Final answer:
The sinus venosus in birds develops into tissue that helps form the sinoatrial node and the coronary sinus in the right atrium, thereby regulating the heart's rhythm and aiding in systemic circulation.
Step-by-step explanation:
In birds, the sinus venosus persists as a specialized tissue within the right atrium of the heart. In adult birds and mammals, including humans, this structure contributes to the formation of the sinoatrial (SA) node and the coronary sinus. The SA node is critical as it establishes the sinus rhythm, which is the normal contractile pattern of the heart, dictating the pace of cardiac contractions.
The coronary sinus, on the other hand, is a large vessel that collects deoxygenated blood from the myocardium and deposits it into the right atrium. Overall, the original sinus venosus plays an essential role in maintaining the heart's rhythm and facilitating systemic circulation by contributing to structures that ensure blood flows efficiently back to the heart.