All the above respondents answered very well, the MONKEY and BAT spend most of their life upside down, why don't they get bubbles/gas in their heart? Remind you both of them are mammals, surprisingly; and many other mammals in the ocean, whales, dolphins etc. Many of us humans are trained to be like them for a limited period with optimum cardiac functions, how and why are 2 basic questions to be answered. Valves agreed.(how) Myocardial ?(Ambiguous) Independent blood supply to the heart.. No body picked up the pen.(Why). How Long? (Ambiguous). Peripheral resistance (because/How & Why). So it because of Independent Blood supply of Myocardium, Heart Valves & peripheral resistance, keeps your Blood Supply smooth like a fiddle. Since the heart has independent Electrical System it maintains its Normal Rythm. No pooling takes places in the heart for specifically small manoevre. After that periord Pooling in the Brain can cause all complications upto Death can take place. Oops. Don't Forget valves in the Arterio- Veinous system too, they ensure smooth flow of blood in our entire system/s, when you upside down. REMEMBER THAT YOUR THROAT AND GASTOCNEMIUS MUSCLES ARE SECONDARY HEARTS. Continuos training is a must on daily basis to walk upside down, upto a particular age and in your youth and teens. Hope answererd your question.