We learn that the stroke volume is the same for the right and left side of the heart. When the stroke volumes are same, does it imply that the volume in the capillary bed and the systemic bed are same? Like, is there 3L of blood in the systemic and 3L in pulmonary bed? What's the distribution? There's also a list of adaptations in the pulmonary vasculature that accommodates the huge volume of blood. But that's the adult. What about the foetus?
My doubt is that, in the presence of a patent ductus arteriosus and a patent defect in the atrial septum in the foetal circulation, doesn't this distribution change? This shunting of blood into the systemic circulation would result in a low pulmonary venous return and hence, a low left ventricular stroke volume. So in the foetus, both blood distribution (in the two circulations) and also the stroke volumes (of either side of the foetal heart) should be differrent. Is this so?
This actually concerns me because the ductus areteriosus joins after the initial three branches (left brachiocephalic, the common carotid, and the subclavian). So, wouldn't such a shunting drastically reduce oxygenated blood flow/perfusion to the brain? As in there would be decreased flow to the carotids when compared to the adults.
Finally, what are the consequences of the differential blood volumes in the two circulations and differential stroke volumes of the two ventricles due to a patent ductus arteriosus in a foetus? (If happens)
the lungs are usually collapsed in the fetal life and all the systemic blood will pass through the ductus down the descending aorta. tha lungs usually inflate immediately after birth following the first cry of the newborn. at this time the ductus usually closes and pulmonary circulation is established . the QP and QS ratio depends on persistant shunts either ASD or VSD or PDA.
In the adult and newborn once the foetal shunts have closed - the stroke volume of the right and left heart are equal and remain so throughout life with only temporary variations that are quickly adjusted in healthy individuals.
This does not mean that the right side (pulmonary) circulation contains an equal volume to the left (systemic) circulation.
Approximately 5L of blood are pumped from the left ventricle each minute in an adult human. If the equivalent volume was not pumped by the right ventricle each minute then the left heart would have insufficient return to maintain the 5L output per minute. If
left ventricular output decreases transiently while that of the right ventricle remains constant, pulmonary blood volume (and therefore pulmonary pressure) rises until the
outputs of the two ventricles equalise.
The pulmonary circulation always contains less blood than the systemic circulation.
Of approximately 5L blood volume with the heart containing 360mL in diastole (filled, just prior to contraction) the pulmonary circulation holds only 440mL compared to 4200mL in the systemic circulation. it may help to understand when you realise that the entire right cardiac output goes to the pulmonary circulation whereas left ventricular output must be divided up amongt the systemic organs: brain; muscle; viscera; etc.This wide distribution contains a lot more blood volume at any one time than the pulmonary circuit but the volume entering and leaving are the same.
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