Fetal Circulation, Leukocytes, Immunity Scenarios

Solution Preview

Hi,
Hope this helps. I also attached one of the resources that has a nice drawing of the fetal circulation.
Thanks for using BrainMass.

1) Description of the fetal circulation.

The fetal circulation presents some unique characteristics because it depends on the placenta for nutrition, oxygen supply and disposal of waste material and carbon dioxide.
The umbilical vein which transports oxygenated blood rich in nutrients from the placenta to the fetus enters through the abdomen (umbilical ring) toward the liver. Just before entering the liver the blood flow is divided in two (Greer, n.d.; CHW, 2010). One part is goes to the liver and the other one enters the ductus venosus where it bypasses the liver and carries 50% of the oxygenated blood to the inferior vena cava (Greer, n.d.).
In the ductus venosus the oxygenated blood is mixed with the deoxygenated blood from the inferior vena cava and together with the blood coming out of the left and right lobe of the liver enters the right atrium (CHW, 2010). The presence of fetal hemoglobin is shifted to the left compared with adult hemoglobin "ensuring the oxygen is delivered despite low oxygen partial pressures" (Mathieu & Dalgleish, 2005, §Circulation in the fetus, ¶5).

Since the fetal lungs are collapse and non-functional most of the blood entering the right atrium will not go the right ventricle but rather will bypass the lungs (CHW, 2010). This is done by the presence of the 'crista divides' in the atrium which forces the blood entering the cavity into two streams, a large one that goes directly to the left atrium through the 'foramen ovale', and the other one that joins the blood from the superior vena cava and goes to the right ventricle (Mathieu & Dalgleish, 2005). The foramen ovale is an opening in the atrial septum that communicates the atria and has a small valve called the 'septum primum' that prevents the blood entering the right atrium from reversing to the left (Greer, n.d.).

The blood that passes directly to the left atrium goes into the left ventricle where it is ejected into the ascending aorta. "This relatively oxygen rich blood passes predominantly to the head and upper extremities" (Mathieu & Dalgleish, 2005, §Circulation in the fetus, ¶7) before it is mixed with the blood coming from the pulmonary trunk (Geer, n.d.).

About one-third of the blood entering the right atrium does not flow through the foramen ovale, but, instead, stays in the right side of the heart, eventually flowing into the pulmonary artery (CHW, 2010).
Since the lungs are collapse and the pulmonary vessels show high resistance to flow, a relatively small amount of blood reaches the lungs and the rest bypasses the lungs through the 'ductus arteriosus', a fetal vessel which connects the pulmonary trunk to the descending portion of the aortic arch (Greer, n.d.) Some of this blood enters the coronary arteries to irrigate the myocardium (Greer, n.d.). The diameter of the ductus arteriosus can be similar to that of the descending aorta and it not only carries blood which bypass the lungs but also prevents blood with relatively ...