Kidneys and Urinary Tract

The Kidneys

The kidneys are two organs located behind the abdomen and near the middle of the back.  Each one is about 4 or 5 inches long and about the size of a clenched fist. With one on each side of the spine, the kidneys are truly sophisticated processing machines.  Each and every day, a healthy person's kidneys process approximately 200 quarts of blood, sifting out around 2 quarts of extra water and waste products. 

The extra water and wastes become urine, which flows through tubes called ureters to the bladder.  The waste matter in the blood is composed of the normal breakdown of active tissues such as muscles and from food.  The body, which uses food for self-repairs and energy, takes what it requires from food and the wastes are sent to the bloodstream.  Without the kidneys to cleanse the blood of these wastes, the wastes would build up and damage the body.

Waste removal actually occurs in small units inside the kidneys which are called nephrons.  Each of the kidneys has about a million nephrons.  In the nephron, a tiny capillary or blood vessel called a glomerulus intertwines with a urine-collecting tube (tubule).  The glomerulus acts as a sieve or filtering unit and retains normal cells and proteins in the blood but allows wastes and extra fluid to pass through.  A complex chemical exchange takes place as the water and waste materials leave the bloodstream and enter the urinary system.

Each kidney is protected from harm by three outer layers:

  • The renal fascia, a tough, external coat of fibrous connective tissue;
  • A layer of fatty tissue called the adipose capsule; and inside this
  • The renal capsule, another fibrous layer.

The main part of the kidney also has three layers.  They are the renal cortex, which is packed full of capillaries and their capsules; the renal medulla, containing the capillaries and urine-forming tubules; and a central area where urine collects which is known as the renal pelvis.

Each nephron has two tubes, one for forming urine and one for carrying blood. 

The tubules first receive a combination of chemicals the body can still make use of as well as waste materials.  The kidneys take out the chemicals such as phosphorous, sodium and potassium and put them back into the bloodstream to return to the body.  This is how the kidneys regulate the body's level of these chemicals, which is an important process as the right balance is required for life to be sustained.

The word "renal" is referring to the kidneys, and "renal function" relates to kidney function.  When a health care practitioner discusses renal function, they are talking about how efficiently the kidneys are filtering an individual's blood.  Those people who have two healthy kidneys are said to have 100% of their kidney function.  Mild declines in kidney function (say, 30- to 40-percent failure) would rarely be noticeable.

The function of the kidneys is now calculated using a blood sample and a formula to come up with the estimated glomerular filtration rate, or eGFR.  This number corresponds to the percent of kidney function available. 

Some people are born with only one kidney.  However, they can still live normal, healthy lives.  Thousands of people every year donate one of their kidneys for transplantation to a friend or family member.

However, for many people with reduced kidney function a disease of the kidneys is also present and will continue to get worse.  Health problems which are serious can occur in those people who have less than 25% of their kidney function.  When kidney function drops below 10 to 15 percent, some form of renal replacement therapy is required, such as dialysis (blood-cleansing treatments) or a kidney transplant.

Other important functions of the kidneys include the regulation of blood pressure and the production of erythropoietin, which controls red blood cell production in the bone marrow.

The Urinary System

The urinary system is comprised of the organs, tubes, nerves and muscles that all work together to create, store and carry urine.  As well as the kidneys which are discussed above, this system also includes two ureters, the bladder, two sphincter muscles and the urethra.

Once the body has taken what it can use from food, the waste products are left behind in the bowel and the blood.  The urinary system then works in conjunction with the lungs, skin, and intestines (all of which also secrete wastes) to retain the chemical and water needed to keep the body in balance.  The amount of urine excreted depends on several factors, including the amount of food and fluids which a person consumes and how much is lost through breathing and sweating.  Certain medications can also have an effect on the amount of urine which leaves the body.

The bladder is a hollow, muscular organ which is balloon-shaped.  The bladder sits in the pelvis and is held in place by ligaments which are attached to the pelvic bones and to other organs.  The bladder stores urine until the individual is ready to empty it.  This organ swells into a round shape when full and becomes smaller when empty.  When the urinary system is healthy, a person can comfortably hold up to 2 cups of urine for 2 to 5 hours.

Nerves in the bladder signal when it is time to urinate.  This sensation becomes stronger as the bladder continues to fill and reaches its fluid limit.  When full, the nerves send a message to the brain and the urge to urinate intensifies.

A few interesting facts about urine include:

  • The amount of urine formed at night is about half that formed during the day.
  • Normal urine is sterile.  It does contain fluids, waste products and salt, but it is free of viruses, fungi and bacteria.
  • The tissues of the bladder are isolated from toxic substances and urine by a coating which discourages bacteria from attaching and growing on the wall of the bladder.

Posted in Kidney & Urinary Tract

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