Water Balance

Question 1

Normal Routes of Water Entry

Answer 1

Oral intake.
Drinking fluids.
Food consumption.
Metabolic water production.
Water is produced as a byproduct of cellular respiration (breakdown of carbohydrates, fats, proteins).

Question 2

Normal Routes of Water Loss:

Answer 2

Urine.
Sweat.
Respiration.
Feces.
Insensible water loss.
Water evaporates from the skin and respiratory tract, even without noticeable sweating.

Question 3

Explain how changes in water intake/loss can disrupt osmolarity homeostasis.

Answer 3

Increased water intake.
Excessive water dilutes body fluids, reducing osmolarity (hypo-osmolarity), leading to cellular swelling.
In extreme cases, hyponatremia can occur—confusion, seizures, or even death.

Decreased water intake/excessive loss:
Such as sweating, diarrhea, vomiting increases osmolarity (hyperosmolarity).
Causes water to leave cells, leading to cellular dehydration and potential dysfunction.

Regulatory mechanisms:
Thirst mechanism: activated by osmoreceptors in the hypothalamus when osmolarity rises.
Antidiuretic hormone (ADH): released form posterior pituitary to promote water reabsorption in the kidneys, reducing water loss.
Aldosterone: promotes sodium and water retention, helping restore osmolarity.

Question 4

Describe behavioral mechanisms that control water intake and loss.

Answer 4

Behavioral Mechanisms Controlling Water Intake:

Thirst sensation:
Triggered by dehydration or increased osmolarity.
Drives the urge to drink water.

Preference for hydrating beverages:
People naturally choose water or electrolyte-rich drinks when thirsty.

Cultural and habitual drinking:
Regular habits like drinking with meals or carrying a water bottle.

Behavioral Mechanisms Controlling Water Loss:

Seeking shade or cooler environments:
Reduces sweating in hot conditions.

Adjusting clothing:
Wearing light, breathable clothes to minimize sweating.

Limiting physical activity:
Reducing exertion to prevent excessive water loss through sweat.

Voluntarily urinating before long trips or events:
Minimizes discomfort and manages water loss.

Question 5

Explain the role of hypothalamic osmoreceptors in regulation of body osmolarity.

Answer 5

Location:

Found in the hypothalamus, near the thirst center and pituitary gland.
Function:

Detect changes in blood osmolarity (concentration of solutes in blood).
When Osmolarity Increases (e.g., dehydration):

Osmoreceptors shrink due to water leaving their cells.
Signal the release of antidiuretic hormone (ADH) from the pituitary gland:
ADH prompts kidneys to reabsorb more water, reducing urine output.
Activate the thirst mechanism to encourage drinking water.
When Osmolarity Decreases (e.g., overhydration):

Osmoreceptors swell due to water entering their cells.
Inhibit the release of ADH:
Kidneys excrete more water, increasing urine output.
Suppress the thirst mechanism to reduce water intake.
Result:

Maintain stable osmolarity for proper cell function.

Question 6

Explain how the cardiovascular, endocrine, and urinary systems monitor blood volume and/or blood pressure.

Answer 6

Cardiovascular System:
Baroreceptors:
Found in arteries (aorta, carotid arteries).
Sense blood pressure changes and signal the brain.
Adjust heart rate and blood vessel diameter to stabilize pressure.
Stretch receptors:
Found in veins and atria.
Detect changes in blood volume by sensing vessel stretch.
Endocrine System:
Renin-Angiotensin-Aldosterone System (RAAS):
Renin (from kidneys) triggers reactions that increase blood pressure.
Aldosterone helps retain sodium and water, increasing blood volume.
Antidiuretic Hormone (ADH):
Released when blood pressure is low or osmolarity is high.
Promotes water retention in the kidneys to boost blood volume.
Atrial Natriuretic Peptide (ANP):
Released by the heart when blood volume is high.
Encourages water and sodium excretion, lowering blood pressure.
Urinary System:
Kidneys:
Monitor blood flow and pressure through the juxtaglomerular apparatus.
Adjust water and sodium excretion based on signals from the cardiovascular and endocrine systems.
Urine production:
Increased during high blood volume/pressure.
Decreased during low blood volume/pressure.

Question 7

Describe changes to body fluid compartment volumes and osmolarity when a person drinks a large volume of pure water, and then explain the compensatory mechanisms that attempt to restore normal volumes and osmolarity.

Answer 7

Changes to Body Fluid Compartment Volumes and Osmolarity:
Intracellular Fluid (ICF):
Volume: Increases as water diffuses into cells due to osmotic gradient.
Osmolarity: Decreases as water dilutes intracellular solutes.
Extracellular Fluid (ECF):
Volume: Increases due to the ingested water being absorbed into the bloodstream.
Osmolarity: Decreases as pure water dilutes extracellular solutes.
Compensatory Mechanisms to Restore Normalcy:
Kidneys:
Increase water excretion by reducing antidiuretic hormone (ADH) secretion, leading to increased urine output.
Produce large volumes of dilute urine to excrete excess water.
Thirst Mechanism:
Suppressed due to reduced plasma osmolarity, preventing further water intake.
Cellular Adjustment:
Temporary osmotic equilibration occurs as water shifts between ECF and ICF to balance osmolarity.
Renin-Angiotensin-Aldosterone System (RAAS):
Downregulated since the blood volume expansion and low osmolarity inhibit renin release.
Atrial Natriuretic Peptide (ANP):
Secreted by the heart in response to increased blood volume, promoting sodium and water excretion to decrease ECF volume.