Which of the following statements about angiogenesis is true?

• A. Angiogenesis reduces capillary density.
• B. Angiogenesis occurs without any relation to hypoxia.
• C. Angiogenesis involves shrinking of small arteries.
• D. Angiogenesis is the formation of new capillaries and arterioles in response to hypoxic stress.

Answer: (D)

Angiogenesis is the growth of new blood vessels to restore oxygen delivery when tissue oxygen levels drop. It involves endothelial cells proliferating and sprouting to form new capillaries, and as the network matures, it can include the formation of small arterioles to improve perfusion. Hypoxia triggers this process through signals like HIF and VEGF, guiding vessel sprouting and stabilization. Because new vessels are formed, capillary density increases rather than decreases, and the growth is driven by hypoxic stress rather than occurring independently of it. It does not involve shrinking arteries; it adds vessels in response to the need for oxygen delivery.