- The primary role of capillaries in the circulatory system is the exchange of gases and nutrients between blood and tissues.
- Capillaries are the smallest blood vessels, with walls that are only one cell thick.
- This thin structure allows for efficient diffusion of oxygen, carbon dioxide, nutrients, and waste products between the bloodstream and surrounding tissues.
- Unlike arteries and veins, capillaries do not primarily function to transport blood over long distances or regulate blood pressure, but rather to facilitate this crucial exchange at the cellular level.

Option 1, which refers to the transport of oxygenated blood from the heart to tissues, describes the role of arteries, not capillaries.
Option 3, prevention of backflow of blood in veins, pertains to the function of venous valves, not capillaries.
Option 4, regulation of blood pressure through vasoconstriction, is primarily a function of arterioles, which are small vessels leading to capillary beds.

Thus, the key function of capillaries is to serve as the site where nutrient and gas exchange takes place between blood and body cells.

Reference: Guyton and Hall Textbook of Medical Physiology, 13th Edition, Chapter 18: Microcirculation and Lymphatic System