Skip to content

Heart Health Center

Image Collection:

Human Anatomy

Illustration of blood

Picture of Blood

back button next button

© 2010 WebMD, LLC. All rights reserved.

Blood is a constantly circulating fluid providing the body with nutrition, oxygen, and waste removal. Blood is mostly liquid, with numerous cells and proteins suspended in it, making blood "thicker" than pure water. The average person has about 5 liters (more than a gallon) of blood.

A liquid called plasma makes up about half of the content of blood. Plasma contains proteins that help blood to clot, transport substances through the blood, and perform other functions. Blood plasma also contains glucose and other dissolved nutrients.

About half of blood volume is composed of blood cells:

• Red blood cells, which carry oxygen to the tissues
• White blood cells, which fight infections
• Platelets, smaller cells that help blood to clot

Blood is conducted through blood vessels (arteries and veins). Blood is prevented from clotting in the blood vessels by their smoothness, and the finely tuned balance of clotting factors.

1 | 2 | 3

Today on WebMD

hdl letters stacked up
How to boost your ‘good’ cholesterol.
headache
Learn the causes.
 
Compressed heart
5 habits to change.
heart rate
What’s normal? What’s not?
 
Lower Cholesterol 02
QUIZ
Heart Foods Slideshow
Slideshow
 
Compressed heart
Article
doctor looking at xrays
Video
 
Heart Disease And Ed
SLIDESHOW
Lowering Cholesterol Slideshow
SLIDESHOW
 
lowering blood pressure
SLIDESHOW
Wide Awake For Heart Surgery
VIDEO
 

WebMD Special Sections