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Fetal Circulation. Fetal Circulation: Overview. Fetal Circulation: Parturition. Uterine contractions squeeze head and body thereby expelling fluid from fetal lung. Subsequent contractions deliver the placenta thereby severing connection between fetal and maternal blood supply.

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fetal circulation parturition
Fetal Circulation: Parturition
  • Uterine contractions squeeze head and body thereby expelling fluid from fetal lung.
  • Subsequent contractions deliver the placenta thereby severing connection between fetal and maternal blood supply.
  • During birthing process, placental capillaries are crushed, and thus the fetal systemic circuit’s resistance goes up almost 2 fold. (remember: placental circ. WAS about HALF of TOTAL fetal circ., thus removing this parallel circuit would DOUBLE resistance)
fetal circulation parturition contd
Fetal Circulation: Parturition Contd.
  • Assuming that CO remains the same, doubling resistance should also double the pressure within fetal circulation (by P=QR)
  • Also, another effect of increasing R is that this briefly DECREASES movement of blood into the IVC, which results in a decrease in RIGHT ATRIAL PRESSURE!
  • Now, pressure in LEFT atrium is higher than right, and so flow of blood through foramen ovale stops! (See pic to visualize this)
fetal circulation parturition contd5
Fetal Circulation: Parturition Contd.
  • Flap of tissue in Left Heart covers up the Foramen Ovale. Now even LEFT to RIGHT blood flow (due to reversal of relative atrial pressure) cannot take place.
  • As a result, RV output can ONLY enter the pulmonary artery.
fetal circulation first breath
Fetal Circulation: First Breath
  • Post-birth, pO2 goes down while pCO2 goes up (this is new to the baby); this is sensed by chemorecs and initiates first breath.
  • Also, breathing may be influenced by gasps due to “startle reflex”. (baby is startled upon seeing new things etc…and so gasps, which helps with first breath too)
  • The loss of “immersion reflex” may influence breathing. (immersion reflex = if underwater, as in utero, drive to breathe goes down. Post birth, this inhibition is removed, thus baby tries to breathe)
fetal circulation first breath contd
Fetal Circulation: First Breath Contd.
  • Need to generate 80cm of water worth of pressure to establish alveolar liquid-air interface upon first breath.
  • Need to therefore decrease INTRATHORAIC (IT) pressure by a lot via contraction of diaphragm and external intercostals.
  • This decrease in IT pressure not only allows enough of a pressure gradient to breathe, but also elicits MAJOR changes in circulation.
fetal circulation first breath contd8
Fetal Circulation: First Breath Contd.
  • When IT pressure is decreased, the pressure in lungs goes down too, SO…there is less pressure surrounding the pulmonary vessels as well (think about this…)
  • Because of this, there is now a greater difference in pressure between right ventricle (we didn’t do much to P here) and pulmonary vessels (P went down here).
  • This difference now allows for more blood to flow through these vessels since they are now more dilated
fetal circulation first breath contd9
Fetal Circulation: First Breath Contd.
  • Now, vessels exhibit HIGH FLOW and LOW RESISTANCE properties characteristic of adult pulmonary circulation.
  • Also, since you are now oxygenating the alveoli, you release the hypoxia-induced vasoconstrictive tendencies which were predominant during fetal circulation.
fetal circulation first breath contd10
Fetal Circulation: First Breath Contd.
  • As breathing continues, a greater % of blood from the RV enters the pulmonary circulation.
  • This increases FLOW to the LEFT ATRIUM, which causes an increase in LA pressure. (this increase in LA pressure also prevents movement of blood from RA to LA via a patent foramen ovale…critical because it prevents venous blood from mixing with arterial blood!!!)
fetal circulation ductus arteriosus da
Fetal Circulation: Ductus Arteriosus (DA)
  • Since blood flow through lung increases after birth, so does return of blood to the LA and, by extension, to the LV!!!
  • Increase in blood flow to LV, via STARLING FORCES, increases LV contractility.
  • This increases aortic systolic pressure.
  • Increase in aortic pressure prevents blood moving from RIGHT to LEFT into the DA (see pic!!!).
fetal circulation ductus arteriosus da12
Fetal Circulation: Ductus Arteriosus (DA)
  • DA acts like other systemic vessels; that is, when introduced to high O2 environments, they CONSTRICT.
  • Thus, when pulmonary circulation improves after birth, P02 increases as well. Thus, the DA constricts!!!
  • Prostacyclins prevent the constriction of the DA.
  • One way to treat patent DA is to administer a prostacyclin synthesis inhibitor such as INDOMETHICIN.
fetal circulation ductus arteriosus da13
Fetal Circulation: Ductus Arteriosus (DA)
  • LAST NOTE: During fetal circ, RV pressure is higher than LV pressure because of reasons already discussed (hypoxic vasoconstriction, we haven’t decreased IT pressure yet, fluid surrounding the lungs in utero etc…)
  • After birth, this all reverses such that LV pressure is greater than RV pressure.
  • You will therefore see a switch from a thick RV wall during fetal life to a thick LV wall during adult life. (ie. Wall thickness reflects pressure levels…since muscle grows in response to work…and work = PRESSURE * Volume)