Which hormone in the body is responsible for water reabsorption from the kidney tubules?
calcitonin
aldosterone
atrial natriuretic peptide
antidiuretic hormone
The Correct Answer is D
Rationale:
A. Calcitonin is a hormone produced by the parafollicular cells of the thyroid gland. Its primary role is to reduce elevated blood calcium levels by inhibiting osteoclast activity and increasing calcium excretion by the kidneys. While calcitonin influences calcium and phosphate balance, it has no significant function in regulating water reabsorption in the kidneys. Disorders affecting calcitonin levels do not cause abnormalities in fluid balance or urine concentration, which makes it unrelated to the physiologic mechanism of water conservation.
B. Aldosterone is a mineralocorticoid hormone released by the adrenal cortex. It acts on the distal tubules and collecting ducts of the kidney to increase sodium reabsorption and potassium excretion. Because water follows sodium osmotically, aldosterone can indirectly contribute to water retention. However, aldosterone does not directly change the kidney’s permeability to water. Water reabsorption in the presence of aldosterone still depends on antidiuretic hormone (ADH) to insert aquaporin channels into the renal tubules. Therefore, aldosterone cannot be considered the hormone responsible for water reabsorption itself; it influences sodium handling rather than direct water movement.
C. Atrial natriuretic peptide (ANP) is a hormone released by the atria of the heart when they are stretched due to increased blood volume. ANP has the opposite effect of hormones that promote fluid retention. It increases sodium and water excretion by dilating the afferent arteriole, increasing glomerular filtration rate, and inhibiting sodium reabsorption in the collecting ducts. It also antagonizes aldosterone and renin, promoting further loss of sodium and water. Because ANP enhances diuresis rather than water conservation, it does not play a physiological role in promoting water reabsorption. Instead, it helps reduce blood volume and blood pressure.
D. Antidiuretic hormone (ADH), also called vasopressin, is the hormone directly responsible for regulating water reabsorption from the kidney tubules. ADH is synthesized in the hypothalamus and stored in the posterior pituitary, where it is released in response to increased plasma osmolality or decreased circulating blood volume. Once released, ADH binds to V2 receptors in the distal tubules and collecting ducts of the kidneys, triggering the insertion of aquaporin-2 water channels into the tubular membrane. This action increases the kidney’s permeability to water, allowing water to be reabsorbed back into the bloodstream independently of sodium. As a result, urine becomes more concentrated and plasma osmolality decreases. Clinically, ADH is essential for maintaining proper fluid balance, and abnormalities in its production or response, such as in diabetes insipidus or SIADH, lead to serious disturbances in hydration and serum sodium levels. ADH is therefore the primary and direct regulator of renal water reabsorption.
Nursing Test Bank
Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is A
Explanation
Rationale:
A. In end-stage renal disease (ESRD), the kidneys lose the ability to produce sufficient erythropoietin (EPO), a hormone that stimulates the bone marrow to produce red blood cells. Without adequate EPO, erythropoiesis is impaired, leading to a reduced number of circulating red blood cells and anemia. This anemia is typically normocytic and normochromic, meaning the red blood cells are of normal size and color but decreased in number.
B. ESRD does not reduce red blood cell destruction. In fact, uremic toxins in ESRD may actually shorten the lifespan of red blood cells, contributing further to anemia rather than preventing it.
C. Increased erythropoietin production is incorrect. In ESRD, erythropoietin production is diminished, not increased. The lack of EPO is the primary reason for decreased red blood cell production in these patients.
D. Elevated hemoglobin levels is incorrect. Hemoglobin levels are typically decreased in ESRD due to the reduced erythropoiesis and shortened red blood cell lifespan. Elevated hemoglobin is not a feature of anemia associated with kidney disease.
Correct Answer is A
Explanation
Rationale:
A. Proteinuria is the hallmark finding in nephrotic syndrome. This condition results from increased permeability of the glomerular basement membrane, allowing large amounts of protein, primarily albumin, to leak into the urine. The loss of protein in urine leads to foamy or frothy urine, which is often one of the first noticeable signs. Proteinuria also contributes to hypoalbuminemia, resulting in decreased plasma oncotic pressure, fluid shifting into interstitial spaces, and subsequent edema, commonly seen in the face, periorbital area, and lower extremities.
B. Nephrotic syndrome typically causes hyperlipidemia, not a decrease. The liver compensates for low plasma oncotic pressure by increasing lipoprotein synthesis, leading to elevated cholesterol and triglyceride levels in the blood.
C. On the contrary, nephrotic syndrome causes hypoalbuminemia due to excessive urinary loss of albumin. Low serum albumin contributes directly to the development of edema.
D. Urine output is often normal or may even be reduced in nephrotic syndrome. The condition is characterized by protein loss, not an increase in volume output. Fluid retention is more common due to hypoalbuminemia and sodium retention.
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