There are two kidneys inside the human body, each about the size of 6 cm wide and 12 cm long, located on either side of the spine at the lowest level of the rib cage. Kidney has two main layers – an inner layer called the medulla and an outer layer called the cortex.
The kidneys filter and return to the bloodstream about 200 litres of blood every 24 hours. About two litres are removed from the body in the form of urine, and about 198 litres are recovered. The urine excreted is stored in the bladder for anywhere from 1 to 8 hours.
The functional units of the kidney are its nephrons, extremely fine blood-filtering tubules, about 1-2 inches long. Each kidney contains over a million nephrons. At the end of each is a cluster of tiny capillaries called the glomerulus. In the glomerulus fluid filters out of the blood, leaving behind the red and white corpuscles, and passes through the inner wall of the capsule into the nephron tubule. As it does, specific substances are secreted into it while others, like water, are selectively reabsorbed. The end product is urine.
The kidneys perform the vital job of maintaining the body’s acid-alkaline balance, called its pH. Blood pH needs to be held in a narrow range, slightly alkaline but close to neutral. The kidneys do this job by providing the blood with a buffer solution that compensates for any rise or fall in the level of acidity or alkalinity caused by diet or stress. This buffer solution contains fluids as well as vital salts, called electrolytes, including sodium, potassium and calcium. In a sense your body is like a car battery, which also depends on electrolytes to produce electrical current. But a battery is static while your body is dynamic. Electrolyte levels of are continually adjusted as needed to maintain pH.
The kidneys also produce hormones, like renin, which helps to control blood pressure, and erythropoietin (EPO), that stimulates the production of red blood cells. The high blood sugar in diabetes are toxic to the capillaries in the glomeruli, creating tiny holes through which toxins escape. Increased fluid pressure due to hypertension causes the walls of the blood vessels to thicken and the glomeruli lose their ability to filter blood.
As kidney function declines, along with proper pH and hormone balance, body cells detect the change. Damaged kidneys can’t produce enough of the hormone EPO to create an adequate number of red blood cells resulting in anemia. The heart tries to compensate by pumping blood harder. Over time, the heart muscle becomes larger but weaker and starts to pump inefficiently. A damaged heart can no longer pump sufficient amounts of blood to the body’s tissues. Edema (fluid waste) begins to back up into the lungs, and it becomes difficult to breathe. The final result is congestive heart failure. All these and many more fine balance of the body is maintained by kidneys.
In short, the kidneys maintain the chemical balance of the blood and regulates the body’s level of sodium, potassium and calcium. The kidneys remove waste products and excess water from the body and so help to regulate blood pressure. Kidneys activate vitamin D, which helps to maintain strong bones, and produce erythropoietin, a hormone that is vital for the production of red blood cells.
The kidneys are powerful chemical factories that perform the following functions:
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- remove waste products from the body
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- remove drugs from the body
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- maintains the body’s fluids and and electrolyte balance
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- release hormones that regulate blood pressure
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- produce an active form of vitamin D that promotes strong, healthy bones
- control the production of red blood cells
Each kidney contains up to one million functioning units called nephrons. A nephron consists of a network of tiny blood vessels called a glomerulus attached to a tubule. The glomeruli produces a solution that flows through the nephrons. When blood enters the nephron, it is filtered and with the help of the solution produced in the glomeruli, the substances that the body wants to retain (such as sodium, potassium, proteins and most of the water) are re-absorbed back into the blood and the remaining fluid then passes along the tubule carrying the waste products from the body in the form of urine. The production of urine involves highly complex steps of excretion and re-absorption necessary to maintain a stable balance of body chemicals.