Your Heart The size of your fist Mostly muscle 4 hollow chambers Atria (plural of Atrium) upper chambers Ventricles lower chambers

Blood Flow Through the Heart One way Atria accept blood and pump it into ventricles Weak pumps Ventricles pump the blood out of the heart. Much stronger Valves force one way flow of blood

Circulatory System

Blood vessels The blood flows through the body in a closed system from arteries to capillaries and back through veins.

Which is which? Why?

Arteries • Carry blood away from the heart to body tissue • Most carry oxygenated blood (red) • Thick walled to respond to blood pressure

Capillaries Located between arteries and veins Gas and nutrient exchange Functional unit of circulatory system

Veins Returns blood from the capillaries to the heart Thin walled Has valves to prevent back-flow of blood

In your science notebook, trace a drop of blood from the heart through the body and back to the heart. Name the structures and functions along the way.