What causes the differences in physical characteristics like hair color among people?

It has to do with pheomelanin and eumelanin

Explanation:

According to explorebiotech.com, "low concentrations of the eumelanin pigment will yield blonde hair, while high concentrations result in darker brown hair. People with natural red hair have much higher pheomelanin levels in their hair than people with more common hair colors like brown, blonde, and black." (Here's the link https://explorebiotech.com/science-behind-hair-colors/)

Hope this helps :)

the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.