Your developing embryo may still be tiny, but is undergoing rapid and complex development.
The heartbeat is now more easily recognized on an ultrasound scan. The heart continues to form from a simple smooth tube which, as it becomes more muscular, loops, folds, and divides to form four chambers. On the left side the upper chamber (left atrium) takes in blood from the lungs. From here blood passes through a one-way valve (the mitral valve) into the main left pumping chamber (the left ventricle). This then pumps blood out of the heart to the body along the main artery (the aorta). On the right-hand side of the heart, the upper chamber (right atrium) collects blood returning from the body and passes it through a one-way valve (tricuspid valve) into the right main pumping chamber (right ventricle). This pumps blood to the lungs and the cycle continues.
At this stage of development, the circulation is very basic with the heart tube simply sending blood around the length of your baby. No blood travels from your baby's circulation to the placenta (see You are 10 Weeks Exactly).