The Anatomy of the Kidneys

The kidneys are the body's filtration system. These fist-sized, bean-shaped organs manage the body's fluid and electrolyte balance, filter blood, remove waste, and regulate hormones. They produce urine to carry the wastes out of the body.

Anatomy

Each person has two kidneys. The kidneys are located on either side of the spine, with the top of each kidney beginning around the 11th or 12th rib space. The kidneys are sandwiched between the diaphragm and the intestines, closer to the back side of the abdomen. Roughly the size of a closed fist, each kidney measures about 10 to 12 centimeters long, 5 to 7 centimeters wide, and 3 to 5 centimeters thick. Each kidney is connected to the bladder through a ureter. The ureter brings waste products—urine—to the bladder, where it is stored until it leaves the body through the urethra. Together, all of these organs make up the renal system.

Structure

Each kidney is covered in a thick layer of connective tissue and fat that helps shape and protect the organ. The kidneys are fed by renal veins, arteries, and nerves. About 20% of the body's cardiac output—or the amount of blood the heart pumps each minute— flows through the kidneys when the body is at rest. Blood flows into the kidneys through renal arteries that originate at the aorta.

As the blood passes through the kidneys, the vessels that carry the blood get smaller and smaller until they deliver blood to the nephrons. Each kidney contains about 1.3 million nephrons, which do the filtering work of the kidneys. Within each nephron, there is a microscopic filtration unit consisting of an exterior capsule—Bowman's capsule—and a network of tiny capillaries called glomerulus.

As blood moves through the capillary network, or glomerulus, larger components are filtered out by tiny finger-like structures and the remaining blood passes to Bowman's capsule. From there, the filtered blood collects in Bowman's capsule until it is transferred to a system of tubules. While in the tubules, liquid and solutes will diffuse through additional layers of filtration. Some liquids and solutes will be reabsorbed and returned to the body via the renal veins to the vena cava, while others will be secreted as waste—urine—through the ureters. The ureters transport urine to the bladder for storage until it is excreted from the body through the urethra.

Anatomical Variations

In some cases, the kidneys do not form correctly during pregnancy, resulting in congenital malformations.

• Ectopic kidney: Kidneys are initially formed in the pelvis and move up into their permanent position as a fetus develops. In some cases, the kidneys never move to their final location. This can result in a blockage in the flow of urine and require surgery to correct.
• Malrotation: Just as kidneys may never fully move to the correct position during development, they may also not reach the correct position. Malrotation can results from kidneys not properly moving into their final position during development. This could also result in blockages that may require surgical correction.
• Horseshoe/fused kidney: As the kidneys move to their permanent position during development, they can sometimes fuse together, forming a horseshoe shape. The result is one large renal mass rather than two separate kidneys. In some cases, there are no symptoms to indicate you would have fused kidneys, but other times, a host of problems can arise, including problems with kidney stones or urine drainage.
• Kidney agenesis: Occasionally, one or both kidneys may never form at all. While missing both kidneys is fatal, a single kidney will usually adapt and enlarge to perform the function of two.

Function

The main purpose of the kidneys is to filter blood and maintain fluid and electrolyte balance in the body. Together, your kidneys filter your body's entire blood volume about 300 times per day. Electrolytes and solutes like sodium and potassium are regulated in the kidneys and transported to different parts of the body. Blood is filtered several times while in the kidneys, returning about 99% of the water in your blood back into the circulation system, and turning the remaining water and any waste products into urine.

In addition to filtering blood and removing waste, one of the vital functions of the kidney is maintaining the body's fluid volume. Electrolytes like sodium play a role in this process, as well as hormones like antidiuretic hormone (ADH), aldosterone, and atrial natriuretic hormone. Electrolytes and hormones respond to the body's needs to increase or decrease fluid volume, maintaining blood pressure and the body's overall homeostasis.

Associated Conditions

A number of diseases and conditions can impact the function of the kidneys. Some are genetic and others develop as a result of other diseases or lifestyle choices.

• Polycystic kidney disease: This is a genetic form of kidney disease that results in the formation of cysts within the kidney and can lead to kidney failure.
• Kidney stones: These are small masses formed by salts or minerals that build up in your kidneys. They may pass from the body on their own or require more invasive removal when they block the passage of urine from the body.
• Acute renal railure: This occurs when the kidneys suddenly stop working. Acute renal failure or acute kidney injury occurs quickly, with fluids and waste products building up and causing a cascade of problems in the body.
• Chronic kidney disease: This is the result of long-term kidney damage that gradually reduces the function of the kidneys. While some loss of function is tolerable, serious problems develop as kidney function drops below 25%, and life-threatening complications can arise as function drops below 10% to 15%.
• Cancer: A number of cancers can affect the kidneys, including renal cell carcinoma. Cancer treatments, as well as other nephrotoxic medications, may also damage the health of your kidneys.

Tests

There are a number of blood tests, urine tests, and scans that can help a doctor determine how well your kidneys are functioning.

• Blood tests: Testing your glomerular filtration rate (GFR) through a blood draw is the best indicator for the ability of the glomerulus to filter the blood. Normal GFR rates are 90 to 120 milliliters (mL) per minute. Kidney disease is staged based on the range of these numbers, with a GFR of less than 15 mL per minute indicating kidney failure, or end-stage renal disease. Other blood tests that can help measure kidney function include creatinine, blood urea nitrogen, Cystatin C, and metabolic panels that test levels of electrolytes.
• Urine tests: Urine sample testing can provide information about kidney function. Tests include a urinalysis, measuring protein and albumin levels, and osmolality.
• Imaging: A number of scans can help detect function and disease in the kidneys. These test can include X-ray, computed tomography (CT) scan, a nuclear imaging kidney scan, or ultrasound. Scans may be used to determine blood flow through the kidneys, or visualize cysts, stones, or tumors.

Treatment

In cases of severe kidney damage and loss of function, the body can no longer maintain its fluid and electrolyte balance. Toxic levels of waste can cause neurologic and/or cardiac problems.

While you can work to prevent risk factors for kidney disease like type 2 diabetes and high blood pressure, kidney failure will require more intensive treatment. Treatments can include medications or, in severe cases, dialysis. Dialysis uses an external process to filter blood in place of your kidneys. Dialysis is usually used until a kidney transplant is possible.

Kidneys can be transplanted from living or deceased donors. Diseased kidneys are sometimes left in place during transplantation, but may be removed in some cases. The new kidney—often from a close family member in the case of a living donor—is then implanted and connected to your blood vessels and bladder. There are a host of standard surgical risks involved, as well as the chance that your body may reject the new organ.

If one kidney fails, is donated, or is surgically removed to treat cancer or other conditions, it is possible to survive with only one kidney, but there are risks, and regular testing is required.