Topic 8: Hormones and Homeostasis

definition of hormone

chemical substances produced in minute amounts by glands, carried in the blood and alters the activity of target organs , and eventually broken down by liver

details of hormones

• made out of either protein or steroids (lipid)

• works with nervous system to coordinate activities in the body

exocrine vs endocrine glands

EXOCRINE GLANDS
- has ducts, produces a secretion
- etc : salivary glands, sweat glands

ENDOCRINE GLANDS
- ductless, produces hormones and secretes them into bloodstream

PANCREAS: secretes pancreatic juice and hormones (insulin and glucagon) (from cells called islet of Langerhans) thus it is both an exocrine and endocrine gland

examples of endocrine glands

1. gland: hypothalamus (produces) + pituitary gland (stores and releases)
   hormone: antidiuretic hormone (ADH)
   function: osmoregulation
   target organ: collecting duct in kidney nephron

2. gland: pancreas
   hormone: insulin and glucagon
   function: blood glucose regulation
   target organ: insulin - liver and muscle cells
   glucagon - liver cells only

definition of homeostasis

maintenance of constant internal environment

importance of homeostasis

• cells function in a narrow range of conditions , large changes causes cells to be inefficient

• homeostasis maintains a constant internal environment

how does homeostasis help with cell function

1. function more efficiently in an internal environment with optimal conditions

2. independence from external environment (not affected by external changes)

what is the reference point

optimal level in a homeostatic control system

components of homeostatic control system

1. receptor

2. control centre

3. effector

what is negative feedback

mechanism that counteracts changes in internal environment and restores it back to reference point

process of negative feedback

1. receptor detects stimulus (change) from reference point

2. relays information to control centre

3. control centre compares information with reference point

4. change present → sends appropriate signal to the effector

5. effector carries out appropriate response to counteract changes from reference point, causing an effect that is picked up by receptor

6. relays information to control centre and returns system back to normal

why is it important to regulate glucose levels

• ideal substrate in cellular respiration

• only fuel molecule in brain

response in rise to glucose levels

1. increase in blood glucose levels above reference point of 90mg/100ml (stimulus) is detected by islets of Langerhans (receptor) in pancreas

2. triggers secretion of insulin (signal) by β cells of islet of Langerhans (control centre)

3. insulin is transported by blood to liver and muscles (effectors)

4. insulin causes:

   • increases permeability of cell membrane to glucose

   • increase rate of cellular respiration

   • stimulates liver and muscle cells to convert excess glucose to glycogen

   • decrease breakdown of glycogen to glucose

5. these actions decrease blood glucose concentration until it returns to reference point

6. return to reference point is detected by β cells in islet of Langerhans which decreases secretion of insulin
   - circulating insulin is broken down by liver and excreted by kidney

response to fall in glucose levels

1. decrease in blood glucose levels below reference point of 90mg/100ml (stimulus) is detected by islets of Langerhans (receptor) in pancreas

2. triggers secretion of glucagon (signal) by α cells of islet of Langerhans (control centre)

3. glucagon is transported by blood to liver(effector)

4. glucagon causes in the liver:

   • stimulates conversion of stored glycogen back to glucose

   • converts non-carbohydrate sources to glucose

5. these actions increase blood glucose concentration until it returns to reference point

6. return to reference point is detected by α cells in islet of Langerhans which decreases secretion of glucagon - circulating glucagon is broken down by liver and excreted by kidney

type 1 vs type 2 diabetes

type 1: cannot produce enough insulin
type 2: cannot respond to insulin produced

signs of diabetes

• increase of blood glucose

• glucose in urine

• excessive thirst / urination

• weight loss

• tiredness

treatment for diabetes

1. insulin injection (FOR TYPE 1 ONLY)

2. controlled diet and exercise

3. medicine

why is temperature regulation important

• enzymes work within certain range of temperature

• change in temperature leads to inactivation or denaturation

response to rise in temperature

1. thermoreceptors in hypothalamus detect rise in blood temperature

2. arterioles in skin DILATE while shunt vessels CONSTRICT to allow more blood to flow through capillaries under skin surface. more heat loss by radiation, convection and conduction

3. sweat glands become more active to increase production of sweat so more latent heat of vaporisation is removed via evaporation

4. metabolic rate slows down so less heat is produced

5. these processes decrease blood temperature until it returns to reference point

6. return to reference point is detected by thermoreceptors. removal of stimuli stops homeostatic action

response to fall in temperature

1. thermoreceptors in hypothalamus detect fall in blood temperature

2. arterioles in skin CONSTRICT while shunt vessels DILATE to allow less blood to flow through capillaries under skin surface. less heat loss by radiation, convection and conduction

3. sweat glands become less active to decrease production of sweat so less latent heat of vaporisation is removed via evaporation

4. metabolic rate increases so more heat is produced

5. body shivers so contraction of the muscles produce heat

6. these processes increase blood temperature until it returns to reference point

7. return to refernce point is detected by thermoreceptors. removal of stimuli stops homeostatic action

importance of blood plasma water regulation

• too much water - water molecules enter cells via osmosis. cells burst and die

• too little water - water molecules leave the cell via osmosis. cells shrink and cannot function

response to a rise in blood plasma water potential

1. rise in water potential in blood plasma is detected by osmoreceptors in hypothalamus

2. triggers pituitary gland to release less ADH into bloodstream

3. decrease in ADH causes:
   - walls of distal convoluted tubule and collecting duct to be less permeable to water
   - kidney tubules reabsorb less water
   - more dilute urine is produced

4. these actions decrease water potential until returns to reference point

5. return of water potential to reference point is detected by osmoreceptors. secretion of ADH returns to norm

response to a fall in blood plasma water potential

1. fall in water potential in blood plasma is detected by osmoreceptors in hypothalamus

2. triggers pituitary gland to release more ADH into bloodstream

3. increase in ADH causes:
   - walls of distal convoluted tubule and collecting duct to be more permeable to water
   - kidney tubules reabsorb more water
   - less urine is produced, urine is more concentrated

4. these actions increase water potential until returns to reference point

5. return of water potential to reference point is detected by osmoreceptors. secretion of ADH returns to norm