Endocrine system

Comparison of endocrine and nervous system

Characteristic	Nervous system	Endocrine system
Nature of message	Electrical impulses and neurotransmitters	Hormones
Transport of message	Along the membrane of neurons	By the bloodstream
Cells affected	Muscle and gland cells; other neurons	All body cells
Type of response	Usually local and specific	May be general and widespread
Time taken to respond	Rapid – within milliseconds	Slower – seconds to days
Duration of response	Brief – stops quickly when stimulus stops.	Long lasting – may continue long after stimulus has stopped

Endocrine system

Influences cell activity by releasing chemical messengers – hormones

Homeostasis

The maintenance of a relatively constant internal environment despite fluctuations in the external environment. The composition and temperature of the fluids around cells. Achieved through dynamic equilibrium – fluctuations around an ideal or optimal state.

Tolerance limits

The upper and lower conditions in which the body can function normally. Dysfunction will occur if the conditions fall outside these limits. The tolerance limit for body temperature is 36.5 – 37.5^oC.

Exocrine glands

Secrete into a duct that carries  secretion to the body or to the body cavities. Target sites of the gland are located close to the exocrine gland. E.g. Sweat, mucous, salivary.

Endocrine glands

Secrete hormones into extracellular fluid surrounding cells that make up the gland. Usually passes into capillaries and transported into blood. target sites of the are far from the gland. E.g. Thyroid, parathyroid, adrenal.

Hormones

Chemicals, secreted by endocrine glands, transported throughout the body in the blood. They alter the functioning of cells by changing the type, activities or quantities proteins produced. May:

-Activate certain genes in the nucleus so a particular enzyme or protein is produced.

-Change the shape or structure of an enzyme so it is turned on or off.

-Change the rate of enzyme or protein production by changing rate of transcription or translation during protein production

May be steroids, proteins or amines.

What can hormones influence?

Can only influence cells with the correct hormone receptor. Thus, hormones may affect all body cells or only particular groups of cells – Target cells.

Different cells have different types and numbers of receptor proteins. Only on particular organs – Target organs.

Steroid hormones

Examples	Solubility	Time of Effect	Process
Oestrogen	Lipid-soluble   Does NOT dissolve in water	Slow to take effect.   Long lasting	1) Once released into blood, the hormones bind to transport proteins, allowing them to travel in blood stream.
Progesterone			2) When they reach target cells, the hormones separate from the transport protein and diffuse across the cell membrane.
Cortisol			3) Inside the cell, they combine with a receptor protein in cytoplasm or nucleus.
Aldosterone			4) Hormone-receptor complex activates the genes controlling formation of particular proteins by binding to the promoter section of a gene, stimulating or inhibiting transcription and protein synthesis.

Protein and amine hormones

Amine hormones are synthesized from the amino acids tryptophan or tyrosine. Composed of an amino acid with a modified group.

Protein hormones are those consisting of a long chain of amino acids (a protein).

Examples	Solubility	Time of Effect	Process
Prolactin (P)	Water-soluble	Quick to take effect	1) As they cannot diffuse across the cell membrane, they attach to receptor proteins in the membrane of the target cell.
Growth hormone(P)
Melatonin (a)	Unable to cross the cell membrane (not lipid-soluble)	Short- lasting effect	2) The combination of the hormone with the receptor causes a secondary messenger substance to diffuse through the cell and activate particular enzymes.

Hormone receptors

Receptor proteins are specific. Each receptor type will bind with only 1 specific molecule.Limited number of receptor proteins in membrane of cell. When each receptor is bound to a molecule, there can be no further increase in the rate of the activity (saturated). Different cells have different types and numbers of receptor proteins. Variation in sensitivities of cells to hormones.

Enzyme amplification

Causes the activation of thousands of enzyme molecules. The hormone triggers a cascading effect where the number of reacting molecules involved is increased 100-1000x for each stop along the metabolic pathway. One hormone could trigger the production of over a billion enzyme molecules. A small stimulus can have a large effect.

Hormone clearance

Once a hormone has produced the effect, it is turned off by breaking down the hormone molecules. Some are broken down in the target cells. Most are broken down in the liver and kidneys. The degraded hormones are excreted in bile or urine.

Control of hormone secretions.

To maintain homeostasis, the amount of hormone produced by an endocrine gland must be closely regulated. Over secretion or under secretion causes abnormal body functioning. Hormones are regulated by a negative feedback system where the response produced by the secretion of the hormones is the opposite of the stimulus that caused the secretion.

Hypothalamus

Connector between nervous and endocrine systems. Regulates many basic bodily functions: Body temperature, water balance, heart rate, increase or decrease secretions of glands etc. Many of its functions are carried out through the pituitary gland.

Hypothalamus processes

Hypothalamus secretes releasing factors, stimulating hormone secretion, or inhibiting factors, which slows secretion.

These factors travel through blood vessels to the anterior lobe of the pituitary gland, affecting secretion. Other hormones produced by the hypothalamus pass along the nerve fibres to the posterior lobe of the pituitary gland where they are released.

Pituitary gland

Lies under the hypothalamus by a stalk- infundibulum.

Consists of an anterior and posterior lobe which functions separately.

Not a true endocrine gland as it doesn’t secrete substances, but stores and releases hormones.

Feature	Anterior lobe	Posterior Lobe
Size	75% of the pituitary gland	25% of the pituitary gland
Where hormones are produced	the six hormones are produced in the anterior pituitary	the two hormones are produced in special nerve cells in the hypothalamus and then stored in the posterior pituitary.
How the release of hormones is controlled	Controls the release of hormones with hormones (releasing factors) from the hypothalamus.	Controls the release of hormones by nerve impulse initiated in the hypothalamus
Connection to the hypothalamus	Connected to hypothalamus by a complex network of blood vessels lying in the infundibulum	joined to the hypothalamus by nerve fibres that come from nerve cell bodies in the hypothalamus and pass through the infundibulum to

Anterior lobe of pituitary gland

Hormone	Target Organ	Main Effects
Follicle stimulating hormone (FSH)	Ovaries (female) Testes (male)	Growth of follicles. Production of sperm.
Luteinising hormone (LH)	Ovaries (female) Testes (male)	Ovulation and maintenance of corpus luteum. Secretion of testosterone.
Growth hormone (GH)	All cells	Growth and proteins synthesis
Thyroid-stimulating hormone (TSH)	Thyroid gland	Secretion of hormones from the thyroid
Adrenocorticotropic hormone (ACTH)	Adrenal cortex	Secretion of hormones from the adrenal cortex
Prolactin (PRL)	Mammary glands	Milk production

Posterior lobe of pituitary gland

Hormones are not manufactured here, but are produced in special nerve cells in hypothalamus. Cells have long extensions that pass-through infundibulum to posterior lobe. Hormones manufactured move down the extensions and are stored for release. The release of the hormones is triggered by nerve impulses in the hypothalamus and conducted along the cell extensions.

Hormone	Target Organ	Main Effects
Antidiuretic hormone (ADH)	Kidneys	Reabsorption of water
Oxytocin (OT)	Uterus Mammary glands	Contractions of uterus during childbirth Release of milk

Pineal gland

Location: Found deep inside the brain.

Function: Known that it secretes the hormone melatonin, involved in the regulation of sleep patterns. Production of melatonin is stimulated by darkness and inhibited by light.

Thyroid gland location and structure

In the neck, below the larynx

Consists of two lobes that lie on either side of the trachea and are joined by a narrow piece of tissue that lies across the front of the trachea.

Thyroid gland function

1) The follicular cells in the gland secretes two hormones in response to thyroid stimulating hormone: thyroxine (t4, 4 iodine , less active but lasts longer.) and triiodothyroxine (T3, 3 iodine . Produced around 80%).T

2)Once released, enzymes convert T4 into T3.

3) Thyroxine controls body metabolism by regulating reactions where complex molecules are broken down to release energy, or are synthesised from simple ones.

4) Brings about the release of energy and, because some energy is in the form of heat, maintain body temperature.

Thyroid gland function - calcium and phosphate

Helps regulate this in the blood through the release of calcitonin by C-cells. When calcium in the blood increases, the gland releases calcitonin reducing reabsorption of calcium by the kidneys and the breakdown of bone. If phosphate concentration is too high, calcitonin moves phosphate into bone reducing reabsorption in the kidneys.

Parathyroid glands

In the rear surface of the lobes of the thyroid gland. Usually 4 of these, the size of a small pea.

Secretes parathyroid hormone (PTH) which increases calcium levels in the blood and phosphate excretion in the urine

Thymus

In the chest above the heart and behind the sternum. Secretes a group of hormones called thymosins. Influences the maturation of disease fighting cells – T-lymphocytes.

Adrenal glands

Immediately above each kidney. 2 total. Each with an inner adrenal medulla and an outer adrenal cortex. Both parts have a different structure and function. 

Adrenal medulla

Produces adrenaline and noradrenaline.

Adrenaline helps prepare the body for a reaction to a threatening situation. Fight or flight response. Noradrenaline increases the rate and force of heartbeat

Adrenal cortex

Produces over 20 hormones known as corticosteroids. Two main ones:

Aldosterone: Acts on the kidney to reduce the amount of sodium and increase the amount of potassium in the urine.

Cortisol: With related hormones, promotes normal metabolism, helping the body withstand stress and repair damaged tissues.

Pancreas

Soft organ around 15cm long. Below the stomach and alongside duodenum. Both an endocrine and exocrine gland. Exocrine part secretes digestive enzymes into the small intestine through pancreatic duct. Endocrine part is made of clusters of specialised cells – islets of Langerhans / pancreatic islets. These secrete:

Insulin – by beta cells of pancreatic islets.

Glucagon – by alpha cells of pancreatic islets.

Pancreas: Insulin function

-Reduces glucose in the blood (sugar) by promoting the uptake of glucose from the blood by the cells of the body.

-In the liver, causes the conversion of glucose to glycogen and fat.

-In skeletal muscles causes conversion of glucose to glycogen

-In fat storage tissue converts glucose to fat.

Pancreas: Glucagon

Acts in opposite way to insulin – increases blood glucose level

Promotes the breakdown of glycogen to glucose in the liver.

Stimulates the breakdown of fat in the liver and fat storage tissues.

Gonads

Androgens: Male sex hormones.

-Responsible for development and maintenance of male sex characteristics.

-In males, produced by the testes.

- In females, produced in the ovaries, adrenal glands and fat cells.

Oestrogen and progesterone: Female sex hormones.

-Responsible for development and maintenance of female sex characteristics.

-Produced by the ovaries

-Together with gonadotropic hormones of pituitary, they regulate the menstrual cycle and are involved in changes occurring during pregnancy.

Table of hormones

Gland	Hormone	Target cells	Main effects
Thyroid	Thyroxin and triiodothyronine Calcitonin	Most body cells   Bones, kidney	Increase metabolic rate and thus oxygen consumption and heat production. Lower blood calcium and phosphate levels .
Parathyroid	Parathyroid hormone	Bones, Kidneys	Increase calcium level in blood
Thymus	Thymosins	T-lymphocytes	Stimulates development and maturation of T-lymphocytes.
Adrenal cortex	Corticosteroids: Aldosterone   Cortisol	Kidneys   Most body cells	Increase reabsorption of sodium ions and excretion of potassium ions.   Promotes normal metabolism; helps body deal with stress; promotes repair of damaged tissue.
Adrenal medulla	Adrenaline and noradrenaline	Most body tissues	Prepares body for flight or flight responses; reinforces the effects of sympathetic nerve system.
Pancreas	Insulin   Glucagon	Most body cells   Liver and fat storage tissues.	Stimulates uptake of glucose; lowers blood glucose level. Stimulates breakdown of glycogen and fat; increases blood glucose level.
Testes	Androgens: E.g. testosterone	Many tissues	Stimulate sperm production, skeleton and muscle growth, development of male sexual characteristics.
Ovaries	Oestrogens   Progesterone	Many tissues   Uterus and mammary glands	Stimulates development of female sexual characteristics; regulate the menstrual cycle. Regulates menstrual cycle and pregnancy; preps mammary glands for milk secretion.