6 HUMAN HEALTH AND PHYSIOLOGY 1 153 known as the synaptic cleft. Action potentials must be transmitted across
this gap for the impulse to pass on its way and this is achieved by the presence of chemicals known as neurotransmitters. Neurotransmitters are held in vesicles in the pre-synaptic cell until an action potential arrives.
They are then released into the synaptic cleft, and diff use across to the post-synaptic membrane. There they can cause another action potential to be produced.
The synapse shown in Figure 6.25 uses the neurotransmitter
acetylcholine (ACh) and is a cholinergic synapse. ACh binds to receptors and causes depolarisation of the post-synaptic membrane and the initiation of an action potential. Once an action potential is generated in the post-synaptic membrane, ACh in the synaptic cleft is deactivated by acetylcholinesterase enzymes and the products are reabsorbed by the pre-synaptic membrane to be remade and repackaged in vesicles.
Monitoring levels and control by negative feedback Feedback systems work by monitoring the level of a substance, or a product, and feeding this level back to aff ect the rate of production or use of the substance. Negative feedback stabilises the internal
environment by reversing the changes within it. For example, if metabolic processes produce heat causing blood temperature to rise, sensors in the hypothalamus in the brain respond and send messages to increase heat loss or slow down heat production.
Negative feedback also controls levels of blood glucose. If a large amount of glucose is absorbed from the intestine, responses are initiated to bring the levels back to normal. The graph shown in Figure 6.27 shows how the level of glucose in the blood changes from its normal level and how the normal level is re-established.
Control of body temperature
Body temperature is monitored and controlled by the hypothalamus in the brain. The ‘set point’ for body temperature is 36.7 °C. The hypothalamus responds to nerve impulses from receptors in the skin
Figure 6.26 The positions of some endocrine glands in the human body. Endocrine glands have no ducts, and secrete directly into the bloodstream, which carries them to target cells.
pituitary gland (secretes many hormones e.g. ADH)
thyroid gland (secretes thyroxine)
adrenal gland (secretes adrenalin) islets of Langerhans in pancreas (secrete insulin and glucagon) ovary (in female) (secretes estrogen and progesterone) testes (in male) (secretes testosterone)
6 HUMAN HEALTH AND PHYSIOLOGY 1 155 and also to changes in the body’s core temperature. If body temperature
fl uctuates above or below the set point, the hypothalamus coordinates responses to bring it back to normal. This is another example of negative feedback. Nerve messages are carried from the hypothalamus to organs that bring about warming or cooling of the body. Table 6.6 (overleaf ) lists some of the body’s responses to changes in temperature.
Control of blood glucose levels
Blood glucose level is the concentration of glucose dissolved in blood plasma. It is expressed as millimoles per decimetre cubed (mmol dm−3).
Normally blood glucose level stays within narrow limits, between 4 mmol dm−3 and 8 mmol dm−3, so that the osmotic balance of the blood remains constant and body cells receive suffi cient glucose for respiration.
Levels are higher after meals as glucose is absorbed into the blood from the intestine and usually lowest in the morning as food has not been consumed overnight.
Glucose levels are monitored by cells in the pancreas. If the level is too high or too low, α and β cells in regions of the pancreas known as the islets of Langerhans produce hormones which turn on control mechanisms to correct it. Table 6.7 (overleaf ) summarises these responses.
Diabetes
The most obvious symptom of diabetes is the inability of the body to control blood glucose level. A diabetic person will experience wide
Figure 6.27 The control mechanism for blood glucose.
If blood glucose rises too high, glucose appears in urine and coma may result.
Insulin secreted into circulation.
Liver and muscle cells take up extra glucose from blood.
Glucagon secreted into circulation.
Liver cells break down glycogen and release glucose.
Release of glucose raises blood glucose level.
Uptake of glucose lowers blood glucose level.
Rise in blood glucose after food ingestion or glucose release from liver.
Fall in blood glucose after glucose uptake by cells.
SENSOR High
Normal blood glucose level
If blood glucose falls too low, coma may result.
Low
Blood glucose level
STIMULUS
SENSOR STIMULUS EFFECTOR
EFFECTOR EFFECT
EFFECT β cells in islets of
Langerhans in pancreas
α cells in islets of Langerhans in pancreas
Fever is a higher than normal temperature caused as the hypothalamus raises the body’s
‘set point’ to over 37 °C. Fever may be a response to toxins from pathogens or to histamines, released by white blood cells at infection. A high temperature is an important part of the body’s defence response to infection.
1 decimetre cubed is the same volume as 1 litre.
fl uctuations in their blood glucose above and below the normal limits (Figure 6.28).
Type I diabetes is caused when the β cells in the pancreas do not produce insulin. This can be a result of autoimmune disease in which the body’s immune system destroys its own β cells. Without insulin, glucose is not taken up by body cells so blood levels remain high, a condition known as hyperglycaemia. Excess glucose is excreted in urine and its presence is used to diagnose diabetes. About 10% of diabetics have type I Responses to a rise in body temperature Responses to a fall in body temperature
Arterioles in the skin dilate (widen) so that more blood fl ows to skin capillaries – excess heat from the core of the body is lost from the skin
narrow to restrict fl ow of warm blood to the skin capillaries – heat is retained in the body
Sweat glands produce more sweat, which evaporates from the skin surface to cool it
cease production of sweat
Muscles remain relaxed muscular activity such as shivering generates heat Metabolic rate may decrease to minimise heat production thyroxine increases metabolic rate
Table 6.6 The body’s responses to changes in core temperature.
Responses to a rise in blood glucose above normal Responses to a fall in blood glucose below normal Pancreas β cells in the pancreas produce the hormone insulin α cells in the pancreas produce the hormone glucagon Glucose
uptake or release
insulin stimulates cells in the liver and muscles to take in glucose and convert it to glycogen and fat, which can be stored;
inside the cells – blood glucose levels fall
glucagon stimulates the hydrolysis of glycogen to glucose in liver cells – glucose is released into the blood
Table 6.7 The body’s responses to changes in blood glucose.
Figure 6.28 Blood glucose and insulin levels following intake of glucose in a normal person and a person with untreated type I diabetes.
glucose ingested
Time / hours
blood insulin in untreated diabetic person blood insulin in normal person
blood glucose in normal person blood glucose in untreated diabetic person
0 0 0.02 0.04 0.06
Blood glucose concentration / g dm–3 Blood insulin concentration
0.08 0.10 0.12 0.14 0.16 0.18 0.20
1 2 3
6 HUMAN HEALTH AND PHYSIOLOGY 1 157 diabetes, which must be controlled by insulin injections. Symptoms
usually begin in childhood, which is why type I diabetes is sometimes known as ‘early onset’ diabetes.
Type II diabetes accounts for 90% of all cases. It is caused by body cells failing to respond to the insulin that is produced. Again, the result is that blood glucose level remains too high. This type of diabetes can be controlled by a change to a low carbohydrate diet. It is often associated with obesity, age, lack of exercise and genetic factors. The pancreas does produce insulin although levels may fall as the disease progresses. Type II diabetes is sometimes known as ‘late onset’ diabetes.
Diabetics must monitor their blood glucose level carefully so that they can control it, since the body’s internal control mechanism is not working properly.
18 Draw a labelled diagram of a motor neuron.
19 Defi ne the term ‘action potential’.
20 List the key events of synaptic transmission.
21 State the role of negative feedback in homeostasis.
22 Draw a table to compare and distinguish between type I and type II diabetes.
6.6 Reproduction
Reproduction is one of the important characteristics of living things.
Human male and female reproductive systems produce the gametes (the sperm cell and egg cell) that must come together to begin a new life.
The two reproductive systems enable the gametes to meet and the female reproductive system provides a suitable place for fertilisation to occur and an embryo to develop. The ovaries and testes also produce hormones that regulate sexual development and reproduction.