process is also known 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 reabsorptionreabsorbed nutrientnotes early proximal tubuleglucose (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 henleh2o reabsorbs via medullary hypertonicity and makes urine hypertonic. thick ascending loop of henlena+ (10–20%), k+, 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 tubulena+, cl− pth causes ca2+ reabsorption. collecting tubulesna+(3–5%), h2o na+ is reabsorbed in exchange for k+, 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 kidneys, 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 kidneys 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 main articles: blood pressure regulation and renin–angiotensin system although the kidney cannot directly sense blood, long-term regulation of blood pressure predominantly depends upon the kidney. 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 kidney'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 main article: acid–base homeostasis the two organ systems that help regulate the body's acid–base balance are the kidneys and lungs. acid–base homeostasis is the maintenance of ph around a value of 7.4. the lungs are part of respiratory system which helps to maintain acid–base homeostasis by regulating carbon dioxide (co2) concentration in the blood. the respiratory system is the first line of defense when the body experiences and acid–base problem. it attempts to return the body ph to a value of 7.4 by controlling the respiratory rate. when the body is experiencing acidic conditions, it will increase the respiratory rate which in turn drives off co2 and decreases the h+ concentration, therefore increasing the ph. in basic conditions, the respiratory rate will slow down so that the body holds onto more co2 and increases the h+ concentration and decreases the ph. the kidneys have two cells that help to maintain acid-base homeostasis: intercalated a and b cells. the intercalated a cells are stimulated when the body is experiencing acidic conditions. under acidic conditions, the high concentration of co2 in the blood creates a gradient for co2 to move into the cell and push the reaction hco3 + h ↔ h2co3 ↔ co2 + h2o to the left. on the luminal side of the cell there is a h+ pump and a h/k exchanger. these pumps move h+ against their gradient and therefore require atp. these cells will remove h+ from the blood and move it to the filtrate which helps to increase the ph of the blood. on the basal side of the cell there is a hco3/cl exchanger and a cl/k co-transporter (facilitated diffusion). when the reaction is pushed to the left it also increases the hco3 concentration in the cell and hco3 is then able to move out into the blood which additionally raises the ph. the intercalated b cell responds very similarly, however, the membrane proteins are flipped from the intercalated a cells: the proton pumps are on the basal side and the hco3/cl exchanger and k/cl co-transporter are on the luminal side. they function the same, but now release protons into the blood to decrease the ph. regulation of osmolality the kidneys 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 kidney and an increase in urine concentration. the two factors work together to return the plasma osmolality to its normal levels. measuring function main article: kidney function various calculations and methods are used to try to measure kidney 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 kidney. this can be defined using the equation. the kidney is a very complex organ and mathematical modelling has been used to better understand kidney function at several scales, including fluid uptake and secretion. clinical significance main article: kidney disease nephrology is the subspeciality under internal medicine that deals with kidney function and disease states related to renal malfunction and their management including dialysis and kidney transplantation. urology is the specialty under surgery that deals with kidney structure abnormalities such as kidney cancer and cysts and problems with urinary tract. nephrologists are internists, and urologists are surgeons, whereas both are often called "kidney doctors". there are overlapping areas that both nephrologists and urologists can provide care such as kidney stones and kidney related infections. there are many causes of kidney disease. some causes are acquired over the course of life, such as diabetic nephropathy whereas others are congenital, such as polycystic kidney disease. medical terms related to the kidneys commonly use terms such as renal and the prefix nephro-. the adjective renal, meaning related to the kidney, is from the latin rēnēs, meaning kidneys; the prefix nephro- is from the ancient greek word for kidney, nephros (νεφρός). for example, surgical removal of the kidney is a nephrectomy, while a reduction in kidney function is called renal dysfunction. acquired diabetic nephropathy glomerulonephritis hydronephrosis is the enlargement of one or both of the kidneys caused by obstruction of the flow of urine. interstitial nephritis kidney stones (nephrolithiasis) are a relatively common and particularly painful disorder. a chronic condition can result in scars to the kidneys. the removal of kidney stones involves ultrasound treatment to break up the stones into smaller pieces, which are then passed through the urinary tract. one common symptom of kidney stones is a sharp to disabling pain in the middle and sides of the lower back or groin. kidney tumour wilms tumor renal cell carcinoma lupus nephritis minimal change disease in nephrotic syndrome, the glomerulus has been damaged so that a large amount of protein in the blood enters the urine. other frequent features of the nephrotic syndrome include swelling, low serum albumin, and high cholesterol. pyelonephritis is infection of the kidneys and is frequently caused by complication of a urinary tract infection. kidney failure acute kidney failure stage 5 chronic kidney disease renal artery stenosis renovascular hypertension kidney injury and failure main articles: acute kidney injury, chronic kidney disease, and kidney failure generally, humans can live normally with just one kidney, as one has more functioning renal tissue than is needed to survive. only when the amount of functioning kidney tissue is greatly diminished does one develop chronic kidney disease. renal replacement therapy, in the form of dialysis or kidney transplantation, is indicated when the glomerular filtration rate has fallen very low or if the renal dysfunction leads to severe symptoms. dialysis a depiction of peritoneal dialysis. main article: kidney dialysis dialysis is a treatment that substitutes for the function of normal kidneys. dialysis may be instituted when approximately 85%–90% of kidney function is lost, as indicated by a glomerular filtration rate (gfr) of less than 15. dialysis removes metabolic waste products as well as excess water and sodium (thereby contributing to regulating blood pressure); and maintains many chemical levels within the body. life expectancy is 5–10 years for those on dialysis; some live up to 30 years. dialysis can occur via the blood (through a catheter or arteriovenous fistula), or through the peritoneum (peritoneal dialysis) dialysis is typically administered three times a week for several hours at free-standing dialysis centers, allowing recipients to lead an otherwise essentially normal life. congenital disease congenital hydronephrosis congenital obstruction of urinary tract duplex kidneys, or double kidneys, occur in approximately 1% of the population. this occurrence normally causes no complications, but can occasionally cause urinary tract infections. duplicated ureter occurs in approximately one in 100 live births horseshoe kidney occurs in approximately one in 400 live births nephroblastoma (syndromic wilm's tumour) nutcracker syndrome polycystic kidney disease autosomal dominant polycystic kidney disease affects patients later in life. approximately one in 1000 people will develop this condition autosomal recessive polycystic kidney disease is far less common, but more severe, than the dominant condition. it is apparent in utero or at birth. renal agenesis. failure of one kidney to form occurs in approximately one in 750 live births. failure of both kidneys to form used to be fatal; however, medical advances such as amnioinfusion therapy during pregnancy and peritoneal dialysis have made it possible to stay alive until a transplant can occur. renal dysplasia unilateral small kidney multicystic dysplastic kidney occurs in approximately one in every 2400 live births ureteropelvic junction obstruction or upjo; although most cases are congenital, some are acquired. diagnosis many renal diseases are diagnosed on the basis of a detailed medical history, and physical examination. the medical history takes into account present and past symptoms, especially those of kidney disease; recent infections; exposure to substances toxic to the kidney; and family history of kidney disease. kidney function is tested by using blood tests and urine tests. the most common blood tests are creatinine, urea and electrolytes. urine tests such as urinalysis can evaluate for ph, protein, glucose, and the presence of blood. microscopic analysis can also identify the presence of urinary casts and crystals. the glomerular filtration rate (gfr) can be directly measured ("measured gfr", or mgfr) but this rarely done in everyday practice. instead, special equations are used to calculate gfr ("estimated gfr", or egfr). imaging renal ultrasonography is essential in the diagnosis and management of kidney-related diseases. other modalities, such as ct and mri, should always be considered as supplementary imaging modalities in the assessment of renal disease. biopsy the role of the renal biopsy is to