Question

ADH causes (a) Increased water absorption from DCT and CT (b) Increased GFR by increasing blood pressure (c) Increases reabsorption of electrolyte from distal tubules (d) All of these

Short answer

Option (a) Increased water absorption from DCT and CT is the correct answer.

Step-by-step solution

Understanding the Role of ADH

ADH stands for antidiuretic hormone. It is also known as vasopressin. It is released by the pituitary gland and played a vital role in the regulation of water, salts, and the overall balance of fluids in the body. Particularly, it works on the kidneys to control the amount of water excreted in the urine. When ADH is present, more water is reabsorbed by the kidneys and returned to the blood, reducing the amount of urine produced. Therefore, releasing more ADH will increase water absorption.

Understanding the Choices

Choice (a) refers to the effect of ADH on water absorption from the distal convoluted tubule (DCT) and collecting tubule (CT) of the kidney, which is accurate. Choice (b) refers to the role of ADH in increasing Glomerular Filtration Rate (GFR) by increasing blood pressure - ADH certainly can cause fluid retention and thus, indirectly increase blood pressure, but it is not its primary effect. Choice (c) refers to the role of ADH in managing electrolyte levels, which is not directly accurate. Choice (d) indicates that all above functions are true, which is partially correct.

Making a Decision

While ADH does play a role in regulating blood volume and indirectly blood pressure, its primary function is to increase water absorption from the DCT and CT to conserve body water by reducing the amount excreted in urine. Moreover, ADH's role in the direct reabsorption of electrolytes is questionable. Therefore, the best answer would be choice (a).

Key concepts

Antidiuretic Hormone (ADH)

The antidiuretic hormone (ADH), also known as vasopressin, is a vital regulator of the body's water balance. Produced in the hypothalamus and released by the pituitary gland, ADH's primary function is to maintain proper hydration levels in the body. When you are dehydrated, ADH levels increase, prompting the kidneys to reabsorb more water back into the bloodstream rather than excreting it as urine.

ADH achieves this by making the renal tubules more permeable to water, particularly in the distal convoluted tubule and the collecting tubule. As water is reabsorbed effectively, the urine becomes more concentrated. This mechanism is crucial not just for regulating fluid balance but also for ensuring that the blood pressure remains stable. It's a finely tuned process that ensures our bodies conserve water effectively in times of need.

Water Absorption in Kidneys

In the kidneys, water absorption is a crucial function that takes place across different segments of the nephron, the kidney's functional unit. The process is dynamically controlled by ADH, which influences the permeability of the kidney tubules. Water is mostly reabsorbed passively following the osmotic gradient created by the reabsorption of salts and other substances.

Thirst, dehydration, or high blood osmolarity trigger sensors in the body, which then release ADH. With an increase in ADH, the walls of the distal convoluted tubule and collecting duct become more permeable, allowing more water to be reabsorbed into the body's circulation. This intricate balance of water absorption helps the body maintain normal fluid volume and osmotic concentration.

Distal Convoluted Tubule (DCT)

The distal convoluted tubule is a segment of the nephron that plays a central role in salt and water balance. It's located in the kidney's cortex and follows the loop of Henle. The DCT fine-tunes salt reabsorption and is responsible for the selective reabsorption of important ions like sodium and calcium, under hormonal control, such as through aldosterone.

When ADH is released, the DCT's permeability to water increases, allowing more water to be recaptured by the surrounding blood vessels. This reduces the urine volume while maintaining electrolyte balance. The DCT doesn't directly reabsorb a significant amount of electrolytes in response to ADH; instead, its role is more in the precision adjustment of the ion content of the urine.

Collecting Tubule

The collecting tubule is the last structure in the nephron where urine formation is refined before the final product is sent to the bladder. It extends from the cortex deep into the medulla of the kidney. Here, multiple distal convoluted tubules converge, and it's a significant site for ADH action.

Under the influence of ADH, the collecting tubule's cells insert aquaporins into their membranes, channel proteins that facilitate the movement of water. This increases the reabsorption of water back into the bloodstream, concentrating the urine further. The balance here is delicate; too much ADH can lead to water retention and potentially dangerous hyponatremia, while insufficient ADH can result in excessive fluid loss and dehydration.