Function of kidney
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Functions of the Kidney
The kidneys are paired organs that perform a remarkable range of vital functions. As stated in Robbins & Cotran Pathologic Basis of Disease, "human kidneys serve to convert more than 1700 L of blood per day into about 1 L of a highly concentrated fluid called urine." Each kidney contains approximately 1 million nephrons, the basic functional units.
1. Excretion of Metabolic Waste Products
The kidney filters the plasma through the glomeruli and excretes waste products of metabolism. These include:
- Urea and ammonium - nitrogenous products of protein catabolism
- Creatinine - waste product of muscle metabolism
- Uric acid - end product of purine metabolism
- Foreign chemicals such as drug metabolites and toxins
Together, the two kidneys produce about 120 mL of ultrafiltrate/min, but only 1 mL of urine/min - more than 99% of the glomerular ultrafiltrate is reabsorbed. - Goodman & Gilman's Pharmacological Basis of Therapeutics
The glomerular filtrate starts at approximately 180 L in 24 hours and is reduced to just 1-2 L of urine per day. - Henry's Clinical Diagnosis
2. Fluid, Electrolyte, and Volume Regulation
The kidneys precisely regulate:
- Fluid volume (extracellular and total body water)
- Electrolyte concentrations - sodium (Na+), potassium (K+), chloride (Cl-), calcium (Ca2+), magnesium (Mg2+), phosphate
- Osmolality of body fluids
The kidney filters the extracellular fluid volume across the renal glomeruli an average of 12 times a day, with the renal nephrons precisely regulating body fluid volume and electrolyte content via secretion and reabsorption. - Goodman & Gilman's
Sodium handling along the nephron:
- Proximal tubule reabsorbs ~65% of filtered Na+ (isotonic)
- Loop of Henle reabsorbs ~25% of filtered Na+ (mostly in the thick ascending limb)
- Distal tubule and collecting duct handle the remaining fine-tuned regulation under hormonal control
3. Acid-Base Balance
The kidneys are a major regulator of acid-base homeostasis. They excrete both organic and inorganic acids and bases, and regulate bicarbonate (HCO3-) reabsorption and secretion. This complements the respiratory system's faster CO2-based buffering. - Ganong's Review of Medical Physiology
4. Blood Pressure Regulation (RAAS)
The kidney contains the juxtaglomerular apparatus (JGA), which includes:
- Macula densa - monitors Na+ concentration in tubular fluid
- Juxtaglomerular (JG) cells - secrete renin in response to low perfusion pressure or low Na+ delivery
Renin triggers the renin-angiotensin-aldosterone system (RAAS):
- Renin converts angiotensinogen → Angiotensin I
- ACE converts Angiotensin I → Angiotensin II (potent vasoconstrictor)
- Angiotensin II stimulates aldosterone release → Na+ and water retention → increased blood pressure
The RAAS is "a multisystem endocrine cascade that regulates electrolyte homeostasis, fluid balance, and blood pressure." - Histology: A Text and Atlas
5. Endocrine Functions
The kidney is an endocrine organ producing several hormones:
| Hormone | Site of Production | Action |
|---|---|---|
| Erythropoietin (EPO) | Peritubular interstitial cells | Stimulates red blood cell production in bone marrow |
| Renin | Juxtaglomerular cells | Activates the RAAS (blood pressure/volume control) |
| 1,25-dihydroxycholecalciferol (active Vitamin D) | Proximal tubule | Regulates calcium and phosphate absorption |
| Kinins | Renal tissue | Vasodilatory effects |
"The kidneys are also endocrine organs, making kinins, 1,25-dihydroxycholecalciferol (active Vitamin D), erythropoietin, and making and secreting renin." - Ganong's Review of Medical Physiology
6. Glucose and Amino Acid Reabsorption
Under normal conditions, the kidneys reabsorb nearly all filtered glucose and amino acids in the proximal tubule. The kidney also participates in gluconeogenesis - during fasting, it can synthesize and release glucose into the blood, producing almost 20% of the liver's glucose output. - Ganong's Review of Medical Physiology
7. Calcium and Phosphate Regulation
The kidney (along with gut and bone) is a key player in calcium-phosphate homeostasis, under the influence of:
- PTH (parathyroid hormone) - increases renal Ca2+ reabsorption, decreases phosphate reabsorption
- Active Vitamin D (1,25-dihydroxycholecalciferol, activated in the kidney itself) - promotes intestinal Ca2+ and phosphate absorption
Summary Table
| Function | Mechanism |
|---|---|
| Waste excretion | Glomerular filtration + tubular secretion |
| Fluid/electrolyte balance | Tubular reabsorption and secretion |
| Acid-base balance | H+/HCO3- handling in tubules |
| Blood pressure regulation | Renin secretion → RAAS activation |
| RBC production | Erythropoietin synthesis |
| Vitamin D activation | 1-alpha-hydroxylation of 25-OH Vit D |
| Gluconeogenesis | Proximal tubule glucose synthesis |
| Ca/Phosphate homeostasis | PTH/Vitamin D-regulated transport |
Sources: Ganong's Review of Medical Physiology, Robbins & Cotran Pathologic Basis of Disease, Goodman & Gilman's Pharmacological Basis of Therapeutics, Henry's Clinical Diagnosis and Management by Laboratory Methods, Histology: A Text and Atlas with Correlated Cell and Molecular Biology
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