Problem 94 - Objective Biology for NEET

Question

The effect of antidiuretic hormone (ADH) on the kidney is to increase the (a) Excretion of water (b) Excretion of \(\mathrm{Na}\) ' (c) Permeability of the distal nephron to water (d) Glomerular filtration rate

Step-by-Step Solution

Answer

(c) Permeability of the distal nephron to water

Step 1: Understand the function of ADH

Antidiuretic hormone (ADH) is a hormone that helps your kidneys manage the amount of water in your body. The primary role of ADH is to conserve body water by reducing the amount of water passed in urine. It does this by increasing the water permeability in the last portion of the distal nephron, which is part of the renal tubule in the kidney. As a result, water is reabsorbed into the body rather than being excreted in urine, which consequently decreases water excretion.

Step 2: Assess the choices

Let's evaluate each of the response options given in light of the function of ADH as identified in Step 1. (a) Excretion of water: ADH works to reduce water excretion, so this choice is contrary to its role. (b) Excretion of Sodium (Na): ADH's primary concern is with the regulation of water levels, not sodium management, so this option is also unlikely. (c) Permeability of the distal nephron to water: Based on the identified function of ADH, this choice aligns well since raising the distal nephron's water permeability is a direct function of ADH. (d) Glomerular filtration rate: ADH does not primarily affect the glomerular filtration rate, which negates this option as well.

Step 3: Select the right choice

Given the analysis of the choices based on the function of ADH, the correct answer is (c): Permeability of the distal nephron to water. This choice directly correlates with the action of ADH in the kidneys.

Key Concepts

ADH and Kidney

Antidiuretic hormone (ADH), also known as vasopressin, plays a pivotal role in regulating the body's water balance. This hormone, which is released by the pituitary gland in the brain, targets the kidneys, specifically affecting their ability to concentrate urine.

When the body's water level is low, ADH secretion increases. This hormone then travels through the bloodstream to the kidneys, where it binds to receptors on the cells lining the collecting ducts and distal convoluted tubules—the final segments in the kidney's urine-forming pathway. Upon ADH binding, a series of intracellular processes are triggered that insert water channels, called aquaporins, into the cell membranes.

These aquaporins allow water to be reabsorbed from the urine back into the bloodstream, concentrating the urine and reducing water loss. The intricate balance that ADH maintains prevents dehydration and ensures that the body retains water during times of low intake or high loss, such as during exercise or when it's hot. Without adequate ADH function, the body could lose a dangerous amount of water, leading to a condition known as diabetes insipidus.

Renal Physiology

The physiology of the kidneys is complex, and their primary task is to filter the blood to remove waste products and excess substances, including water, while also ensuring valuable components like glucose, electrolytes, and amino acids are reabsorbed. This delicate task is carried out within the renal tubules, which are small tubes that process the initial filtrate coming from the glomerulus—a tiny blood-filtering structure in each nephron, which is the functional unit of the kidney.

Renal physiology encompasses multiple mechanisms that regulate blood pressure, blood volume, and blood composition. One part of this is osmoregulation—the control of the levels of water and mineral salts in the blood. ADH is directly involved in this process, its levels fluctuating based on the osmotic pressure sensed by the hypothalamus in the brain. This feedback loop ensures that the body's internal environment remains stable, a concept called homeostasis. Understanding renal physiology is vital to comprehend how hormones like ADH affect kidney function and the systemic impact on the organism's hydration status.

Distal Nephron Permeability

The distal nephron includes both the distal convoluted tubule (DCT) and the collecting duct. Normally, these structures are relatively impermeable to water, meaning they allow only a small percentage of water to be reabsorbed from the urine. However, the presence of ADH dramatically increases the water permeability of these segments.

ADH promotes the insertion of aquaporin-2 channels into the membrane of the renal cells lining the DCT and collecting ducts. With these additional channels, more water can be reabsorbed back into the bloodstream. This results in urine that is more concentrated and lower in volume, reflecting the body's conservation of water to maintain overall fluid balance.

Some diseases and conditions can affect the distal nephron's permeability independent of ADH, but under normal physiological conditions, ADH is a key regulator. The action of ADH on the distal nephron is an excellent illustration of hormone-receptor interaction and cellular response mechanisms that are fundamental to understanding renal and endocrine physiology.