Urinary System
How Urine Is Made And Expelled From The Body

Health Topics


Urinary System


What Is The Urinary System?
Structures Of The Urinary System
How Does The Urinary System Work?
What Is Urine Made Of?
Diseases And Disorders
Other Systems It Links To In The Body

Human Body, Diagrams
Female Body, Diagrams

In This Article

• Structure and function of the urinary system.
• What is urine made of?
• Diseases and disorders of the urinary system.

What Is The Urinary System?

It is the system in your body that creates, stores and transports urine. It consists of the kidneys, ureters, bladder and urethra. The urinary system is one of the human body's waste disposal and filtration units (the digestive system, respiratory system and the skin are others). The kidneys filter blood removing harmful waste products like urea and alcohol. Urine is the end result of this filtering process which is eventually excreted out of the body through the urethra. If toxins were not removed from your blood in this manner, it could be fatal.

Interesting Facts

• You can survive without food for weeks but only a few days without water.
• The average adult loses 1.5 liters of water a day in urine.
• The average adult takes in 2.2 liters of water a day (1.4 liters from drinks and 0.8 liters from food).
• Since 96 percent of urine is composed of water it is no surprise that a common English expression for urination is to pass water.
• 8 large glasses per day is the recommended intake of water for a healthy body.



What Is The Kidney?
A kidney is a bean-shaped organ, about 11cm long. We have two kidneys positioned near the middle of the back, just below the ribcage. All blood in the body flows through the kidneys every 10 minutes, so your blood is filtered for toxins 150 times a day.
Structure: A kidney has 2 distinct parts: the cortex on the outside and the medulla on the inside. The medulla leads into an area called the pelvis (sometimes called the renal pelvis). The concave center of the kidney is known as the hilum and it is at this point that blood vessels, lymphatic vessels, nerves and the ureter enter the organ. Kidney tissue is made up of over a million twisted tubes called nephrons, which do the kidneys' work of filtration and excretion.
Functions: To filter the blood, reabsorb useful materials needed by the body and form urine.
Renal Pelvis
Structure: The renal pelvis is a funnel-shaped cavity which connects the medulla to the ureter.
Function: It collects urine from the tubules in the medulla and passes it into the ureter.

Ureters, Bladder and Urethra


The tubes which connect the kidneys to the bladder.
Function: To take urine from the kidneys to the bladder. The presence of urine inside them stimulates a mechanical contraction which propels the fluid forwards.

Structure: Sometimes called the urinary bladder this is a sac-like organ in the pelvic cavity.
Function: A reservoir (storage place) for urine. When about 200ml of urine has been collected the presence of the liquid stimulates the autonomic nerve endings in the bladder wall and the walls contract. The bladder has an internal sphincter muscle which relaxes when the walls contract, thus opening and emptying the urine into the urethra.

Structure: A narrow tube passing from the bladder to the outside of the body. It has an external sphincter which is voluntarily controlled by the central nervous system. It is shorter in women, thus making them more susceptible to infections (like urinary tract infections).
Function: To take urine from inside the body (the bladder) to outside. In men, the urethra is also the passage for semen.

How Does The Urinary System Work?


3 Stages Of Filtration And Urine Production

1. Filtration in the Bowman's capsule

Blood enters the kidneys via the afferent arterioles. These tiny blood vessels become the glomerulus, a tangle of capillaries surrounded by the glomerular capsule, also known as the Bowman's capsule. The blood in the capillaries is under pressure and since the capillary walls are permeable to water and other substances these pass through into the capsule, whilst blood cells and protein remain in the blood vessel. The Bowman's capsule thus serves as the collection point for the waste products carried in the blood. However, at this point the capsule has also collected other substances which are not waste and these will be reabsorbed as they pass through the nephron.

2. Re-absorption in the convoluted tubules

Once the filtered substances have been collected by the capsule they are passed into a system of twisted tubes, known as convoluted tubules. The tubes of the nephron which lead away from the Bowman's capsule are known as the proximal convoluted tubules. These straighten out into a long loop, called the Loop of Henle, which passes into the medulla and back to the cortex. Finally, there is another series of twists called the distal convoluted tubules.
Reabsorption takes place in the tubules. Cells in the lining of the tubules are able to absorb any water, glucose, salts and ions which the body needs that must not be disposed of as waste. Only 1 percent of the liquid filtered into the Bowman's capsule is actually excreted as urine. The rest is re-absorbed.

3. Collection in the pelvic calyces

The nephron straightens out into a collecting tube in the medulla. These collecting tubes form masses called pyramids of the medulla, the tops of which stick up into the renal pelvis. The branches of the pelvis, or calyces, connect with the tops of these pyramids and collect the waste liquid, funneling it back into the pelvis, from where it will empty into the ureter.


What Is Urine Made Of?

The liquid that results from the processes of filtration and reabsorption is known as urine. This amber-colored liquid is composed of 96 percent water, 2 percent urea, and 2 percent other substances, such as ammonia, sodium, potassium, phosphates, chlorides, sulphates and excess vitamins. The salts must be excreted in order to maintain the correct balance of fluids and electrolytes in the body. The color of urine comes from bilirubin, a bile pigment. Normal urine is acidic, but this varies depending on diet and other factors.

Urine production

About 1.5 liters of urine is produced every 24 hours, which is only a small percentage compared to the amount of liquid filtered from the blood in the glomerulus. Urine production is increased by liquid intake and cold weather and decreased by drinking less and any activity or state that increases sweating (hot weather, exercise). Humans need to consume a minimum of 0.5 liters of water per day for waste removal.

Diseases And Disorders Of The Urinary System

The bladder is inflamed which causes pain when urinating. Cystitis is sometimes caused by infections. It is very common in women because they have a shorter urethra than men. See, how to treat cystitis.

Interstitial Cystitis
Interstitial cystitis (IC) also produces inflammation of the bladder and pain on urination, however it is not considered a urinary tract infection. Although symptoms appear very similar to cystitis or a UTI, a urine sample or culture will not reveal any bacteria. IC is more recently being referred to as bladder pain syndrome (BPS) or painful bladder syndrome (PBS).

Kidney Stones
Solid stones that form from deposits of substances found in urine. Kidney stones can be found in the renal pelvis, bladder or ureters. They are very painful often need to be removed by surgery.

Nephritis or Bright's Disease
Umbrella term to describe inflammation of the kidneys caused by a range of conditions other than infection. Symptoms may include pain on either or both sides of the lower back, fever and nausea. Causes include lupus, measles, pneumonia, mononucleosis and hepatitis.

Diabetes Insipidus
Diabetes insipidus is where the kidneys cannot regulate how much urine it makes. This causes the patient to urinate excessively and become very thirsty. Diabetes insipidus is completely unrelated to regular diabetes (sugar diabetes), despite sharing a similar name.

Polyuria is defined as producing excessive amounts of urine - at least 2.5 to 3 liters over a period of 24 hours (normal urine loss is about 1.5 liters). The most frequent cause is uncontrolled diabetes (type 1 diabetes or type 2 diabetes) or diabetes insipidus. It may also be caused by drinking excessive coffee or alcohol or taking diuretics (water pills); as well as chronic fatigue syndrome, hyperthyroidism, pregnancy, cystitis and interstitial cystitis.

Disease of the glomerular region of the kidneys. The kidneys filters' become inflamed and scarred, slowly losing their ability to remove waste and fluid from the blood to make urine.

Kidney and pelvic inflammation caused by bacterial infection.

Urinary Tract Infections
A urinary tract infection is a bacterial infection that occurs in any part of the urinary tract.

Inflammation of the urethra which results in painful urination.

Pain or difficulties in urinating. It is usually caused by infection or inflammation.

Also known as bed wetting - it is the inability to control urination. The term is usually limited to older children who should be old enough to control bladder movements.

Urinary Incontinence
Urinary incontinence is the loss of bladder control. The severity ranges from a small leak when you sneeze (known as stress incontinence) to an urgent need to urinate (urge incontinence) that you don’t make it to the toilet.

A kidney tumor that typically occurs in children.

Renal Failure
The inability of the kidneys to excrete waste. It results in a situation where the kidneys fail to function adequately.

Renal Colic
A type of abdominal pain that is caused by the obstruction to the flow of urine - the obstruction is often caused by kidney stones.

A term that loosely describes the illness that accompanies renal (kidney) failure. Nitrogenous waste products (urea) that are usually excreted in the urine, accumulate in the blood. If urea builds up in the blood it causes fatigue, shortness of breath, bone pain, seizures and eventually coma.

Other Systems It Links To In The Body

The urinary system links to:

Circulatory system: The kidneys clean up the blood that flows around the body.
Endocrine system: The kidneys produce the enzyme renin which helps to regulate blood pressure as part of a system involving hormones.
Skeletal system: The kidneys help to stimulate the production of bone marrow in the long bones.
Skin system: The urinary system removes waste by excretion and therefore links to the other excretory system - the skin (through sweating).

SUMMARY: What The Urinary System Does

• Filters blood of potentially harmful substances
• Produces urine through the processes of filtration and re-absorption
• Excretes waste (urine).

Other Useful Guides

Lymphatic System: How fluid is drained from cells in the body.
Respiratory System: How we breathe, organs and disorders.
Hospital Departments Explained: Find your way around a hospital.

Back To Homepage: Womens Health Advice

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