The human kidney is made up of about a million nephrons, the filtering units of this complex and highly vascular organ. Each nephron is composed of a highly coiled tubule, one end of which forms a cup-shaped structure.
Inside this cup and forming a network around its walls, is a tuft of capillaries called a glomerulus, with a special fenestrated basement membrane.
Each glomerulus filters out water and solutes from the blood passing through it into the surrounding space, which is the cavity between the two walls of the cup. This part is wholly within the renal cortex.
The next part of the tubule is highly coiled (the proximal convoluted tubule) and empties into a U-shaped loop which carries the filtered fluid deep down into the medulla and then back again into the cortex. This part of the nephron is called the loop of Henle.
Its main function is to reabsorb water and sodium chloride from the filtrate. This conserves water for the organism, producing highly concentrated urine.
Cross section through the kidney with detail of the kidney tubule (nephron) and related blood supply. Image Copyright: Blamb / Shutterstock
The loop of Henle has a thin descending limb, a thick ascending limb and a thin ascending limb.
The fluid entering the descending limb contains sodium chloride and other salts, urea and other chemicals that have been filtered out from the blood.
The cells here are permeable to water and thus the salt and urea concentration rises within the fluid by the time it reaches the bend.
The ascending limb is permeable to sodium chloride, which passes out of the tubule into the medullary tissue surrounding it.
The absorption of water within the descending limb leads to an increasing osmotic gradient within the tubule by increasing the solute concentration.
Simultaneously, the active transport of sodium chloride into the interstitium through the water-impermeable cells of the ascending limb maintains the gradient by ensuring that the concentration declines as the fluid passes towards the distal convoluted tubule.
The loop of Henle is supplied by two vasa recta which are straight vessels closely accompanying the tubule’s hairpin-shaped course.
These carry blood in opposite directions, just like the passage of tubular fluid – the countercurrent mechanism. Thus they absorb water on the one hand and solute on the other.
This is called the countercurrent multiplier system, which is responsible for maintaining an osmotic medullary gradient in the outer medullary tissue. This gradient ensures the vasopressin-driven reabsorption of water from the luminal fluid in the collecting duct.
The thick ascending limb expresses a sodium-potassium-chloride cotransporter and helps reabsorb approximately a third of the filtered sodium and chloride from the fluid in the tubular lumen into the blood.
Other functions of the loop of Henle include:
- Homeostatic mechanisms to regulate the extracellular fluid volume
- Regulating potassium, calcium and magnesium excretion at the lowest energy costs
- Homeostasis of the acid-base balance via bicarbonate and ammonia excretion
- Regulating the composition of urinary protein