Question

The countercurrent exchange in the vasa recta a. removes \(\mathrm{Na}^{+}\)from the extracellular fluid. b. maintains high concentrations of \(\mathrm{NaCl}\) in the extracellular fluid. c. raises the concentration of \(\mathrm{Na}^{+}\)in the blood leaving the kidneys. d. causes large quantities of \(\mathrm{Na}^{+}\)to enter the filtrate. e. does all of these.

Short answer

The correct answer is option b. The countercurrent exchange in the vasa recta maintains high concentrations of \(\mathrm{NaCl}\) in the extracellular fluid, which is essential for the concentration and dilution of urine as well as maintaining the concentration gradient in the medulla.

Step-by-step solution

Introduction

In order to answer this question correctly, it is important to understand the role of the vasa recta and the countercurrent exchange mechanism in the kidney. The vasa recta is a set of capillaries that surround the loops of Henle in the kidneys. The countercurrent exchange mechanism is the process by which the vasa recta allow the exchange of substances necessary for the concentration and dilution of urine, as well as maintaining the concentration gradient in the medulla.

Option a

The countercurrent exchange in the vasa recta does not directly remove \(\mathrm{Na}^{+}\) from the extracellular fluid. This option is incorrect since the removal of \(\mathrm{Na}^{+}\) primarily happens in the nephron.

Option b

The countercurrent exchange helps maintain high concentrations of \(\mathrm{NaCl}\) in the extracellular fluid, especially in the renal medulla. This creates an osmotic gradient which is essential for the concentration of the filtrate in the nephron. Based on this information, option b seems to be a correct answer.

Option c

The countercurrent exchange mechanism does not raise the concentration of \(\mathrm{Na}^{+}\) in the blood leaving the kidneys. In fact, the kidneys help regulate the concentration of \(\mathrm{Na}^{+}\) in the blood by reabsorbing or excreting \(\mathrm{Na}^{+}\) depending on the body's needs. Thus, option c is incorrect.

Option d

The countercurrent exchange does not cause large quantities of \(\mathrm{Na}^{+}\)to enter the filtrate. The primary site for the entry of \(\mathrm{Na}^{+}\) into the filtrate is the renal corpuscle, specifically in the glomerulus where the blood is filtered. Therefore, option d is incorrect.

Option e

Since options a, c, and d are incorrect, option e cannot be the correct answer.

Conclusion

Based on the analysis of each option, the countercurrent exchange in the vasa recta maintains high concentrations of \(\mathrm{NaCl}\) in the extracellular fluid. Therefore, the correct answer is option b.

Key concepts

Renal Physiology

Renal physiology is the study of kidney function, an essential aspect of human biology that ensures the maintenance of fluid and electrolyte balance and the excretion of waste products. The kidneys perform these tasks through various structures and complex mechanisms, one of which is the countercurrent exchange system involving the vasa recta.

The vasa recta are specialized blood vessels that play a crucial role in concentrating urine. They run in parallel alongside the loops of Henle, where their anatomical arrangement and the counterflow of blood allow them to preserve the osmotic gradient established by these loops. Understanding this vital interplay is fundamental to comprehending how kidneys conserve water while eliminating waste, a process pivotal to renal physiology.

Urine Concentration

The concentration of urine is a key function of the kidney and is vital for maintaining homeostasis. It involves the removal of excess substances from the blood and the reabsorption of water to prevent dehydration.

The process is tightly regulated and mediated by the countercurrent mechanism, which includes both the loop of Henle and the vasa recta. This mechanism leverages the osmotic differences between the filtrate inside the loops and the interstitial fluid outside them to efficiently reabsorb water and solutes like sodium and chloride. The countercurrent exchange in the vasa recta ensures that these substances are cycled back into the bloodstream without disrupting the osmotic gradient.

Kidney Function

Kidney function encompasses various tasks, from filtering blood to regulating blood pressure and pH levels. They achieve this elaborate role through a combination of filtration, reabsorption, secretion, and excretion processes.

The kidneys have about a million nephrons each, where blood filtration occurs. Following filtration, the filtrate moves through the tubular system of the nephron. Here, needed substances are reabsorbed back into the blood, and unwanted residues are secreted into the urine. The renal countercurrent mechanism, specifically involving the vasa recta, is particularly crucial for managing the concentration of substances within the kidneys and urine. It is a prime example of the efficiency and intricacy of kidney function.

Osmotic Gradient in Kidney

The osmotic gradient in the kidney, mainly in its medullary region, is essential for the concentration of urine. This gradient refers to the difference in solute concentration between the kidney's interstitial fluid and the filtrate within the nephron.

This gradient is established and maintained by the countercurrent multiplication system of the loops of Henle and preserved by the countercurrent exchange of the vasa recta. It is within this finely tuned environment that the kidneys can reabsorb water to produce urine that is concentrated or dilute, based on the body's hydration needs. Through this process, the kidneys manage the delicate balance of fluid and electrolyte homeostasis.