urology.pngThis unit was on the urinary system. Understanding how the urinary system helps maintain homeostasis by removing harmful substances from the blood and regulating water balance in the body is an important part of physiology. Your kidneys, which are the main part of the urinary system, are made up of millions of nephrons that act as individual filtering units and are complex structures themselves. The ureters, urethra, and urinary bladder complete this complicated system.
The urinary system helps maintain homeostasis by regulating water balance and by removing harmful substances from the blood. The blood is filtered by two kidneys, which produce urine, a fluid containing toxic substances and waste products. From each kidney, the urine flows through a tube, the ureter, to the urinary bladder, where it is stored until it is expelled from the body through another tube, the urethra.


Regulation of Urine Concentration

The loop of Henle of juxtamedullary nephrons is the apparatus that allows the nephron to concentrate urine. The loop is a countercurrent multiplier system in which fluids move in opposite directions through side-by-side, semipermeable tubes. Substances are transported horizontally, by passive or active mechanisms, from one tube to the other. The movement of the transported substances up and down the tubes results in a higher concentration of substances at the bottom of the tubes than at the tops of the tubes.

The descending limb of the loop of Henle is permeable to H2O, so H2O diffuses out into the surrounding fluids. Because the loop is impermeable to Na+ and Cl- and because these ions are not pumped out by active transport, Na+ and Cl- remain inside the loop.

As the fluid continues to travel down the descending limb of the loop, it becomes more and more concentrated, as water continues to diffuse out. Maximum concentration occurs at the bottom of the loop.

The ascending limb of the loop of Henle is impermeable to water, but Na+ and Cl- are pumped out into the surrounding fluids by active transport.

As fluid travels up the ascending limb, it becomes less and less concentrated because Na+ and Cl- are pumped out.

At the top of the ascending limb, the fluid is only slightly less concentrated than at the top of the descending limb. In other words, there is little change in the concentration of the fluid in the tubule as a result of traversing the loop of Henle.

In the fluid surrounding the loop of Henle, however, a gradient of salt (NaCl) is established, increasing in concentration from the top to the bottom of the loop.

Fluid at the top of the collecting duct has a concentration of salts about equal to that at the beginning of the loop of Henle (some water is reabsorbed in the distal convoluted tubule). As the fluid descends the collecting duct, the fluid is exposed to the surrounding salt gradient established by the loop of Henle. Without ADP, the collecting duct is impermeable to water.

Two outcomes are possible: 1.) If water conservation is necessary, ADH stimulates the opening of water channels in the collecting duct, allowing water to diffuse out of the duct and into the surrounding fluids. The result is concentrated urine. 2.) If water conservation is not necessary, ADH is not secreted and the duct remains impermeable to water. The result is dilute urine.

The vasa recta delivers O2, and nutrients to cells of the loop of Henle. The vasa recta, like other capillaries, is permeable to both H2O and salts and could disrupt the salt gradient established by the loop of Henle. To avoid this, the vasa recta acts as a countercurrent multiplier system as well. As the vasa recta descends into the renal medulla, water diffuses out into the surrounding fluids, and salts diffuse in. When the vasa recta ascends, the reverse occurs. As a result, the concentration of salts in the vasa recta is always about the same as that in the surrounding fluids, and the salt gradient established by the loop of Henle stays in place.

The above video is a great explanation of the urinary system.


The ureters, one from each kidney, deliver urine to the bladder. The ureters enter through the back of the bladder, entering at such an angle that when the bladder fills, the ureter openings are forced closed. A cross section of the ureter reveals three layers of tissue:

An inner mucosa consists of transitional epithelium covered by a lamina propria of connective tissue. Mucus secretions protect the ureter tissues from the urine.

A middle muscularis layer consists of longitudinal and circular layer of smooth muscle fibers. The muscle fibers force urine forward by peristalsis.

The outer adventitia consists of areolar connective tissue containing nerves, blood vessels, and lymphatic vessels.

Urinary Bladder

The urinary bladder is a muscular sac for storing urine. The triangular base of the bladder, the trigone, is defined by the two ureters that deliver the urine and the one urethra that drains the urine. When empty, the bladder collapses, and folds (called rugae) from in the bladder wall. As it fills, the folds become distended and the bladder becomes spherical. The wall of the bladder consists of three layers similar to those of the urethra: the mucosa, the muscularis (detrusor muscle), and the adventitia. Circular smooth muscle fibers around the urethra form the internal urethral sphincter.


The urethra drains urine from the bladder to an exterior opening of the body, the external urethral orifice. In females, the urethra is about 3-4 cm long and opens to the outside of the body between the vagina and the clitoris. In males, the urethra is about 15-20 cm long and passes through the prostrate gland, the urogenital diaphragm, and the penis. In these regions, the urethra is called the prostatic urethra, membranous urethra, and spongy (penile) urethra, respectively. In both males and females, a skeletal muscle, the external urethral sphincter, surrounds the urethra as it passes through the urogenital diaphragm.

Micturition, or urination, is the process of releasing urine from the bladder into the urethra. When the bladder fills to about 200 ml to 300 ml, stretch receptors in the bladder trigger a reflex arc. The signal stimulates the spinal cord which responds with a parasympathetic impulse that relaxes the internal urethral sphincter and contracts the detrusor muscle. Urine does not flow, however, until a voluntary nerve impulse relaxes the skeletal muscle of the external urethral sphincter.


A very common ailment that folks go to the clinic for is Urinary Tract Infections or UTIs. This is something that most all of us have suffered from at some point in our lives. I have had personal experience with UTIs since I was a child.

UTIs are infections caused by pathogenic organisms (bacteria, fungi, or parasites) in any of the structures that make up the urinary tract (kidneys, ureter, urinary bladder, and urethra). They are more common in women than men, and they account for approximately 8.3 million doctor visits per year. Even though some infections go unnoticed, UTIs can cause problems that range from dysuria (pain and/or burning when urinating) to organ damage and even death in extreme cases. The kidneys are the active organs that function to keep electrolytes and fluids in balance, assist removal of waste products, and produce a hormone that aids to form red blood cells. If the kidneys are injured or destroyed by infection, these vital functions can be damaged or even lost.

The signs of a UTI vary from person to person. Some individuals will have no symptoms or mild symptoms and may clear the infection in about two to five days. Many people will not spontaneously clear the infection; some of the most frequent signs and symptoms experienced by most patients is a frequent urge to urinate, accompanied by pain or burning on urination. The urine often appears cloudy and occasionally reddish if blood is present. It could also develop an unpleasant odor. Women often have lower abdominal pain or feel bloated and experience sensations like their bladder is full. They may also complain of vaginal discharge, especially if their urethra is infected, or if they have an STD. Men may complain of dysuria, frequency, and urgency also. Other symptoms they may have include rectal, testicular, penile, or abdominal pain. Men with a urethral infection, especially if it’s caused by an STD, may have a pus-like drip or discharge from their penis. Toddlers and children with UTIs often show blood in the urine, abdominal pain, fever, and vomiting along with pain and urgency with urination.

Treatment for a UTI varies for each patient, depending on how sick the patient is, what pathogen is causing the infection, and the susceptibility of the pathogen to treatment. Patients that are extremely ill usually require IV antibiotics and admission to the hospital; they usually have a kidney infection that may be spreading to the bloodstream. Other people may have a milder infection (cystitis) and may get will quickly with oral antibiotics. It is also important to remember that over-the-counter medicines offer relief from the pain and discomfort of UTIs, but they do not cure them! They work to relieve the pain associated with UTIS. These medications may turn urine an orange-red color, so patients should not be worried when this happens. They can also turn other body fluids orange, including tears, and can stain contact lenses, so caution should be taken. They are, however, a good way to help relieve the pain and discomfort of a UTI.



Describe the mechanism by which normal fluid regulation in the body occurs.
The kidneys act to regulate the volume of water in the plasma by producing less urine to conserve water when the body is approaching dehydration, and by removing extra water from the plasma when the body is in a state of over-hydration. This extra water is stored to excretion in the bladder as urine.
The rate of fluid intake is controlled by the thirst mechanism, and the rate of fluid excretion is controlled by antidiuretic hormone (ADH), also called vasopressin. ADH is synthesized in the cell bodies of neurons comprising the supraoptic nuclei in the hypothalamus. After it is produced, ADH is transported the length of the nerve fibers, which end as bulbous knobs in the posterior pituitary. When stimulated by nerve impulses from the supraoptic nuclei, the hormone is released from the nerve endings, absorbed into nearby capillaries, and transported throughout the body.
Sensory neurons located near supraoptic nuclei function as osmoreceptors, monitoring osmolality of the plasma. An increase in plasma osmolality is detected by receptors, which then excite the neurons of the supraoptic nucleus, which in turn transmit an action potential to the posterior pituitary and ADH is secreted.
ADH is transported by the blood to the kidneys, where it acts to increase the permeability of the cells lining the collecting duct to water. Water then leaves the filtrate to equilibrate with the hyperosmolality of the medullary interstitium, while electrolytes continue to be lost in the urine. The extracellular fluid is diluted and the normal range of plasma osmolality is re-established.

What is considered to be excessive thirst and urination in an adult?
An adult who urinates more than 50 ml/kg of body weight every two hours is considered to have polyuria. An adult who drinks more than 1 gallon or about 12 glasses of beverages per day would be considered to have polydipsia.

List and briefly describe the four types of diabetes insipidus.
Pituitary Diabetes Insipidus: The most common type, results from a lack of ADH due to destruction of the posterior pituitary gland. The posterior pituitary can be destroyed by neoplasms, traumatic injury, infectious processes, and inheritable conditions.
Nephrogenic Diabetes Insipidus: Results from a type of ADH resistance. Normal blood levels of ADH are present but the kidneys don’t respond to ADH by producing more concentrated urine.
Gestational Diabetes Insipidus: Occurs during pregnancy and thought to be due to the inappropriate destruction of ADH by the placenta. The condition is treatable, but the ADH deficiency and the symptoms of diabetes insipidus usually disappear four to six weeks after delivery.
Dipsogenic Diabetes Insipidus: Occurs as the result of ADH suppression secondary to the excessive ingestion of fluids. The abnormality lies with the part of the brain responsible for regulating the thirst mechanism.

How is pituitary diabetes insipidus diagnosed?
To diagnose pituitary diabetes insipidus, we must get:
A thorough family history and collection of blood for genetic testing
An MRI of the brain
A therapeutic trial of DDAVP
A water deprivation test

How is diabetes insipidus similar to diabetes mellitus; how do they differ?
Diabetes insipidus resemble diabetes mellitus in that both diseases result in increased frequency of urination and thirst. In terms of their cause and treatment, they are completely dissimilar.

How does the mechanisms by which diuresis occurs with diabetes insipidus differ from that which occurs in diabetes mellitus?
Diabetes insipidus is caused by a lack of, or nonresponse to, the antidiuretic hormone vasopressin. The hormone controls water balance by concentrating urine. Diabetes mellitus is caused by the lack of, or nonresponse to, the hormone insulin. Insulin helps in carbohydrate metabolism.

How does diabetes insipidus compare with a condition called syndrome of inappropriate antidiuretic hormone (SIADH)?
SIADH is diagnosed when excess ADH secretion is associated with hypoatremia without edema or hypovolemia. The ADH excess is therefore inappropriate in the face of plasma hyposmolarity.

What other conditions result in polyuria and polydipsia (PU/PD?)
In addition, polyuria and polydipsia can result from the many other conditions that involve excessive thirst, including endocrine disorders, mineral imbalances, liver disease, head injury, burns, excessive bleeding and drug abuse.

What is DDAVP and how is it administered?
DDAVP (desamino, d-arginine vasopressin) is a synthetic form of ADH. It can be administered by injection, nasal spray, or by tablet at whatever dose or frequency is necessary to completely eliminate the symptoms of diabetes insipidus.

Why should a person who has a pituitary diabetes insipidus and does not feel unreasonable inconvenienced by the symptoms take treatment?
The problem in diabetes insipidus is loss of ability to concentrate urine which results in loss of large amounts of water in the urine, while the whole body water might not be affected by the large amounts of urine per day in someone with diabetes insipidus. Loss of water causes the blood to become thicker and more viscid which causes slower circulation and may lead to stroke. A more rapidly developing problem with loss of water is electrolyte imbalance. The function of nerves in conducting electrical impulses depends on the levels of salts in and outside the nerve cells. Also, the function of muscles (the heart muscle being most important) also depends on exchange of salts with blood. When water is lost, this leads to a higher concentration of salts in blood, most importantly high sodium (hypernatremia) and high postassium (hyperkalemia). Hyperkalemia can cause cardiac arrest.

Why is ADH also known as vasopressin?
ADH at moderate to high concentrations acts as a potent vasoconstrictor, increasing total peripheral resistance to blood flow and increasing systemic arterial blood pressure.

All information was obtained from Human Physiology; Stuart Ira Fox; 12th Edition, Burton’s Microbiology For The Health Sciences; Paul G. Engelkirk and Janet Duben-Engelkirk; 9th Edition, http://www.wikipedia.org/_, http://www.howstuffworks.com/,_ http://www.physioweb.org/, and http://www.webmd.com/.
All sources for photos can be accessed directly by clicking on the photos.