Question

The ability of producing concentrated (hypertonic) urine in vertebrates generally depends on (a) area of Bowman's capsule epithelium (b) length of the proximal convoluted tubule (c) length of Henle's loop (d) capillary network forming glomerulus.

Short answer

The ability of producing concentrated urine generally depends on (c) the length of Henle's loop.

Step-by-step solution

Understanding Kidney Function

To solve this question, first understand that the kidneys function to filter blood, remove waste, and balance electrolytes. Within the kidneys, structures like Bowman's capsule, the proximal convoluted tubule, Henle's loop, and the glomerulus are involved in the urine formation process. Each part plays a specific role in filtration, reabsorption, and concentration of urine.

Identifying the Site of Urine Concentration

Determine which part of the nephron is primarily responsible for creating hypertonic or concentrated urine. This involves the reabsorption of water and electrolytes, making the filtrate more concentrated as it moves through the nephron.

Linking Structure to Function

Recognize that the length of Henle's loop plays a critical role in the concentration of urine. A longer loop provides a greater surface area for reabsorption and descending and ascending limbs that facilitate counter-current multiplication, which is essential for producing concentrated urine.

Eliminating Incorrect Options

Exclude the other listed options as they do not primarily contribute to urine concentration: (a) Area of Bowman's capsule does not directly affect concentration; (b) Proximal convoluted tubule is mainly involved in reabsorption but not in concentrating urine; (d) Glomerulus is responsible for blood filtration, not urine concentration.

Key concepts

Henle's Loop Function

The Henle's loop is a critical segment of the nephron, the basic functional unit of the kidney, chiefly involved in the concentration of urine.

This U-shaped structure comprises a descending limb that plunges into the medulla of the kidney and an ascending limb that returns to the cortex. The primary function of Henle's loop is to create a gradient of salt concentration in the kidney medulla, which is cardinal for urine concentration.

It achieves this through a process called counter-current multiplication. The descending limb is permeable to water but not to salts, allowing water to be reabsorbed into the bloodstream. In contrast, the ascending limb is permeable to salts but not to water, facilitating the reabsorption of ions, particularly sodium and chloride, which increases the osmolarity of the surrounding interstitial fluid while the filtrate itself becomes more dilute.

Due to these unique properties, Henle's loop ensures the reabsorption of vital components and enables the kidneys to produce urine that can be much more concentrated than the original blood filtrate, an essential mechanism for conserving water in terrestrial vertebrates.

Kidney Filtration Process

The kidney filtration process is a sophisticated mechanism that starts in the glomerulus, a network of capillaries enclosed by the Bowman's capsule.

During filtration, blood pressure forces water and solutes out of the blood and into the capsule, forming a filtrate that is essentially free of proteins and cells. This filtrate then travels through various parts of the nephron where different sections, including the proximal convoluted tubule, Henle's loop, and the distal convoluted tubule, each have specialized roles in modifying the filtrate by reabsorption of water, ions, and nutrients as well as secretion of wastes.

The proximal tubule reabsorbs a significant portion of water and solutes from the filtrate, but it is through Henle's loop that the filtrate begins to be concentrated, setting the stage for the creation of urine that is finely tuned to the body's hydration and electrolyte needs.

Counter-Current Multiplication

Counter-current multiplication is a process occurring within Henle's loop that underpins the kidney's ability to concentrate urine. It hinges on the anatomy and differential permeability of Henle's loop's limbs.

The counter-current designation comes from the fact that the filtrate flows in opposite directions in the loop's descending and ascending limbs. This countercurrent flow creates and maintains a gradient of increasing solute concentration as one moves deeper into the medulla, which is crucial for water reabsorption from the descending limb.

Interplay Between Limbs

The ascending limb actively transports salts into the interstitial fluid, which elevates its osmolarity. As the descending limb is permeable to water, water exits the filtrate into the hyperosmotic medulla. This exiting water is reabsorbed into the bloodstream, thus conserving water and allowing for a concentrated urine to be excreted, which is particularly vital for survival in water-scarce environments.

Nephron Structure and Function

A nephron is the kidney's microscopic filtering unit, each kidney containing around a million of them. Each nephron's structure is tailored to its function in processing blood filtrate into urine.

It begins with the renal corpuscle, composed of the Bowman's capsule and the glomerulus. Here, blood pressure forces fluid out of the blood and into the nephron. This filtrate then progresses through the proximal convoluted tubule, where the majority of reabsorption occurs, then into Henle's loop for concentration, followed by the distal convoluted tubule, and finally to the collecting duct where further water reabsorption is regulated by hormones such as antidiuretic hormone (ADH).

Integration of Functions

The integration of the different parts of the nephron is essential for maintaining fluid and electrolyte balance. These structures work together to filter out toxins, waste, and excess ions from blood, and to conserve necessary substances, including water, resulting in urine that is excreted from the body. Efficient nephron function is fundamental to the homeostasis of the body's internal environment and crucial for overall health.