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Acid-base map and compensatory mechanisms
Buffering and Henderson-Hasselbalch equation
Physiologic pH and buffers
The role of the kidney in acid-base balance
Plasma anion gap
Renal system anatomy and physiology
Body fluid compartments
Movement of water between body compartments
Measuring renal plasma flow and renal blood flow
Regulation of renal blood flow
TF/Px ratio and TF/Pinulin
Phosphate, calcium and magnesium homeostasis
Free water clearance
Kidney countercurrent multiplication
Distal convoluted tubule
Loop of Henle
Proximal convoluted tubule
Tubular reabsorption and secretion
Tubular reabsorption and secretion of weak acids and bases
Tubular reabsorption of glucose
Tubular secretion of PAH
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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.
Tubular reabsorption of glucose is an important process that occurs in the kidneys to maintain normal blood glucose levels. When blood is filtered through the glomeruli in the kidneys, glucose is freely filtered into the tubular fluid of the nephron. In a healthy individual, nearly all of this filtered glucose is reabsorbed back into the bloodstream through the proximal tubule of the nephron. The reabsorption of glucose is facilitated by glucose transporters, primarily SGLT2 and SGLT1, which move glucose from the tubular fluid into the cells and then into the bloodstream.
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