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www.GOALias.blogspot.commakes your fist. Your heart is roughlythe size of your fist.What will happen if the blood rich inoxygen and the blood rich in carbondioxide mix with each other? To avoidthis from happening, the heart has fourchambers. The two upper chambers arecalled the atria (singular: atrium) andthe two lower chambers are called theventricles (Fig. 11.4). The partitionbetween the chambers helps to avoidPulmonary arteryAortaPulmonary veinRight atriumLeft atriumPartition completelyseparating the twohalvesLeft ventricleRight ventricleFig. 11.4 Sections of human heartPaheli wonders which side ofthe heart will have oxygen-richblood and which side will havecarbon dioxide-rich blood.mixing up of blood rich in oxygen withthe blood rich in carbon dioxide.To understand the functioning of thecirculatory system, start from the rightside of the heart in Fig. 11.3 and followthe arrows. These arrows show thedirection of the blood flow from the heart124SCIENCE