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Each kidney consists of functional units called nephrons. Following filtration of blood and further processing, wastes (in the form of urine) exit the kidney via the ureters, tubes made of smooth muscle fibres that propel urine towards the urinary bladder, where it is stored and subsequently expelled through the urethra during urination. The ...
Avian kidneys combine so called reptilian-type nephrons, without the loop of Henle, and mammalian-type nephrons, with the loop of Henle. [23] Most nephrons are reptilian-type. [ 84 ] The loop of Henle of birds is similar to that of mammals, the main difference is that the nephron of birds has only a short loop of Henle. [ 77 ]
The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure called Bowman's capsule. The renal tubule extends from the capsule. The capsule and tubule are connected and are composed of epithelial cells with a lumen. A healthy adult has 1 to 1.5 million nephrons in each kidney.
Each adult human kidney contains around 1 million nephrons, while a mouse kidney contains only about 12,500 nephrons. The kidneys also carry out functions independent of the nephrons. For example, they convert a precursor of vitamin D to its active form, calcitriol; and synthesize the hormones erythropoietin and renin.
They perform several homeostatic functions such as:- Maintain volume of extracellular fluid; Maintain ionic balance in extracellular fluid; Maintain pH and osmotic concentration of the extracellular fluid. Excrete toxic metabolic by-products such as urea, ammonia, and uric acid. The way the kidneys do this is with nephrons.
The kidney's ability to perform many of its functions depends on the three fundamental functions of filtration, reabsorption, and secretion, whose sum is called renal clearance or renal excretion. That is: Urinary excretion rate = Filtration rate – Reabsorption rate + Secretion rate [1]
The function of the kidneys is regulated by the autonomic nervous system and hormones. [36] The potential for regeneration in mature kidneys is limited [37] [38] because new nephrons cannot be formed. [39] But in cases of limited injury, renal function can be restored through compensatory mechanisms. [40]
Renal functions include maintaining an acid–base balance; regulating fluid balance; regulating sodium, potassium, and other electrolytes; clearing toxins; absorption of glucose, amino acids, and other small molecules; regulation of blood pressure; production of various hormones, such as erythropoietin; and activation of vitamin D.