Open and closed circulation · Biology Class 11–12 | StemPy Lab
Course 13.2·Lesson 1 of 3
Open and closed circulation
Every animal moves its fluid round the body. In an open system, as in an insect, the blood leaves the vessels and bathes the organs in large spaces. In a closed system, as in an earthworm and in all vertebrates, the blood stays in vessels. Fish have a heart of two chambers, amphibians and most reptiles three, and birds, mammals and crocodiles four.
about 25 min1 simulation1 practice question
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Why a body needs a pump
A single cell takes in oxygen from the water or air around it. In a large body, most cells are far from the surface. So the body needs a circulatory system. It has a fluid, and a pump to move the fluid. Blood or a similar fluid carries oxygen, food and wastes from place to place.
There are two plans.
Open circulation
In an open circulatory system, the heart pumps blood into large vessels. The vessels open into body spaces called sinuses. The blood bathes the organs directly. It does not stay in vessels all the time.
This plan is found in arthropods (insects, prawns, spiders) and in molluscs (snails, mussels).
Closed circulation
In a closed circulatory system, the blood pumped by the heart always goes through a closed network of blood vessels. The blood never leaves them. This plan is found in annelids (the earthworm) and in chordates.
The closed plan is better in one way: the flow of the fluid can be controlled more precisely. The blood can be sent where it is needed.
Two panels. On the left, an open circulatory system: a heart pumps blood into a large vessel, and the blood pours out of the open end into a large body space called a sinus, where it bathes the organs before it is drawn back to the heart. A note says arthropods and molluscs. On the right, a closed circulatory system: a heart pumps blood into arteries that branch into capillaries around the organs and then join to veins that return to the heart, and the blood always stays inside the vessels. A note says annelids and chordates.
The hearts of vertebrates
All vertebrates have a muscular heart with chambers. The number of chambers differs.
Three hearts side by side, with arrows for the flow of blood. Fish: a heart with one atrium and one ventricle receives deoxygenated blood in blue and pumps it to the gills, and the oxygenated blood goes on to the body and returns to the heart, which is single circulation. Amphibian or reptile except the crocodile: two atria and one ventricle, with the left atrium holding oxygenated red blood and the right atrium holding deoxygenated blue blood, which mix in the single ventricle shown in purple. Bird, mammal or crocodile: two atria and two ventricles, the right side blue and the left side red, with no mixing.
Fish: 2 chambers (one atrium, one ventricle). The heart holds deoxygenated blood only.
Amphibians and reptiles (except crocodiles): 3 chambers (two atria, one ventricle). The oxygenated and the deoxygenated blood mix in the ventricle.
Crocodiles, birds, mammals: 4 chambers (two atria, two ventricles). The two kinds of blood stay apart.
Single and double circulation
Fish: single circulation. The heart pumps deoxygenated blood to the gills. The blood takes up oxygen there. It goes on to the body, gives up the oxygen and returns to the heart. In one round, the blood passes through the heart once.
Amphibians and reptiles: incomplete double circulation. The left atrium gets oxygenated blood from the lungs (or gills, or skin), and the right atrium gets deoxygenated blood from the body. Both go into the single ventricle, where they mix. The ventricle pumps out mixed blood.
Birds and mammals (and crocodiles): double circulation. The left side receives oxygenated blood, and the right side receives deoxygenated blood. The ventricles pump them out without mixing. There are two separate pathways, one to the lungs and one to the body. In one round, the blood passes through the heart twice.
You should find that only the heart with four chambers keeps the two kinds of blood apart. You should also find that every animal except the fish sends its blood through the heart twice in one round.
Try it in Python
The program lists some animals with their chambers and their kind of circulation.
Practise with new numbers
Common mistakes
Saying that a fish has a double circulation. A fish has a single circulation, and its heart holds only deoxygenated blood.
Saying that all reptiles have a three chambered heart. The crocodile has four chambers.
Saying that the blood of an insect runs in closed vessels. An insect has an open system, with sinuses.
Mixing up the two words. Open and closed refer to the vessels. Single and double refer to the number of passes through the heart.
Saying that mixed blood goes only to the body. In the three chambered heart, the single ventricle sends mixed blood to the body and the lungs.
Remember these
Open: arthropods and molluscs, with sinuses. Closed: annelids and chordates. Closed flow is easier to control.