Endocrinology: Modern Practical Nursing Series
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Endocrinology - John C Small
Wiltshire
Introduction
The endocrine system consists of a number of specialised organs located in different areas of the body. They are called endocrine glands and each produces individual secretions. The products of endocrine glands are called hormones. They pass directly into the blood stream and not through organised ducts, as bile is passed through the biliary apparatus. Because of this characteristic the endocrine gland is also called the ductless gland.
Hormones are complex chemical substances which affect the body by exerting a specific action on organs or groups of cells. Although their action is difficult to define hormones play an active part in metabolism and the normal function of organs and systems. The glands of the endocrine system are:
1.
Pituitary or hypophysis
2.
Thyroid
3.
Parathyroids
4.
Islet cells of the pancreas
5.
Adrenals
6.
Gonads – testis (male), ovary (female)
Metabolic processes of the body are initiated by stimuli of either chemical or nervous origin. The nerve response acts quickly but is of short duration whereas the chemical response acts slowly and is active for a longer period. For many years it was thought that the nervous and endocrine systems functioned independently but research has discovered much evidence showing that the inter-relationship is closer than previously imagined. This particularly involves the hypothalamus which is a part of the nervous system closely associated anatomically with the pituitary gland.
The most important endocrine gland is the pituitary for among its secretions are a number of hormones affecting other glands of the system. These hormones are called ‘trophic’ hormones because their presence in the circulation stimulates other glands, called ‘target’ organs, at which they are directed. The concentration of hormone secreted by the target gland in the blood, or the variety of body chemistry created by it, passes back to the pituitary gland. This passage of chemical information is called ‘feed back’ and allows the pituitary gland to control the action of the target gland by regulating the output of trophic hormone. This either increases or reduces the target gland secretion, thus balancing circulating hormones constantly. This pituitary effect is not active on all glands in the system.
Intro:1 The endocrine glands
Intro:2 The feed back
mechanism
Disorders of the endocrine system can be divided into three groups.
1.
Oversecretion
2.
Undersecretion
3.
Secretion of abnormal hormones
The body’s ability to store hormones is limited with the notable exception of the thyroid gland. Compensation for this unstable situation is the power of each gland to produce its secretions efficiently when stimulated. Hormones are broken down in the liver and excreted through the normal routes.
The treatment of endocrine disorders include the administration of hormones which are prescribed to create a variety of responses.
1 Replacement
Hormones will be given to patients to replace a deficiency caused by undersecretion or even complete failure of the gland.
2 Stimulation
Trophic hormones can be given to stimulate their target organ in cases of pituitary failure.
3 Retardation
Hormones can be given to retard the effect of abnormal body processes.
4 Reduction of inflammation
Some hormones reduce inflammatory conditions, particularly of allergic origin.
Hormones used therapeutically are obtained either from animal glands or by chemical synthesis. The latter is to be preferred for its greater purity and the reduction of allergic reactions due to protein sensitivity. They can be administered orally, rectally or by injection. Oral hormones are limited to those not inactivated by gastric secretions. Other methods include skin cream, eye drops, snuff and the implantation of pellets for slow absorption.
1
The Pituitary Gland
Publisher Summary
The pituitary gland, or hypophysis cerebri, is a small pea-shaped organ weighing 0.5gm. It lies at the base of the brain behind and below the crossing optic nerve fibers in the pituitary fossa of the sphenoid bone. The gland is attached to the base of the brain by a short stalk, the infundibulum. The pituitary gland is divided into two parts, the anterior and posterior lobes. The anterior lobe develops from the posterior aspect of the pharynx in the roof of the mouth, called Rathke’s pouch. It is glandular in structure and called the adenohypophysis (adeno gland). The posterior lobe develops as a down growth of the brain to which it is attached by the infundubular stalk. It contains a network of nervous tissue and is called the neurohypophysis (neuronerve). This chapter provides an overview of the hormones that are secreted from the adenohypophysis and neurohypophysis. The chapter also discusses the major disorders of the pituitary gland, which are caused due to an undersecretion or an oversecretion of the hormones. A common cause of pituitary dysfunction is a tumor, which can vary in size and type. They may also cause symptoms of raised intracranial pressure such as headache.
The pituitary gland, or hypophysis cerebri, is a small pea-shaped organ weighing 0.5gm. It lies at the base of the brain behind and below the crossing optic nerve fibres in the pituitary fossa of the sphenoid bone. This cavity is also called the ‘sella turcica’ because of its resemblance to the shape of a turkish saddle. Four small protruberances, the clinoid processes, extend over the enclosed gland to complete its protection. The gland is attached to the base of the brain by a short stalk, the infundibulum.
Fig 1:1 Lateral section of skull showing pituitary gland
The pituitary gland is divided into two parts, the anterior and posterior lobes; although anatomically related the two lobes are different in origin, structure and function. In the embryo they are
Fig 1:2 Three stages in the development of the pituitary gland
derived from separate sites. The anterior lobe develops from the posterior aspect of the pharynx in the roof of the mouth, called Rathke’s pouch. It is glandular in structure and called the adenohypophysis (adeno gland). The posterior lobe develops as a down growth of the brain to which it is attached by the infundubular stalk. It contains a network of nervous tissue and is called the neurohypophysis (neuro nerve).
Like other endocrine glands the pituitary is a very vascular organ, secreting its hormones directly into the circulation. Branches of the internal carotid and posterior communicating arteries – the superior and inferior hypophyseal arteries – supply blood to the infundibular stem and the posterior lobe. Other branches of the internal carotid artery form a capillary network in the hypothalamus leading to a portal system supplying the anterior lobe. The whole complex drains into the hypophyseal veins.
The nerve supply of the pituitary gland is concentrated in the posterior lobe; nerve fibres originating in the hypothalamus descend through the hypophyseal tract and infiltrate the tissues of the lobe. It is thought that secretions of the posterior lobe originate in the hypothalamus and pass down into the gland through small vessels. There is no distinct nerve supply to the anterior lobe although the hypothalamus has a relationship to its function by the passage of hormones via a network of capillaries.
Six hormones are known to be secreted from the adenohypophysis. They are produced from the two types of cells, the acidophils and the basophils. They are trophic in character so that they exert a physiological control over specific distant organs.
1. Somatotrophic or human growth hormone (HGH)
2. Mammotrophin or lactogenic hormone
3. Thyrotrophic or thyroid stimulating hormone (TSH)
4. Corticotrophic or adrenocorticotrophic hormone (ACTH)
5. Follicle stimulating hormone (FSH)
6. Luteinising, or interstitial cell stimulating hormone (LH or ICSH)
Two hormones only are secreted from the neurohypophysis.
1. Vasopressin (Pitressin), or the anti-diuretic hormone (ADH)
2. Oxytocin (Pitocin)
In common with other glands of the system the major disorders are due to an undersecretion or an oversecretion of the hormones.
Fig 1:3 Section through the pituitary gland
Fig 1:4 Organs affected by pituitary hormones
Adenohypophysis – anterior lobe
Somatotrophic hormone
One function of the anterior lobe is the secretion of a hormone associated with growth. Although its specific action is not fully understood it exerts a powerful effect on the development of body cells, particularly in bone tissue. It is most effective in youth and once growth has stopped it is thought to play a part in repairing and replacing body tissue. Its action is interrelated with other hormones involved in tissue metabolism, notably insulin; for hypoglycaemia stimulates the secretion of growth hormone.
Disorders of growth hormone cause abnormal changes in body size and structure. Because the hormone is most active in childhood any dysfunction is emphasised in the younger age