Because it is so rational to believe that blood, blood vessels, and the heart all evolved at the same time in synchronization with one another. And a hundred hundred other systems that we take for granted every day.
Example...
Our heart beats 100,000 times a day.
Our body has about 2500 KM of blood vessels. Can you imagine?
The heart sends oxygen and nutrients through all those blood vessels to our extremities .... and at the same time is removing waste products such as carbon dioxide.
There are some amazing engineering feats happening
1. There are actually two circuits of blood vessels.
a) The first circuit of blood vessels takes blood from the body to the lungs, so oxygen can be added, and carbon dioxide removed.
b) The second circuit takes the oxygen rich blood out to all your body parts.
But how does one heart pump in 2 different directions? This is accomplished because our hearts basically is two pumps within one. <b>It is very cool how this happens in the womb</b>, but basically there is a tube that gets kinked forming two compartments that end up functioning as two separate pumps.
2. Why doesn't the friction of a beating heart cause it to over heat? The heart is inside a 2 layered sac, the pericardium. The outer sac attaches to our diaphragm...the inner sac attaches to the heart. Inside this two layered sac is a special lubricant. Without that lubricant our heart would overheat... and we would die.
3. This is so cool..... Our hearts all have a built in pacemaker. Our heart is run by a different type of system than the rest of our body. We become tired and stop... Our senses can get tired or overwhelmed and function less efficiently (for example a putrid smell seems less offensive after a few minutes). Our hearts are not allowed that luxury though of resting. on the upper right hand side of our hearts is a cluster of cells. These cells send an electrical impulse down through the heart, stimulating the muscle. That impulse them reaches a cluster of cells at the bottom of the heart causing it to trigger a impulse upwards... (your heart has just beat once... 99,999 more times in the next 24 hours)
What I think is cool about that system, is it basically operates independently from the brain. However... the brain can overide that 'cruise control'. For example during rigorous exercise you need more blood pumping... more adrenaline etc, The brain then directly controls the heart rate.
What an amazing system! What an amazing God!