Final answer:
Evacuated tubes automatically fill with blood during venipuncture because of capillary action, which is a result of the pressure differential and the relative strength of cohesive and adhesive forces.
Step-by-step explanation:
During venipuncture, evacuated tubes fill automatically as soon as the tube is pierced due to capillary action. This phenomenon is due to the relative strength of cohesive and adhesive forces. The design of the evacuated tube system is such that when the tube's stopper is pierced by the needle, the pressure differential between the inside of the tube and the surrounding atmosphere leads to the automatic draw of blood into the tube. The pressure inside the vein, helped by the venous system's physiological pumps and the low pressure in the atria during diastole, aids in the process by pushing blood towards the area of lower pressure within the tube.
It is important to note that the venous system includes mechanisms like the rhythmic movement of smooth muscle and the skeletal muscle pump to aid in venous blood flow, particularly against gravity. This context helps us understand the dynamics of blood flow that are harnessed during the venipuncture procedure.