Kidney Kidneys are the main vertebrates? emunctories (that lead out all final products of metabolism). Vertebrates for example like snail also have organs fulfilling a similar secretory function and sometimes they are called kidneys but they differ from vertebrates? kidney for their structure and evolutionary descent.
Function. The main function of kidneys is to move out of organism water and final products of metabolism. Vertebrates? the main such product is urea. It is a principal final nitrogenated product of protein disassimilation (protein metabolism). Birds and reptiles? principal product of protein metabolism is uric acid. It is a insoluble matter and it is a white paste in excrement. Human uric acid also forms and moves out by kidneys (its salts are urates).
Human kidneys move out about 1-1.5 liters of urine in 24 hours. Though these numbers can vary strongly. On increase water consumption kidneys answer increasing production of more dilute urine. And so they keep up normal water content in organism. If the water consumption is limited kidneys make for saving it in organism. And they use as little as possible water for urine formation. The urine volume can decrease till 300 ml per day and the concentration of moving out products will be accordingly higher. The urine volume is regulated by antidiuretic hormone (ADH) also known as vasopressin. This hormone is secreted by posterior lobe of pituitary gland (it is a gland that located at the base of brain). If an organism needs to save water ADH secretion increases and the urine volume decreases. And vice versa when there is a plenty of water in organism ADH does not educe and so daily urine volume can reach 20 liters. Though urine excretion doesn?t exceed 1 liter per hour.
Structure. Mammals have two kidneys which located in abdominal cavity on both sides of backbone. Total weight of both human kidneys is about 300 gr. or about 0.5-1 % of body weight. In spite of their small size kidneys have rich blood supply. During 1 minute about 1 liter of blood run through renal artery and come out by renal vein. Thus in 5 minutes blood volume equal to the total amount of blood in the whole organism (about 5 liters) goes through kidneys to eliminate products of metabolism.
A kidney is covered with connective tissue membrane and serous tunic. On kidney?s slit one can see that it is divided on two parts. They are called cortical and medullary substance. The most part of a kidney consists of huge number of superfine gyrose tubule which are called nephrons. Every kidney has about 1 million of nephrons. The total nephrons length of both kidneys is about 120 kilometers. Kidneys are responsible for liquid formation that finally becomes urine. The nephrons structure is the key to understanding his functions. On the one side of each nephron there is a distension. It is a round formation that is called renal corpuscle. It consists of bilaminar so called Bowman's capsule which include capillary net that form a glomerule. The rest part of a kidney is trichotomic. Twisted part that is the nearest to glomerule is called proximal convoluted tubule. Then thin-walled straight part that turns round sharply and makes loop so called Henle's loop. It is distinguished (in series) descending part, turn and ascending part in it. A tortile third part is a distal spiral tubule which is run (with other spiral tubules) into a receiving tube. Urine goes from receiving tubes to renal pelvis (in fact it is a dilated end of ureter) and then further along the ureter in an urinary bladder. Urine is led out from an urinary bladder along an urethra in certain terms. Cortical substance includes all glomerules and all gyrose parts of proximal and distal tubules. Henle's loops and receiving tubes which are located between loops are in medullary substance.
Urine formation. In glomerule water and attenuated in it substances go out from blood through capillary wall under the influence of blood pressure. Interstices of capillaries so small that stop blood cells and proteins. Therefore glomerule works like filter that let pass only water without proteins but with all permeates in it. This liquid is called ultrafiltrate, glomerule filtrate or primary urine. It is undergo control while going through other parts of nephron.
In a human kidney the volume of ultrafiltrate is about 130 ml per minute or 8 liters an hour. As the total blood volume of a man equal about 5 liters it is evidently that the most part of ultrafiltrate should be absorbed back into blood. If to assume that about 1 ml of urine is formed in a human organism per hour so the rest 129 ml (more than 99%) liquid of ultrafiltrate is necessary to return in bloodstream before they will become urine and will go out an organism.
Ultrafiltrate contains a lot of valuable substances (salts, glucose, amino acids, vitamins and so on) which an organism can?t lose on a substantial scale. The majority of them are exposed reabsorption in proportion as filtrate goes along proximal nephron tubules. For example glucose reabsorbes till it disappear from filtrate at all (till it concentration reaches zero). As the glucose transfer back in blood where its concentration is higher goes against the density gradient so the process needs additional energy and it is called active transport.
As a result of glucose reabsorption and salts from ultrafiltrate the concentration of solutes decreases. Blood turned out to be more concentrated solution than filtrate and it is draw water from tubules. It means that water passive follow actively traveling salts. It is called passive transport. With the help of active and passive transport 7/8 of water and solutes are absorbed back into from the contents of proximal tubules. And the speed of filtrate volume decreasing reaches 1 liter per hour. So now inside tubules liquid include mainly slag as urea. But the process of urine formation hasn?t finished yet.
The next segment is the Henle?s loop. It is responsible for the creation of very high salts concentration and urea in filtrate. In ascending part of loop an active transport takes place. At first they are salts which gather round an interstitial fluid of medullary substance where as a result a high salt concentration is created. And because of it part of water from descending part of the loop (pervious to water) is drawn off and immediately goes to capillary. At the same time salts gradually diffuse in it and reach the highest concentration at the curve of loop. This mechanism is called countercurrent concentrating mechanism. Then filtrate goes to distant tubules where other substances can transform to it due to active transport.
Finally filtrate gets into discharging tubules. Here it is determined what quantity of liquid will be made out extra and therefore what is the final urine volume. It means the final or secondary urine volume. This stage is regulated by presence or absence of ADH in blood. Discharging tubules are located between numerous Hentle?s loops and goes parallel to them. Under the influence of ADH their walls become pervious. As the salt concentration in Hentle?s loop is very high and water have a tendency to follow salts the water in fact is drawn out discharging tubules. And it leaves a grout with high concentration of salts, urea and other solutes. This grout is the final urine. If ADH is lacking in blood so discharging tubules stay low-permeability for water. Water doesn?t go out of them and the urine volume stays large. So it turned to be diluted.
The main kidney diseases. Renal calculi are sediments of salts in kidneys. They are formed under high salt concentration in urine or increase of urine acidity. It means under conditions that are conductive to salt crystallization. The main types of calculi are oxalates, phosphate or urates. Small calculi (sand) go out through ureters practically without any harm. Bigger ones may get stuck in ureters and it is accompanied by excruciating pain (renal gripes). Still bigger calculi stay in pelvises and cause pain, infection and kidneys dysfunction. The consumption large quantity of water decreases the probability of formation of calculi.
Renal calculi are eliminated by surgical treatment or with the help of lithotripsy (therapeutic use of ultrasound to shatter calculi to small pieces which can be moved out of the organism). Such method doesn?t make any harm to soft tissues of a kidney.
Renal insufficiency and hemodialysis. There are many causes as renal infection or destruction attached to diseases such as pancreatic diabetes which could lead to dysfunction of kidneys right up to renal insufficiency. On chronic renal insufficiency acid-base disequilibrium and accumulation of nitrogenous slag (urine first of all) in blood takes place.
Those who suffer from renal insufficiency could be treated with the help of hemodialysis (artificial kidney). Its principle lay in that blood from artery (usually from forearm) goes through artificial kidney apparatus and returns to the vein of patient. In apparatus blood run through microscopical tubules which are circled by thin plastic membrane. On the other side of membrane dialysis liquid is. If instead of dialysis liquid there was water this case all solutes (salts, sugar and so on) will be washed out of blood plasma. It means that all solutes went out through membrane to water. To avoid it liquid grout that contains all those components and in those concentrations as blood plasma (but those which should be removed for example urea) is taken for dialysis. At the time of hemodialysis these solutes go out of plasma so in the vein of patient entire blood returns. Hemodialysis could have been run for years. While visiting dialysis center regularly patients still keep on living normally.
Kidneys and urinary infection. Kidneys infection (glomerulonephritis, pyelonephritis) and urinary infection (cystitis) occur more often at women (in view of anatomic peculiarities). More than usually women with slowed immune system suffer.
All about Kidney infection.