World Kidney Day 2016: Sharing the message (II)

The kidneys are bean-shaped organs that serve several essential regulato­ry roles in vertebrates. They re­move excess organic molecules from the blood, and it is by this action that their best-known function is performed: the re­moval of waste products of me­tabolism. Kidneys are essential to the urinary system and also serve homeostatic functions such as the regulation of elec­trolytes, maintenance of acid–base balance, and regulation of blood pressure (via maintain­ing the salt and water balance). They serve the body as a natural filter of the blood, and remove water-soluble wastes which are diverted to the bladder. In pro­ducing urine, the kidneys ex­crete nitrogenous wastes such as urea and ammonium. They are also responsible for the re­absorption of water, glucose, and amino acids. The kidneys also produce hormones includ­ing calcitriol and erythropoie­tin. An important enzyme renin is also produced in the kidneys which acts in negative feedback.
Located at the rear of the ab­dominal cavity in the retroper­itoneal space, the kidneys re­ceive blood from the paired renal arteries, and drain into the paired renal veins. Each kid­ney excretes urine into a ureter which empties into the bladder.
Renal physiology is the study of kidney function, while ne­phrology is the medical spe­cialty concerned with kidney diseases. Diseases of the kid­ney are diverse, but individuals with kidney disease frequent­ly display characteristic clini­cal features. Common clinical conditions involving the kidney include the nephritic and ne­phrotic syndromes, renal cysts, acute kidney injury, chronic kidney disease, urinary tract in­fection, nephrolithiasis, and uri­nary tract obstruction. Various cancers of the kidney exist. The most common adult renal can­cer is renal cell carcinoma. Can­cers, cysts, and some other re­nal conditions can be managed with removal of the kidney. This is known as nephrectomy. When renal function, measured by the glomerular filtration rate, is persistently poor, dialysis and kidney transplantation may be treatment options. Although they are not normally harmful, kidney stones can be extreme­ly painful.
In humans, the kidneys are located in the abdominal cavi­ty, one on each side of the spine, and lie in a retroperitoneal po­sition at a slightly oblique an­gle. The asymmetry within the abdominal cavity, caused by the position of the liver, typically re­sults in the right kidney being slightly lower and smaller than the left, and being placed slight­ly more to the middle than the left kidney.
In adult males, the kidney weighs between 125 and 170 grams. In females the weight of the kidney is between 115 and 155 grams. A Danish study measured the median renal length to be 11.2 cm (4.4 in) on the left side and 10.9 cm (4.3 in) on the right side in adults. Me­dian renal volumes were 146 cm3 on the left and 134 cm3 on the right.
The substance, or paren­chyma, of the kidney is divid­ed into two major structures: the outer renal cortex and the inner renal medulla. Grossly, these structures take the shape of eight to 18 cone-shaped re­nal lobes, each containing renal cortex surrounding a portion of medulla called a renal pyramid (of Malpighi). Between the re­nal pyramids are projections of cortex called renal columns (or Bertin columns). Nephrons, the urine-producing function­al structures of the kidney, span the cortex and medulla. The ini­tial filtering portion of a neph­ron is the renal corpuscle which is located in the cortex. This is followed by a renal tubule that passes from the cortex deep into the medullary pyramids. Part of the renal cortex, a medullary ray is a collection of renal tubules that drain into a single collect­ing duct.
The tip, or papilla, of each pyramid empties urine into a minor calyx; minor calyces empty into major calyces, and major calyces empty into the renal pelvis. This becomes the ureter. At the hilum, the ure­ter and renal vein exit the kid­ney and the renal artery enters. Hilar fat and lymphatic tissue with lymph nodes surrounds these structures. The hilar fat is contiguous with a fat-filled cavi­ty called the renal sinus. The re­nal sinus collectively contains the renal pelvis and calyces and separates these structures from the renal medullary tissue.
The kidney participates in whole-body homeostasis, reg­ulating acid-base balance, elec­trolyte concentrations, extracel­lular fluid volume, and blood pressure. The kidney 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.
Many of the kidney’s func­tions are accomplished by rel­atively simple mechanisms of filtration, reabsorption, and secretion, which take place in the nephron. Filtration, which takes place at the renal cor­puscle, is the process by which cells and large proteins are fil­tered from the blood to make an ultrafiltrate that eventually be­comes urine. The kidney gen­erates 180 liters of filtrate a day, while reabsorbing a large per­centage, allowing for the gen­eration of only approximately 2 liters of urine. Reabsorption is the transport of molecules from this ultrafiltrate and into the blood. Secretion is the re­verse process, in which mole­cules are transported in the op­posite direction, from the blood into the urine.
Excretion of wastes
The kidneys excrete a vari­ety of waste products produced by metabolism into the 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 counter-current exchange by the vessels carrying the blood supply to the nephron is essen­tial for enabling this function.
Reabsorption of vital nutrients
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.
ADH binds to principal cells in the collecting duct that trans­locate aquaporins to the mem­brane, allowing water to leave the normally impermeable membrane and be reabsorbed into the body by the vasa recta, thus increasing the plasma vol­ume of the body.
There are two systems that create a hyperosmotic medulla and thus increase the body plas­ma volume: Urea recycling and the ‘single effect.’
Urea is usually excreted as a waste product from the kid­neys. However, when plasma blood volume is low and ADH is released the aquaporins that are opened are also permeable to urea. This allows urea to leave the collecting duct into the me­dulla creating a hyperosmot­ic solution that ‘attracts’ wa­ter. Urea can then re-enter the nephron and be excreted or recycled again depending on whether ADH is still present or not.