Other meanings of Kidney
Anatomy
The kidney is a paired, bean-shaped organ in vertebrates that filters blood to remove metabolic wastes, regulate fluid and electrolyte balance, and produce urine. In humans, the kidneys lie retroperitoneally on either side of the spine at the level of the T12–L3 vertebrae, each about 11 cm long and weighing 150 g. Beyond excretion, they secrete hormones such as erythropoietin and renin, and activate vitamin D, making them central to blood pressure regulation, red blood cell production, and calcium metabolism.
The kidney is enclosed in a fibrous capsule and surrounded by perirenal fat. Internally, it has an outer cortex and inner medulla, with the medulla organized into renal pyramids that project into the renal pelvis. The functional unit is the nephron, comprising a glomerulus, Bowman's capsule, proximal tubule, loop of Henle, distal tubule, and collecting duct. Filtration occurs at the glomerulus, where blood pressure forces water and solutes through a filtration barrier; the filtrate is then selectively reabsorbed and secreted along the tubules. The loop of Henle establishes a medullary osmotic gradient essential for water conservation. The kidney also produces renin, which triggers the renin–angiotensin–aldosterone system, and erythropoietin, which stimulates red blood cell production.1
In embryonic development, the kidney arises from the intermediate mesoderm through three successive structures: the pronephros, mesonephros, and metanephros. The metanephros, which forms the definitive kidney, appears in the fifth week of human gestation and begins urine production by the eleventh week. The ureteric bud induces the metanephric blastema to form nephrons; defects in this process cause congenital anomalies such as renal agenesis. Evolutionarily, kidneys have adapted to diverse environments: freshwater fish excrete dilute urine, marine fish conserve water, and desert mammals like kangaroo rats produce highly concentrated urine. The mammalian kidney's loop of Henle is a key adaptation for terrestrial life, allowing water reabsorption beyond that of other vertebrates.2
Kidney disease is a major global health burden, affecting over 850 million people worldwide. Chronic kidney disease (CKD) is defined by reduced glomerular filtration rate or markers of kidney damage persisting for three months or more. Common causes include diabetes mellitus, hypertension, and glomerulonephritis. Acute kidney injury (AKI) can result from ischemia, toxins, or sepsis, and is associated with high mortality. End-stage renal disease requires renal replacement therapy: dialysis or kidney transplantation. The first successful kidney transplant was performed in 1954 between identical twins, and transplantation now offers the best outcomes for eligible patients. Preventive strategies focus on controlling blood pressure, blood glucose, and avoiding nephrotoxic drugs.
Beyond their excretory role, kidneys have surprising functions and histories. They are the primary site of gluconeogenesis during fasting, contributing up to 40% of endogenous glucose. The kidney also degrades peptide hormones like insulin and produces the enzyme kallikrein, which influences blood pressure. In some fish, the kidney is the main hematopoietic organ. The ancient practice of uroscopy—diagnosing disease by examining urine—dates back to Hippocrates and persisted for centuries. The term "kidney stone" covers various compositions, with calcium oxalate being most common; however, struvite stones are associated with urinary tract infections. The kidney's remarkable regenerative capacity is limited, but recent research on renal progenitor cells offers potential for future therapies.3
The kidneys are vital organs whose functions extend far beyond urine production, influencing blood pressure, bone health, and red blood cell formation.
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