Stand and let your hands hang next to your sides for a few minutes. Then, look at the backs of your hands. Do you see any bumps on the surface on top of your veins? What might those be, and what are their function?

Given what we know, we can confirm that when you stand and let your hands hang next to your sides for a few minutes, the bumps you see on your veins are the valves that help move blood to the larger veins.

Given that the veins in our bodies are littered with small valves to help the blood return to the heart correctly, we can confirm that these are the small bumps that we can see when letting our hands down for a few mins, as they fill with the blood they try not to let flow back into the smaller veins.

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cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!

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