Water and electrolyte balance practice questions.

A marathon runner loses 3 % of body mass as sweat during a race in warm conditions. Which cardiovascular change occurs while the running speed stays constant?

• Plasma volume rises, so venous return to the heart increases
• Stroke volume rises while heart rate falls to compensate
• Stroke volume falls and heart rate rises
• Cardiac output rises steeply because blood viscosity decreases

Answer: Stroke volume falls and heart rate rises

Sweat is drawn from plasma, so plasma volume and venous return fall and less blood fills the ventricle, lowering stroke volume; heart rate then rises to defend cardiac output, which is why heart rate drifts upwards at an unchanged pace rather than because the heart has become stronger.

Plasma osmolality rises in a cyclist during a hot mountain stage. Which response follows?

• Antidiuretic hormone secretion rises and urine volume falls
• Antidiuretic hormone secretion falls, so urine volume rises sharply
• Aldosterone secretion falls and sodium excretion increases markedly
• Osmoreceptors stop firing and the sensation of thirst is suppressed

Answer: Antidiuretic hormone secretion rises and urine volume falls

Hypothalamic osmoreceptors detect the more concentrated plasma and drive antidiuretic hormone release from the posterior pituitary, which inserts water channels in the collecting duct so water is conserved. The kidney defends fluid volume by concentrating urine; it cannot replace the water already lost as sweat.

A recreational runner finishes a 5 h marathon 1.2 kg heavier than at the start and reports nausea and confusion. Which explanation fits these observations best?

• Severe dehydration caused by an unusually high sweat rate
• Exercise-associated hyponatraemia from overdrinking
• Heat stroke caused by a complete failure of sweating
• Depletion of muscle glycogen in the working legs

Answer: Exercise-associated hyponatraemia from overdrinking

A gain in body mass shows that fluid intake exceeded losses, so plasma sodium has been diluted rather than concentrated. Dehydration always shows as a loss of body mass, which is why the mass measurement distinguishes the two conditions that share the symptoms of nausea and confusion.

Sodium is present in the rehydration solution offered to a hockey squad after training. What is its main physiological function in that solution?

• It provides a rapid source of energy for the muscles
• It increases the rate of sweating during the next session
• It lowers the osmolality of the drink below that of plasma
• It helps the ingested fluid to be retained in the body

Answer: It helps the ingested fluid to be retained in the body

Sodium is the main osmotically active particle of the extracellular fluid, so replacing it keeps the drunk fluid in that compartment instead of being excreted as dilute urine. Sodium supplies no energy; that role belongs to the carbohydrate that such solutions also contain.

Plasma osmolality rises in a cyclist during a hot stage of a tour. Which receptors detect that change and trigger the release of antidiuretic hormone?

• Stretch receptors in the wall of the right atrium
• Chemoreceptors in the carotid and aortic bodies
• Baroreceptors in the carotid sinus
• Osmoreceptors in the hypothalamus

Answer: Osmoreceptors in the hypothalamus

Osmoreceptors sense the concentration of the plasma and are the more sensitive of the two routes, responding to changes of about one per cent. The pressure and stretch receptors also contribute, but they respond to a fall in blood volume, which is a later and larger signal.

A student calculates a basketball player's sweat rate from the fall in body mass alone and leaves out the 1.2 L of fluid the player drank during the session. What is the effect on the calculated value?

• It is overestimated, because the fluid drunk adds to the mass that was recorded as lost.
• It is unaffected, because the fluid drunk leaves the body again as urine within the session.
• It is unaffected, because sweat rate is defined from the change in body mass alone.
• It is underestimated, because the fluid drunk replaced part of the mass lost as sweat.

Answer: It is underestimated, because the fluid drunk replaced part of the mass lost as sweat.

Fluid taken in masks part of the sweat loss, so the mass deficit that remains is smaller than the sweat produced and the calculated rate is too low. The full expression adds intake back and subtracts urine before dividing by time.