Which of the following is an extrinsic factor affecting venous flow?

Atmospheric pressure is considered an extrinsic factor affecting venous flow because it refers to the pressure exerted by the weight of the atmosphere around us. This external force can influence venous return by altering the pressure gradients within the venous system. When atmospheric pressure changes, it can have an impact on the pressure within the thoracic cavity, thereby affecting the overall venous return to the heart.

Other factors could include the way the body is positioned or the state of physical activity, as these changes in environment and activity level directly affect venous flow. However, atmospheric pressure is a consistent external influence that operates independently of the body's physiological mechanisms.

The other options, such as the musculovenous pump, thoracic pressure, and venous contractions, are more related to intrinsic mechanisms within the body. The musculovenous pump pertains to the action of muscles surrounding the veins, thoracic pressure changes are related to respiration, and venous contractions refer to the muscle contractions within the venous structure, all of which are intrinsic factors that facilitate venous return.