Water Balance Mechanisms and Abnormalities

Water Balance

• Water balance is maintained when water intake equals water loss.
• Daily water intake (net): drinking (1.3 L), solid food (0.9 L), metabolism (0.3 L) = 2.5 L
• Daily water loss (net): urine (1.4 L), stool (0.2 L), sweating & insensible loss (0.9 L) = 2.5 L

Objectives

• Explain the mechanism of water balance
• Relate water balance to conditions such as diarrhea, excessive sweating, burns, and diabetes insipidus.

Physiological Abnormalities of Water Balance

• Exercise: Increased water loss (sweating & hyperventilation). Balanced by decreased urine volume and increased intake.
• Fasting: Decreased water intake. Balanced by decreased urine volume and increased intake.
• High temperature: Increased water loss. Balanced by decreased urine volume and increased intake.

Pathological Abnormalities of Water Balance

• Vomiting: Loss of isotonic fluid.
• Diarrhea: Loss of isotonic fluid.
• Diabetes insipidus: Loss of hypotonic fluid.
• Burns: Loss of hypotonic fluid.
• Heat exhaustion/excessive sweating: Loss of hypotonic fluid.
• Hyperventilation: Loss of hypotonic fluid.
• Excessive intravenous (IV) fluid: Excess isotonic fluid.
• Excessive drinking (psychogenic): Excess hypotonic fluid.

Control of Water Intake

• Thirst center: Located in the hypothalamus.
• Stimuli for thirst:
  • Hyperosmolarity
  • Hypovolemia
  • Hypotension
  • Angiotensin II
  • Inhibitors of thirst (e.g., ANP)

Control of Water Intake - Osmoreceptors

• Hyperosmolarity stimulates osmoreceptors in the hypothalamus which initiates a signal to the thirst center.
• These receptors detect changes in osmolarity of the extracellular fluid.
• Increased osmolarity causes dendrites to be pulled, which leads to increased action potentials.
• Triggering osmolarity changes initiates the thirst response.

Control of Water Intake - Volume Receptors

• Hypervolemia stretches volume receptors inhibiting thirst through the vagus nerve.
• Hypovolemia causes less stretch, therefore no inhibition of thirst.

Control of Water Intake - Baroreceptors

• Hypertension stretches baroreceptors inhibiting the thirst center through the vagus and glossopharyngeal nerves.
• Hypotension causes less stretch, therefore no inhibition of thirst.

Formation of Angiotensin II

• Stimuli for angiotensin II formation include hyponatremia (low ECF volume), renal ischemia (reduced blood supply to the kidneys), and hypotension (low blood pressure).
• This process involves sequential activation and conversion of substances in the liver, lungs, and kidneys.
• The final product, angiotensin II, stimulates the adrenal glands to produce aldosterone and ADH in response to the above stimuli.

Atrial Natriuretic Peptide (ANP)

• ANP is released by the atria of the heart in response to hypervolemia.
• ANP inhibits the effect of angiotensin II on thirst.
• ANP inhibits renin secretion.
• ANP inhibits aldosterone production.
• ANP inhibits renal reabsorption of sodium, leading to a decrease in ECF volume.

Control of Water Loss

• Water Loss is primarily regulated by the kidneys and hormones such as ADH.
• Kidneys reabsorb about 99% of the filtered water in the glomeruli.
• The kidneys also reabsorb a smaller percentage of filtered water through ADH. (7-13%)
• The kidneys are involved in controlling the volume of urine output, which is essential for water balance.

Anti-diuretic Hormone (ADH)

• ADH deficiency causes increased urine volume, up to 23 L/day, and excessive thirst.

• This condition is known as Diabetes Insipidus.

• Excess ADH secretion causes a decrease in urine volume, hypertension, and edema due to water retention (Syndrome of Inappropriate ADH secretion, SIADH).

Dehydration

• Water +/− salt depletion
• Types:
  • Isotonic
  • Hypotonic
  • Hypertonic
• Causes of fluid loss:
  • Intestinal tract (diarrhea, vomiting, bleeding)
  • Skin (sweating, burns)
  • Urine (diuretic therapy, diabetes insipidus/mellitus)
  • Other (hyperventilation, decreased intake)

Isotonic Dehydration

• ECF osmolarity: No change.
• ICF volume: No change.
• ICF osmolarity: No change.

Hypotonic Dehydration

• ECF osmolarity: Decreases.
• ICF volume: Increases.
• ICF osmolarity: Decreases.

Hypertonic Dehydration

• ECF osmolarity: Increases.
• ICF volume: Decreases.
• ICF osmolarity: Increases.

General Symptoms & Signs of Dehydration

• Thirst
• Dry mouth
• Rapid heartbeat
• Headache
• Dry skin
• Decreased urination

Treatments of Dehydration

• Depending on the type, treatment may involve:
  • Treating the cause
  • Intravenous (IV) fluids
  • Oral rehydration salts (ORS)

Effects of IV Solutions on Body Fluid Compartments

• Isotonic: No change to ECF or ICF volume or osmolarity.
• Hypotonic: ECF volume increases, ICF volume increases, ICF osmolarity decreases.
• Hypertonic: ECF volume increases, ICF volume decreases, ICF osmolarity increases.

This quiz explores the mechanisms of water balance in the human body and relates it to various conditions like diarrhea, excessive sweating, burns, and diabetes insipidus. Understand both physiological and pathological abnormalities affecting water balance and their implications.