Circulatory System: General Structure

Section 1 of 7A closed system

Closed circulation

• Closed circulation Blood stays inside a continuous network of vessels that is connected to the heart.
• The blood flows in one direction only, which is kept that way by the heart's valves and the valves in veins.

Double circulation

• Double circulation One loop takes blood from the heart to the lungs and back, while the other takes blood to the rest of the body and back.
• Blood therefore passes through the heart twice on each complete circuit.

Lymphatic vessels

• Lymphatic vessels A separate network of lymphatic vessels collects the extra tissue fluid that leaks from capillaries.
• It returns this fluid to the circulatory system so that it is not lost.

Section 2 of 7The journey of blood

1Arteries

• Arteries Carry blood away from the heart to the organs. The blood is usually oxygenated.

2Arterioles

• Arterioles The arteries divide into narrower vessels that spread blood through each organ.

3Capillaries

• Capillaries Respiring cells take oxygen from the blood, so the blood becomes deoxygenated here.

4Venules and veins

• Venules and veins Capillaries join to form venules, which widen into veins that return blood, usually deoxygenated, to the heart.

Section 3 of 7Exceptions to remember

Pulmonary artery

• Pulmonary artery Leaves the heart and takes deoxygenated blood to the lungs, even though it is an artery.

Pulmonary vein

• Pulmonary vein Brings oxygenated blood from the lungs back to the heart, even though it is a vein.

Section 4 of 7Main vessels at each organ

Organ	Vessel bringing blood towards it	Vessel taking blood away
Heart	Vena cava and pulmonary vein	Aorta and pulmonary artery
Lungs	Pulmonary artery	Pulmonary vein
Liver	Hepatic artery and hepatic portal vein	Hepatic vein
Kidneys	Renal artery	Renal vein

Section 5 of 7Using the table

1

Decide which direction the question asks about: towards the organ or away from it.

2

Remember that the vessel name usually begins with the organ it serves, for example renal for kidney and hepatic for liver.

3

Check whether the vessel is an artery or a vein by asking if it carries blood away from the heart or towards it.

4

Note that the liver is unusual because it receives blood from two vessels, the hepatic artery and the hepatic portal vein.

Section 6 of 7Key terms

Term	Meaning
Closed circulatory system	A system in which blood is always contained within vessels.
Aorta	The large artery that carries oxygenated blood from the heart to the body.
Vena cava	The large vein that returns deoxygenated blood from the body to the heart.
Hepatic portal vein	The vessel that carries blood rich in digested food from the gut to the liver.
Lymphatic vessel	A vessel that returns excess tissue fluid to the blood system.

Section 7 of 7Exam technique

1

Saying the blood leaves the vessels to flow around the body — in humans blood stays inside vessels, and only plasma leaks out to form tissue fluid.

2

Assuming every artery has oxygenated blood; the pulmonary artery does not.

3

Mixing up the renal artery and renal vein: the artery takes blood to the kidney and the vein takes it away.

4

Forgetting that the liver has two incoming vessels but only one leaving vessel.