Endocrine System

Hormones :

·       Hormones are chemicals, proteins in nature, secreted by ductless endocrine glands into the blood plasma.

·       They travel in solution in blood plasma to specific target organs.

·       They bring about a specific effect in target organs.

·       They bring about long lasting changes in the body.

·       They act slowly.

Differences between Hormonal and Nervous System :

Hormonal System	Nervous System
Slow reaction	Fast reaction
Chemical	Electrochemical
Long term	Short term
Via blood for transport	Via nerves in nervous pathways
Produced in endocrine glands	Impulse is generated by receptor
General effect as well as target organ	Specific destination, thus, a localised effect
	All or no response

Negative Feedback : A rise / fall in an amount of a substance or a factor is detected and this triggers a mechanism to decrease / increase that substance / factor to normal levels to keep it constant despite intake / output.

This occurs as a result of changes in internal environment and is a result of homeostasis. Changes in hormone levels reduce enzyme / hormone production, returning the system to normal. Example -

Adrenaline : Adrenaline is produced by adrenal glands on top of the kidney. Adrenaline is the flight, fright or fight hormone. It is released in response to excitement.

·                Heartbeat / Stroke Volume and blood pressure are increased. Voluntary muscles are provided with blood faster. More glucose and oxygen are available to muscles. So, the muscles are prepared for fight or flight. So, more energy is released.

·                This reduces blood flow to the skin / gut as more blood from skin and gut is diverted to the muscles.

·                It increases conversion of glycogen to glucose, for respiration, increasing blood sugar.

·                Pupils dilate.

·                Increased sweating occurs.

·                Hair becomes erect.

·                Body is more alert. Mental reactions are faster

·                Metabolism increases.

·                Rate and depth of breathing increase, causing greater oxygen uptake, due to more gaseous exchange.

Pancreas : It lies below the stomach, above the ileum. It is attached to the duodenum.

Functions: It controls blood glucose level.

·                It produces insulin.

·                Permeability of liver cells is increased.

·                Glucose is converted to glycogen.

·                This decreases blood glucose level

·                Alpha cells of pancreas secrete glucagon.

·                Glucagon converts glucose to glycogen.

·                Blood glucose level is increased (due to adrenaline secretion).

A rise in blood glucose concentration :

·                The rise in blood glucose concentration is detected by the beta cells in the Islets of Langerhans in the pancreas.

·                More insulin is secreted by these cells.

·                The hormone, insulin travels in solution in blood plasma to the liver.

·                Insulin causes the liver cells to convert glucose to glycogen to be stored in the liver and in the muscles.

·                Insulin makes cell membranes of body cells more permeable to glucose.

·                Insulin increases the rate of respiration.

·                Glucose is taken up by body cells.

·                Blood glucose level falls.

Blood glucose level increases with diet. It falls with activity and time since last meal, due to respiration.

Thyroxine :

·                Thyroxine is formed in the thyroid gland.

·                Secretion of thyroxine is stimulated by the secretion of TSH by the pituitary.

·                It brings about a rise in the rate of metabolism by increasing the rate of respiration.

·                It leads to rapid weight loss.

·                It allows material for growth to be utilised.

Diabetes :

·                Diabetics usually have glucose in their urine as liver cannot store glucose.

·                Blood glucose rises to high levels.

·                Insufficient insulin is secreted by the pancreas. So, glucose is not converted to glycogen.

·                The blood glucose is not reduced to normal by insulin.

·                Glucose in blood is filtered by Bowman’s capsule.

·                Only some glucose is reabsorbed. Too much glucose is present for all to be reabsorbed.

·                The excess glucose passes out in urine.

·                Less water is reabsorbed.

·                The urine is very dilute.

Pituitarian Hormones :

• FSH / ICSH = Stimulates follicles to produce gametes
• LH = Stimulates follicle to produce progesterone
• ADH = Controls osmotic potential of blood by affecting water reabsorption in the kidney tubule
• Growth hormone = Stimulates cell division
• TSH = Stimulates thyroid gland