THE HUMAN KIDNEY

                                             

      Did you­ know that kidney stone plagued even the ancient Egyptians? Or that 26 million Americans have chronic kidney disease? Hundreds of thousan­ds of people suffer from renal failure each year and undergo dialysis or await a kidney transplant.
But what do your kidneys do? Why are they so important? Don't they just produce urine? In this article, we'll take a close look at our kidneys and find out exactly what they do.
    Your kidneys are two bean-shaped organs, each about the size of your fist. They are located in the middle of your back, just below your rib cage, on either side of your spine. Your kidneys weigh about 0.5 percent of your total body weight. Although the kidneys are small organs by weight, they receive a huge amount -- 20 percent -- of the blood pumped by the heart. The large blood supply to your kidneys enables them to do the following tasks:
    Regulate the composition of your blood: Keep the concentrations of various ions and other important
substances constant; Keep the volume of water in your body constant; Remove wastes from your body (urea, ammonia, drugs, toxic substances); Keep the acid/base concentration of your blood constant.
  Help regulate your blood pressure.
  Stimulate the making of red blood cells.
  Maintain your body's calcium levels.
  Your kidneys receive the blood from the renal artery, process it, return the processed blood to the body through the renal vein and­ remove the wastes and other unwanted substances in the urine. Urine flows from the kidneys through the ureters to the bladder. In the bladder, the urine is stored until it is excreted from the body through the urethra.

 In humans, the kidneys are the organs that produce urine. There are two kidneys, one on each side of your back, just below the ribs. Like most organs in the body, the function of the kidneys is closely tied to its structure.

Each kidney has three distinct areas:

The renal cortex, which is the outer layer.

The renal medulla, which is the middle layer.

The renal pelvis, which becomes a ureter.

Each kidney contains more than 1 million nephrons, which are microscopic tubules that make urine. Each nephron contributes to a collecting duct, which carries the urine into the renal pelvis. From there, the urine flows down the ureter, which is the tube that connects the kidney to the bladder.

Each of the million tiny nephrons in the kidney is a mass of even tinier tubules, as shown in the figure.
The main part of the nephron consists of the proximal (near) and distal (far) convoluted tubules, which become the nephron’s collecting duct.


Structure of the kidneys and the nephrons inside the kidneys.
At the beginning of the proximal convoluted tubule is a ball-like structure made up of the glomerulus, which is the site where the nephron’s tubule intermingles with a capillary, and the glomerular capsule (also called Bowman’s capsule).

In the glomerulus, the transfer of waste products from the bloodstream takes place through the capillary wall into the tip of the proximal convoluted tubule. Also at this site, any materials that are filtered by the nephron and are to be returned to the bloodstream are reabsorbed from the glomerulus through the capillary wall so that they can be recirculated.

Venules (smallest veins) join the capillaries (smallest arteries), and together, they join the renal vein, which carries blood away from the kidney.

Urine is spurted from the ureter into the top of the bladder continuously. The bladder holds a maximum of about 1 pint of urine, but you begin to feel the need to urinate when it is only one-third full. When the bladder is two-thirds full, you start to feel really uncomfortable.