8th February 2024

Proximal tubules

Overview

The proximal tubule is responsible for all glucose and amino acid reabsorption and 90% bicarbonate reabsorption via Na+/H+ antiporters. PTH, vitamin D and FGF-23 act on the proximal tubule to regulate phosphate reabsorption. Urea is also reabsorbed here and is done so in higher volumes during periods of dehydration. Half of the sodium filtered at the glomerulus is reabsorbed here via Na+/K+ ATPase channels. Water is also reabsorbed following the movement of the solutes. 

Image Source: Wikimedia Commons (2009). Proximal convoluted tubule., Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/f/f1/Proximal_convoluted_tubule.jpg (Accessed: 4 January 2024).

The renal tubule has both secretory and resorptive functions that require high levels of energy – this makes the renal tubules particularly susceptible to ischaemia during periods of poor perfusion. In prolonged poor blood flow periods, ongoing ischaemia can lead to cell death and cell shedding from the tubular basement membrane. Cell shedding can lead to the formation of casts, which may then obstruct the nephron. 

Tubular cells are also vulnerable to several substances. Medications, such as vancomycin, gentamicin, antivirals, and contrast mediums, are toxic to the tubular cells. Myoglobin (released during rhabdomyolysis), uric acid (released by cancer cells following chemotherapy), and Bence-Jones Proteins (produced in multiple myeloma) are also directly damaging. When injured, the tubular cells’ ability to secrete and reabsorb declines.

Alongside reabsorption, the proximal tubule is also responsible for secreting uric acid, creatinine and several drugs, including trimethoprim and diuretics.

References

Kumar, P. and Clark, M., 2012. Kumar & Clark’s clinical medicine. 8th ed.

Fairweather, J., Findlay, M., & Isles, C. (2020). Clinical companion in nephrology. Springer International Publishing. 

Tortora, G. J., & Derrickson, B. (2017). Tortora’s principles of anatomy and physiology. John Wiley and Sons.