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II.   How Your Kidneys Work

The urinary system is made up of the kidneys, ureters, bladder and urethra. Together, they make, store and get rid of urine. As you will read in the following discussions, problems that affect one part of this system often have an impact on the other parts.

The kidneys are two of the vital organs in your body. In fact, their work is so important that other major organs cannot function properly if the kidneys become damaged by disease or injury. Their job is to remove waste products from your blood as it passes through each kidney. The waste products and excess water are removed from your blood and passed out of the body as urine.

Healthy kidneys regulate the amount of salts and chemicals in your body and maintain the balance needed for your body to function properly. The kidneys also make a number of hormones that:

In order to understand what can go wrong with your kidneys, it is helpful to understand what happens when the kidneys are healthy. The following sections provide an overview of the anatomy and physiology of the kidneys.

A.   Kidney Anatomy

Normal Anatomy of the Kidneys and Urinal Tract

Most people are born with two kidneys, which are located behind the abdominal organs, just below the ribs. Imagine two giant kidney beans - each about the size of your fist - lying on either side of your spine, right below your rib cage. Each kidney receives blood through the renal artery, which branches off the main artery - the aorta - which carries blood from the heart. When the blood has passed through the kidney and has been filtered, it returns to the main bloodstream through the renal vein, which is connected to the vena cava, the blood vessel that returns blood to the heart.

Inside each kidney is a network of about a million tiny blood-filtering units, known as nephrons. Each nephron contains a glomerulus (a cluster of tiny blood vessels that filters waste and water from the blood), and a tubule (a place where water is subtracted and chemicals are either added to or subtracted from the waste fluid to form urine). The waste products the kidney has filtered from the blood are collected in the renal pelvis of the kidney and pass through the ureter into the bladder as urine.

The body only needs one healthy kidney to function properly, so even people who are born with only one kidney, or who lose the use of a kidney due to injury, illness, or who donate a kidney to another person, can live a normal life.

B.   Kidney Physiology

When most people think of kidneys, they think about urine formation. However, the kidneys have four major responsibilities:

  1. Balancing Body Fluids
    Healthy kidneys know how much fluid to get rid of and how much to save for the body to function properly.

  2. Balancing Body Chemicals
    Healthy kidneys are responsible for maintaining a proper balance of various chemicals your body needs to function. One of the most important of these is potassium, which is needed for nerve and muscle control. Too much or too little potassium can cause weak muscles and heart problems. Healthy kidneys are also active in balancing the levels of calcium, phosphorus and magnesium in your blood.

  3. Removal of Waste Products
    A waste product called urea is formed from the breakdown of protein from the foods you eat. Another waste product called creatinine is formed by normal muscle activity.
  4. Release of Essential Hormones
    Normal, healthy kidneys release several vital hormones into the bloodstream, including:
    1. renin, which helps regulate blood pressure
    2. erythropoietin (EPO), which helps bone marrow make red blood cells an activated form of vitamin D, which regulates calcium absorption from food and helps maintain healthy bones.

Now that you have learned some basic information, let's tie it all together in one smooth process. Every 24 hours, the kidneys filter and clean about 200 quarts of fluid from the blood. Of this, about 198 quarts are absorbed back and retained in the body, and about two quarts are sent to the bladder in the form of urine.

Here is a step-by-step look at how the system works:

  1. Your heart pumps blood into your aorta, which carries it to the artery leading to each kidney (renal artery).
  2. The blood passes through the artery into your kidney where the tiny filters (the glomeruli) and tubules cleanse it of excess fluid and waste products.
  3. The clean blood then returns to the main bloodstream through the renal vein, which connects to the vena cava, and it is re-circulated through the body.
  4. The waste material cleared from the blood passes from the kidney to the bladder in the form of urine.
  5. This is a continuous process that takes place around the clock, 24 hours a day. Once the bladder is full, and you have the urge to urinate, the urine enters the urethra (a tube that drains urine from the bladder) and passes out of the body.

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