It is an advantage to the horse, as with any mammal, to maintain a relatively constant internal environment. Even small fluctuations in the physical and chemical surroundings of cells can disrupt their biochemical processes and in extreme cases, kill them (Roberts et al, 2000).
A constant internal environment enables chemical reactions to take place at a predictable rate, the animal also acquires a degree of independence from the environment. This means that it is not restricted to living in certain regions of the earth, nor does it have to restrict its activities to particular periods of the day, or seasons, when conditions are suitable. The maintenance of a constant internal environment is called homeostasis, which in the horse is largely controlled by the endocrine system (Toole, 1999).
The horse requires a constant input of energy to maintain stable internal conditions; such stability requires control systems capable of detecting any deviation from the usual and making the necessary adjustments to return to the normal condition (Toole, 1999). The presence of a corrective mechanism is an important principle of homeostasis, where deviations from the norm are corrected by negative feedback (Roberts et al, 2000).
Of particular importance to the sports horse are systems that control glucose levels, respiration, heart rate and blood pressure, thermoregulation, electrolyte balance and osmoregulation (Roberts et al, 2000).
All metabolising cells require a constant supply of glucose in order to create ATP, the cellular energy source. A system which maintains constant glucose levels in the blood, despite intermittent supplies from the intestine, is therefore essential (Toole, 1999).
The secretion of insulin and glucagon is controlled by receptors in the pancreas monitoring glucose levels in the blood. If the blood glucose concentration rises above the norm, less glucagon and more insulin is released by the pancreas; conversely, if glucose levels fall, less insulin and more glucagon is released (Roberts et al, 2000).
This system illustrates the principle of ‘negative feedback’, where an opposite effect is instigated if a situation changes. The result is that whether the glucose level increases or decreases, a process is set in motion to return it to the optimum value. This corrective mechanism is controlled by the level of glucose itself, and so is self-adjusting.
Homeostatic scheme for the control of blood glucose concentration (Roberts et al, 2000).
The liver acts as a storage site for glycogen and plays a key role in glucose homeostasis; it can add glucose by the breakdown of glycogen (glycogenolysis), and remove glucose by converting glucose into glycogen (glycogenesis). This process is under the control of two hormones produced by the pancreas (Toole, 1999).
Throughout the pancreas are cells known as the Islets of Langerhans, these contain alpha-cells which produce the hormone glucagon, and beta-cells which produce the hormone insulin, both hormones are discharged directly into the blood. Insulin promotes the conversion of excess glucose into glycogen, and when glucose levels are low, glucagon activates enzymes in the liver to reconvert glycogen into glucose (Toole, 1999).
In times of stress the hormone adrenaline is released from the adrenal glands, overriding the homeostatic control of glucose. This causes the breakdown of glycogen in the liver, immediately raising the blood sugar level (Toole, 1999).