Blood Vessels and Double Circulation
Structure follows function, in three kinds of tube.
Tap any part of the diagram to learn its name and its job. Try: Trace a drop from the right ventricle back to the right ventricle and count how many times it passes through the heart. The answer is why it is called double circulation.
Free play — explore the diagram, or switch to "Label it yourself" and place every label. Switch to when you're ready to test yourself.
What this lesson covers
What you do
- Trace a drop from the right ventricle back to the right ventricle and count how many times it passes through the heart. The answer is why it is called double circulation.
- Follow food absorbed from the intestine: hepatic portal vein into the liver, capillaries again, then hepatic vein into the posterior vena cava. A portal vein is one that starts in capillaries and ends in capillaries.
The parts you label
- Artery — Carries blood away from the heart towards an organ. Its walls are thick, muscular and elastic, it lies deep in the body, it does not collapse when empty, and it can constrict or dilate to control blood flow. The blood inside travels in spurts that match the ventricular contractions — that is what you feel as your pulse. The smallest branch is an arteriole.
- Vein — Carries blood away from an organ towards the heart. Its walls are thin, less muscular and non-elastic, it lies nearer the skin surface, it collapses when empty and it cannot constrict. Blood flows through it steadily and under very little pressure. Veins are formed by venules uniting, and grow larger as they go.
- Capillary — The narrowest vessel, only about 8 micrometres across, with a wall of a single layer of squamous epithelial cells and no muscle at all. Oxygen, glucose, amino acids, urea, hormones and carbon dioxide all diffuse through it, and white cells squeeze out through it by diapedesis. Laid end to end the body's capillaries would run some 100,000 kilometres.
- Wall — The vessel wall is where the whole artery-versus-vein contrast lives. Thick, muscular and elastic in an artery, so it can withstand and smooth out the pressure surges from the ventricle; thin and slack in a vein, where the pressure is very low; reduced to one cell thickness in a capillary, because there exchange, not strength, is the job.
- Lumen — The central bore of the vessel. It is narrow in an artery, which keeps blood moving fast under high pressure, and wide in a vein, which lets a large volume of slow-moving blood return without resistance. Narrow lumen with a thick wall means artery; wide lumen with a thin wall means vein.
- Valve — Thin pocket-shaped valves are found in the inner lining of veins, never in arteries. Their openings face towards the heart, so blood can pass onwards but cannot fall back. They matter because venous blood is under very little pressure and often has to travel upwards against gravity.
Challenges to clear
- Find the vessel whose wall is a single layer of cells.
- Now find the feature veins have and arteries never do.
- Label all three vessels and the parts of the vessel wall.
Hint
Define arteries and veins by direction, never by the gas they carry. An artery carries blood away from the heart, a vein carries it towards the heart — and the pulmonary artery and pulmonary veins are the standing exceptions to the oxygen rule. Blood makes two circuits per round trip: the short pulmonary circulation to the lungs and back, and the long systemic circulation to the body and back. That is what double circulation means.