8th February 2024

Other functions

Erythropoiesis

Erythropoietin is secreted by the peri-tubular cells in response to hypoxia. This stimulates erythropoiesis within the bone marrow to produce red blood cells. Low levels of erythropoietin result in a reduced number of red blood cells produced with abnormal size and iron content. Uraemia also has a toxic effect on precursor cells within the bone marrow; therefore, waste excretion is essential to maintain bodily functions. 

Blood pressure

The kidneys play an integral role in blood pressure control. 

The juxtaglomerular apparatus within the distal convoluted tubule detects periods of reduced blood volume (whether due to dehydration or haemorrhage) and thus reduces blood pressure and renal perfusion. Renin is then released from the kidney, which converts angiotensinogen to angiotensin I. Angiotensin I travels within the bloodstream to the lungs and is, in turn, converted to angiotensin II by angiotensin-converting enzyme (ACE). Angiotensin II results in vasoconstriction, increased thirst, and the release of aldosterone from the zona glomerulosa of the adrenal gland. This pathway can also initiated by hyponatraemia. 

Image Source: Wikimedia Commons (2019). Renal corpuscle-en, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/6/69/Renal_corpuscle-en.svg  (Accessed: 4 January 2024).

Aldosterone promotes sodium reabsorption by activating sodium channels within the distal convoluted tubule and collecting duct. Water then follows by osmosis, increasing blood volume and reducing urine output. Spironolactone causes diuresis by acting competitively against aldosterone to prevent the activation of these sodium channels.

Hyperkalaemia can also trigger the release of aldosterone from the adrenal gland, which attempts to normalise potassium levels by increasing potassium and hydrogen ion loss in the urine.  

ACE inhibitors such as ramipril interrupt this chain, preventing the conversion of angiotensin I and reducing blood pressure.

Image Source: Wikimedia Commons (2018). Renin angiotensin aldosterone system, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/3/36/Renin-angiotensin-aldosterone_system.svg  (Accessed: 4 January 2024).

Electrolyte balance and vitamin D metabolism

As we have seen already, the kidneys can influence the levels of some electrolytes within the body. Low calcium levels stimulate PTH release from the parathyroid glands. PTH acts on the kidneys to reduce calcium and magnesium loss in the urine and increase phosphate excretion. 

PTH also stimulates the activation of vitamin D in the kidneys thanks to the production of alpha-1-hydroxylase by the proximal tubular cells. This enzyme catalyses the conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol (calcitriol). Calcitriol then stimulates absorption of calcium, phosphate and magnesium from the gastrointestinal tract. In CKD, the production of alpha-1-hydroxylase drops. 

Acid-base balance

The kidneys have a significant role in acid-base balance.

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.