Water Reabsorption

Definition

Water reabsorption is the process by which the kidneys selectively retain water from the filtrate in the nephrons, allowing the body to maintain proper fluid balance and concentration of solutes. This mechanism is crucial in the context of antidiuretic hormone regulation and the action of diuretic drugs.

5 Must Know Facts For Your Next Test

1. Water reabsorption occurs primarily in the proximal convoluted tubule and the collecting duct of the nephron, where the filtrate is concentrated by the removal of water.
2. The amount of water reabsorbed is regulated by the hormone antidiuretic hormone (ADH), also known as vasopressin, which increases the permeability of the collecting duct to water.
3. Diuretic drugs, such as thiazides and loop diuretics, work by inhibiting the reabsorption of water and solutes in the nephron, leading to increased urine output and fluid loss.
4. Impaired water reabsorption can result in conditions like diabetes insipidus, where the body is unable to concentrate urine, leading to excessive urination and dehydration.
5. The process of water reabsorption is driven by osmotic gradients created by the active transport of solutes, such as sodium and chloride, out of the tubular lumen and into the surrounding interstitium.

Review Questions

• Explain the role of antidiuretic hormone (ADH) in the regulation of water reabsorption.
  • Antidiuretic hormone (ADH), also known as vasopressin, is a key regulator of water reabsorption in the kidneys. ADH is released from the posterior pituitary gland in response to increased blood osmolarity or decreased blood volume. When ADH binds to receptors in the collecting duct of the nephron, it increases the permeability of the duct to water, allowing more water to be reabsorbed back into the bloodstream. This helps the body maintain proper fluid balance and osmolarity. Without sufficient ADH, water reabsorption is impaired, leading to the condition of diabetes insipidus and excessive urine output.
• Describe how diuretic drugs influence the process of water reabsorption.
  • Diuretic drugs, such as thiazides and loop diuretics, work by inhibiting the reabsorption of water and solutes in the nephron. Thiazide diuretics block the sodium-chloride cotransporter in the distal convoluted tubule, while loop diuretics inhibit the sodium-potassium-2 chloride cotransporter in the thick ascending limb of the loop of Henle. By reducing the reabsorption of these solutes, the osmotic gradient that drives water reabsorption is disrupted, leading to increased urine output and fluid loss. This mechanism is useful in the treatment of conditions like hypertension, edema, and congestive heart failure, where reducing fluid volume can provide therapeutic benefits.
• Analyze the relationship between water reabsorption, osmosis, and the maintenance of fluid balance in the body.
  • Water reabsorption in the nephron is driven by the principle of osmosis, where water molecules move from an area of lower solute concentration (the tubular lumen) to an area of higher solute concentration (the surrounding interstitium). This osmotic gradient is created by the active transport of solutes, such as sodium and chloride, out of the tubular lumen and into the interstitium. As water follows the solutes, it is reabsorbed back into the bloodstream, allowing the body to maintain proper fluid balance and the concentration of essential solutes. Disruptions in this process, either through hormonal imbalances (e.g., ADH deficiency) or the use of diuretic drugs, can lead to imbalances in fluid homeostasis and the development of various clinical conditions.

Related terms

Glomerular Filtration: The process by which blood plasma is filtered through the glomerular capillaries in the kidneys, forming the initial filtrate that will undergo further processing.

Tubular Reabsorption: The selective reabsorption of useful substances, such as glucose, amino acids, and ions, from the filtrate back into the bloodstream as it passes through the renal tubules.

Osmosis: The passive movement of water molecules across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration, in an effort to equalize the concentrations on both sides.