Blood and Vessels
Elementary School
Definition
The blood and the network of vessels that carry it around the body. Arteries carry blood away from the heart, veins return it, and tiny capillaries allow the exchange of oxygen, nutrients, and wastes with the body's cells.
Worked examples
Arteries have thick, elastic walls that withstand high pressure as blood pumps from the heart to organs like the brain and muscles.
Arteries carry oxygen-rich blood away from the heart under high pressure.
Capillaries are one cell thick, allowing oxygen and glucose to diffuse out to tissue cells while carbon dioxide and waste move in.
Capillaries connect arteries to veins and are where gas and nutrient exchange happens.
Veins have thinner walls and valves that prevent backflow as they return oxygen-poor blood to the heart at low pressure.
Veins carry blood back to the heart; valves keep it moving in one direction.
Common mistakes
- Veins always carry deoxygenated blood and arteries always carry oxygenated blood. → Arteries carry blood away from the heart; veins return it. The pulmonary artery carries deoxygenated blood. The pulmonary vessels are exceptions: the pulmonary artery carries blood to the lungs to pick up oxygen.
- Capillaries are just tiny veins. → Capillaries are distinct vessels connecting arteries to veins, specialized for exchange. Capillaries have unique one-cell-thick walls for diffusion; veins are thicker and have valves.
Where you'll use it next
You'll build on blood and vessels when studying the heart and circulatory system in detail, learning about blood pressure, cardiovascular disease, and how the respiratory and circulatory systems work together to deliver oxygen.
See also
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026