Lab 3: Renal Physiology, Water and Sodium Balance

1. How the Kidneys Handle a Water Load

• When the body is given a water load (excess water intake), the kidneys adjust by increasing urine output.

• Mechanism:

  • Filtration: The kidneys filter blood plasma, and water is reabsorbed primarily in the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.

  • If there is excess water, the kidneys will reabsorb less water in the collecting ducts.

  • Dilution of urine: When the body has a water load, the kidneys produce dilute urine, and the concentration of solutes in the urine is low.

  • The hormone antidiuretic hormone (ADH) plays a key role by regulating water reabsorption in the kidneys. When water intake is high, ADH secretion is suppressed, leading to less water reabsorption and increased urine volume.

2. Distinguishing Between Water and Isosmotic Salt Loads

• Water load: Excess intake of water leads to a dilution of blood plasma (low solute concentration), triggering the kidneys to excrete more water through urine (as explained above).

• Isosmotic salt load: A salt load where the concentration of salts (typically sodium chloride) increases, but the osmolarity remains similar to the blood's osmolarity. The body may retain water to balance the sodium concentration and prevent dehydration.

  • Key difference: In the case of water load, the kidneys increase water excretion without losing significant amounts of solute. For isosmotic salt load, water reabsorption is typically maintained to keep osmolarity balanced, and excess salt is excreted, often with water.

3. How ADH Contributes to Water Balance

• ADH (Antidiuretic Hormone), also known as vasopressin, is released from the posterior pituitary gland in response to increased blood osmolarity (higher solute concentration) or low blood volume.

• Action of ADH:

  • Increases the permeability of the collecting ducts to water by promoting the insertion of aquaporins (water channels) into the cell membranes.

  • More water is reabsorbed back into the bloodstream, leading to concentrated urine (less water excreted).

  • Effect: Helps to conserve water and maintain osmolality (balance between water and solutes).

4. How the RAAS Pathway Works and Contributes to Sodium Balance

• RAAS (Renin-Angiotensin-Aldosterone System) is activated in response to low blood pressure, low sodium levels, or sympathetic nervous system stimulation.

  1. Renin is secreted by the kidneys in response to these signals.

  2. Renin activates angiotensinogen (produced by the liver) to form angiotensin I.

  3. Angiotensin I is converted to angiotensin II by the enzyme ACE (Angiotensin-Converting Enzyme) in the lungs.

  4. Angiotensin II has several effects:

     • Stimulates aldosterone release from the adrenal glands.

     • Stimulates vasoconstriction to raise blood pressure.

  5. Aldosterone promotes sodium reabsorption in the distal convoluted tubule and collecting ducts of the kidneys, increasing sodium retention, which leads to water retention and increased blood volume/pressure.

• Result: The RAAS pathway contributes to maintaining sodium balance, blood pressure, and overall fluid balance.

5. Defining Specific Gravity

• Specific gravity is a measure of the concentration of solutes in urine compared to water.

  • It indicates the kidney’s ability to concentrate or dilute urine. A higher specific gravity means urine is more concentrated with solutes (low water content), while a lower specific gravity indicates dilute urine.

• Normal range: 1.005 to 1.030.

• The specific gravity of urine reflects the body's hydration status and the kidneys’ ability to regulate water and solute balance.

6. How Drinking Water or Isotonic Saline Alters Hormone Release

• Drinking water:

  • Increases blood volume and lowers plasma osmolality (dilutes solutes in the blood).

  • This decrease in osmolality signals the hypothalamus to reduce ADH secretion, leading to less water reabsorption by the kidneys and increased urine output.

• Drinking isotonic saline (NaCl solution):

  • Increases extracellular sodium concentration without altering plasma osmolality much.

  • ADH release may not be significantly affected, but the RAAS pathway may be activated to retain sodium and water, maintaining balance.

  • Angiotensin II and aldosterone levels may increase to facilitate sodium retention.

7. How Diuretic and Aquaretic Agents Alter Renal Physiology

• Diuretics: Medications that increase urine production by promoting sodium excretion in the kidneys.

  • Loop diuretics (e.g., furosemide) inhibit sodium reabsorption in the loop of Henle, leading to increased sodium and water loss.

  • Thiazide diuretics (e.g., hydrochlorothiazide) inhibit sodium reabsorption in the distal convoluted tubule.

  • Effect on water and sodium balance: Diuretics lead to sodium and water excretion, reducing blood volume and blood pressure.

• Aquaretics: Medications that increase water excretion without significantly affecting sodium balance.

  • Mechanism: These agents typically work by inhibiting the action of ADH or blocking the water channels (aquaporins) in the kidneys, leading to increased urine volume without a significant change in sodium concentration.

  • Effect on water balance: Aquaretics promote water excretion but not sodium excretion, helping the body eliminate excess water while preserving sodium.

Key Terms to Remember:

• Water load: Excess water intake; kidneys increase urine production to excrete excess water.

• Isosmotic salt load: A load of salts where the osmolarity remains constant; kidneys maintain water balance by reabsorbing water.

• ADH (Vasopressin): Hormone that increases water reabsorption in the kidneys to conserve water.

• RAAS (Renin-Angiotensin-Aldosterone System): Hormonal pathway that helps regulate sodium and water balance, especially under low blood pressure conditions.

• Specific Gravity: A measure of urine concentration relative to water; higher values indicate more concentrated urine.

• Diuretics: Medications that promote sodium and water excretion.

• Aquaretics: Medications that promote water excretion without significantly affecting sodium levels.

Review Questions:

1. How does ADH help maintain water balance in the body?

2. What is the difference between the handling of water load and isosmotic salt load by the kidneys?

3. How does the RAAS pathway affect sodium balance?

4. What does a low specific gravity indicate about urine composition?

5. How does isotonic saline intake affect hormone release in the body?

Use this guide to understand key concepts related to water and sodium balance, kidney physiology, and the hormonal regulation of these processes!