Blood circulates throughout the body delivering all the materials needed to stay alive, but it also acts as a system to recycle materials, return gasses and eliminate waste. In this sense, it works like the post office, the package delivery service, the messenger service and the recycling and trash service. This last part requires the blood to move through an incredible filtering system called the kidneys.
Kidneys are very smart organs because they remove trash and monitor and regulate the pressures of the cardiovascular system and the levels of certain molecules in the system (electrolytes). It has a system of hormones that talk with the brain, heart and all other body systems. When it senses too much fluid in the body, it forms a fluid called urine. It measures blood pressure, osmolality (density) and temperature, to name a few, and the kidneys regulate the other systems through urine production (or lack thereof).
To prevent the constant elimination of urine, a bladder stores the urine for voiding or use as a marker. Pressure receptors in the bladder measure the pressure there and signal the horse’s movement towards a spot, the posturing and the elimination of the urine from the body. There is more.
The urine has molecules that become unique to the individual, and these molecules can mark out territory or attract a mate. The same goes for feces. These molecules link with the sense receptors in the nose that signals the brain’s amygdala to recall an emotion that signals the motor cortex that creates movement of the legs. Wow!
There are a few problems with this system in horses. All mammals have quadruple (4 times) the amount of kidney function they need to survive. In other words, a horse can lose 75% of its kidney function without any outward signs of kidney failure. Beyond this, things become bad. The immune system can cause a blockage of the filters. Too much salt intake can disrupt the gradient that filters the blood. Some antibiotics can destroy kidney tissue. The formation of mineral stones can block the output in the kidney or the tubes to (the ureters) and from (the urethra) the bladder. Incontinence can lead to scalding of the skin where the urine dribbles. Most of these conditions can be life-threatening or ongoing, requiring care throughout the horse’s life.