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The endocrine system

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VOL: 103, ISSUE: 21, PAGE NO: 26

Brendan Docherty, MSc, PGCE, RN is clinical stream manager, cardiology and critical care, South Eastern Sydney and Illawarra Health Service, Australia

This article, the first in a four-part series, explores the role and functions of the endocrine system and its rela…

This article, the first in a four-part series, explores the role and functions of the endocrine system and its related hormones. It also looks at the hypothalamus and pituitary gland, often referred to as the master glands, in detail. Part two of the series will focus on the pineal, thyroid and parathyroid glands; part three on the thymus and adrenal glands; and part four on the pancreas and the sex glands (gonads).

THE ENDOCRINE SYSTEM

The endocrine system releases chemical messengers known as hormones and works mainly in tandem with the nervous system, though the mechanisms by they alter and adjust bodily functions is different. Hormones serve as a ‘signal’ to the target cells, area or organ (either as a regulatory function or in response to a specific need); and the actions are often longer-acting and slower than the nervous system (Wood et al, 2006).

When hormone release is initiated it usually occurs as a sudden peak in production, with either very little or no release between each initiation. Hormones can be released due to an imbalance in blood chemistry, nervous system initiation, or by another hormone (Wood et al, 2006).

The main endocrine glands and their hormones are outlined in Table 1 (Marieb and Hoehn, 2006; Wood et al, 2006).

The mechanism of hormone action varies and depends on the hormone itself and on the target cell or area. For example, some hormones work by altering cell membrane permeability to other substances, some by altering metabolic processes, and others by affecting the muscle strength and heart rate (Saladin, 2006).

Hormones tend to be classified as lipid-soluble and water-soluble. Lipid-soluble hormones include steroid and thyroid hormones. They work by attaching to receptors inside the target cell and altering the genes within the cell nucleus, which then alters the function of that cell to exert the desired response (Marieb and Hoehn, 2006).

 

 

Water-soluble hormones include the amines and protein hormones, which are the largest group and include insulin, growth hormone and parathyroid hormone (Harrower, 2005). They work by attaching to receptors on the plasma membrane on the outside of the target cell that then activate another membrane substance called G-protein. This starts a cascade effect of changes inside the cell (Marieb and Hoehn, 2006). Responsiveness may depend on the hormone amount (or concentration) and receptor amount, as well as the hormone interaction (Wood et al, 2006).

 

 

Some hormones may have a greater effect when combined - for example follicle stimulating hormone and oestrogens are both required for ovarian egg development, and some hormones work in opposition to each other - for example insulin and glucagon, which maintain blood sugar levels (Wood et al, 2006).

 

 

Hormones can also be classified by the manner in which they reach their target cells or organ (Saladin, 2006; Harrower, 2005):

 

 

- Local hormones act in the cell or surrounding cells, for example immune system cells;

 

 

- Circulating hormones travel via the bloodstream to reach their point of action.

 

 

THE HYPOTHALAMUS

 

The hypothalamus is an endocrine gland but is also one of the main regulatory centres of the nervous system. It is therefore an important link between the endocrine and nervous systems (Wood et al, 2006). It controls autonomic nervous activity and receives signals from nervous receptors which require a response or activity (for example fear, pain, temperature regulation, water balance) (Marieb and Hoehn, 2006).

 

 

Hormones produced by the hypothalamus work directly on the pituitary (anterior lobe) as part of a cascade system. They can be split into two main types: releasing hormones, which initiate production of a hormone; and inhibiting hormones, which halt production of a hormone in the pituitary gland (Harrower, 2005).

 

 

Together, the hypothalamus and the pituitary control homeostasis, metabolism, growth and development (Saladin, 2006).

 

 

THE PITUITARY GLAND

 

The pituitary gland (Fig 2) has two sections: the larger anterior and smaller posterior lobes. It sits below the hypothalamus, to which it is connected by a stalk called the infundibulum and thousands of neurones (Marieb and Hoehn, 2006). The anterior lobe derives from epithelial tissue and produces endocrine hormones while the posterior lobe derives from nervous system tissue and produces neurohormones (Saladin, 2006).

 

 

The anterior pituitary lobe produces seven hormones:

 

 

- Growth hormone - promotes childhood growth and maintains metabolic processes, muscle and bone mass in adulthood;

 

 

- Adrenocorticotropin - controls production of cortisol, which maintains blood pressure and blood sugar levels;

 

 

- Thyroid-stimulating hormone - controls the production of thyroid hormones;

 

 

- Prolactin - stimulates milk production following childbirth;

 

 

- Luteinising hormone - controls testosterone and oestrogen levels;

 

 

- Follicle-stimulating hormone - causes sperm production and egg release in the ovaries;

 

 

- Melanocyte-stimulating hormone - increases skin pigmentation.

 

 

The posterior pituitary lobe stores and releases two hormones: oxytocin, which stimulates milk production following childbirth and contractions during childbirth; and anti-diuretic hormone, which decreases urine production in regulating water balance (Wood et al, 2006; Harrower, 2005).

 

 

- This article has been double-blind peer-reviewed.

 

 

GLAND: HORMONE(S)

 

 

Pineal: Melatonin

 

 

Pituitary: Oxytocin; vasopressin; melanocyte-stimulating hormones; corticotropin; lipotropin; thyrotropin; growth hormone; prolactin; luteinising hormone; follicle-stimulating hormone

 

 

Thyroid: Thyroxin; calcitonin; calcitonin gene-related peptide

 

 

Parathyroid: Parathyroid hormone

 

 

Thymus: Thymus hormone

 

 

Heart: Atrial natriuretic peptide

 

 

Adrenal: Glucocorticoids (cortical); mineralocorticoids (cortical); norepinephrine (medullary); epinephrine (medullary)

 

 

Renal: Calcitriol

 

 

Pancreas: Insulin; glucagon; pancreatic polypeptide; somatostatin

 

 

Liver: Angiotensin II

 

 

Gonadal: Oestrogens (ovarian); progestins (ovarian); androgens (testicular)

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