Energy and Heat Loss in a Melbourne Cup Racehorse

Physiology of a Melbourne Cup Racehorse
During the Melbourne Cup race today, you may appreciate the speed and stamina of the equine athletes, but you can also admire the extreme physiological thermoregulatory adaptations, or heat loss efficiency relative to heat/energy produced, in the galloping horses.

A horse running in the Melbourne Cup this afternoon in warm conditions will generate 80% of its energy to gallop using aerobic (oxygen-using) metabolism, burning up around 20% of the total muscle energy stores over the 3,200 metre journey. Aerobic metabolism is a very efficient metabolic pathway, facilitated by the 2,200 litres of air inhaled into the lungs at 140 breaths per minute at approx. 15 litres each breath. Therefore, galloping horses use up to 70 litres of oxygen per minute to fuel muscle energy (glycogen) metabolism.

However, a byproduct of aerobic metabolism is heat. During the race, a Cup horse will generate enough heat to boil 28 litres of cold tap water. This heat must be dissipated before a horse overheats and muscle damage begins to occur. Luckily, horses have multiple physiological mechanisms for heat loss.

A horse loses heat by evaporation of sweat (around 50%), radiation from the skin (15%), convection as air passes over the body (15%) and 20% in expired air in each breath as it exercises. As a horse becomes fitter in training fat reserves under the skin are minimised to reduce the insulating effect and their skin becomes thinner and more densely packed with blood vessels to facilitate heat loss. Horses need to also cool their brain during such intense exercise. This is primarily done by billowing out the 2 guttural pouches (small expandable sacs in the pharyngeal-auditory tubes), with about 200 mL of fresh inhaled air per breath, to cool the carotid artery to the brain located in the wall of each guttural pouch.

However, it is after the race that excess heat must be quickly removed from the exercising muscles to prevent heat induced damage. As the horse pulls up after the race, heat loss by convection decreases and sweat loss increases to make up some of thermoregulatory efficiency. The red blood cells which deliver large amounts of oxygen to the exercising muscles also act as 'heat carriers' to remove heat from the muscle cells as large volumes of blood (350 litres per minute) pass through the body. The red cells deliver heat to the gut cavity, skin and lungs. After the race, a 'heat flash' response occurs as heat rapidly builds up to increase body temperate by around 1°C as convection and lung heat loss reduces initially.

The heat carrying red blood cells then dump heat into the hind gut content, which contains up to 60 litres of water in the digestive mass. The hind gut becomes a useful 'heat sink' or storage site, allowing excess heat to be quickly removed from the muscles and then slowly dissipated during the 4 hour cool down period after the race. Up to a third of the heat retained is 'blown off' from the lung surface during the post race recovery period, supported by heat radiating from the skin and sweat loss.

A quick tip to cool a horse after any strenuous exercise
Hosing a horse down to assist skin and sweat loss and then quickly 'scraping off', or removing the warm water from the surface of the coat, allows moisture evaporation from the horse's heated body, an important post-race practice to assist cooling. Hosing a hot horse under the belly after strenuous exercise will target the 'heat sink' in the hindgut for a quick cool down after intense exercise. Walking the horse to redistribute warm blood from the muscles throughout the body and skin also aids thermoregulation after galloping or strenuous exercise.


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