The Urinary System


The Urinary System consists of a group of organs which filter out the excess fluid and other waste products from the blood. Many nitrogenous substances, ions, carbon dioxide, water, etc. that accumulate in the body must be eliminated. The urinary system plays an imperative role in maintaining the concentrations of water and electrolytes within the body by removing metabolic waste products, toxins and excess ions from the body in the form of urine. Simultaneously it retains the essential substances in the blood. Each day about 45 gallons, of fluid is filtered, which means fluid from the blood is filtered by the kidneys 20-25 times a day. Along with the kidneys, lungs and skin also participate in excretion, but the kidneys are the main excretory organs of the body. The urinary system also helps in the maintenance of homeostasis, as do the other systems of the body.  The kidneys are the major organs of homeostasis for the acid-base balance, as well as the water-salt balance of the blood.

Organs of the urinary system

The Urinary System is made up of the following structures:

  1. 2 Kidneys that filter blood
  2. 2 Ureters that transport the urine formed in the kidneys to the urinary bladder
  3. Urinary bladder that collects and temporally stores the urine
  4. Urethra is a small tube, which expels the urine from the urinary bladder

Kidneys: These are paired, bean-shaped, reddish-brown organs. They are located in the lumbar region, just above the waist, behind the peritoneal cavity in the abdomen and are thus known as a retroperitoneal organ. Kidneys are protected by a covering known as the renal capsule. The right kidney is placed slightly lower than the left because the liver occupies a large area on the right side. The renal capsule has a depression i.e. hilum on the concave side through which Renal blood vessels and lymph vessels, the ureter and nerves enter the kidney while renal vein and ureter exits the kidney.

Ureters: These are small muscular tubes that carry urine from kidneys to the urinary bladder. Their length is about 25 cm and diameter is 5 mm. Peristalsis helps in the transport of urine.

Urinary Bladder: It acts as a storage for urine until it is expelled from the body. Urinary bladder has three openings, two for the ureters and one for the urethra. It has an expandable wall for storage purposes. Two sphincter muscles help to control the release of urine into the urethra. Urine is stored in the bladder and excreted by the process of micturition.

Urethra: It is a small tube extending from the urinary bladder to an external opening. Its main function is to expel urine from the body. It is longer in males than females. In males, it also transports the semen.

Detailed structure of the kidney:

Longitudinally a section of the kidney reveals three main areas; renal cortex, renal medulla, and renal pelvis. Renal medulla is made up of cone-shaped tissue masses known as the renal pyramids. Renal pelvis has a central cavity for collection of urine formed by the kidney and is continuous with the ureter.

The kidney is made up of nearly 1 million functional units, known as the nephrons and a lesser number of collecting ducts. The collecting ducts transport the formed urine through the pyramids to the renal pelvis. The nephrons are formed by a cup- shaped structure known as the glomerular or Bowman’s capsule. A tube connects it to a collecting tubule. The Bowmen’s capsule is entirely covered with a network of arterial capillaries. The remainder of the nephron is about 3 cm long. This part of the nephron consists of three sections known as the proximal convoluted tubule, the medullary loop (loop of Henle) and the distal convoluted tubule. As the name indicates the distal convoluted tubule is the distal portion that ultimately leads to a collecting duct.

Urination or micturition

Micturition is basically the passage of urine by emptying the bladder. Urinary bladder wall has a smooth muscle layer, the detrusor muscle. Detrusor muscles contract on stimulation because it has a parasympathetic nerve supply. Urination is triggered when the urinary bladder fills up to 200 – 400 ml, because of stretching of the detrusor muscles of the bladder. In infants where micturition cannot be controlled voluntarily, this is the stimulus for the micturition reflex. However, in adults micturition can be inhibited by voluntary contraction of the external urethral sphincter for a limited period of time beyond which if the bladder continues to fill and be stretched, voluntary control is lost.

Functions of the urinary system

The main function of the kidney is to filter blood and form urine.

  1. Formation of urine – The kidneys form urine which passes through the ureters to the bladder for storage prior to excretion.
  2. Excretion of metabolic waste products – Most of the waste products formed are nitrogenous products. Kidneys play an important role in their excretion. The liver breaks down amino acids and simultaneously it releases ammonia. The liver then quickly combines that ammonia with carbon dioxide, creating urea which is much less toxic and is the primary nitrogenous end product of metabolism in humans. Humans also excrete ammonia, creatinine and uric acid. Excess amounts of uric acid accumulate in the joints in the form of crystals, thus causing gout as it is insoluble.
  3. Regulation of volume and electrolyte composition of blood – Kidneys control plasma volume by controlling the amount of water excreted. This has a direct effect on the total blood volume and eventually the blood pressure. Salt (NaCl) causes osmosis i.e. diffusion of water into the blood.
  4. Regulation of plasma osmolarity: Osmolarity is regulated by the kidneys as they directly monitor the amount of excretion of ions and water. Ions such as sodium, potassium, calcium, magnesium, chloride, bicarbonate, and phosphates are regulated by the plasma volume that the kidney excretes.
  5. Maintenance of acid – base balance: The kidneys with the help of lungs maintain the pH or the acid – base balance. Kidneys perform the chief role by controlling the amount of bicarbonate excreted. Blood pH is maintained mainly by excretion of hydrogen ions and reabsorption of bicarbonate ions as needed by the kidneys. These balances are essential to maintain life.
  6. Hormone secretion: Kidneys help the endocrine system for release of hormones by secreting renin. Renin further leads to the secretion and release of aldosterone from the adrenal cortex. Aldosterone promotes the kidneys to reabsorb the sodium (Na+) ions. Renin is an important enzyme in the control of blood pressure. Erythropoietin, which stimulates red blood cell production by the bone marrow is also secreted by kidneys. The Vitamin D from the skin is also metabolized to its activated form by kidneys.

Diseases of the Kidney

Kidney disorders are mostly secondary to underlying diabetes, hypertension or certain autoimmune diseases. These underlying conditions tend to damage the glomeruli, eventually resulting in impaired kidney functioning and kidney failure.

  1. Diabetic nephropathy is a progressive kidney disease due to longstanding diabetes mellitus. Here the nephrons of the kidneys are affected thus leading to failure of normal functioning of kidneys.
  2. Kidney stones or renal calculi are an insoluble mass of crystallized salts (oxalates, etc.) formed within the kidney. They normally pass through the urine. But, if they grow relatively large before passing, they can cause obstruction of the ureter, which further distends due to the accumulation of urine. This gives rise to severe pain in the flank, lower abdomen and groin.
    Blausen 0595 KidneyStones
  3. Pyelonephritis: When infection of the renal pelvis spreads and involves the entire kidney it is known as pyelonephritis. Infection with E. coli from the anal region is considered as the main causative factor. In advanced cases, there is kidney swelling. Scars and abscesses formation within the renal pelvis is also seen. Early diagnosis and administration of proper antibiotics usually help in complete recovery.
  4. Glomerulonephritis: It is the inflammation of the glomeruli of the kidneys, which can be caused by immunologic abnormalities, drugs, toxins, vascular disorders, and systemic diseases. It is characterized by the presence of blood and proteins in the urine. Also, the volume of urine excreted is decreased.
  5. Kidney failure: When the urine output reduces to less than 400 ml of urine output per day, it is indicative of kidney failure. Blood urea and creatinine levels are elevated. Early diagnosis and intervention can help in the complete treatment of acute renal failure. When the renal function decreases to less than 25%, it is considered to be chronic renal failure.
  6. Diabetes Insipidus: It is a condition wherein there is a deficiency of the antidiuretic hormone (ADH). These patients have an inability to concentrate their urine thus they expel about 3 to 20 liters urine per day. Diabetes insipidus can be neurogenic, and nephrogenic. In nephrogenic type, kidneys do not respond to ADH. The cause may be a genetic trait, electrolyte disorder, or side effect of drugs like lithium. Neurogenic type is caused by a head injury near the hypophyseal tract.

Disorders of the Bladder and Urethra

  1. Infections of the bladder and urethra – It is a common infection encountered by the health care providers. Females are more commonly affected, especially those who have a shorter and broader urethra. Urine from the bladder is normally bacteria-free and it is the distal urethra that is colonized with bacteria. These harmful bacteria from urethra gain access to the urinary bladder. Escherichia coli is the most common bacteria causing urinary tract infection. Menarche, manipulation between intercourse, and menopause are three stages that predispose females to urinary tract infections. Nonetheless, men in older age group also get urinary tract infections due to the prostate gland growth.
  2. Urinary Bladder Stones –These occur because of bladder infections with associated inflammation or prostate enlargement in men and can be seen in any age group. They could also be kidney stones which got carried to the bladder. They can be removed surgically or broken apart by lithotripsy.
  3. Bladder Cancer – This can be a malignant cancer and is the most common type of cancer affecting the urinary system. Smoking is a high-risk factor for the development of cancer. Removal of the bladder can be required for limiting the spread of cancer.

Clinical significance

  1. Diuretics: Diuretics, also known as the water pills are a diverse group of compounds which cause stimulation or inhibition of naturally occurring hormones for regulation of the urine production by kidneys. It is a drug that increases the excretion of urine i.e. dieresis which ultimately decreases the extracellular fluid volume. Caffeine and cranberry juice are naturally occurring weak diuretics. Diuretics are used for treatment purposes in case of heart failure, liver cirrhosis, hypertension and certain kidney diseases. They just help to decrease the symptoms of these diseases by increasing the excretion of sodium and therefore water through the urine. As urine is produced by the kidney, sodium, and water (which are responsible for the pathologic edema) move into the blood to replace the volume that is lost as urine, thereby reducing the edema. Some diuretics reduce blood pressure independent of the diuretic effect.
  2. Dialysis: Dialysis is a supportive treatment for impaired kidney functioning. If started early, reversal of chronic kidney failure due to diabetes or high blood pressure might be possible. Dialysis is a medical procedure wherein there is filtration of blood outside of the body. Malfunctioning of kidneys can lead to accumulation of waste products in the blood, which is extremely harmful and can lead to kidney failure. These waste products can be removed with the help of dialysis.
  3. Kidney transplantation: It is the ultimate treatment method for end-stage kidney failure. Usually one can lead a normal life with just one kidney but when most of the functioning kidney tissue is significantly diminished kidney failure develops. A kidney transplant can be from a living donor or a cadaver (dead donor). Compatibility of the donor and recipient is of utmost importance for a successful transplant. Only healthy and medically fit individuals can act as donors. In some countries, it is essential to obtain legal consent from the family of the donor for the organ donation. In both cases, the recipient of the new organ needs to take drugs to suppress their immune system and minimize the chances of rejection of the organ. Modern clinical procedures have increased the success rate of such a complicated technique.