Red blood cells - the cells that help you breath

Adult humans carry around 5 liters of blood in their circulatory system. Roughly half of that is cells, and the other half is a pale yellow liquid called plasma. The vast majority of these cells are erythrocytes, more commonly known as red blood cells (RBCs). Our red blood cells are the most abundant cell type in the human body accounting for approximately 70% of all your cells. They measure roughly 7 microns in diameter and around 1-2 microns in thickness.

The function of our RBCs is to carry oxygen from the lungs to all of our cells. When the red blood cells pass by the lungs, they pick up inhaled oxygen and hold on to it with a small molecule called hemoglobin. This molecule contains iron and is the reason why the cells and our blood is red. When they have delivered the oxygen, they pick up carbon dioxide and transport it back to the lungs to be exhaled. They then pick up some more oxygen and take another trip around the body. Your red blood cells will make this trip around 1500 times each day and will keep repeating this cycle for 120 days until they are retired by the spleen.

The cells appear yellowish when viewed in a single or a few layers and the deep red 

color appears when a lot of cells are present in multiple layers.

RBCs are donut shaped to increase their surface area. This allows them to carry more oxygen, and at the same time gives them incredible flexibility. As mentioned the cells are around 7 microns in diameter, but the smallest blood vessels (called capillaries) can be even smaller than that (around 5 microns). However, due to the cells design they are able to fold and squeeze through these tight tubes without getting stuck.

The cell membrane of these cells are semipermeable, which means that water is able to cross freely but salts is not. This can be observed by raising the salt concentration in the fluid around the cells. This pulls the water from inside the cells via a process called osmosis, leaving the red blood cells shrunken and spiky. On the other hand, if you were to lower the salt concentration the cells would ballon up and be completely spherical before bursting due to the building pressure inside.