The respiratory system of the horse is well adapted to athletic exercise, with unrestricted upper airway diameters, and a large lung capacity afforded by 18 ribs. These combine to enable air intakes of up to 1800 litres per minute in a galloping horse. Volumes of up to 300 litres of blood are pumped at high pressure through small lung capillaries surrounding 10 million air sacs to take up and deliver over 70 litres of oxygen per minute to the working muscles at the gallop.
As a result, any restriction in upper airway diameter, obstruction of the airways, diseases or stress related conditions that reduce efficiency of oxygen uptake from the air sacs, can have a great influence on athletic capacity.
The large lung surface and high blood flow rates also provide the additional function of heat loss during and after exercise, with up to 20% of the muscle heat generated during exercise being exchanged across the lung surface to supplement sweating and other skin surface heat loss mechanisms.
The respiratory system is continually challenged by a large amount of foreign material, including viruses, bacteria and fungi inhaled in air from track and arena surfaces during exercise, or from dusty bedding, feed and stable environments.
The horses circulatory system is a very large and complex system made up of veins and arteries. The blood is the pumped under enormous pressure from the heart along the arteries which have thick muscular walls to deal with the pressure. It oxygenates the body and the internal organs
The circulatory system is based upon the heart – a hollow, muscular organ in the chest cavity. It pumps the blood around the body and is divided into four separate compartments .
Blood from the right ventricle goes to the lungs to be oxygenated and then is returned to the left ventricle.
Blood from the left ventricle is pumped all through the body in arteries.
Arteries repeatedly branch and diminish in size until they become microscopic capillaries.
Capillaries permit necessary interchange between blood and tissue. They eventually join up to produce veins, which convey blood to the right atrium and from there to the right ventricle.
A horse of average size has approximately 50 pints (28 litres) which circulate through his system every 40 seconds.
Depending on size, age, and productive status (work, sport, pleasure, breeding, pregnancy, lactation, retirement), a horse will digest about 60% of most feedstuffs. Feed that is 60% digestible indicates that if a horse is fed 25 pounds of dry feed, 15 will be digested and 10 pounds will be excreted as manure (feces). This will vary by feed. Feeds that are higher in fiber such as hays and grasses have a lower digestibility. Conversely, concentrate feeds that contain grains such as corn, oats, and/or barley, usually have a higher efficiency of digestion and less fecal excretion.
Nitrogen (N) is a major component of protein. Horses need protein for maintenance, growth, reproduction, lactation, and work. Phosphorus (P) is a macromineral needed for maintenance, growth, and other physiologic functions. Water is also essential for bodily functions. Water is lost from the body primarily in the excretion of feces and urine, sweat, evaporation from the lungs and skin, and in the case of lactation, from milk. It also affects the consistency of manure.
All nutrients that are digested (absorbed) are metabolized in the horse’s body. Some of these, especially nitrogen in proteins, are excreted in the urine. After being digested and metabolized in the body, waste nitrogen is converted to urea in the liver and excreted in the urine. Additional undigested nitrogen is excreted in the feces. Overfeeding protein will increase the excretion of nitrogen.
Overfeeding phosphorus will increase the excretion of phosphorous, most of which is excreted in the feces. Horses should be fed a diet that is digestible and formulated to meet nutritional requirements, while avoiding excesses. Overfeeding can result in higher levels of nitrogen and phosphorus in the manure. Horse farmers should feed horses according to their nutritional needs. Specific recommendations for nitrogen (protein) and phosphores.