Oxygen-poor blood is dark red; oxygen-rich blood is bright red. It's never blue. Veins look bluish because of optics: red light penetrates deep and gets absorbed by the blood inside the vein, while blue light bounces back from nearer the surface. Compared with the skin around it, the vein simply looks blue.
Every day, your kidneys filter around 180 liters of blood plasma, about a bathtub's worth. Yet you only have about 3 liters of plasma, so it passes through your kidneys some sixty times a day. Over 99% of the filtered water is reabsorbed, leaving just one to two liters of urine.
A 70 kg adult is made of about 7 × 10²⁷ atoms, or seven billion billion billion. The observable universe is thought to hold on the order of 10²² to 10²⁴ stars. Even with the high estimate, you contain thousands of times more atoms than there are stars in the entire visible cosmos.