Homeostasis

Question 0

What is the difference between negative and positive feedback? Give an example of each.

Answer 0

NEGATIVE:
Effector opposes stimulus (return to set point)
e.g. hyperglycaemia stimulates insulin to return the blood [glucose] to a set point

POSITIVE:
Effector increases response of stimulus
e.g. ovulation ……………….

Question 1

What are the different components of homeostatic controls?

Answer 1

COMMUNICATION: 
Autocrine = self-regulation 
Nervous = neurotransmitters & APs 
Endocrine = hormones 
Paracrine = local hormones 

CONTROL CENTRE:
Hypothalamus = determines set point, analyses input, determines response

RECEPTOR:
Stimuli —> Sensor —> Signal afferent pathway —> Control centre

EFFECTOR:
Control centre —> efferent pathway —> effector

Question 2

Give some examples of how set points can vary according to biological rhythms.

Answer 2

[cortisol] varies throughout the day (therefore measuring cortisol will depend in the time the sample was taken)

melatonin (secreted from pineal gland) increases as light decreases

hormones in menstrual cycle (28 days)

Question 3

Where and what is the biological clock?

Answer 3

Suprachiasmatic nuclues (hypothalamus)

Question 4

Approximately how much water is in the average 70kg male? What are the proportions in the intracellular fluid, extracellular fluid, and blood plasma?

Answer 4

~ 42l of water

INTRACELLULAR FLUID = ~ 28L

EXTRACELLULAR FLUID = ~ 9.4L

BLOOD PLASMA = ~ 4.6L

Question 5

How is the water content of the body controlled?

Answer 5

Increased osmolality of blood plasma (reduced water) = increased release of ADH (from posterior pituitary) —> increased resorption of water in kidneys

Decreased osmolality of blood plasma (increased water) = decreased release of ADH —> more water excreted in urine

Question 6

What is the hypothalamic-pituitary axis?

Answer 6

(releasing factors) (trophic hormones)
Hypothalamus ——-> Anterior pituitary ———> Endocrine glands

Anterior pituitary (endocrine) 
Posterior pituitary (neuro-endocrine)

Question 7

Outline the regulation of cortisol.

Answer 7

HYPOTHALAMUS —> Corticotrophin releasing hormone —-> ANTERIOR PITUITARY —> Adrenocorticotrophic hormones —> ADRENAL GLAND —> Cortisol

Cortisol inhibits the release of the previous two hormones

Stress activates the pathway

Question 8

What factors usually affect hormone secretion?

Answer 8

Change in a parameter regulated by a hormone

Change in concentration of hormone or another hormone

Question 9

How can hormones be transported in the blood? Which hormones are transported each way?

Answer 9

Soluble = free in blood
(peptide & amide hormones)

Insoluble = free + binding globulin = bound hormone
(steroid and thyroid hormones)

Note: free hormone is biologically active

Question 10

What are the classes of peptide hormones?

Answer 10

POLYPEPTIDE: single chain peptides varying in chain length e.g. insulin, TRH, growth hormone

GLYCOPROTEINS: two polypeptide chain + carbohydrate side chains e.g. TSH, FSH, LH, hCG

PROHORMONES: e.g. insulin

Question 11

What are some examples of steroid hormones?

Answer 11

Cholesterol —> Steroids

Progestins —> Progesterone

Calciferols

Corticosteroids (adrenal cortex)

Androgens e.g. testosterone

Oestrogens e.g. estradiol

Question 12

What are some examples of amine hormones?

Answer 12

Tyrosine —> Hormones e.g. thyroxine, adrenaline, T3 & T4

Histidine —> Histamine

Tryptophan —> 5-HT

Question 13

What does hormone action depend on?

Answer 13

• [free hormone] at active tissue
• number of receptors on target cell
• affinity of hormone for receptor
• degree of signal amplification

Question 14

How do non-lipid soluble hormones affect target cells?

Answer 14

Hormone binds to receptor

G-protein activated

Secondary messenger causes metabolic effects in cell

Question 15

How do lipid-soluble hormones affect target cells?

Answer 15

Diffuse through plasma membrane and through the nuclear membrane.

Steroid dimer binds to hormone response element of DNA

Switches transcription on or off

Translation of mRNA produces altered protein which causes metabolic effects

Question 16

What is the definition of homeostasis?

Answer 16

Control of internal environment in a dynamic equilibrium between certain set limits