Physiology

The Four Main Compartments

The horse's body has four main repositories for water:
Intravascular space: The blood vessels hold about 5% of the horse's total body fluids, a level that is subject to rapid fluctuation as the body's needs change.
Extracellular space: About 15% of the body's water surrounds the blood vessels and cells
Intracellular space: Up to 50% of the body's water is contained inside the cells. This fluid level is critical to the specialized performance of cells and is less adaptable to change than any other bodily water reservoir. Only about 10% of the water in intracellular space can be moved without serious consequences to the horse's health, and it can take two to three days to replace water lost from this area.
The gut: The remaining 30% of water is stored in the large intestine and cecum, providing a holding tank of sorts that's unique to horses. About half of the water in the cecum is available to replenish losses elsewhere in the body.

Brain Function

Water plays a role in both hearing and vision. The inner ear contains fluid that vibrates in response to sound waves, sending nerve impulses to the brain. The eyeball, which is predominantly water, is continually bathed in fluids from the lachrymal gland

The hypothalamus, a gland located at the base of the brain, controls the body's use of water by secreting antidiuretic hormone (ADH). When ADH is released, the cells retain water; when it is suppressed, they release it more freely.

Water makes up 65 to 70% of the average horse's body. A human body is 65% water, an apple is 85% and a jellyfish is 95%.
Water accounts for 85% of a horse's brain, 75% of his muscle mass and 30% of his bone.
Water bathes and cushions the brain and central nervous system, and lubricates all of the body's moving parts. It also plays a critical role in regulating the body's temperature through sweating.
If you drained the water from a 1,000lb horse, you would have removed 70 gallons, and he would weigh a mere 440lbs.

Consumption

The equine stomach holds three to four gallons of water, which is as much as a very thirsty horse can drink at a single time.
A horse drinks water as he nursed from his dam: by pursing his lips together, leaving only a small crack between them, and using his tongue as a suction pump to draw water up and into the esophagus. Waves of involuntary muscle contraction, know as peristalsis, move the water down to his stomach.
It takes a horse 12 swallows to drink a quart of water.
"Binge" drinking is an uncommon vice, but bored horses have been known to drink too much water. Kidney damage and excessive salt intake can also cause overconsumption of water.
Bands of wild horses head toward water only after the alpha mare decides it's time for a drink. Dominant bands get first "dibs" on a watering hole; others wait their turn.
In the wild, a horse can survive by filling up on water just once every other day.
A 1,000lb horse consumes approximately 10 gallons of water a day. A 150lb person needs just 2 1/2 quarts.

Water Needs

On a hot, humid day, a working horse can lose up to 4 gallons of water an hour - a rate that can't be replaced by drinking.
To avoid dangerous water loss in summer, limit your horse's activity when the temperature is over 80 degrees and the humidity is 70% or higher.
Horses ingest water in two ways: Through intake of food and liquid and through metabolism, the breakdown of carbohydrates and other organic molecules in reactions that yield water as a by-product.
Horses lose water in four ways: Excretion in urine and manure; evaporation by breathing; evaporation through the skin and sweating.
As horses age, their bodies contain less water, making them more susceptible to dehydration and impaction colic.
The increased metabolic demands of extremely cold weather may lead a horse to drink even more than he would in warm weather.
A horse who consumes lush grass, which is 50 to 90% water, will need to drink less than one who eats hay, which is only 5 to 8% water.
Lactation dramatically increases a mare's need for water. Most foals do not drink much water before weaning; youngsters who seek out water may not be getting sufficient milk from their dams.
Exhausted or severely dehydrated horses often lose interest in drinking - this is a situation that calls for immediate veterinary attention to replace body water and electrolytes.
A dehydrated horse's urine and feces are dark because his body is retaining what little water it has left. His skin may lose its elasticity, and he may appear "tucked up" in his belly and flanks.

Environment

Groundwater in heavily polluted industrial areas can contain such contaminants as alkali, lead, arsenic, organophosphates and chlorinated hydrocarbons.
Several waterborne contaminants can lead to serious illness, and even death in horses:
Botulism - usually caused by the decomposition of dead rodents in the water source. The infected horse suffers paralysis of the swallowing muscles and dies of dehydration.
Anthrax - spread when infected carcasses come in contact with the water source. Signs include acute colic, fever and swelling of the throat, neck and chest. Fortunately, anthrax is rare in the United States.
Toxic Algae - sometimes found in nutrient-rich drainage ponds exposed to bright sunlight. Contact with these generally rare species can lead to swelling of the head and legs, erratic movements and loss of appetite.
A variety of equine diseases, including rhinopneumonitis, influenza and strangles, can be transmitted via communal watering troughs. As a precaution, use your own buckets when visiting an unfamiliar farm or when your horse travels to shows.
Horses can be picky about unfamiliar water sources, but they usually won't allow themselves to become dehydrated.
Muddy water is harmless to horses, though they probably prefer clean water.

Horses in Water

Horses find water a great source of amusement. A bored horse may pass the time by blowing bubbles in his bucket or flipping his nose on the surface of the trough, splashing anyone who happens by. In summer, horses with access to natural bodies of water make the most of it by pawing, rolling and wallowing in shallow spots, enjoying the cool water and the respite from insects. More adventurous horses will actually swim through deeper bodies of water. All horses are natural swimmers when riderless; they simply dogpaddle along, with noses held high, "blowing" rhythmically as they swim.

Reference: Equus Magazine