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Even though molecules, proteins, viruses, and cells are all tiny, there are significant size differences between them. The diameter of a water molecule is roughly 0.28 nanometers. The diameter of the protein hemoglobin is roughly 5 nanometers. The diameter of the HIV virus is roughly 120 nanometers. A red blood cell is 6-8 micrometers.

Skeletons support and protect our bodies. Bugs have exoskeletons outside their bodies, while humans have endoskeletons inside. Our bones are in axial (skull, ribcage, spine) and appendicular (arms, legs) groups. Bone marrow makes blood cells; more specifically, red marrow makes blood and yellow marrow stores fat. 

Micturition (peeing) starts with urine flowing from the kidneys to the bladder via ureters. The bladder expands using transitional epithelium. Urine exits through the urethra, controlled by internal and external urethral sphincters. Ureters prevent backflow, thereby reducing infection risk