Question

Which of the following causes an increase in \(\mathrm{Na}^{+}\) reabsorption in the distal tubule and cortical collecting duct? a. an increase in aldosterone concentration b. an increase in antidiuretic hormone concentration c. a decrease in aldosterone concentration d. a decrease in antidiuretic hormone concentration e. Both b and c cause this increase in reabsorption.

Short answer

The correct answer is a. An increase in aldosterone concentration causes an increase in \(\mathrm{Na}^{+}\) reabsorption in the distal tubule and cortical collecting duct.

Step-by-step solution

Understanding the Role of Aldosterone

Aldosterone is a hormone that signals the cells in the kidneys (particularly in the distal tubules and cortical collecting duct) to reabsorb more sodium ions (\(\mathrm{Na}^{+}\)) and water back into the bloodstream. This means that an increase in aldosterone leads to an increase in sodium reabsorption.

Understanding the Role of Antidiuretic hormone

Antidiuretic hormone, also known as vasopressin, mainly affects the reabsorption of water, but does not have a significant effect on \(\mathrm{Na}^{+}\) reabsorption.

Analyzing the Options

Based on the roles of aldosterone and antidiuretic hormone, one can determine that an increase in aldosterone concentration (Option a) would increase \(\mathrm{Na}^{+}\) reabsorption in the distal tubule and cortical collecting duct. On the other hand, changes in the concentration of the antidiuretic hormone (Options b and d) would not have a significant effect on sodium reabsorption. A decrease in aldosterone concentration (Option c) would lead to less \(\mathrm{Na}^{+}\) reabsorption. Thus, Option e cannot be correct since both aldosterone and antidiuretic hormone need to cause an increase in reabsorption, which is not the case.

Key concepts

Sodium Reabsorption

Sodium reabsorption is a crucial process taking place in the kidneys, where sodium ions (\(\text{Na}^+\)) are returned to the bloodstream, conserving water and salts essential for the body's functioning. This process occurs mainly in the distal tubules and the cortical collecting ducts of the nephrons—the basic working units of the kidneys.

Aldosterone, a steroid hormone, plays a significant role in regulating sodium reabsorption. It signals kidney cells to increase the uptake of sodium ions from the filtrate and move them into the blood. This action effectively raises blood pressure and blood volume by retaining water through osmotic balance. In contrast, without sufficient sodium reabsorption, the body could lose too much salt, leading to low blood pressure and dehydration.

Understanding sodium reabsorption is crucial as it underpins many physiological processes, helping maintain fluid balance, muscle function, and nerve signaling. Proper function of this process ensures that the body's homeostasis is preserved, allowing all organs to work optimally.

Kidney Physiology

The kidneys are remarkable organs responsible for filtering blood and forming urine. Each kidney contains about a million nephrons, which are the functional units where blood filtration occurs. Kidney physiology encompasses several complex functions beyond just urine production. These functions include:

• Electrolyte balance - regulating levels of sodium, potassium, and other essential ions.
• Acid-base balance - maintaining the pH level of the blood.
• Fluid balance - ensuring the right amount of water in the body.
• Removal of waste products and toxins from the bloodstream.

Working seamlessly, the kidneys help stabilize the internal environment of the body, commonly referred to as homeostasis.

In terms of managing sodium reabsorption, the distal tubule and the cortical collecting duct are specifically vital components. They precisely adjust the reabsorption of sodium, bicarbonate, and other substances under hormonal control. They adjust reabsorption levels based on the body's needs at any given time, which is crucial in maintaining blood pressure and volume. The interaction between sodium reabsorption and overall kidney physiology echoes the sophisticated balance controlled by these organs.

Hormonal Regulation

Hormonal regulation in the kidneys involves several hormones, but aldosterone and antidiuretic hormone (ADH) play pivotal roles. Each hormone has a unique influence on kidney function and, consequently, on systemic homeostasis.

Aldosterone primarily regulates sodium reabsorption. It is secreted by the adrenal glands and acts on the distal tubule and cortical collecting duct of the nephrons, boosting sodium uptake and potassium excretion. This hormonal action is essential for controlling fluid volume in the body, affecting blood pressure regulation.

• Increased aldosterone leads to more sodium—and hence water—being reabsorbed.
• Decreased aldosterone results in less sodium reabsorption, affecting blood pressure and volume.

Alternatively, the antidiuretic hormone (ADH), or vasopressin, influences the reabsorption of water rather than sodium. It works by making the renal tubules more permeable to water, allowing water to be reabsorbed back into the blood. This helps reduce urine output and conserve water, especially important during dehydration or excessive fluid loss.

These hormones exemplify how the body precisely adjusts kidney function according to its needs. Although they target different substances, aldosterone and ADH together orchestrate the fine-tuning of volume and electrolyte balance.