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 descending and ascending limb, and the distal convoluted tubule which empties into the collecting duct, which collects the urine.
So the proximal convoluted tubule comes right after the glomerulus and it’s where most of the reabsorption happens in the kidney. It’s called “convoluted” because it has a twisting path.
Zooming in on the proximal convoluted tubule, it’s lined by tubule cells which are also known as brush border cells.
On one side is the apical surface which faces the tubular lumen and is lined with microvilli.
Microvilli are tiny projections that increase the cell’s surface area to help it reabsorb more solutes or water.
On the other side is the basolateral surface, which faces the interstitium or the space between the tubule and the peritubular capillaries.
The peritubular capillaries run alongside the nephron and return solutes and water that were reabsorbed into the interstitium back into the circulation.
Various solutes like Na+, K+, Ca2+, Cl-, and Mg2+ get reabsorbed in the proximal convoluted tubule, and sodium is one of the most important ones because it helps during the reabsorption of other solutes as well as water.
Around 67% of the sodium that gets filtered into the tubular fluid is reabsorbed in the proximal convoluted tubule, and the remaining 33% goes on to the rest of the nephron.
The tubular fluid has about the same composition as the plasma, so it contains more sodium than the tubule cells, and its flows down its concentration gradient into the tubule cells using various channels.
Some of these channels are cotransporters, meaning they move two or more different solutes at a time.
Like the Na+-glucose cotransporter which moves Na+ in the direction of its concentration gradient, and uses that energy to move glucose into the cell up or against its concentration gradient, meaning there’s a higher concentration of glucose already in the cell than in the lumen.
Once inside, the glucose quickly leaves the tubule cell via two channels called GLUT1 and GLUT2 on the basolateral surface.
That movement is passive and doesn’t require energy because the glucose concentration in the interstitium and blood is relatively low.
Almost all of the glucose, lactate, and amino acids, and most of the phosphate and citrate that’s filtered by the glomerulus is reabsorbed with the help of sodium in the proximal convoluted tubule.
Meanwhile, Na+ is pumped out through the basolateral side into the interstitium by a solute pump called the Na+/K+ ATPase.