Glucose is found in almost every food we eat, like bread, potatoes, or fruit. Once it’s absorbed by the body, it’s converted into a source of energy.
The body needs the plasma glucose levels to remain within a pretty narrow range, between 70 mg/dl to 110 mg/dl, when you’ve had nothing to eat and less than 140 mg/dl after a meal.
Now, the entire blood volume is about 5 liters, and the plasma volume is about 3 liters of that.
The kidneys filter the entire plasma volume 60 times a day, which means that means our kidneys filter approximately 180 liters of plasma each day!
If each liter of plasma contains about 1 g of glucose, this means about 180 g of glucose get filtered by the kidneys per single day. That’s the filtration rate of glucose.
If you had a blood glucose concentration of 1.5 g of glucose per L, you’d end up with a filtration rate of glucose of 270 g / day. Essentially, the higher the plasma glucose concentration, the more glucose will get filtered.
If we wanted to illustrate that in a graph, with plasma glucose concentration on the x axis and glucose filtration rate on the y axis, we would see that as the plasma glucose concentration increases, the filtered load of glucose increases linearly.
Now, looking at the kidney, specifically inside the kidney, there are two main parts, the outer cortex and the inner medulla.
If we zoom in, there are millions of tiny functional units called nephrons which go from the outer cortex down into the medulla and back out into the cortex again.
Each nephron is made up of the glomerulus, or a tiny clump of capillaries, where blood filtration begins.
When glucose enters the glomerulus, some of it gets filtered into the renal tubule.
Zooming in on one of these renal tubules, each one is lined by brush border cells which have two surfaces.
One is the apical surface which faces the tubular lumen and is lined with microvilli, which are tiny little projections that increase the cell’s surface area to help with solute reabsorption.
The other is the basolateral surface, which faces the peritubular capillaries, which run alongside the nephron.
Now, the body needs glucose and doesn’t want glucose getting lost in the urine, so it tries to reclaim this filtered glucose right away, in the first segment of the renal tubule, known as the proximal convoluted tubule.
Now, more than 99% of the filtered load of glucose is reabsorbed back into the circulation. But, that doesn’t just happen, there are, obviously, a couple steps to take to to accomplish this.
First, the glucose needs to crosses the apical surface of the renal tubule cells. But normally, the glucose concentration inside the cells is much higher than that inside the tubule, so for glucose to cross the apical surface requires energy.
Fortunately, the electrochemical gradient of sodium drives it to move inside the cell, and that sodium gradient is sufficient to pull glucose into the tubule cell as well.