22nd June 2023

Functions of the kidney

The kidney has several important functions, including:

• Filtration and excretion of waste products via the urine.
• Regulation of electrolytes, acid-base balance, serum osmolality and blood volume to ensure adequate conditions for cell function.
• Production of hormones affecting haemodynamics (renin, prostaglandins), red blood cell production (erythropoietin) and calcium homeostasis (1,25 dihydroxyvitamin D3).

Renal excretion of waste products

The kidneys eliminate metabolic waste products such as:

• Urea – from amino acids
• Creatinine – from muscle creatine
• Uric acid – from nucleic acids

Products of haemoglobin breakdown e.g. bilirubinThe kidneys also eliminate other substances which may have been ingested, such as some pharmaceutical drug metabolites, recreational drug metabolites and food additives.

Regulation of electrolytes and blood volume

The kidneys aim to maintain homeostasis so that the excretion of water and electrolytes matches the body’s intake. The kidneys can alter the excretion of water, sodium, and most other electrolytes to maintain this homeostasis. The kidneys are also an important contributor in maintaining acid-base homeostasis by excreting acids and regulating bicarbonate levels.

Hormone production

The kidneys are the main source of erythropoietin (EPO) production, a hormone which stimulates the production of red blood cells. In cases of severe kidney disease, anaemia can result from reduced EPO production.

Renin is an enzyme stored in the juxtaglomerular cells of the kidney in its inactive form and is activated and released in response to low arterial blood pressure. This triggers the renin-angiotensin-aldosterone system, a cascade of vasoactive substances and hormones which work to ultimately increase arterial blood pressure.

The kidney also has a role in the hydroxylation of inactive vitamin D to its active form, 1,25-dihydroxyvitamin D3, also known as calcitriol. Calcitriol has an important role in calcium homeostasis by increasing the deposition of calcium into the bones and increasing the reabsorption of calcium in the gastrointestinal tract.