Re: How does our heart rate get affected by sitting up or lying down?

Nice observation!!!!
During the day time, there is not much difference in our heart 
rate when we lie down or sit up. The difference comes (and that too a 
minor one) when we wake up in the morning, after a nice and sound sleep. 
During the night, our body is functioning, but at a minimum. During that 
time, the heart rate tends to fall because of what is known as the 
stimulation of the parasympathetic system. When we wake up, the body 
starts all its functions in full gear, and during that time the 
parasympathetic system decreases its effect and the sympathetic system(its 
opposite)kicks in. This causes the heart rate to increase thus supplying 
the requisite amount of blood to the body.

When you inhale, your chest expands, and so do your lungs. With that, the negative 
pressure in your rib cage increases, pulling blood from the blood vessels
into the heart. Due to the increased blood flow, the heart rate increases. 
The opposite happens when you exhale. The rib cage, lungs and chest 
contract, increasing the pressure, and decreasing the blood flow, and 
thereby the heart rate. All of this process is absolutely normal, and the 
heart beat variation is not very much. In medical terms, it is known as 
sinus arrhythmia.