The Circulatory System -- The Parts

Heart

The heart is the pump that sends blood to the lungs (to get oxygen and get rid of carbon dioxide) and then to the whole body.
It two pairs of chambers.
The top ones (which receive blood into the heart) are called the atrium (singular: atrium).
The bottom ones (which pump the blood out of the heart) are called ventricles.

The right side of the heart receives blood from the body (through the veins). It is deoxygenated, meaning that it has little or no oxygen and lots of carbon dioxide.
The right side of the heart sends blood to the lungs to get oxygen and get rid of carbon dioxide.
The left side of the heart receives blood from the lungs. It is now oxygenated, meaning that it is rich in oxygen.
The left side of the heart sends blood to the rest of the body.

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Blood Vessels

Arteries

Arteries are the blood vessels which carry blood away from the heart (either to the lungs or to the rest of the body). Think "A" for away.
Smaller arteries are called arterioles.

Veins

Veins are the blood vessels which bring deoxygenated blood to the heart from the body.
Smaller veins are called venules.

Capillaries

Capillaries are the smallest blood vessels in the body.
It is hear that all Diffusion takes place between the blood and surrounding cells.
Oxygen is delivered to the cells; carbon dioxide is removed.
Nutrients are delivered to the cells; waste is removed.
In the alveoli these tiny blood vessels release carbon-dioxide to the lungs in exchange for oxygen.

Blood and its Components

Blood is the tissue that circulates throughout the body within blood vessels, with the primary purpose of carrying materials (especially oxygen and carbon-dioxide) to and from the cells of the body.

Red Blood Cells

These cells carry oxygen.
The oxygen is attached by a molecule called hemoglobin.

White Blood Cells

These are the cells that fight off disease.
The White Blood Cells "eat" alien invaders (see The Immune System).

Platelets

Platelets are essential fragments of cells in the blood stream that help to form clots, so bleeding does not occur continuously.

Plasma

Blood plasma is the liquid part of the blood.