If we take a cross-section of the kidney, there are two main parts, the outer cortex and the inner medulla.
If we zoom in, there are millions of tiny tubes called nephrons which go from the outer cortex down into the medulla and back out into the cortex again.
Nephrons filter out harmful substances in the blood so that we can excrete them into the urine.
Each nephron is made up of the glomerulus, or a tiny clump of capillaries, where blood filtration begins.
These capillaries have very thin walls and they act like a coffee filter. Red blood cells and proteins are large and stay in the capillaries whereas blood plasma and smaller particles get filtered out.
This filtrate, called tubular fluid, collects in a cup shaped structure containing the glomerulus called the Bowman's capsule.
Together, the glomerulus and the Bowman’s capsule make up the renal corpuscle.
The Bowman’s capsule is connected to the renal tubule which has a few segments: the proximal convoluted tubule, the U- shaped loop of Henle with a thin descending, a thin ascending limb and a thick ascending limb, and finally the distal convoluted tubule which empties into the collecting duct, which collects the urine.
Zooming in on the U shaped loop of Henle, it’s lined by epithelial cells.
On one side is the apical surface which faces the tubular lumen, and on the other side is the basolateral surface, which faces the interstitium between the tubule and the peritubular capillaries.
The peritubular capillaries run alongside the nephron, and the capillaries that run along the loop of henle are called vasa recta.
Solutes and water that are reabsorbed into the interstitium go into the vasa recta and re-enter circulation.
Now, when tubular fluid leaves the proximal tubule, it has an osmolarity of around 300 mOsm/L which is the same as the osmolarity of the interstitial fluid around the tubule.
The tubular fluid mainly contains water, sodium, potassium, chloride, calcium, and urea.
When it travels down the descending limb, the osmolarity in the medullary interstitium increases rapidly, from 300 mOsm/L at the top to 1200 mOsm/L at the bottom, where the loop bends.
Most of that osmolarity in the interstitium is due to the large quantities of sodium as well as some urea.
Now the squamous epithelial cells that line the thin descending loop have a lot of aquaporin proteins on the apical and basolateral sides.
These aquaporin proteins are channels that only allow water to pass through.