Key terms

TermMeaning
Circulatory systemThe body system responsible for carrying blood, nutrients, and waste throughout the body
CardiacRelated to the heart
PulmonaryRelated to the lungs
ArteryBlood vessel that moves blood away from the heart
VeinBlood vessel that moves blood toward the heart
AortaMajor artery that carries blood to the systemic circulatory system
CapillarySmall blood vessel that allows nutrient exchange
AtriumUpper chamber of the heart
VentricleLower chamber of the heart

The circulatory system

The circulatory system is a network consisting of blood, blood vessels, and the heart. This network supplies tissues in the body with oxygen and other nutrients, transports hormones, and removes unnecessary waste products.

The heart

The heart is made of specialized cardiac muscle tissue that allows it to act as a pump within the circulatory system.
The human heart is divided into four chambers. There are one atrium and one ventricle on each side of the heart. The atria receive blood and the ventricles pump blood.
The human circulatory system consists of several circuits:
  • The pulmonary circuit provides blood flow between the heart and lungs.
  • The systemic circuit allows blood to flow to and from the rest of the body.
  • The coronary circuit strictly provides blood to the heart (not pictured in the figure below).
Diagram showing the flow of blood from the heart to the rest of the human body.
Image credit: Blood flow from the heart by OpenStax, CC BY 4.0

Blood and blood vessels

Blood from the heart is pumped throughout the body using blood vessels. Arteries carry blood away from the heart and into capillaries, providing oxygen (and other nutrients) to tissue and cells. Once oxygen is removed, the blood travels back to the lungs, where it is reoxygenated and returned by veins to the heart.
Diagram labeling the major arteries (red) and veins (blue) in the human body
Image credit: Arteries and veins of the body by OpenStax, CC BY 4.0
The main artery of the systemic circuit is the aorta which branches out into other arteries, carrying blood to different parts of the body.

Common mistakes and misconceptions

  • Arteries usually carry oxygenated blood and veins usually carry deoxygenated blood. This is true most of the time. However, the pulmonary arteries and veins are an exception to this rule. Pulmonary veins carry oxygenated blood towards the heart and the pulmonary arteries carry deoxygenated blood away from the heart.
  • Blood is always red. Veins can appear blue as we see them through our skin, leading some people to believe that deoxygenated blood is blue. However, this is not the case! Blood only appears blue because of the way tissues absorb light and our eyes see color. Although oxygen does have an effect on the brightness of the blood (more oxygen makes a brighter red, less makes it darker), blood is never actually blue.
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