Day 14 of 30
Circulatory system: blood vessels
Lessons are AI-drafted and being reviewed and improved over time.
Learning Objective
Compare arteries, veins, and capillaries by structure and function, and trace the path of blood through the vessels.
The heart pumps, but blood needs roads to travel on. Your blood vessels form a network so large that, if laid end to end, they would stretch for tens of thousands of kilometers. Three kinds of vessels carry out three different jobs, and each is built to match its job.
Arteries carry blood away from the heart. Blood leaves the heart in strong spurts under high pressure, so artery walls are thick, muscular, and elastic. They stretch with each heartbeat and spring back, which keeps blood moving. This stretching is the pulse you can feel at your wrist or neck. The largest artery, the aorta, branches into smaller and smaller arteries, and the smallest are called arterioles.
Capillaries are the tiniest vessels, so narrow that red blood cells pass through single file. Their walls are only one cell thick. This is where the real work of the circulatory system happens: oxygen and nutrients leave the blood through the thin walls to reach body cells, and carbon dioxide and wastes enter the blood. Nearly every cell in your body is close to a capillary.
Veins carry blood back toward the heart. After the capillaries, pressure is low, so vein walls are thinner than artery walls. Many veins, especially in the legs, contain valves, small flaps that stop blood from flowing backward. Veins also get help from nearby skeletal muscles: when your leg muscles contract, they squeeze the veins and push blood upward. Blood from the capillaries first enters tiny venules, which join to form veins.
Putting it together, blood flows from the heart through arteries, arterioles, capillaries, venules, and veins, and back to the heart. Blood pressure is the force of blood against vessel walls. It is written as two numbers, such as 120/80: the first is pressure when the heart beats, and the second is pressure between beats. Fatty deposits can narrow arteries and raise the risk of heart disease, which is one reason exercise and a healthy diet matter.
Common mistake: thinking all arteries carry oxygen-rich blood and all veins carry oxygen-poor blood. The pulmonary artery carries oxygen-poor blood from the heart to the lungs, and the pulmonary veins carry oxygen-rich blood back. The real rule is direction: arteries carry blood away from the heart, and veins carry it toward the heart. Next lesson: blood itself.
A thick-walled vessel that carries blood away from the heart.
A vessel that carries blood back to the heart; many contain valves.
A tiny vessel with walls one cell thick where materials pass between blood and cells.
The stretch and recoil of an artery with each heartbeat.
The force of blood pushing against the walls of blood vessels.
Instructions
Materials Needed
- Clock with seconds
- Paper and pencil
Why do arteries have thicker, more elastic walls than veins?
Why are capillary walls only one cell thick?
What are valves in veins for, and what else helps blood return to the heart from the legs?
Put these in order along the path of blood leaving the heart: capillary, artery, vein, arteriole, venule.
A student says all arteries carry oxygen-rich blood. Give a counterexample and state the real rule.
Teaching Tip
Students often define arteries by oxygen content. Have them label the pulmonary artery and pulmonary veins on the Day 13 diagram to show why direction is the true rule.
Go Deeper
Research how a blood pressure cuff works, or what happens inside an artery when plaque builds up.
The AI tutor is not currently available.