Veins, arteries and other blood vessels are essential to our body's survival. To deliver blood around the body, a sophisticated circuit of blood vessels provides oxygen, removes waste, and maintains pressure so that the vascular network can operate ceaselessly.

Every cell in the human body requires oxygen to produce energy, but most of our cells cannot access it directly. A single–celled life form interacts directly with its environment, exchanging nutrients and waste products as required, according to the journal Environmental Microbiology Reports. But our ancestors gave up that lifestyle hundreds of millions of years ago when they evolved to become multicellular.

As this multicellular life grew more sophisticated, our ancestors' cells became specialised and compartmentalized, according to NASA. 

Many millions of years later, their descendants migrated from the ocean to the land. Instead of external cells exchanging nutrients with their environment, our ancestor's cells were encased inside a protective barrier of skin, enabling them to retain their water and maintain consistent internal temperatures. This meant that precious few cells interacted with the environment, and therefore precious few cells could access much–needed oxygen and sugars for energy. 

Fortunately, our species, just like our land–treading ancestors, possesses an interwoven network of tissues and organs dedicated to ensuring our cells acquire the nutrients that they need, according to the book "Regulation of Tissue Oxygenation" (Biota Publishing, 2011). We call this network the circulatory system.