THE ENDOCRINE SYSTEM

Function: The co-ordination of body organs, so as to operate as part of an integrated system.
The endocrine system is often compared with the nervous system, which also has the function of co- ordination and passing "instructions", but by an independent mechanism.

This is achieved by the production of HORMONES ("chemical messengers"):
- organic compounds
(i.e. fairly complex molecules, based on carbon - often proteins, peptides, steroids/sterols [lipids] )
- produced by various glands in different parts of the body - endocrine glands, also called ductless glands
(because they have no duct or tube to pipe their secretions to a release point)
- instead they are transported in the blood
- so they travel at the "speed of blood": slower than nervous impulses
- cause longer lasting effects: gradually eliminated from body in urine
- delivered by blood circulation to all parts of the body
- each has a specific target organ/organs with specific receptors on their cell surfaces which detect the presence of the hormone
- produced in small quantities (mg/µg/ng/pg)
- often have profound physiological effects
- used to stabilise the body's internal environment by regulating its physiology
(i.e. have a rôle in homeostasis)
- also co-ordinate longer term processes, e.g. sexual development, growth.

Hormones have the function of controlling body processes which require several organs of the body to interact for a combined effect. For example, sexual development at puberty, and events during the menstrual cycle and pregnancy need general co-ordination over a long period of time, but the body's response to variation in levels of carbohydrate in the body, and reaction to stressful situations are much quicker responses.

Hormonal responses are not as quick as nervous responses, but they can be almost instantaneously communicated to many parts of the body - as fast as the heart can pump!

Several endocrine glands produce their hormones in response to stimulation from the nervous system.

Furthermore, some hormones have the function of interacting with other glands to stimulate or inhibit production of other hormones, and some work in opposition to others to switch some processes on and off.

Over- or under-activity of certain endocrine glands may cause functional disorders.

Examples of hormones for special study:

insulin, produced in the pancreas (actually only in a small part of the pancreas - the islets of Langerhans)
testosterone from the testes (actually the cells between the sperm-producing tubules)
oestrogen and progesterone from the ovaries (actually the follicle from which an egg develops) and from the placenta when a woman is pregnant
adrenalin(e) from the adrenal glands - above the kidneys

Examples of endocrine organs:

(several more besides!)

Pituitary gland/body

- at base of forebrain - "master gland"
- produces several hormones, some of which act on other endocrine glands : "trophic hormones".

Being near to the brain, it can react to nervous stimuli.
The hypothalamus, which is very close, functions as the body's control centre for osmoregulation and temperature control.

Examples of pituitary hormones:

a) growth hormone - promotes bone and muscular growth - acts on protein synthesis, and other glands, e.g. thyroid & pancreas
- excess causes gigantism
- deficiency causes dwarfism
b) antidiuretic hormone (ADH) - controls reabsorption of water into blood by kidneys
(refer to previous work on excretion and osmoregulation)
c) thyroid stimulating hormone (TSH) - trophic
d) follicle stimulating hormone (FSH) causes follicle in ovary to develop, and secrete oestrogen
e) luteinising hormone (LH), acting in combination with FSH and other hormones, causes ovulation
f) other hormones in pregnancy, e.g. oxytocin (causes contractions of uterus in birth, and expulsion of milk during suckling) & prolactin (milk production hormone).

Thyroid

produces thyroxin(e): amino acid with 3 or 4 Iodine atoms attached
- regulates rate of metabolism (mitochondrial energy release - all cells)
- excess causes overactivity, "pop-eyes"
- deficiency causes overweight, sluggishness
In child: deficiency causes (goitrous) cretinism: mental & physical retardation - curable by early administration of thyroxin
Parathyroid - embedded in thyroid
produces hormone - parathormone - regulates Ca and P metabolism - promotes release of Ca²⁺ from bones into bloodstream (Ca is necessary for bone formation, nervous and muscular action).

Pancreas

(n.b. One NON-HORMONAL function is secretion of digestive juices, which pass out via pancreatic duct)

Part of the pancreas - the islets of Langerhans - also functions as a ductless gland, producing the hormones INSULIN and GLUCAGON.

Assuming that these hormones passes out of the pancreas and into the vena cava, list the blood vessels they passes through on their way to the liver.

pancreatic vein - vena cava - heart/atrium/ventricle (RHS) - pulmonary artery -lungs - pulmonary vein - heart/atrium/ventricle(LHS)- aorta - hepatic artery - liver

Insulin controls the interconversion of soluble blood sugar - glucose - to insoluble glycogen, which is stored in the liver. It also increases metabolic rate by stimulation of cellular glucose uptake, and increases protein synthesis in some cells.

Another hormone produced by the islet cells of the pancreas - glucagon - does the reverse, causing the breakdown of glycogen and releasing glucose as a result.

carbohydrate interconversions in the liver

The combined action of these 2 hormones keeps the blood glucose level fairly constant, or at least within reasonable limits: Under the influence of insulin, extra glucose is removed from the blood stream and stored as glycogen, and glucagon causes the addition of more glucose when the level falls.

Via which blood vessel does glucose enter the liver after a meal?
> hepatic portal vein
Via which blood vessel does glucose leave the liver, when glycogen is broken down?
> hepatic vein
What does protein synthesis mean?

> making protein from amino-acids

Lack of insulin causes "sugar diabetes", in which blood sugar level is unregulated. After a meal, blood contains excess glucose because it is not stored in the liver as glycogen (glycogen is also metabolised).
Hence the kidneys' power to reabsorb glucose is exceeded, and glucose is excreted in urine.
When glucose is used up, coma and convulsions may occur.

Regular insulin injections and careful diet can correct the condition.

Gonads

(female: ovaries male: testes)

Ovary produces OESTROGENS (several hormones, e.g. oestradiol, oestrone)
Effects: a) control development of female secondary sexual characteristics
b) regulates menstrual cycle - causes thickening of uterus lining prior to ovulation

PROGESTERONE, produced by corpus luteum after ovulation, has further effects on the uterus - thickening and vascularisation - and prevents uterine contractions until baby is being born.

What does "ovulation" mean?
>release of an egg/eggs (ovum/ova) from a follicle in the ovary

Testis - interstitial cells produce TESTOSTERONE - promotes development of male secondary sexual characteristics

Adrenal glands

- these have external cortex & internal medulla.
Cortex

several hormones, including cortisone (accelerates conversion of protein

glucose)
Medulla

ADRENALIN ("fight or flight" hormone):
The brain interprets a potentially dangerous situation, and a motor nerve causes the secretion of adrenalin (another link with the nervous system)

Complete the following table about the effects of adrenalin on the body.
Note that THESE ARE ADAPTIVE TO THE SITUATION!

Physiological effects of adrenalin	causing the following effects on the body to be observed
quickens heart beat	`chest/heart thumping - pulse racing`
diverts blood to muscles, away from alimentary canal and possibly skin	`ready to react physically (fight/flight) hollow feeling /"butterflies" in stomach - turning pale`
dilates airways, increases breathing rate	`more air allowed into lungs / deeper breathing /faster breathing!`
increases rate of oxidation of carbohydrates	`greater energy for activity`
consequently, more blood with extra oxygen and glucose flows to muscles and brain	`speed and power of reactions to events boosted - clearer thinking? - insensitive to pain?`

What does "physiological" mean?
> to do with how the parts of the body carry out their functions