Fetal and Neonatal Circulation Flashcards
What are the four bypasses in the fetal circulation?
Placenta, ductus venosus, foramen ovale, ductus arteriosus
How much blood goes to and from the placenta in fetal circulation?
50%
Is the blood oxygenated or deoxygenated as it arrives from the placenta into the right atrium?
It is initially oxygenated, and mixes with deoxygenated blood before going into the IVC
What does the ductus venosus allow for?
Bypass of the liver before going to the IVC
How much of the blood in the IVC (69%) goes from the RA to the RV? RA to the LA?
42 and 27% respectively
How much blood goes to the RV in total in fetal circulation? The LV?
66% (21% from the SVC, 3% that nourished the heart, 42% from the IVC); 34% (27% from the LA, 7% from the developing lungs)
Where do the RV and LV both pump blood into? Where does most of the blood go through from the RV to get to that destination? What is the output from the RV and LV called?
Descending aorta;
ductus arteriosus (3/4 of blood from RV)
combined cardiac output
Where does most of the blood from the descending aorta go to?
To the two umbilical arteries and later the placenta
Why does the lung have high resistance in fetal circulation?
- Lung hypoxic leading to vasoconstriction
2. Blood vessels not inflated
Compared to adult blood flow, how does blood flow in the fetus?
Mostly parallel in the fetus, in series in the adult!
Where is the greatest O2 saturation and O2 partial pressure in the fetal circulation?
In the umbilical veins
As we go from the umbilical veins to the RV and LV, why do the O2 sat and pO2 levels decrease?
Because of the low levels in the lungs for the LV; and the heart and SVC blood for the RV
What does blood being pumped in parallel mean for organs?
Organs are able to get some oxygenated blood
What happens when a newborn loses placental circulation?
Newborn breathes on its own:
Increased pulmonary blood flow
Ductus venosus, foramen ovale, ductus arteriosus close
What triggers the first breath?
Mild hypoxia, hypercapnia, tactile stimuli, cold skin
How much pressure is needed for first breath?
60 mm H20, compared to 2.5 mm H20 for the adult
What makes breathing easier? What is a consequence if one doesn’t have this?
Alveoli open and type II alveolar pneumocytes produce surfactant;
Respiratory distress syndrome (premature births)
At birth, what three things happen?
Pulmonary vascular resistance decreases, pulmonary blood flow increases, mean pulmonary arterial pressure decreases
At birth, why does PVR decrease?
- Pulmonary bv’s not crushed
- Breathing leads to higher pO2, more vasodilation
- Prostaglandins cause vasodilation
What four functions does the placenta perform?
Equivalent of:
1. Lungs (gas exchange) 2. GI tract (nutrition) 3. Liver (nutrition, waste removal) 4. Kidneys (fluid and electrolyte balance, waste removal)
What happens with insufficiency of the placenta?
Type II intrauterine growth restriction (IUGR)
What if gas exchange is interfered with?
Fetal asphyxia, or baby smaller than normal
Where does maternal blood build up in the placenta?
Intervillous space
What enables gas exchange in the placenta?
Chorionic villus (capillaries)
From the mother back to the mother, trace out the blood flow; how is gas exchanged?
Spiral arteries go to intervillous space and back to maternal veins
O2 to the fetus, CO2 back to the mother
Even with similar pO2, why does the umbilical vein have a higher O2 sat than the uterine vein?
Because fetal Hg binds O2 tighter than the mother Hg (ensures enough O2 sent to the organs;
Babies with higher HR (CO) than mother
Concentration of fetal Hg > mother Hg
What happens in the neonatal circulation once the placenta doesn’t have blood flow?
systemic vascular resistance doubles; increased aortic, LV, LA pressures
With respiratory activity by neonate, what happens to the pulmonary vascular resistance? When then happens with pulmonary vessels?
Greatly decreased;
No longer compressed
arteriolar vasodilation seen with higher blood O2 and prostaglandin release
With lung expansion, what happens to the right and left heart?
Pulmonary arterial pressure, RV, RA pressures go down
Increased LA pressure (more blood through lung)
What leads to the foramen ovale closing?
Increased LA pressure, decreased RA pressure
What happens if you don’t seal the foramen ovale?
Patent foramen ovale
What happens within three hours after birth to ductus venosus?
Vascular smooth muscle of ductus venosus constricts, becomes ligamentum venosum
Consequence of ductus venosus not closing
Portosystemic shunt
What happens to the ductus arteriosus?
Closes with constriction with increased plasma pO2, lower blood prostaglandins, bradykinin involvement
What happens if the ductus arteriosus does not close?
Patent ductus arteriosus (pulmonary hypertension, maybe CHF and arryhthmias)
What happens with aortic pressure exceeding pulmonary artery pressure?
Blood flow through ductus arteriosus reverses
What do the elimination of the shunts mean for O2 sats and PO2 in the circulation?
They increase!
What are some birth defects besides the patent ductus arteriosus?
- Coarctation of the aorta (ductus arteriosus invaded the descending aorta)
- Ventricular septal defects (blood flow from LV to RV and CHF)
- Atrial Septal Defects (LA to RA, leading to pulmonary hypertension, right heart enlargement, heart failure)
- Tetralogy of Fallot (pulmonary stenosis, dextroposition of aorta, RV hypertrophy, VSD) baby blue!
