Overview of the endocrine glands

Student activity

Now complete Activity 9.1 in the resources for this book on Dynamic Learning Online.

Endocrine gland

Location

Pituitary gland

Attached by a stalk to the hypothalamus of the brain

Thyroid gland

In the neck on either side of the trachea

Parathyroid glands

Four small glands situated on the posterior of the thyroid gland

Adrenal glands

Two triangular-shaped glands which lie on top of each kidney

Pancreas

Situated behind the stomach between the duodenum and the spleen

Ovaries

Situated in the lower abdomen below the kidneys

Testes

Situated in the groin in a sac called the scrotum

study tip_

When studying the endocrine system, it is helpful to think of not only where the hormone is secreted from but also the target organ it has an effect on. Sometimes this information may be apparent from the name of the hormone/s, for example thyroid-stimulating hormone (TSH) is secreted by the anterior lobe of the pituitary gland and the target it has an effect on is the growth and activity of the thyroid gland.

Pituitary gland

This is a lobed structure attached by a stalk to the hypothalamus of the brain. For many years the pituitary gland has been referred to as the 'master' endocrine gland because it secretes several hormones that control other endocrine glands. However, the pituitary itself has a master — the hypothalamus.

The hypothalamus is a small region of the brain that is the major integrating link between the nervous and endocrine systems. Hormones of the pituitary are controlled by releasing or inhibiting hormones produced by the hypothalamus.

The hypothalamus initiates the process by producing its own set of hormones (releasing or inhibiting hormones) as a result of stimulation in the brain. This has a cascading effect on the pituitary which in turn produces its own hormones that stimulate other glands. An example is thyrotrophin releasing hormone from the hypothalamus that promotes the pituitary to secrete thyroid-stimulating hormone which controls the growth and activity of the thyroid gland.

The pituitary gland consists of two main parts — an anterior and a posterior lobe.

Anterior lobe

The principal hormones secreted by the anterior lobe of the pituitary are as follows:

Growth hormone

Controls the growth of long bones and muscles

Thyroid-stimulating hormone (TSH)

Controls the growth and activity of the thyroid gland

Adrenocorticothrophic hormone (ACTH)

Stimulates and controls the growth and hormonal output of the adrenal cortex

(Continued )

Gonadotrophic hormones

The gonads or sex hormones include:

a) Follicle-stimulating hormone b) Luteinising hormone

Control the development and growth of the ovaries and testes

In women this stimulates the development of the graafian follicle in the ovary which secretes the hormone oestrogen

In men it stimulates the testes to produce sperm

In women this helps to prepare the uterus for the fertilised ovum In men it acts on the testes to produce testosterone

Prolactin

Stimulates the secretion of milk from the breasts following birth

Melanocyte-stimulating hormone (MSH)

Stimulates the production of melanin in the basal cell layer of the skin

Bone and muscle growth

Growth hormone (GH)

Bone and muscle growth

Growth hormone (GH)

Adrenal cortex Adrenocorticotropic hormone (ACTH)

Anterior pituitary

Posterior pituitary

Kidney tubules

Antidiuretic hormone (ADH)

Adrenal cortex Adrenocorticotropic hormone (ACTH)

Thyroid gland Thyroid-stimulating u ^ hormone

Thyroid gland Thyroid-stimulating u ^ hormone

Testis and ovary

Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH)

Skin

Melanocyte-stimulating hormone (MSH)

Mammary glands Prolactin (PRL)

Skin

Melanocyte-stimulating hormone (MSH)

Fig 9.2 The pituitary and its master control

Anterior pituitary

Posterior pituitary

The pituitary gland and its master control

Kidney tubules

Antidiuretic hormone (ADH)

Uterus smooth muscle

Oxytocin (OT)

Uterus smooth muscle

Oxytocin (OT)

Mammary glands

Oxytocin (OT)

Mammary glands

Oxytocin (OT)

Posterior lobe

The posterior lobe of the pituitary secretes two hormones which are manufactured in the hypothalamus but are stored in the posterior lobe:

Anti-diuretic hormone (ADH)

Increases water reabsorption in the renal tubules of the kidneys

Oxytocin

Stimulates the uterus during labour and stimulates the breasts to produce milk

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