By Jared Dashevsky, M.Eng. | Last Updated: March 15, 2021

Image by: Brett Dashevsky

What is Blood?  

Blood is a bodily fluid rich in cells, nutrients, oxygen and proteins and circulates constantly throughout your body. 7% to 8% of your body is blood. 


Key Takeaways

  • Blood is composed of plasma, white blood cells, red blood cells and platelets 

  • Blood primarily functions as a mode of transportation for nutrients, molecules and proteins throughout the body


What is Blood Composed of?

  1. Plasma

  2. White blood cells

  3. Red blood cells 

  4. Platelets 


Plasma is the liquid component of blood, rich in fats, sugars, proteins (e.g., antibodies) and salts. This is the largest component of blood — 55%! Its main job is to transport nutrients, cells, hormones and proteins throughout the body. 

Red Blood Cells

RBCs account for 40%-45% of your blood volume. These abundant cells, which kind of look like a donut, are in charge of transporting oxygen from your lungs to your tissues. RBCs also carry carbon dioxide (a waste product) from your tissues back to your lungs to be exhaled from your body. 

White Blood Cells

WBCs are your fighter cells — they protect your body from infection. These cells only make up 1% of your blood — so, there are not that many of them. However, when you have an infection, the number of WBCs in your blood increases to fight the foreign invader. 


Platelets are the toilet paper of your blood. Their main job is to clog up any cuts you have as soon as possible to minimize blood loss. 

Function of Blood

The main role of blood is to provide a highway by which important molecules, proteins and nutrients can travel. RBCs, for example, transport oxygen from your lungs all the way down to the tips of your toes. When you have an infection, antibodies travel through your blood to attack the foreign invaders. 


Outside the Huddle


Reviewed by Geetika Rao, MPH | Fact checked by Julia Radossich, PA-C