How do I copy a specific shape on to a different layer's layer mask?

Question

I have heart shape, below, that is on a separate layer to the two lungs. How do I take this shape and use it as a layer mask on the two lungs?

I have created the layer mask on the lungs layer, but I'm not sure how to copy the shape over. It's bitmap, but I'd also be interested to know how to do it for a vector shape also.

Answer 1

Command-click (Mac) or Ctrl-click (Win) the layer thumbnail for the heart. This will load the shape as a selection.

Click the layer mask.

Fill the selection with black.