In at least 500 words:
Discuss the function of capillaries in the body and how their structure facilitates their functions. You should introduce the different types of capillaries. Then you may choose to elaborate on the capillaries that are found in the liver, or the kidney, or the brain, or the alveoli, or the small intestine (or several of them). You should discuss how the structure of each type of capillary that you discuss facilitates the function that they perform.