The earliest evidence in the fossil record of a blood-like substance, found on the remains of an armored arthropod, dates back at least 520 million years.

Blood has evolved several times across the animal kingdom, giving rise to a rainbow of color, composition, and purpose. Some creatures are able to skip having blood entirely, a nearly heretical approach to humans who can't live without it.

What is it made of?

Human blood has three distinct parts that become visible once it's been sitting out for a while or swung around a centrifuge: plasma, white blood cells and platelets, and red blood cells.

So, what exactly does blood do? In general, blood ferries oxygen, nutrients, and hormones to cells in need while regulating body temperature and collecting cellular waste and byproducts, such as carbon dioxide, for removal.

White blood cells, known as leukocytes, are part of the immune system and float around in the bloodstream, responding to injuries or invaders. Platelets and plasma help stop bleeding by creating clots.

The iron-rich protein hemoglobin in erythrocytes (red blood cells) allows oxygen to hitch a ride and colors the cell carmine.

Where does it come from?

Red blood cells don't have nuclei, so they don't have the genetic information to produce more blood cells by mitosis or copying themselves. They rely solely on hematopoietic stem cells—primitive cells whose purpose is developing into other types of cells—from inside the fatty, jam-like marrow inside the porous recesses of certain bones.

These stem cells are constantly crafting new platelets and red and white blood cells (the latter of which has nuclei). The newly born blood cells then travel out of bone marrow through small blood vessels called capillaries and enter the bloodstream, where they circulate the body.

In adult humans, bone marrow produces all the body's platelets and red blood cells, while other organs, including the spleen and lymph nodes, assist white cell production. Some of these organs also run quality control on blood cells.


Red blood cells are covered in antigens, protein molecules that let the immune system know to boot anything that doesn't match.

In humans, there are two main blood groups. Blood group ABO refers to the presence of A and/or B antigens. The Rh blood group describes Rh antigens' presence (positive) or absence (negative).

More blood groups exist, with an average of one discovered each year for the past decade or so, and blood types can change.

Groups describe the type of antigens that determines blood type, leading to four main types (A, B, AB, and O) with positive and negative variations. The combination of these types determines how the immune system responds to foreign blood, such as in the case of transfusions.

Understanding blood types is critical to successful blood transfusions. If someone were to receive a transfusion of blood for which they have antibodies, their immune system would respond by sending out antibodies to clot the invaders' progress, leading to possible kidney failure and death.

Dive Deeper

Relevant articles, podcasts, videos, and more from around the internet - curated and summarized by our team

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Blood has been seen as a semi-magical substance since ancient times. Alchemists theorized it was a mixture of the four elements. Others proposed it was one of four bodily fluids whose balance dictated health (the other fluids were yellow bile, black bile, and white phlegm). This 2019 New Yorker essay by a hematologist—a medical blood specialist—overviews blood's reputation across cultures and centuries.

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Blood's importance to humanity is summarized by the fact that, without it, we die. This importance naturally begets assumptions about its medicinal properties, many of which appear accessible through drinking or eating the fluid connective tissue. This article details the history of blood drinking, from ancient Roman gladiators to modern-day sanguinarians.

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About 55% of your blood volume is plasma, the liquid part of blood created from water, proteins, minerals, and salts absorbed through the digestive tract. This 49-minute podcast episode explores the business of plasma donations, which differ from blood donations in that they are usually compensated and are part of a global industry expected to be worth $50B by 2029.

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We're trying. Scientists are currently working on creating viable synthetic blood, an alternative to the presently relied-upon donations that manage to fall short of demand. More than half of the world's nations struggle to meet their population's need for blood. This article details current and historical efforts to develop synthetic blood, a venture that started more than 400 years ago.

Photo of medical professional putting vials of blood into a centrifuge, a device that can spin blood at such a speed as to separate it into its various components.
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The antigens (proteins that tell the immune system to attack mismatched variations) that dot the outside of a red blood cell determine one's blood type, which is why the four main blood types are A, B, AB, and O, each with positive and negative variations. Watch this six-minute video to better understand the physiology of blood types and how they factor into blood donations.

Photo of anatomical model of human heart sitting on a flat surface.
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Blood is pumped around the body via the circulatory system, also known as the cardiovascular system. This system comprises the heart, arteries, veins, and capillaries. This 10-minute video is the first in a multi-part series on the circulatory system and focuses on the role of the heart, the pump that powers the entire system.

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