The blood subsequently returns to the heart via the veins (vayns), and the cycle starts all over again. The human body is a very complex system, so it's no surprise that it needs many different types of cells to work properly. Cells are the building blocks of living matter and without them, we would not be able to survive more than a few days. In addition to the various types of white blood cells that fight infection, there are also red blood cells that transport oxygen through our bodies. This article focuses on the relationship between blood vessels and red blood cells because they play such an important role in maintaining a healthy body. Red blood cells are responsible for carrying oxygen throughout the body. They do this by utilizing a protein called hemoglobin that is contained within the cell membrane. Hemoglobin is an iron-containing molecule that binds to oxygen molecules at sites where red blood cells pass through narrow spaces or capillaries. This allows oxygen to be transported to tissues that need it and prevents its release into areas where it could cause damage. Once inside a red blood cell, oxygen binds to hemoglobin, giving off carbon dioxide as a byproduct.