Question

Which of the following statements is/are incorrect regarding the collecting duct? (i) It extends from the cortex to medulla. (ii) Large amount of water could be reabsorbed by it to produce concentrated urine. (iii) Small amount of urea diffuses into it from the medulla to keep up the osmolarity. (iv) It plays a role in maintaining \(\mathrm{pH}\) and ionic balance of blood by the selective secretion of \(\mathrm{H}^{+}\)and \(\mathrm{K}^{+}\)ions. (a) Only (i) (b) Only (iii) (c) (ii) and (iii) (d) (i) and (iv)

Short answer

The incorrect statement is (i), as the collecting duct does indeed extend from the cortex to the medulla, which is a fundamental aspect of its function.

Step-by-step solution

Understanding the Structure of Collecting Duct

Know that the collecting duct is part of the kidney's nephron that extends from the cortex through the medulla. It serves to concentrate urine by reabsorbing water when antidiuretic hormone (ADH) is present.

Analyzing Water Reabsorption

Recognize that statement (ii) is correct because the collecting duct reabsorbs a large amount of water, which is regulated by ADH, leading to the production of concentrated urine.

Examining Urea Diffusion

Understand that statement (iii) refers to the counter-current exchange mechanism which maintains the osmolarity gradient in the medulla. A small amount of urea does indeed diffuse into the collecting duct.

Evaluating pH and Ionic Balance

Statement (iv) is correct as the collecting duct plays an essential role in the body's acid-base balance by selectively secreting \(H^{+}\) ions and in potassium balance by the selective secretion of \(K^{+}\) ions.

Identifying the Incorrect Statement

Since statements (ii), (iii), and (iv) are all correct, statement (i) must be the incorrect one. It should also be known that 'only' is a strong word and that its presence implies that all other options would be correct, which matches our analysis.

Key concepts

Nephron Structure

The nephron is the fundamental functional unit of the kidney, crucial for purifying blood and producing urine. Each nephron consists of two main parts: the renal corpuscle and the renal tubule. The renal corpuscle is made up of the glomerulus, a tangled cluster of blood capillaries, and the Bowman's capsule, which encloses it. This structure is where the initial filtration of blood happens, capturing water, ions, glucose, and waste products.

From here, the filtered fluid enters the renal tubule, which is a long, convoluted structure that reabsorbs necessary substances back into the bloodstream and leaves waste materials to form urine. The tubule is divided into several segments: the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and finally, the collecting duct. The collecting duct is not a part of the nephron itself but is connected to many nephrons. It channels the urine to the ureters, leading to proper waste elimination from the body. This intricate structure facilitates complex processes of waste removal and reabsorption essential for maintaining the body's homeostasis.

Urine Concentration

Concentrating urine is a vital kidney function, allowing the body to retain water and maintain hydration balance. This process occurs primarily in the collecting duct, where the urine's final concentration is adjusted. The presence of antidiuretic hormone (ADH) plays a key role in this mechanism. When ADH levels are high, the walls of the collecting duct become more permeable to water, leading to greater water reabsorption into the surrounding blood vessels and, consequently, more concentrated urine.

Conversely, when ADH is low, less water is reabsorbed, resulting in more dilute urine. This ability to concentrate urine is vital for survival, particularly in situations where water intake is limited or the body is losing excess water, such as during heavy sweating. Effective urine concentration requires an intricate balance of solute and water reabsorption, all orchestrated with precision within the nephron and collecting duct.

Counter-Current Exchange Mechanism

The counter-current exchange mechanism is a sophisticated process occurring within the kidneys that efficiently conserves water and salt. It involves the interaction between the flow of filtrate through the loop of Henle and the flow of blood through the vasa recta, the capillary network surrounding the loop.

The loop of Henle's descending limb is permeable to water but not to salt, allowing water to be reabsorbed into the blood. Meanwhile, the ascending limb is impermeable to water but actively pumps salt into the surrounding tissue, creating a high osmolarity in the kidney medulla. This osmotic gradient encourages water reabsorption from the collecting duct, facilitated by the presence of ADH.

The counter-current multiplier intensifies the concentration gradient, and the counter-current exchanger, involving the vasa recta, preserves this gradient without washing away the accumulated salt. This tandem action of loop and capillaries ensures maximum efficiency in urine concentration, waste removal, and conservation of vital solutes.

Acid-Base Balance in Kidneys

Maintaining the acid-base balance is another crucial kidney function, primarily handled by the collecting duct. Kidneys regulate blood pH levels by either reabsorbing bicarbonate (HCO3^-) from the urine or secreting hydrogen ions (H^+) into the urine. These mechanisms effectively neutralize excess acids or bases, thus stabilizing the body's pH.

When blood pH drops and becomes more acidic, the kidneys respond by increasing (H^+) secretion and reabsorbing more HCO3^-. Conversely, an alkaline pH prompts the kidneys to withhold HCO3^- reabsorption and diminish H^+ secretion. The kidneys also handle electrolyte balance, particularly with potassium (K^+) ions, which is tightly linked to the body's overall acid-base homeostasis. The precise orchestration of these processes in the collecting duct underscores the organ's critical role in sustaining the delicate chemical equilibrium necessary for normal bodily function.