Which of the following has the greatest influence on tubular reabsorption? a. Filtration b. Hormones c. Blood flow d. Secretion
The correct answer and explanation is:
Correct Answer: b. Hormones
Explanation:
Tubular reabsorption is a key process in the formation of urine in the nephron of the kidney. It involves the movement of water and solutes from the tubular fluid back into the blood via the peritubular capillaries. Among the given options, hormones exert the greatest influence on the regulation and extent of tubular reabsorption.
Several hormones tightly regulate how much water, sodium, potassium, calcium, and other substances are reabsorbed:
- Antidiuretic Hormone (ADH): ADH increases the permeability of the distal convoluted tubule and collecting ducts to water, promoting water reabsorption. This concentrates the urine and helps maintain body water balance.
- Aldosterone: Produced by the adrenal cortex, aldosterone promotes sodium reabsorption (and potassium secretion) in the distal tubule and collecting duct. Sodium reabsorption leads to water reabsorption, affecting blood volume and pressure.
- Atrial Natriuretic Peptide (ANP): This hormone opposes the actions of aldosterone, promoting sodium and water excretion to lower blood pressure and volume.
- Parathyroid Hormone (PTH): PTH increases calcium reabsorption in the distal tubule and reduces phosphate reabsorption in the proximal tubule.
While filtration (a) is the first step in urine formation and sets the stage for what can be reabsorbed, it is a passive process and does not directly regulate reabsorption. Blood flow (c) supports overall kidney function, and secretion (d) is a separate process that moves substances from blood into the tubule.
Therefore, hormonal regulation is the most dynamic and influential factor in determining how much and what substances are reabsorbed, allowing the body to maintain homeostasis in varying conditions.