One of the best ways to demonstrate and discuss the functioning of the human heart is through the use of a three-dimensional model. Although the heart is a complex and vital organ, its basic shapes lend themselves well to being rendered in clay, whether you want to create a model with only basic shapes or one with a high degree of detail.
Pull a piece of clay about the size of your fist and condition it. Work the clay around in your hands to get it soft and malleable. Work some water into it if you need to.
Shape the basic form of the heart. Form the fist-sized piece of clay into a round ball by rolling it between a flat hand and your work station. Slightly flatten the circular shape by pressing on it gently. Squeeze and roll the resulting shape against your work station to make it more oval, then form one end into a sharp point. Try to imitate the shape of a heart-shaped human face rather than a Valentine heart; round the top, but make the bottom narrow with a triangular, but rounded, point.
Create veins. Pull and soften some small pieces of clay and roll them between your hands to create long, thin snakes. The snakes should vary in thickness from the smallest you can make to about the diameter of a bamboo skewer. Look to your photograph to imitate the placement of these pieces as you lay some branching vein patterns on the heart. Blend the edges of these veins into the surface of the heart by smoothing them with a wet finger.
Make the major veins, arteries and aorta on the top of the heart. Form these by rolling clay into pieces about the thickness of your thumb, then cut short pieces and attach them to the top of the heart. Use your photo as a guide for placement and positioning. Blend and smooth the edges of each piece into the heart.