Mammals have a closed double circulation: blood goes through the heart once on its way to the lungs and again on its way to the body. Arteries, arterioles, capillaries, venules and veins each have a wall built for the pressure they face.
The mammalian circulatory system is a closed double circulation made of a heart, blood and blood vessels: arteries, arterioles, capillaries, venules and veins. Closed means the blood stays inside vessels. Double means it passes through the heart twice per complete circuit: the pulmonary circulation (right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium) and the systemic circulation (left ventricle, aorta, body, vena cava, right atrium). Elastic arteries near the heart stretch and recoil to smooth the flow; muscular arteries have thick smooth muscle to control flow to organs; veins carry low-pressure blood in a wide lumen with thin walls and valves; capillaries have walls one endothelial cell thick for rapid exchange.
Click each vessel in the model: pulmonary artery, pulmonary vein, aorta and vena cava, then the arterioles, capillaries and venules. Watch the colour of the blood moving in each one and where it goes.
| Vessel | From | To | Blood |
|---|---|---|---|
| pulmonary artery | right ventricle | lungs | deoxygenated |
| pulmonary vein | lungs | left atrium | oxygenated |
| aorta | left ventricle | body tissues (systemic circulation) | oxygenated, high pressure |
| vena cava | body tissues | right atrium | deoxygenated, low pressure |
Blood leaves the heart in arteries, which branch into arterioles, then capillaries; capillaries join into venules, which join into veins. A double circulation lets the blood return to the heart after the lungs to be pumped again at high pressure to the body, while the lungs receive blood at a lower pressure that will not damage their capillaries.
Arteries carry blood away from the heart and veins carry blood towards it. Do not define them by oxygen: the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood.
Choose the elastic artery, muscular artery, vein and capillary in turn, then click each layer. Compare the thickness of the tunica media and the size of the lumen.
An artery has a small lumen relative to its thick wall; a vein has a wide lumen and often looks squashed or irregular on a slide because its thin wall does not hold its shape.
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In the double circulation model, which vessel carries deoxygenated blood from the right ventricle to the lungs?
According to the model, what do the muscular walls of the arterioles do?
In the blood vessels tab, which layer makes up most of the tunica media of the elastic artery?
Which vessel in the lab has valves and a wide lumen with a thin wall?
On slides, photomicrographs and electron micrographs, decide which vessel is which from the wall and the lumen, then draw plan diagrams.
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| wall thickness relative to lumen | thick | thin | one cell thick |
| shape of lumen in TS | small and round | wide, often irregular | about the width of one red blood cell |
| tunica media | thick: elastic fibres and smooth muscle | thin | absent |
| valves (seen in LS) | none (except at the heart) | present | none |
A plan diagram of a vessel in TS or LS shows the outline of each layer (tunica intima, media, externa) and the lumen, with no individual cells, no shading and correct proportions. In an LS of a vein, draw the valve flaps pointing in the direction of flow.
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Which sequence shows the path of blood in the systemic circulation of a mammal?
Why is it an advantage that the mammalian circulation is double rather than single?
How does the structure of an elastic artery such as the aorta relate to its function?
A section shows a vessel with a wide, irregular lumen and a thin wall containing little muscle. Which vessel is it most likely to be?
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