If you were in the bloodstream, you would follow the steps below one by ... returns to the heart and enters the left atrium. Blood flows through the left atrium into the left ventricle.

It opens due to a build-up of pressure in the right atrium, and prevents back flow ... blood is carried to the heart from the lungs in the pulmonary vein. It goes into the left atrium, through ...

only allows blood to pass through one cell at a time Other features Thick muscular walls to withstand blood flowing at high pressure as it leaves the heart; the largest artery is the aorta Contain ...