Every cell needs a delivery service: oxygen and nutrients in, carbon dioxide and wastes out. The circulatory system is that service, and its pump is made of the cardiac muscle from Muscles.
Key idea
The heart pumps blood through three kinds of vessels: arteries carry blood away from the heart, capillaries are tiny vessels where materials pass between blood and cells, and veins carry blood back to the heart.
Why it works
The heart is two pumps side by side. The right side sends oxygen-poor blood to the lungs, where it picks up oxygen and drops off carbon dioxide. The left side, with the thickest wall, pumps that oxygen-rich blood out to the whole body. Capillary walls are only one cell thick, so oxygen and nutrients can diffuse out to the cells and carbon dioxide can diffuse in.
Worked example: Artery or vein?
The vessel that carries blood from the heart to the lungs carries oxygen-poor blood. Is it an artery or a vein?
- Arteries and veins are named by direction, not by oxygen.
- This vessel carries blood away from the heart.
Answer: An artery (the pulmonary artery).
Worked example: Trace the blood
Blood leaves a toe and returns to the toe. Name the main places it passes.
- Veins → right atrium → right ventricle.
- Lungs (oxygen in, carbon dioxide out).
- Left atrium → left ventricle → arteries → capillaries in the toe.
Answer: Veins, right side of the heart, lungs, left side of the heart, arteries, capillaries.
Common mistake
Students think blood in veins is blue, because diagrams color veins blue and veins look bluish through the skin. Blood is always red. Blood with less oxygen is a darker red. Veins look blue because of how skin scatters and absorbs light.