Nephron Components and Structure
📌 The nephron, the functional unit of the kidney, consists of the renal corpuscle, proximal convoluted tubule, Loop of Henle, distal convoluted tubule, and collecting duct.
⚖️ Each kidney contains approximately 1.25 million nephrons, totaling about 2.5 million nephrons per pair of kidneys.
🗺️ Nephrons are categorized as cortical (about 75%, with short Loops of Henle) or juxtamedullary (about 25%, with long Loops of Henle).

Loop of Henle Anatomy and Function
⬇️ The Loop of Henle is divided into the descending limb and the ascending limb (further divided into thin and thick segments).
💧 The primary function of the Loop of Henle is urine concentration, achieved by creating osmotic gradients in the medulla.
📈 Osmolarity gradients increase from the cortex (300 mOsmol) towards the innermost medulla, reaching up to 1200 mOsmol.

Mechanism for Creating Osmotic Gradients
🔄 The gradient is established via the countercurrent multiplier mechanism within the Loop of Henle.
➡️ Solutes ($\text{Na}^{+}$, $\text{K}^{+}$, $\text{Cl}^{-}$) are actively pumped out of the thick ascending limb into the medullary interstitium via the $\text{Na}^{+}-\text{K}^{+}-2\text{Cl}^{-}$ cotransporter (NKCC).
💧 The descending limb is highly permeable to water ($\text{H}_{2}\text{O}$) via aquaporins, causing water to exit into the hypertonic interstitium, concentrating the filtrate.
🛑 The ascending limb is impermeable to water but permeable to solutes, which move out, maintaining the required concentration difference.

Role of Vasa Recta (Countercurrent Exchanger)
🩸 The Vasa Recta (capillaries surrounding the loop) acts as a countercurrent exchanger to maintain the medullary osmotic gradient.
🔄 In the descending Vasa Recta, solutes ($\text{Na}^{+}$, $\text{Cl}^{-}$) move into the blood, and $\text{H}_{2}\text{O}$ moves out due to the high medullary concentration.
⬆️ In the ascending Vasa Recta, solutes are pumped out back into the interstitium, and $\text{H}_{2}\text{O}$ moves back in from the interstitium, preventing the loss of solutes and the washing out of the gradient.

Urea Recycling and Final Concentration
♻️ Urea recycling further contributes to medullary concentration, especially when $\text{ADH}$ is present.
🌊 When $\text{ADH}$ promotes water reabsorption in the distal tubule, concentrated urea ($\text{UT}1$ transporter) is diffused into the interstitium to help maintain the high osmotic gradient necessary for maximum urine concentration.

Key Points & Insights
➡️ Filtrate entering the proximal tubule is isotonic at approximately 300 mOsmol.
➡️ Hypertonic solutions have a higher solute concentration; hypotonic solutions have a lower solute concentration relative to plasma.
➡️ Urine concentration is the main purpose of the Loop of Henle, relying on the countercurrent multiplier to build steep medullary gradients.

📸 Video summarized with SummaryTube.com on Dec 21, 2025, 14:08 UTC