Which vascular access needle can be placed against the flow of blood?

The arterial needle can be placed against the flow of blood, which is why it is the correct answer. In the context of vascular access for procedures such as dialysis, the arterial needle is designed to draw blood from the artery, and therefore it must be oriented in the direction opposite to that of blood flow. This allows for efficient extraction of blood and the proper functioning of the system during treatment.

In contrast, venous needles are placed in accordance with the flow of blood returning to the heart, which is essential for obtaining a proper venous blood return. When working with vascular access, understanding the direction of blood flow is crucial for effective and safe procedures. This fundamental knowledge helps ensure that the correct needle type is used in the appropriate manner.