So within our thoracic cavity, we have our two lungs. So if this is the right side of the body and this is the left side of the body, then this is the left lung and this is our right lung. Now, because of the position of the heart, it turns out that our left lung is slightly smaller than our right lung. And that's because our heart, which is found within this particular portion of the body, is actually accommodated inside the left lung. So the left lung has to use some of its space to basically accommodate our heart. And that's exactly why our left lung is slightly smaller by volume and by mass than our right lung.