The urinary system is a bodily system responsible for producing, storing, and secreting urine (waste). It includes the kidneys, urinary bladder, urethra, and ureters. It also serves several important roles other than filtering out wastes; including maintain the water levels in the body, and control blood pressure and pH levels. After taking nutrients and vitamins from digested food and converting it to energy for the body to use, certain waste materials are left behind that need to be excreted from the body. Waste materials, both solid and liquid, are left in the bowel and in the blood. This is where the urinary system comes into play.
Urea, which is the primary waste removed by the kidney, is the result of the breakdown of foods and vegetables that have protein. The urea is carried by the bloodstream to the kidneys where the filtration process begins.
The urinary system is a critical system in the body. The average adult excretes almost a quart and a half of urine per day, which varies depending on the amount of liquid consumed. This waste must be removed from the body and the urinary system provides that mechanism. If kidney failure occurs, death will usually result in 2-3 days.
The central organs of the urinary system are the kidneys. There are two kidneys, each possessing it's own ureter. A ureter removes fluid from the kidney and transports it to the urinary bladder. The main function of the urinary bladder is to collect and hold urine. While the amount of urine increases, the bladder stretches due to it's makeup of mucosa which comes from stratified transitional epithelium. The bladder drains through the urethra which directs outside the body. 
The kidneys exist as the most important organs of the urinary system. They perform the filtration process, waste removal, and waste extraction into the urine. Although there are various accessory organs that assist in the functions of the urinary system, the kidneys are the primary organs that perform all functions. There are two kidneys in the body that can be found on each side of the vertebral column. The kidney on the right side of the vertebral column is positioned slightly lower than the left kidney because the liver pushes it downwards. Since the kidneys are protected by the lower rib cage, they are held in slight depressions against the abdominal wall, which is found behind the parietal peritoneum. Connective tissue holds both kidneys in place, and are surrounded by adipose (fatty) tissue for protection.In the average adult, the kidney size is around 3 cm. thick, 6 cm. wide, and 12 cm. long. It has the shape similar to a bean. Each kidney contains more than a million functional units named nephrons. The urine passed through the nephrons right into collecting ducts. 
There are two ureters in the body, each one connecting to either the right or left kidney. A ureter is a small tube that transports urine away from the renal pelvis and carries it to the urinary bladder. Each ureter is approximately 25 centimeters long. It is connected to the renal pelvis, lines the posterior abdominal wall, and connects to the urinary bladder on the posterior inferior surface. The walls of the ureters are made up of three different layers. The outer layer is made of a fibrous connective tissue that is used as a supporting layer. The second layer is the muscular layer that is made up of inner circular and outer longitudinal smooth muscle. The purpose of the middle layer is to propel the urine towards the urinary bladder. Then the innermost layer is made up of transitional epithelium and is often known as the mucosa. The role of this layer is to secrete mucus to line and protect the cells' surface layers. 
The urinary bladder acts as a storage unit for urine. It can be found in the pelvic cavity. The size of an average bladder can vary depending on the amount of urine inside the bladder. It possesses an inside lining that is a mucous membrane composed of transitional epithelium that continues down into the ureters. Once the bladder is emptied out, the inner lining (mucosa) possesses several folds that are called rugae. The rugae and transitional epithelium permit the bladder to expand in size when it begins to fill with urine.  The lining of the bladder is made up of special tissues that prevent bacteria, viruses, and fungi from growing on the walls of the bladder. 
The uretha is the last stage of transportation for the urine. It is a tube that has thin walls, that transports urine after emptying the bladder to the final stage of excretion. The uretha is made of mucosa lining that is also known as transitional epithelium. It also possesses smooth muscle fibers that gains support from connective tissue. The uretha is encompassed by the internal urethal sphincter. The sphincter is made of smooth muscle. The other sphincter, also known as the external urethal sphincter is made of skeletal muscle. It surrounds the uretha as it passes through the pelvic floor. The two sphincters manage the rate of flow of urine by the uretha. In adult females, the size of the uretha is anywhere between three to four centimeters long. However, in adult males, the size differs greatly from females. The average size for the male uretha is anywhere around twenty centimeters long. 
Urine formation is the main function of the urinary system. There are four general steps of urine formation. Urine formation first starts with filtration. This is when fluid will exit the blood and go through a filtration membrane so it can be carried to the lumen of the Bowman's capsule. Filtration takes place in the glomerulus, therefore calling the process, glomerular filtration. The role of the filtration membrane is to prevent blood cells and proteins from leaving the blood vessels. This way, the blood fluid and its non-protein contents are the only substances that enter the nephron. The blood fluid with its non-protein contents is called filtrate, and is very similar to blood plasma, with the exception of no proteins it can be found in the nephrons of the kidneys. The filtration process is strictly to prevent the blood cells and proteins from leaving the blood vessels. The filtrate that circulates through the filtration barrier carries both good and bad chemicals. These chemicals flow into the proximal tubule of the nephron. Even though now, the filtrate is located in the functional unit of the kidney, it is still not the same substance as urine. When the filtrate enters the proximal tubulue, the fluid is technically still the same as interstitial fluid.
The next step in urine formation is known as reabsorption. When blood passes into and out of the blood vessels that are found lining the nephron, the important chemicals which the body needs are reabsorbed through the wall of the nephron. This process is controlled from a sequence of intricate transport processes. These processes guarantee that only the important and needed chemicals are being reabsorbed back into blood circulation and that they are received in healthy amounts. For example, sodium is an important element of blood plasma, however, if too much sodium exists in the nephron, there is no chance that all of it will be reabsorbed back into the blood. Reabsorption will only take place if the sodium levels in the blood plasma are at their correct levels. Thus, it is important that each element and substance being reabsorbed is at it's proper level and correct amount. 
The third step in urine formation is secretion. This takes place when specific chemicals from the nephron flow into the blood and must be removed from the blood plasma. These specific chemicals can not reside in the plasma and need to be secreted back into the nephron. Secretion removes many of the useless, harmful, and excess chemicals which pass back into the blood during reabsorption and filtration. The amount of secretion that actually takes place is much smaller in comparison to the reabsorption process in the kidneys. Despite it's minimal role, secretion is vital because the kidneys are able to alter the blood's pH levels through secreting Hydrogen ions. 
The last and final step of urine formation is water reabsorption. This step is very similar to step three, however, it is more complicated and intricate than secretion. Each of the mechanisms that take place in the nephron coordinate the water volume levels throughout the body. The water will insinuate into the nephron where the blood will reabsorb the exact amount of water required and leave the rest. The abiding water is what makes up the central factor of urine. 
Here is a video of the urine formation process.
There are several ways for the urinary system to become problematic for the human body. Issues with the urinary system are caused by injury, sickness, disease, and aging. As a human grows older, the structures of the kidneys changes, which leads to the kidneys losing their ability to extract wastes from the blood. In addition, the muscles found in the ureters, bladder, and uretha become worn and aren't quite as strong as they used to be. This can lead to urinary infections since the muscles in the bladder are loose, which leaves the bladder with some remaining urine that is unable to be expelled from the body. When the muscles in the urinary system weaken, the sphincters and pelvis have also lost strength, which causes incontinence. Various sorts of injury and sickness can also prohibit the kidneys from filtering the blood thoroughly which will create a barrier for the urine passageway. 
- Benign Prostatic Hyperplasia (BPH): BPH is a disorder that typically affects older males. BPH enlarges the prostate gland which conflicts with the urinating process. The enlargement of the prostate gland creates blockage because it constricts the urethra, which causes difficulty and pain in urinating. Typically, with BPH, males usually have other bladder symptoms such as incontinence and the need to frequently empty the bladder. Several treatment options exist in treating BPH. 
- Kidney Stones: Kidney stones are typically referred to as stones or calculi which can be found in the urinary system. They are more typically found in males than in females. Stones are made in the kidneys and can be located anywhere within the entire urinary system. Due to their variation in size, stones can cause both large amounts of pain or small amounts of discomfort. The goal of treating kidney stones are to completely remove the stone, avert infection, and avert stones from reoccurring. 
- Urinary Tract Infections (UTI): A urinary tract infection is when bacteria is found in the urinary tract. When any part of the urinary system is infected, doctors typically diagnose it as a UTI. A UTI is the second most widespread form of infection in the body. Symptoms include: severe burning or pain while urinating, fatigue and shakiness, frequency in urination, bloating in the lower stomach area, discolored or putrid smelling urine, pain in the lower back, and nausea. 
- Urinary Retention: Urinary retention can also be recognized as bladder emptying problems. It is a widespread problem that can result from various circumstances. Typically, the urination problem is involuntary and the bladder will completely empty on it's own; however, urinary retention is the complete opposite situation. It's the process of aberrantly holding urine in the bladder. Acute urinary retention is the abrupt inability to empty the bladder. Both acute urinary retention and urinary retention are extremely painful and cause much discomfort. Chronic urinary retention is the constant existence of urine left in the bladder when the bladder is not fully emptied. Causes of urinary retention, both acute and regular stem from muscle failure in the urinary system- particularly the bladder, damaged nerves, or blockages in the urinary tract. The kinds of treatment available for urinary retention depend on the cause of the retention. 
- Urinary Incontinence: Urinary incontinence is the complete loss of bladder control, meaning someone who suffers from incontinence can maintain no control over when they urinate. The male or female possesses no control and the urination process becomes involuntary. Various types of urinary incontinence exist and several causes lead to incontinence, however, there are various treatments available. Treatments such as exercises and surgery are available for those suffering from urinary incontinence. This ailment is typically found more in women than in men.
- Kidney Failure: Kidney failure is a result of many different causes. It can stem from kidney damage, low blood pressure, blood clotting near the kidney's blood vessels, kidney infections, and pregnancy mishaps. It can be known as acute kidney failure which is the acute loss of the ability to excrete wastes from the body and concentrate urine while trying to prevent the loss of electrolytes. Treatments such as diet change and dialysis can help aid the symptoms of kidney failure. Symptoms can include bloody bowel movements, seizures, extreme fatigue, tremors, and changes in look and smell of urine. 
- Anatomy of the Urinary System The Ohio State Medical Center, 2007.
- Wile, Jay. The Human Body: Fearfully and Wonderfully Made!. 2001, Apologia Educational Ministries, p.458
- The Urinary System National Cancer Institute, 2000.
- Wile, p.458
- Your Urinary System and How It Works National Institute of Diabetes and Digestive and Kidney Diseases, 2007.
- Urinary Tract Infections Medline Plus, 2011.
- Kidney Failure Yahoo! Health, 2008.