🏃 The lung loop
The pulmonary artery carries blood from the heart to the lungs. In the lungs the blood picks up oxygen. The pulmonary vein then carries this blood back to the heart.
Transport Systems » The General Plan of the Circulation
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.69
- The main blood vessels to and from the heart and lungs
- The vessels that serve the liver and the kidneys
- Why the circulation is called a double circulation
- How to sketch and follow the whole plan
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Unlock This CourseThe big picture
Your blood never leaves your blood vessels. It goes round and round in a loop of tubes, with the heart as the pump. In this lesson we zoom out and look at the general plan: which big vessels go where. How the inside of the heart works is in the lesson Heart Structure and Function, and how arteries, veins and capillaries are built is in the lesson Arteries, Veins and Capillaries. Here we only follow the route.
The easiest way to learn a route is to think of a city map. The heart is the central station. The big vessels are the main roads, and each organ is a town on the way.
Key terms:
- Vena cava: the large vein that brings blood from the body back to the heart.
- Aorta: the large artery that carries blood from the heart out to the body.
- Pulmonary artery: the artery that carries blood from the heart to the lungs.
- Pulmonary vein: the vein that carries blood from the lungs back to the heart.
- Pulmonary circuit: the loop from the heart to the lungs and back.
- Systemic circuit: the loop from the heart to the rest of the body and back.
Vessels to and from the heart and lungs
There are four big vessels attached to the heart. "Pulmonary" always means "to do with the lungs".
💪 The body loop
The aorta carries blood from the heart out to the body. After the organs have used some of the oxygen, the vena cava brings the blood back to the heart.
Names are an exam favourite
Remember the direction, not the oxygen content. Arteries go away from the heart and veins go to the heart. That is why the pulmonary artery carries blood with little oxygen, while the pulmonary vein carries blood full of oxygen. Both names still follow the direction rule.
Vessels to and from the organs
From the aorta, branches carry blood to every organ. Two organs are named in the spec, and each has its own vessels.
🍳 The liver
The liver gets blood from two sources. The hepatic artery brings blood from the aorta. The hepatic portal vein brings blood from the gut, carrying the food that has just been digested and absorbed. The hepatic vein takes blood away from the liver to the vena cava.
💧 The kidneys
The renal artery brings blood from the aorta to each kidney. The renal vein takes blood away from the kidney to the vena cava.
"Hepatic" means "to do with the liver" and "renal" means "to do with the kidneys". Once you know these two words, the names are easy to guess.
The odd one out
The hepatic portal vein is a vein, but it does not carry blood to the heart. It carries blood from the gut to the liver. Its job is to bring newly absorbed food to the liver first.
Double circulation
Blood passes through the heart twice on every full trip round the body. This is called a double circulation. It is made of two circuits joined at the heart.
- The pulmonary circuit: heart, lungs, heart.
- The systemic circuit: heart, body, heart.
Why does blood go through the heart twice? Blood loses pressure as it squeezes through the tiny capillaries in the lungs. If it went straight on to the body, it would flow too slowly. Instead, it goes back to the heart, which pumps it again at high pressure. This means the body gets oxygen-rich blood quickly.
A plan you can sketch
You can follow the whole route in order. Start at the vena cava and read down the list.
- Vena cava brings blood from the body to the heart.
- The heart pumps it into the pulmonary artery, which goes to the lungs.
- The blood picks up oxygen in the lung capillaries.
- The pulmonary vein brings it back to the heart.
- The heart pumps it into the aorta, which sends it to the body.
- Branches of the aorta supply the organs. The hepatic artery goes to the liver and the renal artery goes to each kidney.
- The hepatic vein and renal vein drain into the vena cava, and the loop starts again.
How to draw it
Draw a heart in the middle. Draw the lungs on one side and the other organs below. Put an arrow on every vessel to show which way the blood flows. Label each vessel with its name. Add the gut above the liver, with the hepatic portal vein joining them.
Worked example
Follow a drop of blood from the gut to the heart. Step 1: after a meal it leaves the gut in the hepatic portal vein, carrying digested food. Step 2: it passes through the liver. Step 3: it leaves in the hepatic vein. Step 4: the hepatic vein joins the vena cava, which takes it to the heart.
Common mistakes
Many students mix up the pulmonary artery and the pulmonary vein. Use direction: the artery goes away from the heart. Another slip is to say the hepatic portal vein goes to the heart. It goes to the liver. Students also write "renal vein" when they mean "renal artery". The artery is the one that arrives with blood from the aorta. Finally, do not say the blood goes round the body twice. It goes through the heart twice.
Exam-style question
(a) Name the blood vessel that carries blood from the heart to the lungs. [1 mark]
(b) Name the vessel that carries blood from the gut to the liver. [1 mark]
(c) Explain why the circulation in humans is called a double circulation. [2 marks]
Model answer
(a) Pulmonary artery (1). [1 mark]
(b) Hepatic portal vein (1). [1 mark]
(c) Blood passes through the heart twice (1) on each complete circuit of the body, once on the way from the lungs and once on the way from the body, in two separate circuits (1). [2 marks]
Exam tip
If a question asks for a vessel, give its full name, such as "renal vein". Just writing "vein" will not get the mark.