Kidney

The kidneys are a pair of reddish-brown, bean-shaped organs situated high in the posterior abdominal cavity. As vital components of the urinary system, they filter the blood, regulate body fluid balance and contribute to numerous homeostatic processes essential for human physiology. Adult human kidneys are multilobar and multipapillary, typically smooth externally and located retroperitoneally on either side of the vertebral column. Each kidney measures roughly 11–12 cm in length and is supplied by the renal arteries and drained by the renal veins. Urine formed within the kidneys passes into the ureters and onward to the urinary bladder.

Location, Structure and Gross Anatomy

The kidneys lie between vertebral levels T12 and L3, with the right kidney positioned slightly lower than the left due to the presence of the liver. Their upper poles lie deep to the 11th and 12th ribs, offering partial protection. Superior to each kidney sits an adrenal gland. The organs are enclosed by a fibrous renal capsule, surrounded in turn by perirenal adipose tissue, the renal fascia and pararenal fat. This layered arrangement stabilises the kidneys and cushions them against mechanical stress.
A medial concavity, the renal hilum, serves as the entry and exit point for the renal artery, renal vein and ureter. The internal cavity beyond the hilum—the renal sinus—contains fat, lymphatics, the renal pelvis and the branching calyces.
The kidney parenchyma is organised into two primary regions:

• Renal cortex: the outer granular layer.
• Renal medulla: the inner region, arranged into 8–18 conical renal pyramids.

Cortical tissue projects between pyramids as renal columns. The pyramidal tips, or renal papillae, drain urine into minor calyces, which merge into major calyces and then into the renal pelvis, continuous with the ureter.
Median measurements show that the left kidney is slightly longer and larger in volume than the right.

Blood Supply and Lymphatics

The kidneys receive a substantial share—around 20–25%—of the resting cardiac output. Each renal artery, branching directly from the abdominal aorta, divides into segmental, interlobar, arcuate and interlobular arteries before reaching the glomerular afferent arterioles. Venous return follows a parallel pathway, with interlobular, arcuate and interlobar veins converging into the renal veins, draining into the inferior vena cava.
Lymphatic drainage occurs via the renal sinus into para-aortic lymph nodes, while renal nerves accompany the renal vessels. Sympathetic fibres regulate vascular tone and can reduce renal blood flow, whereas parasympathetic innervation from the vagus nerve remains less well understood. Sensory fibres refer pain to the T10–T11 dermatomes, explaining flank pain patterns in renal disorders.

Microscopic Structure and Cellular Organisation

The functional unit of the kidney is the nephron, of which each adult human kidney contains about one million. A nephron comprises the glomerulus, proximal tubule, loop of Henle, distal tubule and connecting elements that lead to the collecting ducts. The kidney contains at least twenty-six distinct cell types, including epithelial, endothelial, stromal and smooth muscle cells, each contributing to filtration, reabsorption or secretion.
Gene expression studies indicate that roughly 70% of human protein-coding genes are active in the kidneys. Around 300 genes exhibit kidney-enriched expression, with about fifty being highly kidney-specific. Key proteins include:

• Tamm–Horsfall protein (uromodulin), abundant in urine and protective against calcification and bacterial adhesion.
• Podocin and nephrin, located in glomerular filtration structures.
• Transporter proteins such as SLC22A8 in proximal tubules.
• Calbindin in distal tubules and aquaporin-2 in collecting ducts.

Development

Kidney development arises from the intermediate mesoderm and proceeds through three stages: pronephros, mesonephros and metanephros. The metanephros forms the permanent kidneys. Nephrogenesis involves complex interactions between the ureteric bud and metanephric mesenchyme, culminating in the formation of functional nephrons and collecting systems.

Functions of the Kidneys

The kidneys perform a spectrum of homeostatic and endocrine functions:

• Filtration: Approximately one-fifth of renal blood flow is filtered through the glomeruli.
• Reabsorption: Recovery of water, sodium, bicarbonate, glucose, amino acids and other essential solutes.
• Secretion: Elimination of hydrogen ions, ammonium, potassium, uric acid and other waste substances.
• Fluid and electrolyte balance: Regulation of plasma osmolality and electrolyte concentrations.
• Acid–base homeostasis: Modulation of body pH through reabsorption and secretion of acid–base constituents.
• Endocrine activities: Production of erythropoietin, renin and the activation of vitamin D into calcitriol.

Kidney Disease and Clinical Evaluation

Chronic kidney disease (CKD) affects an estimated 13–14% of the global population. As renal failure develops, patients may require renal replacement therapy, including dialysis or kidney transplantation, especially when filtration capacity drops below 15%.
Diagnostic tools for renal disorders include:

• Urinalysis for chemical and microscopic assessment.
• Estimated glomerular filtration rate (eGFR), calculated using serum creatinine.
• Renal imaging via CT scan or ultrasound.
• Renal biopsy for histopathological evaluation.

Specialties differ in their focus: nephrology studies kidney function and related diseases such as nephritic and nephrotic syndromes, acute kidney injury and CKD, while urology addresses structural problems such as tumours, stones, cysts and urinary tract obstruction.

Terminology

The term renal—originating from French and post-classical Latin—remains widely used in medical contexts, though some advocate substituting it with “kidney” in scientific nomenclature. Both forms continue to appear in clinical and anatomical descriptions.
The kidneys, through their intricate structure and multifaceted physiological roles, remain indispensable to the maintenance of internal balance and overall health in the human body.