How Blood Circulates Through the Heart

This page is part of a series of entries about the science and medicine behind my novel, The Stars That Govern Us. 
In The Stars That Govern Us, Alec provides Scarlett, an eleven-year-old girl with a ventricular septal defect, with a basic overview of how blood circulates through the human heart. As Alec explains it, the heart is a double-sided pump:
The atria, or upper chambers, collect blood. The lower chambers, or ventricles, pump blood. Blue blood that has delivered oxygen to the body drains through capillaries into veins to collect here in the right atrium. From the right atrium, the blood passes through the tricuspid valve into the right ventricle. The right ventricle nudges the blood into the lungs where microscopic air sacs add oxygen. 
The now oxygen-rich red blood passes through the pulmonic valve into the pulmonary veins where it collects in the left atrium. The mitral valve opens to allow the blood into the left ventricle. The left ventricle drives the blood through the aortic valve and out to the aorta. The blood then travels through smaller and smaller arteries to the rest of the body, carrying oxygen.
In visual terms, here is what Alec is describing. Blood flows through the heart in a “Figure-8” pattern:

The pattern described here by Alec is repeated over and over, approximately sixty to seventy times per minute, every hour, every day, as blood flows continuously to deliver oxygen and nutrients throughout the body.
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