A hormone is a secreted molecule that is carried in the bloodstream to where it acts on target cells that bear specific receptors for that hormone.

This is because anatomy and physiology are closely related branches of biology that study the structure and function of living organisms respectively.
Anatomy describes the shape, size, location, and relationships of body parts, while physiology explains how those parts work together to maintain life.
For example, the anatomy of the heart determines how it pumps blood, and the anatomy of the lungs determines how they exchange gases.
Choice B is wrong because our understanding of both anatomy and physiology is constantly changing as new discoveries are made in the field of biology.
Choice C is wrong because body parts take up space regardless of their physiological functions.
Choice D is wrong because physiological functions are not limited to an organism, but can also occur at the cellular, tissue, organ, and system levels.

The systemic circuit sends oxygen-rich blood to the tissues.

It is part of the circulatory system that carries blood away from the heart, delivers it to most of the organs and tissues, and returns it to the heart again.
Choice A is wrong because it describes the pulmonary circuit, which brings oxygen-poor blood from the heart to the lungs.
Choice B is wrong because it is the opposite of what the systemic circuit does.
The systemic circuit brings oxygen-rich blood from the heart to the tissues, not from the tissues.
Choice C is wrong because it is also the opposite of what the systemic circuit does. The systemic circuit sends oxygen-poor blood to the heart, not from the heart.