An Introduction to Water Balance
1. The dude in the sauna is sweating. Under what conditions do humans sweat?
2. Sweating is a mechanism by which the body attempts to regulate what variable?
3. Where does sweat come from? What body system(s) are “losing” water?
4. Can sweating cause, or promote, dehydration? What is dehydration?
5. Is the guy in the sauna losing water by any other mechanism(s) other than
sweating? If so, list them.
6. The dude in the sauna proceeded to jump into some ice-cold water. Did jumping
into the water replace the water lost by his sweating? Did he “re-hydrate?” Explain.
7. What’s more effective at re-hydrating; drinking a glass of water or jumping into a
lake? Explain.
8. When a person is losing water their blood becomes more concentrated.
Physiologists use the term osmolarity when describing the concentration of liquids
such as blood. Normally, human blood has a concentration of about 280 to 300
mOsm / L (milli-osmoles / Liter of Blood) or mOsm. When mildly dehydrated, the
blood osmolarity is about 320 mOsm and when severely dehydrated the osmaoairty
is about 350 or greater. As a group, predict how drinking water affects the
osmolarity of blood.
9. Humans rehydrate by drinking water. Make a flow chart to trace a water
molecule as it moves from the oral cavity to the blood. What organs and body
systems are involved?
Model 2. Example Water Homeostatic Feedback Loop.
10. List the organs, and the associated organ systems, in the above feedback loop.
11. The above image shows a homeostatic feedback loop for water balance. Like all
homeostasis mechanisms, this loop includes sensor, control center, effector and a
variable.
What is the variable? What units are used to measure this variable?
In what organ can you find the sensors?
What organ, and organ system serves as the effector?
12. Vasopressin is a hormone produced and released by the pituitary gland and it
targets the kidneys. Under what conditions is vasopressin released?
13. Brief, how does vasopressin (a hormone) move from the pituitary to the
kidneys?
14. What happens to the kidneys once stimulated by vasopressin?
15. If a person is dehydrated, will the kidneys produce more urine or less urine?
16. Where in the body can you find hypothalamic osmoreceptors? Predict their
function in the homeostasis of water.
17. On a separate piece of paper, construct a feedback loop showing the response to
over hydration. For example, what happens to the blood osmolarity? Vasopressin
levels? Kidney function? Etc.
18. List a few functions of the kidney and the urinary system.
19. Explain the mechanism by which the pituitary communicates with the kidneys.
Extension Questions
20. Dehydration may eventually lead to death. What organ or organs in the body
is/are most affected by severe dehydration?
21. Predict the cause of death in someone who dies of dehydration.
22. Predict the meaning of the phrase “drink to thirst.”
23. Construct two graphs to predict what happens to vasopressin levels and urine
output respectively during prolonged exercise.
24. Vasopressin output normally increases during the night. However, some
children’s levels of vasopressin stay flat (no increase.) What common childhood
inconvenience might these individuals experience?
25. The feedback loop illustrated in Model 2 includes the phase “less water lost in
the urine.” Can water conservation alone cause a decrease in blood osmolarity?
26. Along with vasopressin and kidney function, what other factors and
mechanisms are integral to maintaining water homeostasis?
27. Is water balance regulated by positive, or negative feedback? Explain.