6. Control systems, biological rhythms & body water homeostasis

Question 1

Describe the 5 key features common to all homeostatic control systems.

Answer 1

1. communication (between the different components)
2. control centre (determine reference set point, analyse afferent input and determine appropriate response)
3. receptors (sensors required to detect stimuli, e.g. changes in environment)
4. effectors (agents that cause change)
5. feedback (the output has an effect on the control centre)

Question 2

Describe the cycle in a homeostatic control system.

Answer 2

1. Stimulus detected by receptor.
2. Receptor communicates with control system via afferent pathway.
3. Control centre communicates with effector via efferent pathway.
4. Effector causes change.
5. Change/output usually has negative feedback effect on control centre.

Question 3

What are the 2 main communication pathways in the body?

Answer 3

1. Peripheral nervous system: action potentials
   - afferent branch (sensory input towards the CNS)
   - efferent branch (motor output away from the CNS)
2. Endocrine system: hormones
   - (or autocrine and paracrine communication)

Question 4

Give 2 examples of important control centres in the brain.

Answer 4

1. hypothalamus
   - control of endocrine system
   - located in diencephalon
2. medulla oblongata
   - control of ventilation and cardiovascular system
   - located in brainstem

Question 5

What are control system receptors? Give examples of the different types.

Answer 5

Usually specialised nerve endings, e.g.

• chemoreceptors
• thermoreceptors
• proprioreceptors (self-awareness)
• nociceptors (pain)

Question 6

What is the function of negative feedback in control systems?

Answer 6

• Output inhibits the function of the control centre and effector acts to oppose the stimulus.
• Gives stability to control systems and allows set point to be controlled within fine limits.

Question 7

Give an example of negative feedback.

Answer 7

Hyperglycaemia stimulates insulin release from beta cells in Islets of Langerhans in pancreas… acts to decrease blood glucose level… glucose level returns to normal range.

Question 8

What is meant by the term ‘hunting behaviour’?

Answer 8

feature of negative feedback: tendency to overshoot set point several times until system returns to rest at set point

indicative of dynamic equilibirum

Question 9

What is the effect of positive feeback loops? Give an example.

Answer 9

Stimulus produces a response that increases its effect - output adds on to the input.

Blood clotting: positive feedback effect of thrombin to stimulate clotting

Question 10

What causes biological rhythms? Give an example.

Answer 10

Set point of control centre can vary over time.
E.g. woman’s core body temp. varies during menstrual cycle - sudden increase in core body temp. can be used as ovulation marker.

Question 11

What is the circadian/diurmal rhythm and how is this controlled?

Answer 11

Natural “biological clock” of 24hrs 11min dependent on a small group of neurons - suprachiasmatic nucleus - in the hypothalamus.

Cues from the environment - Zeitgebers - keep body on cycle:

• light
• temp
• social interaction
• exercise
• eating/drinking pattern

Release of melatonin from pineal gland also involved in setting clock.

Question 12

What causes jet lag?

Answer 12

mismatch between environmental cues (Zeitgebers) and body clock

Question 13

What is the normal total body water content, and in which compartments is this water contained?

Answer 13

Total body water = 50-60% for males, 45-50% for females.
I.e. approx. 42L in average 70kg male.

Contained in:

• intracellular fluid (35%)
• extracellular/interstitial fluid (12%
• extracellular/blood plasma (4-5%)

Question 14

What controls the mov. of water between body fluid compartments?

Answer 14

osmotic pressure

Question 15

How is blood osmotic pressure monitored and regulated?

Answer 15

Osmolality and sodium ion conc. of blood plasma monitored by osmoreceptors in supraoptic and paraventricular nuclei of hypothalamus.

Cells in these nuclei stimulate feelings of thirst and release of ADH from posterior pituitary gland.

Question 16

Describe the control system stimulated by high blood osmolality.

Answer 16

1. High blood osmolality detected by osmoreceptors in hypothalamus.
2. a) Stimulate thirst… drinking reduces osmolality.
   b) Stimulate release of more ADH from posterior pituitary… increased reabsorption of H2O from urine into blood in collecting ducts in kidney… small volume of concentrated urine.
3. Normal blood osmolality