The nephron, shown here, is the functional unit of the<unk> s. Its parts are labelled except the (gray) connecting tubule located after the (dark red) distal convoluted tubule and before the large (gray) collecting duct (mislabeled collection duct). The<unk> s excrete a variety of waste products produced by metabolism into the urine. The microscopic structural and functional unit of the<unk>  is the nephron. It processes the blood supplied to it via filtration, reabsorption, secretion and excretion; the consequence of those processes is the production of urine. These include the nitrogenous wastes urea, from protein catabolism, and uric acid, from nucleic acid metabolism. The ability of mammals and some birds to concentrate wastes into a volume of urine much smaller than the volume of blood from which the wastes were extracted is dependent on an elaborate countercurrent multiplication mechanism. This requires several independent nephron characteristics to operate: a tight hairpin configuration of the tubules, water and ion permeability in the descending limb of the loop, water impermeability in the ascending loop, and active ion transport out of most of the ascending limb. In addition, passive countercurrent exchange by the vessels carrying the blood supply to the nephron is essential for enabling this function. The<unk>  participates in whole-body homeostasis, regulating acid–base balance, electrolyte concentrations, extracellular fluid volume, and blood pressure. The<unk>  accomplishes these homeostatic functions both independently and in concert with other organs, particularly those of the endocrine system. Various endocrine hormones coordinate these endocrine functions; these include renin, angiotensin II, aldosterone, antidiuretic hormone, and atrial natriuretic peptide, among others. Formation of urine Four main processes are involved in the creation of urine. Filtration Filtration, which takes place at the renal corpuscle, is the process by which cells and large proteins are retained while materials of smaller molecular weights are filtered from the blood to make an ultrafiltrate that eventually becomes urine. The<unk>  generates 180 liters of filtrate a day. The process is also<unk> nown as hydrostatic filtration due to the hydrostatic pressure exerted on the capillary walls. Reabsorption Secretion and reabsorption of various substances throughout the nephron Reabsorption is the transport of molecules from this ultrafiltrate and into the peritubular capillary. It is accomplished via selective receptors on the luminal cell membrane. Water is 55% reabsorbed in the proximal tubule. Glucose at normal plasma levels is completely reabsorbed in the proximal tubule. The mechanism for this is the Na+/glucose cotransporter. A plasma level of 350 mg/dL will fully saturate the transporters and glucose will be lost in the urine. A plasma glucose level of approximately 160 is sufficient to allow glucosuria, which is an important clinical clue to diabetes mellitus. Amino acids are reabsorbed by sodium dependent transporters in the proximal tubule. Hartnup disease is a deficiency of the tryptophan amino acid transporter, which results in pellagra. Location of Reabsorption Reabsorbed nutrient Notes Early proximal tubule Glucose (100%), amino acids (100%), bicarbonate (90%), Na+ (65%), Cl− (65%), phosphate (65%) and H2O (65%) * PTH will inhibit phosphate reabsorption. * AT II stimulates Na+, H2O and HCO3− reabsorption. Thin descending loop of Henle H2O * Reabsorbs via medullary hypertonicity and makes urine hypertonic. Thick ascending loop of Henle Na+ (10–20%),<unk> +, Cl−; indirectly induces para cellular reabsorption of Mg2+, Ca2+ * This region is impermeable to H2O and the urine becomes less concentrated as it ascends. Early distal convoluted tubule Na+, Cl− * PTH causes Ca2+ reabsorption. Collecting tubules Na+(3–5%), H2O * Na+ is reabsorbed in exchange for<unk> +, and H+, which is regulated by aldosterone. * ADH acts on the V2 receptor and inserts aquaporins on the luminal side Examples of substances that are reabsorbed in the<unk> s, and the hormones that influence those processes. Secretion Secretion is the reverse of reabsorption: molecules are transported from the peritubular capillary through the interstitial fluid, then through the renal tubular cell and into the ultrafiltrate. Excretion The last step in the processing of the ultrafiltrate is excretion: the ultrafiltrate passes out of the nephron and travels through a tube called the collecting duct, which is part of the collecting duct system, and then to the ureters where it is renamed urine. In addition to transporting the ultrafiltrate, the collecting duct also takes part in reabsorption. Hormone secretion The<unk> s secrete a variety of hormones, including erythropoietin, calcitriol, and renin. Erythropoietin is released in response to hypoxia (low levels of oxygen at tissue level) in the renal circulation. It stimulates erythropoiesis (production of red blood cells) in the bone marrow. Calcitriol, the activated form of vitamin D, promotes intestinal absorption of calcium and the renal reabsorption of phosphate. Renin is an enzyme which regulates angiotensin and aldosterone levels. Blood pressure regulation Although the<unk>  cannot directly sense blood, long-term regulation of blood pressure predominantly depends upon the<unk> . This primarily occurs through maintenance of the extracellular fluid compartment, the size of which depends on the plasma sodium concentration. Renin is the first in a series of important chemical messengers that make up the renin–angiotensin system. Changes in renin ultimately alter the output of this system, principally the hormones angiotensin II and aldosterone. Each hormone acts via multiple mechanisms, but both increase the<unk> 's absorption of sodium chloride, thereby expanding the extracellular fluid compartment and raising blood pressure. When renin levels are elevated, the concentrations of angiotensin II and aldosterone increase, leading to increased sodium chloride reabsorption, expansion of the extracellular fluid compartment, and an increase in blood pressure. Conversely, when renin levels are low, angiotensin II and aldosterone levels decrease, contracting the extracellular fluid compartment, and decreasing blood pressure. Acid–base balance Two organ systems, the<unk> s and lungs, maintain acid–base homeostasis, which is the maintenance of pH around a relatively stable value. The lungs contribute to acid–base homeostasis by regulating carbon dioxide (CO2) concentration. The<unk> s have two very important roles in maintaining the acid–base balance: to reabsorb and regenerate bicarbonate from urine, and to excrete hydrogen ions and fixed acids (anions of acids) into urine. Regulation of osmolality The<unk> s help maintain the water and salt level of the body. Any significant rise in plasma osmolality is detected by the hypothalamus, which communicates directly with the posterior pituitary gland. An increase in osmolality causes the gland to secrete antidiuretic hormone (ADH), resulting in water reabsorption by the<unk>  and an increase in urine concentration. The two factors work together to return the plasma osmolality to its normal levels. Measuring function Various calculations and methods are used to try to measure<unk>  function. Renal clearance is the volume of plasma from which the substance is completely cleared from the blood per unit time. The filtration fraction is the amount of plasma that is actually filtered through the<unk> . This can be defined using the equation. The<unk>  is a very complex organ and mathematical modelling has been used to better understand<unk>  function at several scales, including fluid uptake and secretion.
Kidney
Liver
Lung
Heart
Peripheral nervous system
Central nervous system
Brain
Adrenal gland