The parathyroid glands

There are four small parathyroid glands, two embedded in the posterior surface of each lobe of the thyroid gland (Fig. 9.9). They are surrounded by fine connective tissue capsules. The cells forming the glands are spherical in shape and are arranged in columns with channels containing blood between them.

Function

The parathyroid glands secrete parathyroid hormone (PTH, parathormone). Secretion is regulated by the blood level of calcium. When this falls, secretion of PTH is increased and vice versa.

The main function of PTH is to increase the blood calcium level when it is low. This is achieved by indirectly increasing the amount of calcium absorbed from the small intestine and reabsorbed from the renal tubules. If these sources provide inadequate supplies then PTH stimulates osteoclasts (bone-destroying cells) and resorption of calcium from bones.

Parathormone and calcitonin from the thyroid gland act in a complementary manner to maintain blood calcium levels within the normal range. This is needed for:

• muscle contraction

• blood clotting

• nerve impulse transmission.

Adrenal (suprarenal) glands

There are two adrenal glands, one situated on the upper pole of each kidney enclosed within the renal fascia. They are about 4 cm long and 3 cm thick.

The arterial blood supply to the glands is by branches from the abdominal aorta and renal arteries.

The venous return is by suprarenal veins. The right gland drains into the inferior vena cava and the left into the left renal vein.

The glands are composed of two parts which have different structures and functions. The outer part is the cortex and the inner part the medulla. The adrenal cortex is essential to life but the medulla is not.

ADRENAL CORTEX

The adrenal cortex produces three groups of steroid hormones from cholesterol. They are collectively called adrenocorticocoids (corticosteroids, corticoids). They are:

• glucocorticoids

• mineralocorticoids

• sex hormones (androgens).

The hormones in each group have different characteristic actions but due to their structural similarity their actions may overlap.

Figure 9.10 Negative feedback regulation of glucocorticoid secretion.

F igure 9.11 Negative feedback regulation of aldosterone secretion.

ADRENAL MEDULLA

The medulla is completely surrounded by the adrenal cortex. It develops from nervous tissue in the embryo and is part of the sympathetic division of the autonomic nervous system. It is stimulated by its extensive sympathetic nerve supply to produce the hormones adrenaline (epinephrine) and noradrenaline (norepinephrine).

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RESPONSE TO STRESS

When the body is under stress homeostasis is disturbed. To restore it and, in some cases, to maintain life there are immediate and, if necessary, longer-term responses. Stressors include exercise, fasting, fright, temperature changes, infection, disease and emotional disturbances/ situations.

The immediate response is sometimes described as preparing for 'fight or flight'. This is mediated by the sympathetic part of the autonomic nervous system and the principal effects are shown in Figure 9.12.

In the longer term, ACTH from the anterior pituitary stimulates the release of glucocorticoids and mineralocorticoids from the adrenal cortex and a more prolonged response to stress occurs (Fig. 9.12).

PANCREATIC ISLETS

The cells that make up the pancreatic islets (islets of Langerhans) are found in clusters irregularly distributed throughout the substance of the pancreas. Unlike the exocrine pancreas, which produces pancreatic juice (p. 304), there are no ducts leading from the clusters of islet cells. Pancreatic hormones are secreted directly into the bloodstream and circulate throughout the body.

There are three main types of cells in the pancreatic islets:

• a (alpha) cells, which secrete glucagon

• P (beta) cells, which secrete insulin

• 5 (delta) cells, which secrete somatostatin (GHRIH, pp. 214 and 223).

The normal blood glucose level is between 3.5 and 8 mmol/litre (63 to 144 mg/100 ml). Blood glucose levels are controlled mainly by the opposing actions of insulin and glucagon:

• glucagon increases blood glucose levels

• insulin reduces blood glucose levels.

F igure 9.12 Responses to stressors that threaten homeostasis. CRH = corticotrophin releasing hormone. ACTH = adrenocorticotrophic hormone.

PINEAL GLAND

The pineal gland is a small body attached to the roof of the third ventricle and is connected to it by a short stalk containing nerves, many of which terminate in the hypothalamus. The pineal gland is about 10 mm long, is fed-dish brown in colour and is surrounded by a capsule.

(Melatonin)

THYMUS GLAND

The location and structure or the thymus gland are described on page 136.

(thymosin)

LOCAL HORMONES

A number of body tissues not normally described as endocrine glands secrete substances that act locally. Some of these are described below.

Histamine

Serotonin (5-hydroxytryptamine, 5-HT)

Prostaglandins (PGs)

Gastrointestinal hormones