When the lungs are upright, ventilation is greatest at the bases. Also, the blood is most concentrated at the bases (just as veinal volume is greatest in the feet). This is the main difference between keeping the lungs upright and laid down.1)
However, this shouldn't make a practical difference for us because during fetal development the baby is upside down, and even adults often lie down and breath normally.
On of the most difficult and most important things to mindful of is that the alveoli cannot burst. If they do, the blood will go into the bronchioles and gas into the blood vessels, so the lung will seize to function. On the one hand, our perfusion and ventilation has to be strong enough to reach every narrow passage and keep the alveoli from collapsing. On the other hand, it cannot be so strong as to burst the alveoli. Bursting is immediately noticeable because fluids will exit out of vessels that they did not enter into.
The average adult human lung air capacity is 6 liters for males and 4.2 for females
More detailed specs:
Vital capacity 4.8
Inspiratory capacity 3.8
Functional residual capacity 2.2
Total lung capacity 6.2)
The amount of air pumped through a ventilator with each breath breath varies with the individual. The general rule is 8cc per Kg of body weight. Also, even though normal human breathing is 12-20 bpm, we can probably provide only 6 or so, because we are not trying to sustain the body, but rather providing flow through the bronchioles and alveoli.3)
During fetal development, lungs start to form in the second trimester. However, the baby only simulates breathing until birth. While the lungs are developing, the lungs are “inhaling” and “exhaling” amniotic fluid, not air. So in our bioreactor, we should pump sterile fluid in and out of the trachea with similar properties (viscosity, makeup) to amniotic fluid.4)
The lungs inhales air when the diaphragm (beneath the lungs and above the stomach) and intercostal muscles (between the ribs) contract, creating negative pressure in the bronchioles, forcing air from the mouth into the lungs. For our lung to inhale and exhale in the bioreactor, we can either have an external syringe pump to force fluid in and out of the trachea, or we can expand the lung chamber (bag) using a bellows-like system. There doesn't seem to be any inherent value to either except that it may be difficult to syringe at a sufficient rate because it uses a ball screw.
Because the blood goes through the lungs between the right and left ventricles, the rate of blood flow through the heart should exactly equal that of the lungs. Healthy stroke volume (blood pumped with each heart beat) ranges from 55-100 mL. At a typical heart rate of 72 bpm, the normal range of cardiac output is from 4-8 L/min.
We may want to halve this value because we are only pumping into “one lung” at a time.