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Learning Resources - Endocrine System- Revision Notes

 

Endocrine System

Endocrine system consists of glands widely separated from each other with no direct links

 

These glands consist of groups of secretory cells surrounded by an extensive network of capillaries that facilitates diffusion of hormones (chemical messengers) from secretory cells to the bloodstream

 

Ductless glands – hormones diffuse directly into the bloodstream

 

Hormones are then carried by the blood to target tissues and organs – they influence cellular growth and metabolism

 

Homeostasis of the internal environment is maintained partly by the autonomic nervous system and partly by the endocrine system

 

Autonomic nervous system = rapid changes

 

Hormones of endocrine system = slower and more precise adjustments

 

This system consists of a number of distinct glands and some tissues in other organs

 

The hypothalamus is classified as part of the brain – it controls the pituitary gland and has an indirect effect on many other glands within the body

 

Endocrine glands:

Pineal body

Pituitary glans

Thyroid gland

Parathyroid gland

Thymus gland

Adrenal glands

Pancreatic islets

Ovaries (female)

Testes (male)

 

When a hormone arrives at its target cell it binds to a specific area (receptor)

Here it acts as a switch – influencing chemical/metabolic reactions inside the cell

Receptors for peptide hormones are situated on the cell membrane

Receptors for lipid-based hormones are inside the cell

 

The level of a hormone in the blood is variable and self-regulating within normal range

 

Hormones are released in response to a specific stimulus

Usually action reverses the stimulus through a negative feedback system

Can be controlled

Indirectly – release of hormones by the hypothalamus and anterior pituitary glans

Steroid

Thyroid hormones

Directly – by blood levels of the stimulus

Insulin

Glucagon

 

Effect of a positive feedback mechanism is amplification of the stimulus

Increase of a specific hormone until a particular process is complete and the stimulus ceases

Oxytocin during labour

 

Pituitary Gland and Hypothalamus

 

Pituitary gland and the hypothalamus act as a unit

Regulate activities of most other endocrine glands

 

Pituitary gland lies in the hypophyseal fossa of the sphenoid bone below the hypothalamus

Attached to hypothalamus by a stalk

Size of a pea

Weight about 500mg

Consists of three distinct parts that originate from different types of cells

Anterior pituitary (adenohypophysis)

Up-growth of glandular epithelium from the pharynx

Posterior pituitary (neurohypophysis)

Down-growth of nervous tissue from the brain

Network of nerve fibres between the hypothalamus and posterior pituitary

Between these lobes there is a thin strip of tissue (intermediate lobe) – function is unknown in humans

 

Blood Supply

 

Arterial blood

Supplied by branches from the internal carotid artery

Anterior lobe is supplied indirectly by blood that has already passed through a capillary bed in the hypothalamus

Posterior lobe is supplied directly

 

Venous drainage

Comes from both lobes

Containing hormones

Leaves the gland in short veins that enter the venous sinuses between layers of dura mater

 

The Influence of the Hypothalamus on the Pituitary Gland

 

Anterior pituitary

Supplied with arterial blood that has already passed through a capillary network in the hypothalamus

This network forms part of the pituitary portal system

Transports blood from the hypothalamus to the anterior pituitary where it enters thin-walled vascular sinusoids

Is in very close contact with the secretory cells

This blood provides

Oxygen

Nutrients

Releasing and inhibiting hormones

Secreted by the hypothalamus

Influence secretion and release of other hormones formed in the anterior pituitary

Some hormones secreted by the anterior lobe stimulate/inhibit secretion by other endocrine glands (target glands)

Other hormones have a direct effect on target tissues

Growth hormone (GH)

Regulates metabolism

Promotes tissue growth – especially of bones and muscles

Thyroid stimulating hormone (TSH)

Stimulates growth and activity of the thyroid gland

Adrenocorticotrophic hormone (ACTH)

Stimulates the adrenal cortex to secrete glucocorticoids

Prolactin (PRL)

Stimulates milk production in the breasts

Follicle stimulating hormone (FSH)

Stimulates production of sperm in testes

Stimulated secretion of oestrogen by the ovaries

Maturation of ovarian follicles

Ovulation

Luteinising hormone (LH)

Stimulates secretion of testosterone by testes

Stimulates secretion of progesterone by the corpus luteum

 

The release of an anterior pituitary hormone follows stimulation of the gland by a specific releasing hormone produced by the hypothalamus and carried to the gland through the pituitary portal system of blood vessels

 

Whole system is controlled by a negative feedback mechanism

When there Is a low level of a hormone in the blood supplying the hypothalamus it produces the appropriate releasing hormone

Stimulates release of a trophic hormone by the anterior pituitary

This then stimulates the target gland to produce and release its hormone

Results in the blood level of that hormone rising – inhibiting the secretion of releasing factor by the hypothalamus

 

Posterior pituitary

Formed from nervous tissue and consists of nerve cells surrounded by supporting cells (pituicytes)

These neurones have their cell bodies in the supraoptic and paraventricular nuclei of the hypothalamus and their axons

Then stored in vesicles within the axon terminals within the posterior pituitary

Their release is by exocytosis is triggered by nerve impulses from the hypothalamus

Oxytocin

Stimulates uterine smooth muscle and muscles of lactating breast during childbirth

Antidiuretic hormone (ADH)

Reduces urine output

 

Thyroid Gland

 

Situated in the neck in front of the larynx and trachea at the level of the 5th-7th cervical and 1st thoracic vertebrae

 

Highly vascular gland

 

Weighs about 25 grams

 

Surrounded by a fibrous capsule

 

Resembles a butterfly in shape

 

Consists of two lobes – one either side of the thyroid cartilage and upper rings of the trachea

 

The lobes are joined by a narrow isthmus

Lies in front of the trachea

 

Lobes are roughly cone shaped

About 5cm long

About 3cm wide

 

The arterial blood supply to the gland is through the superior and inferior thyroid arteries

Superior thyroid artery is a branch of the external carotid artery

Inferior thyroid artery is a branch of the subclavian artery

 

Venous return is by the thyroid veins

Drain into the internal jugular veins

 

Two parathyroid glands lie against the posterior surface of each lobe and are sometimes embedded in thyroid tissue

 

Recurrent laryngeal nerve passes upwards close to the lobes of the gland and on the right side it lies near the inferior thyroid artery

 

Gland is composed of cuboidal epithelium that forms spherical follicles

Secrete and store colloid

Thick sticky protein material

Between the follicles are other cells found either singly or in small groups

Parafollicular cells (C-cells)

Secrete the hormone calcitonin

 

This glad secretes

Thyroxine

Tri-iodothyronine

Calcitonin

 

Parathyroid Glands

 

Four small parathyroid glands

 

Embedded in posterior surface of each lobe of the thyroid gland

 

Surrounded by fine connective tissue capsule

 

Cells forming the glands are spherical in shape and are arranged in columns with channels containing blood between them

 

Function

 

Parathyroid glands secrete parathyroid hormone (PTH parathormone)

 

Secretion is regulated by the blood level of calcium

When this falls secretion of PTH is increased

Opposite happens if levels increase

 

Main function of PTH is to increase the blood calcium level when it is low

Achieved by indirectly increasing the amount of calcium absorbed from the small intestine and reabsorbed from the renal tubules

If these sources are inadequate then PTH stimulates osteoclasts (bone-destroying cells) and reabsorption 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

Needed for

Muscle contraction

Blood clotting

Nerve impulse transmission

 

Adrenal Glands

 

Two adrenal glands

 

One situated on the upper pole of each kidney

 

Enclosed within the renal fissure

 

About 4cm long

 

About 3cm thick

 

Arterial blood supply to the glands is by branches from the abdominal aorta and renal arteries

 

Venous return is by suprarenal veins

Right gland drains into the inferior vena cava

Left gland drains into the left renal vein

 

The glands are composed of two parts which have different structures and functions

Outer part is the cortex

Inner part is the medulla

Adrenal cortex is essential to life – the medulla is not

 

Adrenal Cortex

 

Produces three types of steroid hormones from cholesterol

 

Collectively called adrenocorticocoids

Glucocorticoids

Mineralocorticoids

Androgens (sex hormones)

 

Hormones in each group have different characteristic actions

Due to structural similarity actions may overlap

 

Adrenal Medulla

 

Completely surrounded by the adrenal cortex

 

Develops from nervous tissue in the embryo and is part of the sympathetic division of the autonomic nervous system

 

Stimulated by its extensive sympathetic nerve supply to produce the hormones

Adrenaline (epinephrine)

Noradrenaline (norepinephrine)

 

Pancreatic Islets

 

Cells that make up the pancreatic islets are found in clusters; are irregularly distributed in the substance of the pancreas

 

There are no ducts leading from the cluster of islet cells

 

Pancreatic hormones are secreted directly into the bloodstream and circulate throughout the body

 

Three main types of cells in the pancreatic islets

Alpha cells – secrete glucagon

Beta cells – secrete insulin

Delta cells – secrete somatostatin

 

Normal blood glucose level is between 3.5 and 8mmol/litre

 

Blood glucose levels are controlled mainly by the opposing actions of insulin and glucagon

Glucagon – increases blood glucose levels

Insulin – reduces blood glucose levels

 

Insulin

 

Polypeptide

 

50 amino acids

 

Main function – to lower raised blood nutrient levels

Glucose

Amino acids

Fatty acids

 

When these nutrients are in excess of immediate needs insulin promotes their storage by

Acting on cell membrane and stimulating uptake and use of glucose by muscle and connective tissue cells

Increasing conversion of glucose to glycogen (glycogenesis) especially in the liver and skeletal muscles

Accelerating uptake of amino acids by cells, and the synthesis of protein

Promoting synthesis of fatty acids and storage of fat in adipose tissue (lipogenesis)

Decreasing glycogenolysis (breakdown of glycogen into glucose)

Prevents breakdown of protein and fat and gluconeogenesis (formation of new sugar from e.g. protein)

 

Secretion of insulin is stimulated by increased blood glucose levels and to a lesser extent by parasympathetic, raised blood amino acid and fatty acid levels, and gastrointestinal hormones

Gastrin

Secretin

Cholecystokinin

 

Secretion is decreased by

Sympathetic stimulation

Glucagon

Adrenaline

Cortisol

Somatostatin

 

Glucagon

 

Increase blood glucose levels by stimulating

Conversion of glycogen to glucose in the liver and skeletal muscles (glycogenolysis)

Glucogenolysis

 

Secretion of glucagon is stimulated by a low blood glucose level and exercise

 

It is decreased by somatostatin and insulin

 

Somatostatin (GHRIH)

 

Also produced by the hypothalamus

 

Effect of this hormone is to inhibit the secretion of both insulin and glucagon in addition to inhibiting the secretion of GH from anterior pituitary

 

Pineal Gland or Body

 

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

 

About 10mm long

 

Reddish-brown in colour

 

Surrounded by a capsule

 

Melatonin is the hormone secreted by this gland

 

Secretion is controlled by daylight and levels fluctuate during each 24-hour period – highest at night and lowest around midday

 

Secretion is also influenced by the number of daylight hours

Seasonal variations

 

Functions of this hormone are not fully understood it is believed to be associated with

Coordination of the circadian and diurnal rhythms of many tissues – possibly by influencing the hypothalamus

Inhibition of growth and development of the sex organs before puberty – possibly preventing synthesis or release of gonadotropins

 

The gland tends to atrophy after puberty and may become calcified in later life

 

Thymus Gland

 

Thymosin – hormone secreted by the thymus gland

 

Required for the development of T-lymphocytes for cell-mediated immunity

 

Local Hormones

 

Histamine

Synthesised by mast cells in the tissues and basophils in blood

Released as part of the inflammatory process

Increasing capillary permeability and causing vasodilatation

Also causes contraction of smooth muscle of the bronchi and alimentary tract

Stimulates secretion of gastric juice

 

Serotonin (5-hydroxytptamine, 5-HT)

 

Present in platelets, in brain, and in intestinal wall

 

Causes intestinal secretion and contraction of smooth muscle

 

Has a role in haemostasis (blood clotting)

 

Prostaglandins

 

Lipid substances found in most tissues that act as local hormones and have wide-ranging physiological effects in

Inflammatory response

Potentiating pain

Fever

Regulating blood pressure

Blood clotting

Uterine contractions during labour

 

Gastrointestinal Hormones

 

Gastrin, secretin and cholecystokinin (CCK) influence the secretion of digestive juices

 

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