Evaluation of Starling forces in the equine digit.

This study examines the forces that affect blood flow in horse’s hooves, using an artificial setup to simulate natural conditions. The findings suggest that blood capillary pressure in horse’s hooves is higher than in similar tissues in dogs or humans, but this high pressure is balanced out by high tissue pressure and low permeability of the microvasculature to fluid.

Experiment Set-up and Procedure

• The researchers used an extracorporeal digital preparation – an artificial setup that allows for controlling and manipulating the conditions under which the horse’s digit (hoof) is studied.
• The scientists evaluated various parameters related to blood circulation in the hoof, such as blood flow, arterial pressure, venous pressure, capillary pressure, and vascular compliance in six normal horses.
• They measured the capillary filtration coefficient, which is a measure of how easy it is for fluid to pass through the capillary walls.
• They also estimated certain pressures, derived from the protein concentrations in plasma and lymph. This was used to calculate the oncotic pressure, which is the osmotic pressure in the blood vessels due to proteins.

Data Analysis

• From the observed data, the researchers calculated pre- and postcapillary resistances and resistance ratios. These calculations show how much resistance there is to the blood flow before and after it passes through the capillaries.
• They used the collected and calculated data to find the tissue pressure in the digit using the Starling equation, a formula that predicts fluid movements in capillaries based on pressures and resistances.
• They found that the average capillary pressure in an isolated equine digit was 36.7 mmHg, plasma and lymph oncotic pressures averaged 19.12 and 6.6 mmHg respectively, the interstitial fluid pressure averaged 25.6 mmHg, and the capillary filtration coefficient averaged 0.0013 ml.min-1.mm-1.100 g-1.

Interpretation of Results

• The researchers found that digital capillary pressure in the horse’s hoof, when the animal is lying on its side, is much higher than in similar tissues in dogs or humans.
• Despite the high capillary pressure, potential edema (swelling due to fluid accumulation) is counteracted by two factors: a high tissue pressure which can push back against the incoming fluid, and a low microvascular surface area or low microvascular permeability which restricts fluid exchange across the capillary walls.