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What is the endocrine system?

Influences the activity of cells by the release of chemical messengers known as hormones


What is homeostastis

Maintaining a stable internal enviroment


What are exocrine glands?

Secrete into a duct that carries the secretion to the body surface or one of the body cavities EG. Sweat glands, mucous glands, salivary glands and the glands of the alimentary canal


What are endocrine glands?

Secrete hormones into the extracellular fluid that surrounds the cells that make up the gland. They are then usually passed into the capillaries which are then transported by blood.
Can also be called ductless glands


What are the major endocrine glands?

- Hypothalamus
- Pituitary
- Pineal
- Thyroid
- Pancreas
- Thymus
- Parathyroid
- Adrenal glands
- Ovaries in females
- Testes in males


What are hormones?

Any secretion from an endocrine gland
- May be proteins, steroids or amines
- Transported by blood
- May affect all the cells of the body, or only a particular group of cells (target cells) or particular organs (target organs)


What are paracrine hormones?

- May be released in order for cells to communicate within other cells in the same tissue
- Are secreted by all cells in a tissue
- Move through the extracellular fluid


Protein and amine hormones

Attach to receptor proteins in the membrane of the target cell
This causes a secondary messenger substance to diffuse through the cell and activate a particular enzyme.

EG. The hormone insulin binds to a receptor protein and this leads to increased glucose absorption by the cell. However, because there are only a limited number of specific receptor proteins, once each insulin receptor in the cell membrane is bound to insulin, the rate of glucose uptake cannot be increased further.


Steroid hormones

Enter a target cell and combine with a receptor protein inside the cell.
The receptor may be on the mitochondria, or on other organelles or in the nucleus


What is the hormone-receptor complex?

Activates the genes controlling the formation of particular proteins


List three activities that hormones carry out?

- Activate certain genes in the nucleus so that a particular enzyme or structural protein is produced
- Change the shape (active site) or structure of an enzyme so that it is turned ‘off’ or ‘on’
- Change the rate of production of an enzyme or structural protein by changing the rate of transcription or translation


Enzyme amplification

Means that one molecule does not cause the manufacture or activation of just one enzyme molecule, but thousands of enzyme molecules
- Hormone triggers a cascading effect
- The number of reacting molecules involved increased hundreds or thousands of times for each step along the metabolic pathway


When and where are hormones broken down?

Hormones are broken down after producing the desired effect
- Some hormones are broken down by the target cell
- Most are broken down in the liver and kidneys
- Degraded hormones are then excreted either in bile or urine


The hypothalamus

- Located at the base of the brain
- Regulates functions such as body temperature, water balance and heart rate
- Many of these functions are carried out through the pituitary


What joins the hypothalamus and the pituitary?

A stalk called the infundibulum


The pituitary

- Lies just under the hypothalamus
- Consists of an anterior lobe and a posterior lobe


The anterior lobe of the pituitary

It has no nerves connecting to the hypothalamus, but instead is connected by a complex network of blood vessels.


How are the secretions of the anterior pituitary controlled?

Secretions are controlled by releasing and inhibiting factors secreted by the hypothalamus
- These factors are hormones in themselves
o Secreted into extracellular fluid around the cells of the hypothalamus
o Carried by the blood to the anterior lobe


What are the main hormones secreted by the anterior lobe of the pituitary?

- gonadotrophins
- growth hormone
- thyroid stimulating hormone
- Adrenocorticotropic hormone
- prolactin



Are hormones that affect the gonads
- Follicle stimulating hormone (FSH)
o In females: it stimulates development of the follicle that contains eggs
o In males: it stimulates the production and maturation of sperm in the testes
- Luteinising hormone (LH)
o In females: it brings about ovulation, leading to the development of the corpus luteum
o In males: it stimulates the interstitial cells in the testes to secrete male sex hormones


Growth hormone (GH)

- Stimulates body growth, particularly growth of the skeleton
- Increases the rate at which amino acids are taken up by the cell and built into proteins
- Secreted throughout life, helping to maintain the size of organs once maturity is reached


Thyroid-stimulating hormone

Stimulates production and release of hormones from the thyroid gland


Adrenocorticotropic hormone (ACTH)

Controls production and release of some of the hormones from the cortex of the adrenal glands



Works with other hormones to initiate and maintain milk secretion in females


Posterior lobe of the pituitary

Releases oxytocin and anti-diuretic hormone
(neither is manufactured in the posterior lobe)

Both hormones are produced in special nerve cells in the hypothalamus of the brain
- These cells have long extensions that pass through the infundibulum to the posterior lobe
- These hormones move down the extensions and are stored ready for the release into the blood stream

The release of hormones is triggered by the nerve impulses which are initiated by the hypothalamus



- Stimulates contraction of the muscles of the uterus
- Released in large quantities during labour
- Also stimulates the contraction of cells in the mammary glands, resulting in release of milk during breastfeeding


Anti-diuretic hormone

- Causes the kidneys to remove water from urine that is forming
- Water is returned to blood stream
- Helps to retain fluid within the body
- In high concentrations- cause constriction of small arteries


Pineal gland

Found deep inside the brain, about the size of a pea in children
- After puberty, it gradually decreases in size

Secretes Melatonin
- Involved in the regulation of sleep patterns
- The production of melatonin is stimulated by darkness and inhibited by light


The thyroid gland

Located in the neck and consists of two lobes that lie on either side of the trachea. Joined by a piece of tissue in front of the trachea

Main hormone = thyroxine
- Made from iodine and amino acid
- Controls body metabolism
- Regulates reactions in which complex molecules are broken down to release energy, other reactions in which complex molecules are synthesised from simpler ones
- Due to this release of energy, which is the overall effect of thyroxine, some energy is released in the form of heat, maintaining body temperature

Secreted in response to TSH from the anterior lobe of the pituitary


Parathyroid glands

There are usually four parathyroid glands (some people have more)
- Size of a small pea
- Embedded in the rear surface of the thyroid glands

Secrete parathyroid hormone (PTH) which controls calcium and phosphate levels in the blood