1. Regulating Mechanisms
Chapter 24

2. Homeostasis Maintainence of the body’s internal environment
- cells & tissue fluid
Water concentration, blood sugar, & temperature must be maintained
Blood flow must also be maintained
If any deviate from norm, change is detected by receptors
Nerve or chemical signals sent to effectors
This counteracts original deviation = negative feedback

3. Control of heart rate Pacemaker initiates each heartbeat
Autonomic Nervous Control:
Control centre in medulla regulates heart rate
Cardio-accelerator centre sends impulses via sympathetic nerves
Increased impulses = increased heart rate
Cardio-inhibitor centre sends impulses via a parasympathetic nerve (vagus)
Increased impulses = decreased heart rate
Two pathways are antagonistic
Hormonal Control:
Adrenaline from adrenal glands also increases heart rate
Adrenaline release stimulated by sympathetic nerves

4. Autonomic nervous system

5. Exercise on respiratory system
Increases oxygen & glucose demand
Increases CO2 production
Chemoreceptors detect blood CO2 concentration
If CO2 elevated
- impulses sent to the respiratory control centre in the medulla
Medulla sends impulses to intercostal muscles & diaphragm
Result – increased breathing rate & depth

6. Exercise on Cardio-vascular system
Heart Rate: Heartbeats (cardiac cycles)/min
Stroke Volume: Vol of blood expelled by each ventricle on contraction
Cardiac output: Vol of blood from ventricle/min
- heart rate X stroke volume
All 3 increased by exercise (more sympathetic activity)
Strenuous exercise = x5 increase of cardiac output

7. Blood Distribution with exercise
Organ demand for blood varies greatly
Rest
– ‘vegetative’ functions promoted
- blood diverted to intestine & kidneys
Exercise
– blood diverted to skeletal muscles
Nerve impulses from medulla constrict arterioles into abdominal organs
Decreased blood flow to abdominal organs
Nerve impulses make arterioles into muscles dilate
Increased blood flow to muscles
Effects on skin, brain, kidneys

8. Blood sugar control Islets of Langerhans detect blood sugar levels
Cells in this area produce insulin
Excess glucose converted to glycogen
Liver stores 100g of glucose as glycogen
If blood sugar too low, Islet cells release glucagon instead
Glycogen broken down into glucose
Adrenaline release can override
– blocks insulin secretion
Allows for an emergency burst of glucose

9. Diabetes Mellitus Disfunction of insulin secreting cells
Glucose levels elevated
millimoles/l (normally 5/l)
Glucose often present in urine
Cells inefficient at using glucose
Fat stores are therefore depleted for energy
- weight loss
Glucose tolerance test is a diagnostic test
– measured on glucose tolerance curve
If blood glucose doesn’t return to norm within 2.5 hrs
- irregular insulin production

10. Control of body temperature
Hypothalamus acts as temperature-monitoring centre
Responds to nerve impulses from skin thermoreceptors
Indicates surface temperature
Also has central thermoreceptors
– sensitive to blood temperature
Indicates body core temperature
Hypothalamus responds with impulses to effectors

11. Effectors - skin Prevents overcooling by: 1) Decreased sweating
2) Vasoconstriction
3) Contraction of erector muscles
– skin hairs raised
- (layer of air trapped = insulation)
Corrects overheating by:
1) Increased sweating – water vapour lost
2) Vasodilation – Blood flows to skin surface, heat radiates out

12. Effectors - other Shivering by skeletal muscles – rapid contraction
- more heat generated
Liver
– Metabolic rate increases
Hormones
– Adrenaline & thyroxine increase metabolic rate
- Slower, long-term response
Voluntary response
Thermoreceptors also send impulses to cerebrum
Individual ‘feels’ too hot or cold, and takes action

13. Temperature regulation in infants
Infants suffer more heat loss than adults:
- Larger surface area
- Involuntary mechanisms poorly developed
- Unable to avoid cold by voluntary responses
In cold, brown fat tissue actively generates heat
Critical temperature – temp where vasoconstriction can maintain body temp
27oC in adults, higher in babies
Below this, metabolism maintains body temp
Body food reserves used up for heat energy
If prolonged cold, reserves used up, temp falls rapidly

14. Hypothermia When body temperature is subnormal Infants:
- Far less able to tolerate cold conditions
- Risk greater in premature babies (often kept in incubators)
Elderly:
Vasoconstriction often fails
Shivering fails
Slower metabolism = less heat generated
Less active = less muscle activity = less heat
Prolonged exposure to extreme temps can break down homeostatic mechanisms
At this stage voluntary responses can also fail