Circulatory System Trivia
0Loading crowd target
0Loading crowd target
7correct to beat the crowd
110 free Circulatory System trivia questions with answers — science & nature quiz, new questions added Oct 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 110 questions with answers and explanations. Play the quiz
Q 01Blood travels around the human body in which two major circuits?
Pulmonary and systemic
Pulmonary and systemic loops are joined end to end, so every full lap takes blood through the heart twice, once bound for the lungs and once for the rest of the body.
Q 02What is the largest artery in the human body?
The aorta
The aorta has a stretchy wall that swells with each surge from the heart and then springs back, which keeps blood moving between beats and smooths the heart's spurts into a steadier flow.
Q 03Which two large veins return deoxygenated blood from the body to the right atrium?
The superior and inferior vena cava
The superior and inferior vena cava are named for position, not rank: 'inferior' only means lower, and the inferior vena cava, which drains the legs and abdomen, is in fact the largest vein in the body.
Q 04Blood on its way out from the heart reaches the capillaries through which small vessels?
Arterioles
Arterioles have muscular walls that tighten or relax to control how much blood reaches each capillary bed, and that squeeze makes them the main place where the body adjusts its blood pressure.
Q 05Most capillaries are so narrow that red blood cells can only pass through them how?
In single file
In single file is the only way through a vessel barely wider than one red cell, and the narrowest are tighter still, so the flexible cells have to bend out of shape to squeeze along.
Q 06A 1920s estimate put human muscle capillaries at 100,000 km end to end; a 2021 recalculation gave roughly what?
10,000 km
10,000 km is the right scale: the recalculation gave 9,000 to 19,000 km. August Krogh's famous figure had assumed a huge man with 50 kg of muscle and far denser capillaries than real muscle has.
Q 07Which structures in medium and large veins prevent blood from flowing backwards?
One-way valves
One-way valves got their first clear description from the anatomist Fabricius, who never grasped their purpose; a pupil of his later used them as key evidence that blood travels in a circuit.
Q 08A heart bypass often reroutes blood through a vein borrowed from which part of the body?
The leg
The leg gives up a length of its great saphenous vein, which is lifted out and stitched in as a detour, so blood can flow around the blocked stretch of a heart artery.
Q 09Which vessels break the rule that blood returning to the heart is deoxygenated?
The pulmonary veins
The pulmonary veins carry freshly oxygenated blood from the lungs to the left atrium; a vein is defined by flow toward the heart, not by how much oxygen its blood holds.
Q 10Instead of returning gut blood to the heart, the portal vein takes it where?
Into a second capillary bed in the liver
Into a second capillary bed in the liver: the portal vein sends gut blood there first, so the liver can process absorbed nutrients and detoxify what was swallowed before it reaches the rest of the body.
Q 11The heart's own blood supply comes from two arteries that branch off where?
Near the origin of the aorta
Near the origin of the aorta, just above the aortic valve, the two coronary arteries branch off; they fill mostly while the heart relaxes, because each contraction squeezes the vessels inside its own wall.
Q 12The arteries supplying the brain join up in a ring called what?
The circle of Willis
The circle of Willis lets blood reroute if one feeding artery is blocked; Thomas Willis described it in a 1664 book on the brain illustrated by the future architect Christopher Wren.
Q 13Which component of blood clumps at a wound to start forming a clot?
Q 21What do mature human red blood cells lack, unlike almost every other cell?
A nucleus
A nucleus is pushed out as each red cell matures, leaving more room for hemoglobin and a flexible disc that can fold over to squeeze through the narrowest capillaries.
Q 22Which hormone, a notorious doping agent, orders the body to make more red blood cells?
Erythropoietin (EPO)
Erythropoietin (EPO) is released by the kidneys when they sense low oxygen, which is why time at altitude raises red cell counts and why injecting it became a favourite of endurance cheats.
Q 23Which gas grips hemoglobin over 200 times more tightly than oxygen?
Carbon monoxide
Platelets
Platelets are not whole cells but fragments shed by giant cells called megakaryocytes, each releasing thousands; they stick to a damaged vessel wall and to each other to plug the gap.
Q 14What sets the rhythm of the heartbeat?
Pacemaker cells in the sinoatrial node
Pacemaker cells in the sinoatrial node fire on their own, with no signal from the brain; nerves and hormones only speed up or slow down the rhythm the node sets.
Q 15A healthy adult human heart is usually described as about the size of what?
A clenched fist
A clenched fist is the usual comparison for a heart of about 250 to 350 grams, and like any muscle it responds to training: a well-trained athlete's heart can be considerably larger.
Q 16The heart sits in a double-membraned sac called what?
The pericardium
The pericardium holds a thin layer of lubricating fluid between its membranes, which cuts friction so the heart can slide freely inside the sac with every beat.
Q 17How much blood does an average adult have?
About 5 litres
About 5 litres, roughly 8% of body weight, is the usual adult figure; a standard blood donation takes about a tenth of it, and the body replaces the lost fluid and cells on its own.
Q 18Which protein, the most abundant in plasma, holds water inside the blood vessels?
Albumin
Albumin is the most abundant protein in plasma, and its pull keeps water inside the blood vessels; when levels drop, fluid seeps into the tissues and the ankles and belly swell.
Q 19What colour is blood plasma on its own?
Straw yellow
Straw yellow: blood's red comes entirely from the hemoglobin packed into its red cells, so when a sample is spun and the heavy cells sink, the liquid left on top is pale yellow.
Q 20After about 120 days in circulation, dying red blood cells are filtered out of the blood by which organ?
The spleen
The spleen threads blood past beds of macrophages that swallow dying red cells; it also fights blood-borne germs, which is why people who lose theirs are more prone to certain infections.
Carbon monoxide takes the same four sites oxygen uses and holds on far harder, so even a little in the air starves the body of oxygen; it has no smell, which is why homes need detectors.
Q 24Which metal sits at the heart of hemoglobin's oxygen-binding heme groups?
Iron
Iron at the centre of each heme group is what actually grips the oxygen, which is why too little of the metal in the diet leads to anaemia: less hemoglobin, less oxygen delivered, and constant tiredness.
Q 25Arthropods such as insects have what kind of circulatory system?
Open, with hemolymph
Open, with hemolymph: the heart pumps this blood-like fluid into the body cavity, where it bathes the organs directly before being drawn back into the heart through pores called ostia.
Q 26What is the main oxygen-carrying molecule in the blood of crabs, lobsters and octopuses?
Hemocyanin
Hemocyanin grips oxygen with copper atoms, which is why the blood of these animals turns blue when it is loaded with oxygen and fades almost to colourless when it is not.
Q 27Which of these animals has a closed circulatory system like vertebrates?
Earthworms
Earthworms keep their blood sealed inside vessels, as vertebrates do, which lets it be steered to the hardest-working organs instead of washing loosely through the body cavity.
Q 28Where does blood leaving a fish's heart go first?
The gills
The gills come first: a fish's heart pushes blood through them to load up with oxygen, and it then flows straight on around the body without returning to the heart in between.
Q 29Which of these animals has a three-chambered heart?
Frogs
Frogs have two atria feeding one ventricle, but mixing is limited: the heart's inner structure steers oxygen-poor blood toward the lungs and oxygen-rich blood out to the body.
Q 30Which reptiles have a four-chambered heart, fully separated into two pumps?
Crocodilians
Crocodilians have the full four-chambered heart plus a trick of their own: during long spells underwater, valves steer blood away from the lungs and straight back around the body.