Keywords endocrinology , endocrine , hormones , negative feedback , gland
Introduction
The endocrine system is a complex anatomical system involved in maintaining homeostasis within the body . It consists of various ductless glands located throughout the body , each of which secretes specific hormones . These hormones are chemical messengers that are secreted in response to stimuli and travel , via the bloodstream , to a specific site of action where they will have an effect . Hormonal secretion is stimulated and inhibited by :
• Chemical changes in the blood , for example , changes in calcium or glucose levels
• Nerve impulses
• Other hormones
Endocrine glands
The major endocrine glands in the canine and feline body ( moving cranial to caudal through the body ) are :
• Pituitary – anterior and posterior
• Thyroid
• Parathyroid
• Pancreas
• Adrenal
• Gonads – ovaries or testes
Pituitary gland
The pituitary gland ( or hypophysis ) is located at the base of the brain and is connected to the hypothalamus , which controls its activity . The pituitary is sometimes referred to as the ‘ master gland ’ – it is an important site of producing , storing and secreting hormones and it links with the nervous system ( the second system involved in maintaining homeostasis ) via the hypothalamus . The pituitary gland coordinates the effects of all the other endocrine glands in the body . It is divided into two lobes , the anterior and posterior , and both act independently of each other .
Conditions in body change from a set point
In normal , healthy patients , the level of hormones in the body will fluctuate while maintaining homeostasis . In diseased animals , these levels may be abnormal – either very low or very high – and measurement of these hormone levels can aid diagnosis of clinical conditions .
Negative feedback loop
Corrective mechanisms switch off and hormones no longer released
This change is detected by receptors located throughout the body
The level of some hormones in the blood is controlled and managed by a negative feedback loop . These self-regulating systems prevent oversecretion of hormones by endocrine glands and will reduce the level of secretion once the necessary effect has been achieved in the body ( Figure 1 ). It can be helpful to think of it like a home heating thermostat – if the temperature falls below the desired limit , the system clicks on to increase the heating until the desired temperature is reached . Once the desired limit is met , the system clicks off to prevent overheating . If the temperature falls again , the system will detect this and click on .
Conditions return to set point
Corrective mechanisms are activated and hormones released
Figure 1 . Diagrammatic representation of how the negative feedback loop works .
54 Veterinary Nursing Journal