# data[].attributes.body
A drop of blood contains millions of red cells, and each cell contains 250 million molecules of a substance called haemoglobin. In the lungs, oxygen binds to haemoglobin, but in the tissues the oxygen is released again. Several types of white blood cell exist, and all are important to the body’s immune system. Platelets are tiny cells that are needed for blood clotting.

# data[].attributes.body
A drop of blood contains millions of red cells, and each cell contains 250 million molecules of a substance called haemoglobin. In the lungs, oxygen binds to haemoglobin, but in the tissues the oxygen is released again. Several types of white blood cell exist, and all are important to the body’s immune system. Platelets are tiny cells that are needed for blood clotting.

# data[].attributes.body
If a blood vessel is damaged, a clot forms to stop blood leaking. First, platelets stick together to form a plug that stops the leak. At the same time, a complex sequence of chemical events in the blood leads to the production of long strands of a protein called fibrin. These bind the blood cells and debris together to form a gel-like clot that gradually solidifies. The solid clot remains until the blood vessel has been repaired.

# data[].attributes.body
Blood is composed of a straw-coloured fluid, plasma, and huge numbers of blood cells that float in the plasma. Of the two main types of blood cell, red blood cells carry oxygen to the body’s tissues, and white blood cells help defend the body against infection. Blood also transports nutrients, proteins needed for blood clotting, and waste products.

# data[].attributes.body
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.

# data[].attributes.body
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.

# data[].attributes.body
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.

# data[].attributes.attachment_link.readable_text
If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

To log in and use all the features of Khan Academy, please enable JavaScript in your browser.

Circulatory system

The body system responsible for carrying blood, nutrients, and waste throughout the body

Cardiac

Related to the heart

Pulmonary

Related to the lungs

Artery

Blood vessel that moves blood away from the heart

Vein

Blood vessel that moves blood toward the heart

Aorta

Major artery that carries blood to the systemic circulatory system

Capillary

Small blood vessel that allows nutrient exchange

Atrium

Upper chamber of the heart

Ventricle

Lower 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 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).

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.

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.

# data[].attributes.attachment_link.title
The circulatory system review (article) | Khan Academy

# data[].attributes.attachment_link.description
Learn for free about math, art, computer programming, economics, physics, chemistry, biology, medicine, finance, history, and more. Khan Academy is a nonprofit with the mission of providing a free, world-class education for anyone, anywhere.