Capillary permeability to endogenous macromolecules in the equine digit.

This research focused on examining the characteristics of tiny blood vessels in horse feet. It found that the horse’s digit can retain about 67% of large molecules within its blood vessels, suggesting it has a relatively high permeability.

Research Methodology

• The study was conducted on the digits (hoof region) of 7 adult horses. To investigate the microvascular permeability, i.e. how easily substances can pass through the tiny blood vessels, capillaries in the digits were isolated.
• An extracorporeal perfusion circuit, a system to circulate fluid outside of the body, was established for the digit. This allowed the precise control and measurement of fluid passing through the isolated capillaries.
• A lymphatic vessel – a vessel through which excess fluid in body tissue (known as lymph) drains – was cannulated, or inserted with a tube, at the coronary band level. This was performed to drain the distal, or far, portion of the phalangeal (hoof) region.

Data Collection

• Several measures were made to understand the capillary’s permeability. The venous pressure within the digit was gradually increased, after which the lymph flow, lymph protein concentration (Cl), and plasma protein concentration (Cp) were determined.
• These measurements were only made after the system reached a steady state, meaning these variable measures were consistent over time.
• The ratios of protein concentration in the lymph compared to that in the plasma (Cl/Cp) were calculated. Also, the oncotic pressures (pressure exerted by proteins) in the lymph and plasma were determined.

Findings

• The osmotic reflection coefficient, a measure of how a capillary or membrane’s permeability range against large molecules or “macromolecules”, was determined and recorded at 0.67 for the horse’s digit. This measurement was made once the Cl/Cp ratio became constant despite increases in lymph flow.
• This 0.67 osmotic reflection coefficient suggests that the microvasculature bed of the horse’s digit retained only 67% of macromolecules within it, thus implying relative permeability.