Take a look at the inside of your elbow. Chances are, you can see a vein—one of the body’s large vessels that routes blood around the body.

But there are lots of blood vessels that cannot be seen with the naked eye—the arterioles, venules, and capillaries that branch off from veins and arteries. Collectively known as the microcirculation, these vessels play a vital role in carrying oxygen and nutrients to tissues and cells, and carrying away waste products.

A recent study led by Deborah Silverstein, an associate professor in Penn’s School of Veterinary Medicine, took a careful look at how the microcirculation was affected by intravenous fluids given during minor surgical procedures in dogs. She found that keeping animals hydrated during these surgeries has a direct effect on blood circulation, and may lead to better short- and long-term health outcomes.

Silverstein, an emergency and critical care veterinarian, has long been interested in what the microcirculation can reveal about an animal’s health, often studying it in the context of sepsis.

“When we monitor a patient’s blood pressure or oxygen levels, we’re not always able to discern what is happening at the tissue level,” she says. “Looking at the microcirculation could help explain why some otherwise healthy animals end up with unexplained adverse outcomes after surgeries.”

In many veterinary hospitals outside of Penn Vet, healthy animals undergoing minor surgical procedures, such as spays and neuters, do not receive IV fluids. Silverstein and her colleagues were interested in finding out whether giving IV fluids would have an impact on the microcirculation of dogs during surgery, and also to see what level of fluid delivery was optimal.

Their study, supported by Abbott Animal Health, included 49 client-owned dogs in good health. They assigned the dogs to one of three groups: one received no fluid and two others received either 10 or 20 milliliters per kilogram weight per hour of an IV solution.

Using a video microscope held against the gums of the dogs to assess blood flow during spay procedures, the researchers were somewhat surprised to see no differences among the groups in the proportion of vessels with blood flow or the amount of flow in the vessels. They also saw no differences in the tiniest blood vessels—the capillaries that are less than 20 micrometers in diameter.

They did, however, see a difference in the blood vessels larger than 20 micrometers: dogs that received the greatest amount of fluids had greater densities of these blood vessels than dogs receiving no fluids or 10 milliliters per kilogram weight per hour of an IV solution. The animals that received the highest fluid rate also had greater densities of these larger vessels with blood flow compared to the control group.

The results demonstrate that fluids are indeed having an effect on circulation, but more research is needed to understand the implications of the study and the optimal rate of fluid delivery.