL CIRCULATION
The foetal circulation differs mainly from the adult one by the presence of 3 major vascular shunts:
1. Ductus venosus : between the umbilical vein and inferior vena cava.
2. Foramen ovale: between the right and left atrium.
3. Ductus arteriosus: between the pulmonary artery and descending aorta.
The rationale of these shunts is to divert the oxygenated blood from the less functioning organs as lungs, liver, kidney and intestine as placenta carries their functions, to the brain, heart and other parts of the body.
The circulation is as follow:
- Oxygen and nutrients are carried from the placenta to the foetus in a single large umbilical vein.
- The oxygen saturation in the blood of the umbilical vein is reduced from 95% in the maternal arterial blood to 80% due to its consumption by the placental metabolism.
- As the umbilical vein entering the foetal body most of its oxygenated blood passes to the inferior vena cava (IVC) through the ductus venosus. While the remainder communicates the portal vein to supply the liver.
- The liver drains into the IVC through the hepatic veins.
- The blood in the IVC is a mixture of the oxygenated blood from the umbilical vein and the desaturated blood from the lower limbs and abdominal organs including the liver so its O2 saturation is reduced to 65% when it enters the right atrium.
- Most of the blood is directed to the left atrium through the foramen ovale and from it to the left ventricle and descending aorta.
- The remainder of the blood in the right atrium passes with that coming from the head and upper limbs via the superior vena cava to the right ventricle ® pulmonary artery where most of it passes also to the aorta through the ductus arteriosus because of the high resistance of the unexpanded foetal lungs.
- The blood passes finally from the aorta to the hypogastric arteries ® umbilical arteries ® placenta.
Changes At Birth:
- With clamping of the umbilical cord, the pressure in the ductus venosus drops leading to its closure to form the ligamentum venosum.
- The initiation of respiration creates a negative intrathoracic pressure which is transmitted to the right ventricle and atrium, while the pressure in the left atrium is increased due to returning blood from the lungs this leads to closure of the foramen ovale.
- With diversion of most of the blood into the lungs, no further blood passes through the ductus arteriosus so it is closed.
- The umbilical vein is obliterated to form the ligamentum teres in the falciform ligament of the liver.
- The hypogastric arteries are obliterated to form the hypogastric ligaments.
THE FOETAL BLOOD
- Site of hematopoiesis (formation of blood cells) : first in the yolk sac then fetal liver and lastly in the bone marrow.
- Erythrocytes : are all nucleated but at term only about 5-10% of them are nucleated. The number is 6 millions/ mm3.
· Concentration at term is 15-20 gm/dl.
· 10-45% of it is adult hemoglobin (HbA), the remainder is foetal hemoglobin (HbF).At age of one year , less than 2% is HbF.
· HbF has more affinity to oxygen than HbA as it contains less 2,3 diphosphoglycerate (2,3 DPG) than that in HbA. 2,3 DPG competes for oxygen binding sites in the hemoglobin molecule , so the less 2,3 DPG contents the more affinity to oxygen.
- Serum iron: At term it is 150 m g/dl.
- Leucocytic count: At term it is 2-3 times the adult one.
- The Rhesus factor: can be detected in the foetal blood from the 10th week.
- Anti-A and Anti-B: appear in the fetal blood at about 4-8 months after birth. Those present at birth are acquired form the maternal blood and usually disappear 2 weeks after birth.
- Fetal respiratory movement can be detected by ultrasound as early as 11 weeks as a chest wall movement. From the beginning of the 4th month, this respiratory movement is sufficient to move the amniotic fluid in and out the respiratory tract.
- Pulmonary surfactant :
· It is formed by type II pneumocytes that line the alveoli.
· This starts at the 20th week and level increases gradually up to term.
· Detection of lecithin/ sphingomyelin (L/S) ratio of 2 or more or detection of phosphatidyl glycerol in the amniotic fluid indicates lung maturity.
· Pulmonary surfactant facilitates distension of the alveoli thus preventing the development of neonatal respiratory distress syndrome.
- Initiation of respiration:
During intrauterine life, the foetal respiratory centre in the medulla is inhibited by cortical impulses. Anoxia at birth affects the cortical centers leads to release of the respiratory centre from its inhibition and becomes sensitive to cutaneous stimuli, muscle stretch and biochemical changes in fetal blood as CO2 concentration.
The small intestine undergoes peristalsis by the 11th week of gestation.
(2) Fetal swallowing:
Starting from the 2nd trimester, the foetus swallows and absorbs amniotic fluid.
It consists of undigested debris from the swallowed amniotic fluid, secretions and desquamation from the gastrointestinal tract.
a. Glucuronidation: i.e. conjugation of free bilirubin is limited.
b. Glycogen: appears in low concentration in fetal liver during the 2nd trimester but near term it is 2-3 times those in adult liver.
c. Clotting factors: fibrinogen, factors II, VII, IX, XI and XII are produced by the liver in a low level at birth .
d. Vitamin K stores in the liver are deficient at birth as vitamin K is formed by bacteria in the intestine.
e. Gall bladder : It secretes bile from the 3rd month of gestation.
The foetal pancreas responds to hyperglycemia by increasing insulin secretion. However, the alpha cells of pancreas do not respond to hypoglycemia by secreting glucagons.
- By the end of the first trimester, the kidneys can excrete urine which is hypotonic due to low electrolytes concentrations.
- The full fetal bladder, seen by ultrasound, indicates functioning kidneys.
CENTRAL NERVOUS SYSTEM
- At full term is partially developed and functioning.
- By the end of the first year of life the brain doubles its weight and triple it by the end of the fifth year.
Before the end of the 17th week, the fetal pituitary is able to synthesize and store all pituitary hormones.
Thyroid gland and parathyroid glands:
They are capable of function by the end of the first trimester.
The outer (adult) zone of the fetal adrenal cortex produces cortisol.
The inner ( foetal ) zone produces dehydroepiandrosterone, the precursor of estrogen.
The adrenal medulla produces small amount of catecholamines.
Testosterone is synthesized by the fetal testis from progesterone and pregnenolone by 10 weeks of gestation.
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