|MadSci Network: Anatomy|
Dear Arthur, In order to really understand the topic you are inquiring about you need to read a book about pulmonary physiology, but I think I can help you answer your question in a basic way. The short answer is that when a person inspires the diaphragm lowers thus the intrathoracic cavity is larger. Following Boyle's law, which states that pressure is inversely related to volume, every time the intrathoracic cavity becomes larger, the intrathoracic pressure becomes lower. When intrathoracic pressure lowers then the heart rate increases because of pressure sensors in the right side of the heart. This is a safety mechanism, when the pressure in the right heart decreases the heart beats faster in order to maintain blood pressure and make sure blood gets to all the organs, but this also happens in a minor way with each inspiration. At the same time, each time intrathoracic pressure lowers, venous return to the heart increases according to Poiseulles's law, Q = p1-p2/R, with Q being flow, p being pressure and R being resistance. Restated this means that the flow of a fluid increases when it goes from a high pressure are to a low pressure area because there is less resisitance to flow. Of course the pulmonary blood pressure gets even more complicated because the blood is going from the high pressure systolic system to the very low pressure pulmonary system. And the issue is further complicated by the fact that blood vessels are distensible and compressible and not simply stiff tubes. However, pulmonary blood flow can equal systemic blood flow because of the large amount of branching (remember that resistance in series adds directly, and resistance adds reciprocally and branching means a high degree of parallel circuitry.) The issue is further complicated by the fact that there are two types of pulmonary blood vessels, those exposed to the air pressure inside the lung and those exposed to the air pressure in the intrathoracic cavity. And when one is under low pressure, the other is under high pressure, since on inspiration the air pressure becomes more negative in the intrathoracic cavity, and the vessels exposed to alveolar pressure are compressed by the expanding alveoli(think of them being pressed between two adjacent expanding alveoli.) Fortunately because of these opposing effects of inspiration and because of the high degree of branching (and thus very low resistance), there is very little change in pulmonary blood pressure for any reason, except severe various diseases of the vessels in the lungs (pulmonary embolus being an extreme example.) However, the lungs do act as a reservoir for the left ventricle. Because of its high compliance the pulmonary blood volume is about 500ml, so if left ventricular ouput is greater than venous return the pulm. circulation can make up for it for a few strokes. I hope I have given you a starting point in your investigation. My reference and a great book (and short book!) to read on the subject is Pulmonary Physiology by Michael G. Levitsky, published by McGraw-Hill. good luck in your research. Sincerely, Sarah Martin Mason, Mad Scientist
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