Which system is the first major system to function in the embryo?

The cardiovascular system is the first major system to begin functioning in the embryo. During early embryonic development, the formation of the heart and blood vessels occurs relatively early in the process, around the third week of gestation. This is crucial because the cardiovascular system is responsible for transporting oxygen, nutrients, and hormones throughout the developing embryo, enabling proper growth and development.

The heart starts to beat and pump blood, which is essential for the delivery of nutrients from the yolk sac and later from the placenta. This system's early function supports not just the embryo itself but also lays the groundwork for the development of other systems by ensuring that tissues receive adequate oxygen and nutrients.

The other systems mentioned, such as the respiratory and nervous systems, develop later in the process. Specifically, the respiratory system begins functioning after birth, while the nervous system develops progressively but does not fully function until more advanced stages of development. The early establishment of the cardiovascular system underscores its critical role in supporting embryonic life.