As well as strengthening your breathing muscles (there are very many I guess) restringing air flow on an in breath, like in Ujjayi Breathing, brings more blood into the alveoli because of the low pressure in the lungs. I guess this would have some effect like enhanced gas exchange.

Additionally to the parasympathetic changes, there are mechanical effects. the lungs are a negative pressure system. When you breathe, you drop the diaphragm, increasing the negative pressure in your chest while also increasing the positive pressure in your abdomen. Blood flows through the vena cava (the main vein bringing blood back to the heart which runs through the abdomen and chest) towards the negative pressure. This means more blood gets back to the heart (“increased preload”) and so the right heart can pump more blood to the lungs, and more blood gets to the left heart to be sent around the body without the heart muscle actually having to do more work.