Maintaining homeostasis practice questions.

An open-water swimmer in cold water shows marked vasoconstriction of the skin arterioles. What is the main effect of this response?

• More heat is produced by the skeletal muscles of the limbs
• The set point of the hypothalamus is lowered to match the water
• Less warm blood reaches the skin, so heat loss falls
• Sweat production rises to remove the excess heat produced

Answer: Less warm blood reaches the skin, so heat loss falls

Vasoconstriction conserves heat by keeping warm blood in the deep vessels away from the surface. It generates no heat of its own; that is the job of shivering and of a rising metabolic rate.

A cross-country skier travels from sea level to a training camp at 2700 m. Which change has already taken place within her first hours at that altitude?

• Ventilation has risen, and the extra carbon dioxide lost raises blood pH
• Ventilation has fallen, because the central chemoreceptors detect less carbon dioxide
• Haemoglobin concentration has risen as the bone marrow responds to the hypoxia
• Maximal heart rate has risen, so maximal cardiac output is unchanged from sea level

Answer: Ventilation has risen, and the extra carbon dioxide lost raises blood pH

The low partial pressure of inspired oxygen is detected by the peripheral chemoreceptors in the carotid bodies, and the hypoxic ventilatory response begins within minutes. Blowing off carbon dioxide makes the blood more alkaline, while the increase in red cell mass takes weeks and maximal heart rate falls slightly rather than rising.

A distance squad spends three weeks sleeping at 2500 m and travelling down to 800 m for every hard session. What is the physiological reasoning behind this arrangement?

• Sleeping high prevents the fall in maximal heart rate that hard work at altitude would otherwise cause in the squad
• The hypoxic stimulus at rest raises red cell mass while the low sessions keep training speed high
• Training low increases the hypoxic stimulus, because ventilation is lower at the smaller altitude
• Sleeping high expands the plasma volume, which raises the oxygen carried in each litre of blood

Answer: The hypoxic stimulus at rest raises red cell mass while the low sessions keep training speed high

Hypoxia has to be endured for many hours a day to drive erythropoietin release, but training intensity collapses in thin air. Splitting the two exposures keeps the adaptive stimulus at rest and the mechanical quality of the sessions near sea level.

A swimming squad prepares for a championship in a hot country by training for ten days in a heated chamber at home rather than travelling early. Which term describes the adaptation produced by that chamber work?

• Acclimatisation, because the stressor is heat
• Acclimation, because the environment was controlled artificially
• Habituation, because the response fades with repetition
• Adaptation, because the change would be passed to the next generation

Answer: Acclimation, because the environment was controlled artificially

Acclimation describes an adjustment produced in an artificially controlled environment and acclimatisation one produced in the natural environment; the physiological changes are broadly the same, so the distinction is about method rather than mechanism. Habituation is a reduced response to a repeated stimulus, and adaptation in the evolutionary sense applies to populations across generations, not to one swimmer over ten days.

Within hours of arriving at a training camp at 3000 m, a cyclist's resting ventilation is higher than at sea level. Which receptors initiate this response?

• Central chemoreceptors detecting a rise in the carbon dioxide level
• Stretch receptors in the walls of the airways
• Peripheral chemoreceptors detecting the low arterial oxygen
• Thermoreceptors detecting the cold dry mountain air

Answer: Peripheral chemoreceptors detecting the low arterial oxygen

The carotid and aortic bodies respond to a fall in the partial pressure of oxygen in arterial blood, and at sea level that drive is almost silent because arterial oxygen is already high. The central chemoreceptors act in the opposite direction here: the extra breathing blows off carbon dioxide, which restrains the response until the kidneys have compensated over the following days.

In the fourth hour of a long training ride a cyclist's liver glycogen store is nearly empty, yet her blood glucose concentration is still being defended. Which process is now the main source of that glucose?

• Gluconeogenesis from lactate, glycerol and amino acids
• Continued breakdown of the remaining liver glycogen
• Release of glucose from the glycogen stored in the muscles
• Direct conversion of stored triglyceride into glucose

Answer: Gluconeogenesis from lactate, glycerol and amino acids

The liver can build glucose from non-carbohydrate precursors, which is what keeps the blood concentration up once its own store is gone. Muscle glycogen cannot be released as blood glucose because muscle lacks the enzyme required, and the fatty acid part of a triglyceride cannot be converted to glucose at all, only its glycerol backbone.