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The Kidney is the principal organ of the urinary system. It is essential in keeping the body healthy, clean, and chemically balanced. They are two bean-shaped organs, each about the size of a persons fist, which are located near the middle of a humans back, just below the rib cage. The kidneys are not at congruent locations that are side by side; instead the right kidney is lower than the left kidney. The kidneys are retropertioneal unlike the liver,spleen, pancreas, stomach and other parts of the urinary system. The visceral peritoneum secretes fluid to lubricate the kidneys. This helps the kidneys stay into place, and have little friction as the body moves. Urine flow could ultimately be effected if the kidneys were to move out of place. This is the reason for the visceral peritoneum.(Wile ,p454) Inside each kidney about a million tiny structures called nephrons filter blood. Which remove waste products and extra water, this becomes urine. The urine flows through tubes called ureters to the bladder, which stores the urine until flowed out of the body, when a person goes to the bathroom. Damage to the nephrons results in kidney disease. This damage may leave kidneys unable to remove wastes. Which damages the filtration of the blood. The kidneys extrete toxins, control ion concentrations, the H2O volume and pH of the blood cell production. Filtering units within the kidneys are called nephrons, throughout the nephron the kidneys cause filtration, reabsorption, secretion, reabsorption, and water reabsorption.(Wile, p458)
The kidneys are two bean-shaped organs, each consisting of the Renal Capsule, Cortex, Medulla, and Renal Pelvis. The Renal capsule is a thin, membrane around the kidneys that helps protect it. The cortex is part of the outer region of the kidneys. The medulla is the inner region. The renal pelvis is a flat and funnel-shaped cavity which will collect the urine into the ureters.  Each kidney is surrounded by the renal capsule, this protects the components within the kidney. The first 'layer' of the kidney begins with the cortex leading inward towards the medulla. The medulla subsist with renal pyramids, when urine is formed it enters the renal pyramids. The minor calyces are funnel-shaped and are the tip of the renal pyramid, which leave the major calyces which lead to be the renal pelvis. The renal pelvis leads the exit to the body, towards the renal ureter. The ureter transports the urine out of the body.(Wile, p455)
The primary functional unit of the kidney is the nephron, which is the site of filtration, site of the tubular secretion/absorption and urine processing takes place. A network made up of capillaries are within the Bowman's capsule. Plasma levels are controlled by the pores of these capillaries. By increase of GCP the capsule creates movement of the plasma level components. Backup of blood rises the increase of blood pressure. Filtration is maintained partly by the cells in the inner visceral layer, which are called podocytes. Active transport by large molecules such as glucose, ions, sodium, also known as proteins requite ATP. Excess amounts of waste are secreted by the tubular maximum (number of carriers) for these proteins to be transported. Blood is carried into the nephron beginning at the Glomerulus, leading down to the outer medulla, inner medulla, the descending medulla, the loop of Henle, the ascending limb, up towards the distal tubule, and out of the nephron.(Wile, p455)
It keeps the volume in your body constant by removing substances that are unwanted (ex: urea, ammonia, drugs, alcohol, etc.). The body takes what is needed like nutrients to help the body function correctly, it then gets rid of unwanted waste.  Your kidneys constantly adjust, control, handle and improve the composition of your blood. The kidney helps keep concentrations of a few ions and other elements constant in the body. This helps to regulate your blood stream and blood pressure by it's basic and acidic levels (pH), as well as rousing the making of red blood cells in your body.
The kidney cleans out your blood by taking the wastes out of your body. Without the kidneys your body would build up wastes and poisons. In order to survive and produce functions that the body needs to live, a person needs at least one kidney. The blood is carried by the Renal Artery to 1 million tiny filters in the kidney called nephrons, which cleans the body and excretes it as urine. The urine then travels down to the ureter, which carries the urine to the bladder. The bladder is a sac-like 'bag' that holds the urine. Then comes the urethra, it carries out the urine out of the body.
Filtration of blood, occurs in the glomerulus. The glomerulus capillaries help the exchange of filtrate. The filtrate leaves the capillaries and enters the Bowmans capsule. The kidneys produce about 125 mL/minute. Thus, the blood is filtered every 25 minutes each day.(Wile, p460)
Reabsorption is controlled by frequent transport processes. Different levels are needed for the body to survive, this helps insure the proper levels of these chemicals. If the body is given too much sodium, the kidneys will not reabsorb all of the sodium processed in the body, although it is important. Only a proper level of sodium is needed for the body. Climatically, the nephrons within the kidney transport in and out the useful and unuseful, or needed chemicals for the body. Such as water, sodium, vitamins, minerals, or proteins. The nephrons clean out the blood, and give what is needed for the blood, at a fit level.(Wile, p458)
The chemicals that are unwanted or needed in the body, are removed out of the body. The chemicals that are adjunct to the blood or plasma are secreted by the nephron. Useless things, that are no longer needed or lessened are taken away from the body. Excess chemicals can become harmful to the body, that is the purpose of secretion. The kidneys adjust the blood pH by secreting H+ ions.(Wile, p458)
Step 4:Water reabsorption
This step is also, part of the reabsorption step. Water reabsorption regulates the water volume found in the body. The required measure of water needed for the body is reabsorbed into the blood. Excess water is secreted from the body, and is put aside as part of urine.(Wile, p458)
Kidneys alter the volume and balance of chemicals in your body. This process, called "homeostasis", occurs constantly. The way the kidneys carry on this process is by filtering, reabsorbing, and secreting. About 20 percent of plasma and non-cellular fundamentals from the blood is taken up by the nephrons. This is thefiltration process. It may need some substances back from the lumen so it will reabsorb these back into the blood. Then, it will secrete the unnecessary components out of the blood straight to the nephrons and lumen.
Body fluids levels are corrected by water intake and excretion. Receptor cells found in the hypothalamus activate thirst when exposed to hypertonic conditions - such as when water loss has been excessive or salt intake high. Excessive intake of fluid will also be corrected by the specialized receptors in the nervous system that monitor the changes in blood volume, resulting in an increase in urine production. When blood volume increases the pressure is increased within the atria of the heart which then activates stretch receptors, which in turn send a signal for reduction of anti-diuretic hormone release in the posterior pituitary, leading to less fluid to be reabsorbed by the kidneys. It secondly reduces the vasomotor tone of the blood vessels, which leads to dilation of the blood vessels, causing an increase of the glomerular blood pressure and increased filtration is achieved, with less water being reabsorbed by osmosis.
- Main Article: Kidney disease
Some common factors that may lead to a kidney disease are a person with Diabetes, High blood pressure or hereditary.  Some symptoms that occur in effects of a kidney disease could be change in urination, swelling, fatigue, skin rash or itching, metallic taste in mouth or ammonia breathe, nausea or vomiting. As well as, Feeling cold, dizziness or trouble concentrating, leg or flank pain. A combination of calcium combined with oxalate or phosphate, is a common cause of stones in the kidney. 
A kidney stone is also called "Nephrolithiasis". A kidney stone is a solid mass that comes from crystals. Once these stones are evolved, if small enough they may follow down in to the urine causing no pain to the person. However, these crystals have jagged edges to them and if large enough will cause unmanageable pain. They can sometimes get stuck in the interior of the kidneys, and are caused by infection in the urine. This is a common disease that affects not only the kidney but also the, ureter, the bladder, as well as the urethra. These are part of the urinary system, which is used after the kidneys have done their job in separating the nutrients need and waste. This results in having a kidney stone in your bladder may cause the urine not to be able to flow. Some common signs of a kidney stone(s) may be, blood in urine, fevers/chills, urine that smells terrible or looks hazy, vomiting, or a burning sensation during urination.  With too much of one type of substance, the substance may make tiny small crystals, if this continues the crystals will turn in to stones. This can inflate, the kidney and swelling which follows unattainable pain. This pain can move to groin pain or even testicle pain. The most common type of kidney disease are Calcium stones.
There is a more likely chance found in men in receiving a kidney stone three times more than a woman may. A person that is over weight. Gender and diabetes are all more likable people who have a chance in having kidney stones, some time in their life. Caucasian men over right kidney stones VS. African-Americans. Age is also a big factor. People between the ages of 20-70 are much more likely to get kidney stones. There's a smaller percentage of kidney stones found in children or young adults.
Other risk factors may be from Lack of fluids. This can result in high concentration in your urine. It can at times be hereditary from immediate family, or if you've had it prior to having it again. There is a more likely chance of getting kidney stones depending on your diet. If you have high-protein, high-sodium or low-calcium diet. If you are more active you have a smaller chance of getting any type of kidney stone. People who are less active produce more calcium in their bones which can lead to kidney stones. After having, Gastric bypass surgery, inflammatory bowel disease or chronic diarrhea can also lead to kidney stones. 
Kidney stones found in most people are commonly able to pass them on their own, with no surgery needed. In some cases people have great difficulty with kidney stones, and surgery turns to be the only substitute. There are three types of surgery for the removal of kidney stones. They are extracorporeal shockwave lithotripsy, percutaneous nephrolithotomy, and ureteroscopy. Some reasons a person may need surgery are that the stones block the urine from leaving the bladder. Or constant urinary tract infections. Lastly, if it damages the kidney tissue or creates constant bleeding. 
Extracorpeal shockwave lithotripsy -This type of surgery consists of shock waves that break up the kidney stones. After this reaction from the shock waves, the kidney stones then break up and can now easily travel down back in to the urinary tract. Some patients may feel bruising, and little discomfort in the posterior of the abdomen. 
Percutaneous nephrolithotomy- This type of surgery consists of a doctor or surgeon making an incision through your back. A tube is then inserted and the stone is removed by the tubing. During this type of surgery, the doctor can break up the stones and then take the stones out(with the tube). 
Ureteroscopy- This type of surgery, unlike the percutaneous nephrolithotomy surgery, does not require an incision. Instead, the doctor will usually insert a ureteroscope into the urethra, to the bladder, then to the ureter, which is ultimately where the kidney stones are located.  The ureteroscope has lenses like a telescope or microscope, which let the doctor focus on the inner surfaces of the urinary tract.
- The Kidneys and how they work author, NIDDK, February 2009.
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