Homeostasis And The Endocrine System

Question 1

Feedback

Answer 1

NEGATIVE FEEDBACK
- response in a way to reverse the direction of change
- most common form of feedback in physiological systems
POSITIVE FEEDBACK
- response to change the variable even more. In the direction of change - used when rapid change is desirable
e.g. blood clotting and ovulation

Question 2

Biological rhythms 
(set point of control centre can vary)

Answer 2

Suprachiasmatic nucleus controls the biological clock (circadian rhythm)

• environmental cures (Zeitgebers) keep body on a 24hr cycle - can be disrupted with night shifts and long haul flights
• melatonin from pineal gland is involved in setting biological clock

Cortisol peaks at 7/8 in the morning - highest level when just woken up, decreases through the day (when taking a blood sample for cortisol levels the time of day must be noted)

Question 3

Antidiuretic hormone (ADH) 
- synthesised in the hypothalamus and secreted by the posterior pituitary

Answer 3

Blood osmolality is detected by osmoreceptors in the hypothalamus

High blood osmolality (hypertonic)
- causes posterior pituitary to secrete MORE ADH
- increased reabsorption of H20 from urine in the collecting ducts of the kidney
- small volume of concentrated urine
Low blood osmolality (hypotonic)
- causes posterior pituitary to secrete LESS ADH
- decreases reabsorption of H20 from urine in the collecting ducts of the kidney
- large volumes of dilute urine

Question 4

The endocrine system

Answer 4

• a collection of glands located throughout the body

e. g. hypothalamus, parathyroid gland, adrenal gland, ovary/testis

Question 5

Communication via hormones

Answer 5

AUTOCRINE
- hormonal signal acts on the cell of origin
PARACRINE
- hormonal signal carried to adjacent cells of the same type via interstitial fluid
ENDOCRINE
- hormone signal released into bloodstream and carried to distant target cells
NEUROCRINE
- hormone originates in neurone and after transport down axon is released into bloodstream and carried to distant target cells

Question 6

The endocrine and nervous systems work in parallel to control homeostasis

Answer 6

ENDOCRINE SYSTEM
- hormonal signals of chemical nature conveyed in the bloodstream are recognised by receptors
- relatively slow
NERVOUS SYSTEM
- neurotransmitters and action potential signals of chemical and electrical nature conveyed over synapses and axons are recognised by receptors
- relatively fast

Some molecules act as both neurotransmitters and as hormones

Question 7

Classification of hormones

water/lipid soluble determines the type of receptor

Answer 7

Peptide/polypeptide
- all water soluble (e.g. Insulin, Glucagon, Growth Hormone)
Glycoproteins
- all water soluble (e.g. Follicle stimulating hormone, thyroid stimulating hormone)
Amines (derived from amino acids)
- adrenal medulla hormones are water soluble (e.g. Adrenaline and noradrenaline)
- thyroid hormones are lipid soluble
Steroids (derived from steroids)
- all are lipid soluble (e.g. Cortisol, testosterone, aldosterone)

Question 8

Hormone transport

Answer 8

Most bind to specific proteins
Dynamic equilibrium between bound and free forms of hormone in plasma
- only free form of the hormone is biologically active)
Roles of carrier proteins
- increase solubility of hormone in plasma
- increases the half life
- act as a readily accessible reserve

Question 9

Water soluble hormones

Answer 9

• can’t pass through the plasma membrane independently
• bind to cell surface receptors
  2 major classes
• G protein coupled receptor
• tyrosine kinase

Question 10

Lipid soluble hormones

Answer 10

• can diffuse through the plasma membrane
  Type I
• cytoplasmic receptor - receptor hormone complex enters the nucleus and binds to DNA
  Type II
• hormone enters the nucleus and binds to pre-bound receptor on DNA
  They affect the transcription of certain genes - expression of new protein mediates the effects of hormone
• tend to have longer lasting effects

Question 11

The hypothalamic pituitary axis

- Major link between the endocrine and nervous systems

Answer 11

The pituitary gland sits beneath the hypothalamus in a socket of bone called the sella turcica

Question 12

Posterior pituitary

NEUROCRINE function

Answer 12

Oxytocin (milk let down and uterus contractions during birth) and ADH (regulation of body water volume) produced by neurones in the hypothalamus

• transported down nerve cell axons to the posterior pituitary
• stored and released from posterior pituitary into the circulation to act on distant targets

Question 13

Anterior pituitary function

- tropic hormones - affect the release on other hormones

Answer 13

• hormones synthesised in the hypothalamus (stored in median eminence and released into hypophyseal portal system)
• hormones stimulate or inhibit target endocrine cells in the anterior pituitary gland - NEUROCRINE function - endocrine cells then secrete hormones into the bloodstream - ENDOCRINE function
• can also effect neighbouring cells - autocrine and paracrine function

Question 14

The hypothalamic-pituitary-adrenal axis

Answer 14

Stress –> hypothalamus releases corticotropin CRH releasing hormone –> anterior pituitary releases ACTH –> adrenal cortex releases cortisol which acts on target tissues

• CRH (+ve feedback on anterior pituitary) and ACTH (+ve feedback on the adrenal cortex) and cortisol (+ve feedback on the tissues)
• cortisol negatively feeds back on the anterior pituitary and the hypothalamus

Question 15

A control system

Answer 15

1) stimulus
2) receptor (detects change) e.g. Chemoreceptors (hormones, neurotransmitters, ligand detection)
AFFERENT COMMUNICATION PATHWAY (leads to the CC)
- nervous system (action potentials) or endocrine systems (hormones)
3) control centre CC
(determines set point, analysis of afferent input determines response)
EFFERENT COMMUNICATION PATHWAY (away from the CC)
4) effector (causes change e.g. Muscle, kidney)
NEGATIVE FEEDBACK back to stimulus