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The kidneys are the main organs of the urinary system. They function chiefly to filter blood in order to remove wastes and excess water. The waste and water are excreted as urine. The kidneys also reabsorb and return to the blood needed substances, including amino acids, sugar, sodium, potassium, and other nutrients. The kidneys filter about 200 quarts of blood per day and produce about 2 quarts of waste and extra fluid. This urine flows through tubes called ureters to the bladder. The bladder stores the urine until it is excreted from the body.
Kidney Anatomy and Function
The kidneys are popularly described as being bean-shaped and reddish in color. They are located in the middle region of the back, with one on either side of the spinal column. Each kidney is about 12 centimeters long and 6 centimeters wide. Blood is supplied to each kidney through an artery called the renal artery. Processed blood is removed from the kidneys and returned to circulation through blood vessels called the renal veins. The inner portion of each kidney contains a region called the renal medulla. Each medulla is composed of structures called renal pyramids. Renal pyramids consist of blood vessels and elongated portions of tube-like structures that collect filtrate. The medulla regions appear darker in color than the outer surrounding area called the renal cortex. The cortex also extends between medulla regions to form sections known as renal columns. The renal pelvis is the area of the kidney that collects the urine and passes it to the ureter.
Nephrons are the structures that are responsible for filtering blood. Each kidney has over a million nephrons, which extend through the cortex and medulla. A nephron consists of a glomerulus and a nephron tubule. A glomerulus is a ball-shaped cluster of apillaries that acts as a filter by allowing fluid and small waste substance to pass while preventing larger molecules (blood cells, large proteins, etc.) from passing through to the nephron tubule. In the nephron tubule, needed substances are reabsorbed back into the blood, while waste products and excess fluid are removed.
Kidney Function
In addition to the removal of toxins from the blood, the kidneys perform several regulatory functions that are vital to life. The kidneys help maintain homeostasis in the body by regulating water balance, ion balance, and acid-base levels in fluids. The kidneys also secrete hormones that are necessary for normal function. These hormones include:
- Erythropoietin, or EPO - stimulates the bone marrow to make red blood cells.
- Renin - regulates blood pressure.
- Calcitriol - active form of vitamin D, which helps maintain calcium for bones and for normal chemical balance.
The kidneys and brain work in conjunction to control the amount of water excreted from the body. When blood volume is low, the hypothalamus produces antidiuretic hormone (ADH). This hormone is stored in and secreted by the pituitary gland. ADH causes the tubules in the nephrons to become more permeable to water allowing the kidneys to retain water. This increases blood volume and reduces urine volume. When blood volume is high, ADH release is inhibited. The kidneys don't retain as much water, thereby decreasing blood volume and increasing urine volume.
Kidney function can also be influenced by the adrenal glands. There are two adrenal glands in the body. One located atop each kidney. These glands produce several hormones including the hormone aldosterone. Aldosterone causes the kidneys to secrete potassium and retain water and sodium. Aldosterone causes blood pressure to rise.
Kidneys - Nephrons and Disease
The kidneys filter waste products such as urea from the blood. The blood arrives in an arterial blood vessel and leaves in a venous blood vessel. The filtration occurs in the renal corpuscle where a glomerulus is encased in a Bowman's capsule. Waste products drain through the convoluted proximal tubules, the loop of Henle (where water is reabsorbed), and into a collecting tubule.
Nephron Function
The kidney structures that are responsible for the actual filtering of blood are the nephrons. Nephrons extend through the cortex and medulla areas of the kidneys. There are over a million nephrons in each kidney. A nephron consists of a glomerulus, which is a cluster of capillaries, and a nephron tubule that is surrounded by an additional capillary bed. The glomerulus is enclosed by a cup-shaped structure called the glomerular capsule that extends from the nephron tubule. The glomerulus filters waste from the blood through the thin capillary walls. Blood pressure forces the filtered substances into the glomerular capsule and along to the nephron tubule. The nephron tubule is where secretion and reabsorption take place. Some substances such as proteins, sodium, phosphorus, and potassium are reabsorbed into the blood, while other substances remain in the nephron tubule. The filtered waste and extra fluid from the nephron are passed into a collecting tubule, which directs the urine to the renal pelvis. The renal pelvis is continuous with the ureter and allows urine to drain to the bladder for excretion.
Kidney Stones
Dissolved minerals and salts in urine can sometimes crystallize and form kidney stones. These hard, tiny mineral deposits can become larger in size making it difficult for them to pass through the kidneys and urinary tract. The majority of kidney stones are formed from excess deposits of calcium in urine. Uric acid stones are far less common and are formed from undissolved uric acid crystals in acidic urine. This type of stone formation is associated with factors, such as a high protein/low carbohydrate diet, low water consumption, and gout. Struvite stones are magnesium ammonium phosphate stones that are associated with urinary tract infections. Bacteria that typically cause these types of infections tend to make urine more alkaline, which promotes the formation of struvite stones. These stones grow quickly and tend to get very large.
Kidney Disease
When kidney function declines, the ability of the kidneys to filter blood efficiently is reduced. Some kidney function loss is normal with age, and people can even function normally with only one kidney. However, when kidney function drops as a result of kidney disease, serious health problems may develop. Kidney function of less than 10 to 15 percent is considered kidney failure and requires dialysis or a kidney transplant. Most kidney diseases damage nephrons, reducing their blood filtering capacity. This allows dangerous toxins to build up in the blood, which can cause damage to other organs and tissues. The two most common causes of kidney disease are diabetes and high blood pressure. Individuals with a family history of any kind of kidney problem are also at risk for kidney disease.
