The Endocrine System

 

 

Introduction

 

Internal body communication may be nervous or chemical

 

Hormone - a chemical messenger

 

Hormones are signals - they bind to receptors on target tissue cells

 

A signal receptor complex triggers a secondary messenger system

 

Secondary messenger systems are sometimes based on AMP, (adenosine monophosphate)

 

Gland      -    adeno

 

 

Types of gland

 

*           Endocrine  - no ducts

 

*           Exocrine   - ducts

 

 

Main components of the endocrine system

 

Pituitary gland

 

Thyroid gland

 

Parathyroid glands

 

Adrenal gland (suprarenal)

 

Pancreas

 

Kidney

 

Gonads - testis and ovaries

 

 

Pituitary gland

 

Master gland

 

Regulates the other endocrine glands and has direct  target tissues

 

Attached to hypothalamus by a stalk

 

Anterior lobe  - adenohypophysis

 

Posterior lobe - neurohypophysis

 

 

Posterior lobe - neurohypophysis

 

Hormones are produced in the hypothalamus and pass along nerve fibres to the posterior lobe for release

 

 

Oxytocin

 

*           stimulates uterine muscle contraction

 

*           secretion of milk in lactation           

 

 

Antidiuretic hormone (vasopressin)

 

*           increases reabsorption of water from the nephron

 

 

Anterior lobe  - adenohypophysis

 

Regulates the other endocrine glands and has direct  target tissues

 

 

Thyroid stimulating hormone  (TSH)

 

*           stimulates thyroxine release

 

 

Growth hormone  (GH)

 

*           stimulates growth of the body

 

 

Prolactin

 

*           stimulated by suckling

 

*           initiation and maintance of lactation

 

 

Gonadotrophic hormones

 

i.          Follicle-stimulating hormone

 

ii.         Luteinizing hormone

 

 

Males

 

FSH    - stimulates sperm production

 

LH       - stimulates testosterone production

 

 

Females

 

FSH    - stimulates development of ovarian follicle

 

LH       - promotes final maturation of ovarian follicle and ovulation

 

 

Adrenocorticotrophic hormone (ACTH, corticotrophin)

 

*           stimulates secretion of steroid hormones from adrenal cortex

 

 

Control of hormone serum levels

 

Hormones released by stimulation of releasing factors via the pituitary portal system from hypothalamus

 

Release of trophic hormones by negative feedback

 

 

 

Eg.      GHRF             GH

            GHIF               GH

            TSHRF           TSH

            CRF                ACTH

            LHRF              FSH

                                    LH

 

 

Thyroid gland

 

Two lobes joined by an isthmus

 

Follicles in the gland contain colloid which is thyroid hormone and protein

 

C-cells found between the follicles

 

Release stimulated by TSH

 

Iodine required for thyroid hormone

 

 

Two thyroid hormones

 

*           T3 Triiodothyronine

 

*           T4 Thyroxine

 

 

Functions

 

*           Control of metabolic processes

 

*           Involved in growth and development eg nervous system

 

 

Thyroid C - cells

 

Produce calcitonin

 

Acts on

 

*           bone

*           kidneys

*           lowers blood calcium levels

Parathyroid glands

 

4 small glands embedded in the thyroid gland

 

Secrete parathyroid hormone, (PTH)

 

Serum calcium falls - PTH secretion increases

 

Increases amount of Ca absorbed from small intestine

 

Increases tubular reabsorption

 

 

Adrenal glands (suprarenal)

 

Cortex and medulla

 

 

Cortex

 

 

Glucocorticoids

 

*           hydrocortisone

 

*           corticosterone

 

Secretion stimulated by ACTH

 

Functions

 

*           formation of glycogen

 

*           gluconeogenesis from proteins

 

*           regulates carbohydrate metabolism

 

*           promotes sodium and water reabsorption

 

 

 

 

 

Mineralocorticoids

 

Aldosterone

 

*           fall in serum sodium - increased aldosterone production

 

*           increases tubular reabsorption of sodium

 

The cortex also produces some androgens

 

 

Adrenal medulla

 

Related to sympathetic nervous system

 

Catecholamines

 

*           adrenaline

 

*           noradrenaline

 

 

Noradrenaline

 

maintains blood pressure by vasoconstriction

 

 

Adrenaline

 

fight or flight response

 

*

 

*

 

*

 

*

 

 

Islets of Langerhans

 

Alpha - glucagon

 

Beta    - insulin

 

Delta   - somatostatin (GHIH)

 

Kidney

 

Renin

 

Converts angiotensinogen to angiotensin

 

 

Erythropoetin 

 

Stimulates erythropoesis

 

 

The testes and ovaries

 

Testes

 

Interstitial cells - testosterone

 

Ovaries     

 

*           oestrogen

*           progesterone

 

 

The menstrual cycle

 

 

Hormonal control

 

 

Day 1 - 14

Ovum develops in Graafian follicle, the cells of which produce oestrogen

 

The high level of oestrogen cause the development of endometrium, a "developing hormone"

 

 

Day 14 - 28

After ovulation the Graafian follicle becomes the Corpus luteum, which secretes progesterone.

 

Progesterone maintains the endometrium, a "maintaining hormone".

 

When levels of progesterone falls the endometrium is no longer "maintained", causing menstruation.

 

If conception occurs progesterone levels continue to rise to "maintain" the pregnancy.

 

In pregnancy progesterone is produced by the placenta

 

Role of pituitary hormones

 

FSH    -           causes development of the follicle

LH       -           stimulation causes ovulation.

 

 

 

 

 

Fill in the columns on product and function, ie. what the gland secretes and the function of the secreted hormone

 

Gland                                     Product                                                          Function 

 

Anterior

Pituitary

 

 

Posterior

Pituitary

 

 

Thyroid

gland

 

 

Parathyroid

glands

 

 

Adrenal cortex

 

 

Adrenal medulla

 

 

Pancreas

 

 

Kidney

 

 

Testes

 

 

 Ovaries