Capillaries are the smallest blood VESSELS. They are unique in that:

They don't have any muscular component like arterioles or venules do.They are lined by a single layer of endothelium (cells lining blood vessels are called endothelial cells) overlying the basement membrane.

Due to these reasons the capillaries are very permeable facilitating nutrient and gas exchange.

The capillaries don't expand as they do not have muscular or elastic components. However exposure to heat do increase the circulation. This is due to the following facts:

Exposure to heat dilates the pre-capillary arterioles - this WOULD inevitably cause improved capillary circulation in capillaries that are already OPEN.Increase in pre-capillary flow opens up collapsed capillaries. In the human BODY roughly only one-fourth of the capillaries are open during rest1. As the pre-capillary arterioles dilate, the collapsed capillaries open up thus further improving circulation.

To improve capillary perfusion

I HOPE it's help U..:-)