How do the blood cells exit the closed tube that is your vein, and enter the organs through a physical passage?
Well they don’t leave veins into the organs..it’s the other way around. Veins are the return path. Arteries are the path from the heart. Either way, every part of our body is crisscrossed with tiny tiny blood vessels called capillaries. Some of the small spaces are no bigger than one red blood cell around. The small size and large number means an increased surface area for the blood to transfer it’s oxygen and nutrients as well as pick up wastes. Capillaries are the transition point, they’ll eventually connect and form a vein flowing back to the heart.
In reptiles and birds and mammals, it doesn’t. Our circulatory is a closed loop: blood only enters when it’s first made, and unless we’re injured, it doesn’t leave at all.
What enters and leaves the system are the gases we exchange: oxygen when we inhale, and carbon dioxide when we exhale (and a few other substances, but the principle is the same). This happens at very small blood vessels called capillaries, which run around and through our organs. They also mark the transition where blood vessels stop being arteries and start being veins. The capillary walls are so thin and delicate that the substances carried around the body by blood can pass out of the blood cells and through the capillary walls, where they can then be used by our organs. Waste products can also be passed back through the capillaries, so that the blood can deliver it to the excretory system.
Not all animals work this way. Insects and arthropods, for example, do use *open circulation* like you describe: blood is pumped out of blood vessels by the heart and bathes the organs directly, then is collected and pumped back to the heart. It’s just not the way we do it.
Arteries take the blood from the heart. They branch out until they go everywhere in your body.
Very small veins lead to capillaries, which are really tiny blood vessels with walls so thin that things can pass through them. These deliver the blood to every living cell, and the thin walls allow things like nutrients to enter the cell or leave the cell to go to other cells or for the cells to get rid of their waste.
The other end of the capillary connects to veins, which essentially reverse the process, connecting together over and over until they reach the heart.