Re: What happens in respiration when a person holds their breath?

Your question is great. You seem to be very interested in the process and understanding it quite well. Let me make one correction: you have used the word osmosis for movement of oxygen or carbon dioxide across the membranes e.g. in lung. The term osmosis is specific for movement of water across a biological membrane. For the movement of CO2 and O2 we may use the term diffusion. Secondly, the respiratory system and heart (cardiovascular system) work in coordination with each other as you have under stood but it is not in a 1:1 ratio as you think. We take 12 breaths each minute while the heart pumps on the average 72 times each minute. But these figures are adjusted in a coordinated manner so that the Oxygen we inhale in 12 breaths is delivered to the body tissues in 72 beats of the heart every minute. This normally occurs when we are breathing restfully and are not conscious of our breathing. Now coming to the point you want to understand: when one holds his or her breath, the breathing rate is zero whereas the heart goes on pumping; carbon dioxide accumulates in the blood as you would suspect and the level of oxygen gets lowered due to continued usage by body tissues. This is felt by us as consciousness of breathing and the sense of getting saturated or filled up and subsequently the desire to breathe. Try to sense it practically by holding your breath for some time and you’ll understand the sensation I’m talking about. I hope this has cleared your question. If you have any confusion still, feel free to ask me.