The Cardiovascular System

This lesson introduces the anatomy of blood and its connection to the cardiovascular system. Explore the parts that make up the cardiovascular system and how this system functions.

Anatomy of Blood

Blood is a type of fluid connective tissue that circulates throughout the body, carrying substances to and away from bodily tissues. It has a pH of about 7.4 and is more viscous than water. Blood consists of three types of formed elements, an extracellular matrix called plasma, molecules, cell fragments, and debris. The formed elements consist of red blood cells, white blood cells, and platelets. They are also referred to as erythrocytes, leukocytes, and thrombocytes, respectively. The following table details key characteristics of these elements.


Red Blood Cells

White Blood Cells


Scientific Name




Size (Diameter)

0.008 mm

0.02 mm

0.003 mm


Participate in gas exchange, primarily with oxygen and carbon dioxide

Protect the body from foreign substances by eliciting an immune response

Aid in blood clotting and wound healing

Plasma is different from other types of connective tissue because it is a fluid. Consisting of about 92% water, formed elements remain suspended in the matrix where they are circulated throughout the body.

Did You Know?

The average volume of blood in the human body, for a 70-kilogram person, is 5 liters. Blood accounts for roughly 8% of a person’s body weight.

Consider the following image, which illustrates the composition of blood in a person’s blood sample. When a blood sample is spun in a centrifuge, less-dense plasma floats on top of a reddish mass that consists of red blood cells. There is also a thin white layer called the buffy coat that consists of white blood cells and platelets. This layer is found between the reddish mass and plasma layers.

Keep In Mind

Blood viscosity is indirectly proportional to blood flow throughout the body. If the viscosity of blood is high, blood flow decreases. When blood viscosity is low, or blood is thin, blood flow increases.

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