Question

when we breath in ,the oxygen we take diffusses with blood but what happens when we do not breath in and the heart still pump blood for purificationto lungs?

Answer 1

Surprisingly the answer to your question is quite involved. I will answer it from a freedivers point of view.

First of all, holding your breath has different effects depending on what circumstances you do it under.

When you exercise normally your breath will increase as a response to the increased levels of CO2 in your blood. This generally tells the body that you're using your O2, and it tries to wash out the excess CO2 while providing fresh O2. Your heart rate will also increase in order to pump your blood and properly "ventilate" your muscles.
Holding your breath while exercising will mean that you cannot wash out the CO2 in your blood, and the urge to breathe will quickly become strong. your heart will try to work even harder at pumping O2 to your muscles, and so your heart rate will go up.

However, when you're not exercising (e.g. lying on a sofa) the result of holding your breath will be that you are able to relax more of your body muscles (if you're used to holding your breath - practice should quickly take you to 3-4 minutes). This means that holding your breath might actually reduce your heart rate, as the relaxed muscles no longer need a lot of oxygen, and your CO2 levels are low. After holding your breath for a while though, oxygen will run low in your blood, CO2 will be increased, and once again the urge to breathe will be very large. Along with it the ability to relax reduces, and your heart rate will start increasing again.

While these two methods both occur when you're surrounded by air, immersion of your face and/or body into water provokes the so called mammalian effect. This is what freediving is all about, and here you take advantage of the mammalian effect in order to hold your breath for long times under water. This is obviously something that could be dangerous, and let me say right away that holding your breath under water should ONLY be done when somebody is watching you, and is able to help in case of an emergency.
The mammalian effect sets in when especially your face is immersed in cool water and is an oxygen-conserving effect that restricts the amount of blood in your extremities. This will reduce the oxygen consumption and can thus lower your heartrate. It is this effect that allows some freedivers to hold their breath in excess of 10 minutes (https://www.aidainternational.or...) just on atmospheric air (record-attempts have been made with pure oxygen as well, doubling the breath-hold time).

The concluding answer to your question is that whether you heart-rate goes up or down depends on whether you're in need of oxygen or not. If you're relaxed, holding your breath can relax you even further thus lowering your heart rate and taking advantage of certain physiological effects can further this decrease it.