Proximal convoluted tubule

Definitions & Key takeaways

The proximal convoluted tubule (PCT) is a segment of the renal tubule responsible for the reabsorption and secretion of various solutes and water. The PCT is located in the renal cortex, the outer part of the kidney, and is the first segment of the renal tubule, where it receives the filtrate from the renal corpuscle.

The main function of the PCT is to reabsorb water and solutes like sodium, which is continuously pumped into the interstitium to create a gradient that allows many other solutes and even water to be reabsorbed by tubule cells. Certain metabolic byproducts and medications are also secreted into the tubular fluid by the proximal convoluted tubule and excreted in urine.

Chapters:

Watch video only

Introduction0:00–0:25

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.

Nephron0:25–1:30

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.

Proximal convoluted tubule1:30–3:34

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.

Glucose Reabsorption3:34–3:46

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.

Sodium3:46–5:26

Meanwhile, Na+ is pumped out through the basolateral side into the interstitium by a solute pump called the Na+/K+ ATPase.
The Na+/K+ ATPase pumps 3 Na+ out of the cell and lets 2 K+ ions into the cell from the interstitium. Since there’s more Na+ in the interstitium and more K+ in the cell, this goes against both of their concentration gradients and that’s why the pump requires ATP.
So the Na+/K+ ATPase on the basolateral surface keeps the intracellular Na+ level low, and that allows more Na+ to flow into the tubule down its concentration gradient on the apical surface.
In fact, some Na+ also moves directly from the tubular fluid into the interstitium without going through the cell at all.
This is called the paracellular route, which is where solutes and water travel through the lateral intercellular space, which is the narrow space between adjacent tubule cells.
Normally, tight junctions are made of proteins called claudin proteins that fuse part of the membranes of two adjacent cells together, forming a junction impermeable to solutes and water.
However, in the proximal convoluted tubule, there are leaky tight junctions within this space. Leaky tight junctions in the proximal tubules have fewer claudine proteins and as a result the membranes are not as tightly fused which allows solutes to slip between two cells.
When sodium ions slip through cells, they get into the interstitium, and from there, they diffuse into the capillary down its concentration gradient.

Bicarbonate5:26–6:48

Another important cotransporter on the apical surface is the Na-H+ exchanger which allows one sodium ion into the cell in exchange for one hydrogen ion out of the cell.
And this process ends up helping with bicarbonate (HCO3-) reabsorption. When a molecule of bicarbonate approaches the apical surface of the brush border cell it binds to hydrogen to form carbonic acid.
At that point, an enzyme called carbonic anhydrase type 4 which lurks in tubule among the microvilli like a shark, swims along and splits the carbonic acid into water and carbon dioxide.
The overall equation looks like this: H+ + HCO3- <-> H2CO3 <-> H2O + CO2 The water and carbon dioxide happily diffuse across the membrane into the cells where carbonic anhydrase type 2 facilitates the reverse reaction - combining them to form carbonic acid, which dissolves into bicarbonate and hydrogen.
A sodium bicarbonate cotransporter on the basolateral surface snatches up the bicarbonate and a nearby sodium, and shuttles both into the blood.
Meanwhile, that sodium-hydrogen exchanger on the apical surface, pulls sodium into the cell, while pushing hydrogen back into the tubule.
So at the end of the day, there’s a movement of bicarbonate from the tubule to the blood. Around 85% of the filtered bicarbonate is reabsorbed in the proximal convoluted tubule.

Urea6:48–7:03

Some solutes like Urea and water can simply diffuse across the cell down its concentration gradient into the interstitium and then into the capillaries.
About 50% of the filtrated urea is reabsorbed this way. Besides reabsorption of important material from the tubular fluid, the proximal convoluted tubule also secretes many substances into the tubular fluid.

Ammonium7:03–7:31

One of the major ones is NH4+ (ammonium) which is a harmful byproduct when the amino acid glutamine is broken down inside tubule cells to create new HCO3-.
The NH4+ is secreted into the tubular fluid while the HCO3- is reabsorbed back into the blood. Other harmful material like organic acids and medication like penicillin diffuse out of the peritubular capillary, gets transported into the tubule cells by transport proteins and gets secreted into the tubular fluid to be excreted.

Other materials7:31–7:46

Review7:46–8:22

Alright, as a quick recap, each nephron is made up of the glomerulus, a proximal convoluted tubule, a loop of Henle, a distal convoluted tubule and collecting ducts.
In the proximal convoluted tubule, the main function is to reabsorb water and solutes like sodium. Sodium is continuously pumped into the interstitium to create a gradient which allows many other solute and even water to be reabsorbed by tubule cell.
Certain metabolic byproducts and medications are also secreted into the tubular fluid by the proximal convoluted tubule and excreted in urine.