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60 free Circulatory System trivia questions with answers — science & nature quiz, new questions added Aug 2026.
This circulatory system trivia quiz covers the network of heart, vessels and blood that keeps every cell supplied, plus the long history of working out how it does it. The easy questions handle the two circuits, the largest artery, the number of heart chambers, how much blood you have and what colour plasma really is. From there it moves into the plumbing: why veins need valves, where the heart gets its own blood, the portal vein's odd detour through the liver, and how capillaries can add up to thousands of kilometres. The harder end is for biology students and history-of-medicine fans: the fish heart's two chambers, the reptiles with four, hemocyanin in insects, Galen's imaginary pores, the Damascus physician who beat Harvey to the pulmonary circulation, the city where De Motu Cordis was printed, the man who first saw capillaries, and how blood pressure came to be measured in millimetres of mercury. Every answer was checked against encyclopaedia entries on the circulatory system, blood, the heart, the vessels and the scientists involved, and the supporting sentence is stored with each question. If you enjoy this one, our human heart, blood and human body quizzes are natural next stops.
30 of 60 questions with answers and explanations. Play the quiz
Q 01The circulatory system is divided into which two major circuits?
Pulmonary and systemic
The pulmonary loop carries blood from the right heart to the lungs and back to the left heart; the systemic loop serves everything else.
Q 02What is the largest artery in the human body?
The aorta
It leaves the left ventricle, makes a hairpin turn called the arch and ends in the abdomen by splitting into the two common iliac arteries.
Q 03Which two large veins return deoxygenated blood from the body to the right atrium?
The superior and inferior vena cava
From the right atrium the blood is pumped into the pulmonary circuit to pick up oxygen.
Q 04Arteries branch into which small passages before reaching the capillaries?
Arterioles
On the way back, capillaries merge into venules, which merge into veins.
Q 05Roughly what is the diameter of a capillary?
5 to 10 micrometres
Their walls are a single layer of squamous endothelial cells, thin enough for gas and nutrient exchange.
Q 06The total length of muscle capillaries in a 70 kg human is estimated at between what?
9,000 and 19,000 km
That is enough to stretch a quarter of the way around the Earth even at the low estimate.
Q 07Which structures in medium and large veins prevent blood from flowing backwards?
One-way valves
Fabricius described them in the 16th century without grasping their purpose; his most famous pupil worked out what they were for.
Q 08At any moment roughly what share of the body's blood is in the veins?
Nearly 70%
Veins act as the body's blood reservoir; most of that venous blood sits in small veins and venules.
Q 09Which vessels break the rule that blood returning to the heart is deoxygenated?
The pulmonary veins
They bring freshly oxygenated blood from the lungs to the left atrium; the fetal circulation is another exception.
Q 10Instead of returning gut blood to the heart, the hepatic portal vein takes it where?
Into a second capillary bed in the liver
It collects nutrient-rich blood from around the gut so the liver can process the products of digestion first.
Q 11The heart's own blood supply comes from two arteries that branch off where?
Near the origin of the aorta
The right and left coronary arteries arise from the aortic sinuses just above the aortic valve.
Q 12The arteries supplying the brain join up in a ring called what?
The circle of Willis
The anastomosis means the brain keeps its supply even if one route is blocked.
Q 13How many chambers does the human heart have?
4
One atrium and one ventricle serve each of the two circuits.
Q 21What do mature human red blood cells lack, unlike almost every other cell?
A nucleus
It is expelled during development to make room for hemoglobin, leaving each cell as a flexible biconcave sack of the protein.
Q 22Roughly how many new red blood cells does an adult produce every second?
2.4 million
Red cells account for about 84% of all the cells in the human body, some 20 to 30 trillion of them.
Q 23How many oxygen molecules can one mammalian hemoglobin molecule carry?
4
Each red cell holds about 270 million hemoglobin molecules; Max Perutz worked out the protein's structure in 1959.
Q 14What sets the rhythm of the heartbeat?
Pacemaker cells in the sinoatrial node
The adult resting rate is close to 72 beats per minute.
Q 15What is the approximate mass of an adult human heart?
250 to 350 grams
It is often described as fist-sized, about 12 cm long, though that description is disputed as slightly small.
Q 16The heart sits in a double-membraned sac called what?
The pericardium
The sac holds a small amount of fluid and attaches to the mediastinum, the middle compartment of the chest.
Q 17How much blood does an average adult have?
About 5 litres
Plasma makes up around 55% of it, and the plasma itself is 92% water.
Q 18What percentage of blood is plasma?
About 55%
Red cells make up most of the rest, around 45%, with white cells under 1%.
Q 19What colour is blood plasma on its own?
Straw yellow
The average person carries 2.7 to 3 litres of it, and it is 92% water.
Q 20How long does a human red blood cell circulate before being recycled?
100 to 120 days
Macrophages break them down in the spleen and liver.
Q 24Which metal sits at the heart of hemoglobin's oxygen-binding heme groups?
Iron
In 1825 Engelhart used the iron content to make the first-ever estimate of a protein's molecular mass.
Q 25Arthropods such as insects have what kind of circulatory system?
Open, with hemolymph
The fluid bathes the organs directly in a cavity called the hemocoel and is drawn back into the heart through pores called ostia.
Q 26What is the main oxygen-carrying molecule in arthropod hemolymph?
Hemocyanin
Free-floating hemocytes in the fluid also play a role in the arthropod immune system.
Q 27Which of these animals has a closed circulatory system like vertebrates?
Earthworms
Annelids and cephalopods such as squid and octopus keep their blood inside vessels; sponges and comb jellies have no circulatory system at all.
Q 28A fish heart has how many chambers, driving a single-circuit circulation?
2
Blood goes through the gills and on to the body in one loop, so it is never repressurised after the gills.
Q 29Amphibians have a heart with how many chambers?
3
Reptiles also have an incomplete ventricular septum that lets them divert blood away from the lungs, useful for cold-blooded temperature control.
Q 30Besides mammals and birds, which reptiles have a heart fully separated into two pumps?
Crocodilians
Their hearts are thought to have reached this design independently of mammals, as did those of birds.