Loop of Henle Function and Urine Concentration

Establishes a concentration gradient in the kidney's medulla, enabling the production of concentrated or dilute urine.

The difference in solute concentration (osmolarity) between the cortex and the medulla of the kidney.

Permeable to water but not solutes, allowing water to move out of the tubule into the surrounding interstitial fluid.

Impermeable to water, but actively pumps out solutes (e.g., salt) into the interstitial fluid, creating low-water concentration.

The process where the opposite flow of fluid in the ascending and descending limbs of the loop of Henle amplifies the concentration gradient.

A protein that transports sodium, potassium, and chloride into the cells, essential for solute reabsorption in the ascending limb and generating the gradient.

Reducing the concentration of urine by reducing the water reabsorption process.

Increasing urinary concentration by reabsorbing water due to high interstitial osmolarity in the medulla.

Protect the concentration gradient by minimizing exchange of solutes from the capillaries into the medulla.

Solute reabsorption rate, flow rate of tubular fluid, and blood flow rate through vasa recta impact the corticomedullary gradient.

A kidney condition where medication allergies cause fluid buildup around the kidney tubules and blood vessels, impacting urine concentration and tubular function.

Cells in the kidney's outer medulla that can contract, found in the vasa recta.

Drugs that inhibit NKCC2, reducing solute reabsorption in the loop of Henle, impacting urine concentration and dilution.

A disease causing electrolyte imbalances (K+ and HCO3-), low blood pressure, and difficulty concentrating urine, due to mutations in ion channels.

Urea helps create a medullary gradient in the kidney, essential for concentrating urine, and removing waste.

The measure of solute concentration in urine (mOsm/kg).

A hormone that controls water reabsorption in the collecting ducts of the kidney, affecting urine volume and concentration.

A protein in cell membranes that allows for water reabsorption in the kidney.

Permeable to water, allowing it to move out into the hypertonic interstitium, increasing the tubular fluid's osmolality.

Impermeable to water, but actively pumps out solutes, creating a hypotonic tubular fluid.

ADH increases water permeability in the collecting ducts, allowing for further water reabsorption and concentrating urine.

The thick ascending limb of Henle (TALH) actively reabsorbs sodium, potassium, and chloride ions. This process is driven by the basolateral Na+/K+ ATPase, which pumps sodium out of the cell, creating a concentration gradient. Sodium and chloride then move down this gradient from the tubular fluid into the cell, while potassium is actively transported into the cell and then leaks back into the tubular fluid.

Loop diuretics like furosemide block the NKCC2 transporter in the TALH, preventing the reabsorption of sodium, potassium, and chloride. This inhibits the formation of a concentrated medullary gradient, resulting in dilute urine output.

The vasa recta capillaries in the outer medulla contain pericytes, which can contract to regulate blood flow. Deeper in the medulla, vasa recta have fenestrations (pores) to facilitate the exchange of solutes between the interstitial fluid and the lumen, maintaining the medullary concentration gradient.

Urea plays a crucial role in establishing the medullary concentration gradient. It is freely filtered at the glomerulus, reabsorbed by the proximal tubule and inner medullary collecting duct, and secreted by specific segments of the nephron. This movement of urea helps to create an even steeper gradient, particularly in the inner medulla, which is essential for urine concentration.

Uosm refers to the concentration of solutes in urine, measured in mOsm/kg. V represents the volume of urine produced. The distal tubule and collecting duct have the greatest influence on both Uosm and V.

When vasopressin (ADH) is high, the distal tubule and collecting duct become permeable to water due to the insertion of aquaporin 2 (AQP2) channels. This allows water to be reabsorbed from the tubular fluid, resulting in concentrated urine and minimal volume. Uosm can reach 1200-1500 mOsm/kg, while V drops to ~0.5 L/day.

When ADH levels are low, the distal tubule and collecting duct are impermeable to water, leading to minimal reabsorption of water. This results in diluted urine and maximal volume. Uosm drops to 40-50 mOsm/kg, while V increases to ~28 L/day.

Several factors influence the steepness of the medullary concentration gradient, including the rate of solute reabsorption in the TALH, flow rate of tubular fluid, and blood flow through the vasa recta. These factors collectively contribute to the efficiency of urine concentration.