Structure and Function of the Urinary Tract in Dogs

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What Is the Urinary Tract?

The urinary tract is a system made up of the kidneys, ureters, urinary bladder and urethra. These organs work together to produce, transport, store and excrete urine. The urinary tract also rids the body of many fluid waste materials and products and has other vitally important functions, including controlling the volume and composition of the body fluids.

Where Is the Urinary Tract Located?

Dogs and cats have two kidneys, both located in the abdominal cavity under the backbone, close to where the last rib meets the spine. The ureters (one from each kidney) extend from the kidneys to the bladder. The bladder is located in the abdomen just in front of the pubic bone of the pelvis. The urethra, the tube that carries urine from the bladder to the outside, passes along the floor of the pelvis. It ends in the vestibule of the vagina in the female dog and at the tip of the penis in the male dog.

What Is the General Structure and Function of the Urinary Tract?

The kidneys are paired, bean-shaped organs. The indentation of the "bean" is called the hilus, which is the area where the blood vessels, nerves and ureters enter and leave the kidney.

The structural and functional unit of the kidney is the nephron. There are hundreds of these microscopic filtering units, and each has the ability to form urine by itself.
Each nephron consists of a circular, ball-shaped cluster of small blood vessels called a glomerulus, and a small tube called a renal tubule. Nephrons are responsible for removing urea, which is combined with water and other waste products to produce urine.

The urine is emptied into the pelvis of the kidney, a collecting chamber located in the middle of the kidney in the area of the hilus. Urine from the kidney passes into the ureters, which are very thin tubes that extend from the kidney to the urinary bladder. Muscles in the walls of the ureters cause the urine to travel towards the bladder where it enters in small spurts.

The bladder is a collapsible muscular organ shaped like a balloon found in the back portion of the abdomen. It provides a means for temporary urine storage. The bladder is held in place by ligaments that are attached to other organs and the pelvic bones. A circular muscle called a sphincter controls the outlet opening of the bladder, and keeps the urine from leaking out of the bladder. Once the bladder reaches a certain point of distention, nerves in the bladder wall notify the brain that it is time to empty the bladder.

When appropriate, the animal voluntarily relaxes the sphincter and allows the release of urine. The released urine enters the urethra, a tube that carries urine to the outside. The male urethra is long and thin, and terminates at the tip of the penis. The female urethra is much shorter and wider, and opens just in front of the opening of the vagina.

What Are the Functions of the Kidneys?

Although the kidneys work in tandem to perform many vital functions, dogs can live a normal healthy life with just one kidney. The kidneys and urinary tract regulate many important body functions.

  • The kidneys perform their most important functions by filtering and removing waste products of metabolism. The kidneys "clear" unwanted substances by excreting them in the urine while returning substances such as water and certain salts back into the blood stream. The kidneys monitor and maintain the balance of body water, ensuring that body tissues receive enough water to remain hydrated, to function properly and to remain healthy.

  • The kidneys help regulate the acid-base balance (the pH) of the blood and body fluids. Certain ions, such as hydrogen, are either retained or lost from the body to help maintain this balance.

  • By regulating salt levels through the production of an enzyme called renin (as well as other substances), the kidney ensures that blood pressure is regulated. The amount of sodium salt retained in the body greatly influences blood pressure.

  • The kidneys influence certain hormonal functions in the body. They secrete a hormone called erythropoietin, which stimulates the bone marrow to produce red blood cells. The kidneys also make a substance that converts vitamin D to its most active form, calcitriol.

  • What Are Common Diseases of the Urinary Tract?

    Like all other systems in the body, the urinary tract is subject to diseases, disorders, and injuries. The more common of these in dogs include the following:

  • Urinary tract infections of the bladder are very common and may be referred to as cystitis. Female dogs, which have a wider and shorter urethra than male dogs, are affected more often. Male dogs, however, do get bladder infections, especially when they are intact (non-neutered). Typical signs of bladder infections include straining to urinate, increased frequency of urination, blood in the urine, and sometimes a change in the smell of the urine.

  • Pyelonephritis refers to infections of the kidney. Most of these infections are due to bacteria and may be secondary to severe, chronic, or recurrent bladder infections. With time, bacteria associated with bladder infections may travel up the ureters to infect the kidneys. Animals with pyelonephritis may become very ill and may need to be hospitalized for intensive therapy.

  • Stones or calculi may develop anywhere in the urinary tract; however, they are most common in the bladder. Affected dogs have different symptoms, treatment options, and prognoses depending upon the type, number and location of the stones. Pets with bladder stones may have no symptoms or may show blood in the urine, painful urination, and straining to urinate. Male dogs with bladder stones are prone to obstruction of the urethra because the urethra is usually smaller than the diameter of the stones. Stones in the kidney and ureters may cause severe damage to these organs, with rupture of the ureter and sometimes kidney failure. Stones may develop because of chronic infection, defects in the metabolism and excretion of certain salts, or dietary factors.

  • Renal or kidney failure results when the kidneys are not able to remove toxins from the body and can no longer regulate water and electrolyte levels in the body. Dogs can lose up to 2/3 of their functional kidney capacity without showing signs of kidney failure. It is only when 3/4 of the capacity is gone that the kidneys can no longer compensate completely, and toxins and abnormal levels of minerals and electrolytes begin to build up in the blood stream.

    Kidney failure may occur in dogs of all ages and breeds. It is classified as either acute (sudden onset) or chronic (slow, gradual course). Acute kidney failure arises most often with exposure to certain drugs or poisons. One of the most common causes of acute kidney failure in young animals is the ingestion of antifreeze or ethylene glycol. The most common type of kidney failure in older dogs is chronic kidney disease secondary to years of degeneration and deterioration of certain components of the kidney.

  • Urinary incontinence is the involuntary passage of urine that arises with loss of bladder sphincter control. There are many possible causes of urinary incontinence, although low estrogen levels in female spayed dogs is perhaps the most common cause.

  • Tumors can arise anywhere in the urinary tract. One of the most common tumors arises in the bladder and is called transitional cell carcinoma. Tumors of the urinary tract are often malignant.

  • The urinary tract may become traumatized when a dog is hit by a car, falls from a height or is attacked by another dog. Fractures of the pelvis may cause disruption of both the urethra and bladder. Hemorrhages or rupture of the kidneys, ureters or bladder may occur with trauma to the abdomen. The urethra of the male dog is more exposed than the female and may be subjected to bite wounds or other penetrating injuries.

    What Types of Diagnostic Tests Are Used to Evaluate the Urinary Tract?

    There are several diagnostic tests that are particularly helpful when evaluating the urinary tract.

  • A urinalysis is examination of the urine for the presence of certain abnormal substances, such as blood, protein, sugar and white blood cells. It also evaluates the urine concentration, which measures the ability of the kidneys to function and retain water. Urine samples may be obtained by having the patient urinate into a container, or by retrieving a sample directly from the bladder. Urine can be removed from the bladder via catheterization or cystocentesis (drawing urine directly from the bladder with a needle). These latter methods allow the urine to be collected in a sterile fashion.

  • Bacterial urine culture and sensitivity testing are performed to identify the presence of bacteria, to help confirm urinary tract infection, and to select appropriate antibiotic therapy.

  • A biochemical profile may reveal elevations in kidney enzymes, electrolyte abnormalities, or may show changes suggestive of other metabolic or endocrine disorders that predispose the individual to recurrent cystitis or kidney failure.

  • A complete blood count, measurement of blood pressure, and other tests provide valuable information about the function of the kidneys.

  • Abdominal radiographs (x-rays) may reveal changes in kidney size and shape, the presence of urinary tract calculi, changes in bladder size and shape, and abnormalities in structures around the urinary tract organs.

  • Abdominal ultrasonography is very helpful in evaluating the internal structures of the kidney and bladder. An abdominal ultrasound may help differentiate upper and lower urinary tract infections. Ultrasonography is also helpful in evaluating the presence of stones, tumors, and hemorrhages throughout the abdominal urinary tract. An ultrasound is a noninvasive procedure that often necessitates the expertise of a veterinary specialist and may require referral of your dog to a hospital with ultrasound capabilities.

  • A contrast cystourethrogram is an x-ray dye study that evaluates the urinary bladder and urethra. It is performed by inserting a dye that shows up white on an x-ray into the urethra and the bladder. It may confirm the presence of a tumor, stone or structural abnormality of the urethra and bladder. It is used primarily to assess the lower urinary tract.

  • Excretory urography or IVP is an intravenous dye study that examines primarily the the upper urinary tract (kidneys and ureters). A dye that shows up white on an x-ray is injected into a vein. The dye travels to the kidneys and is excreted along the same route that urine is produced. An IVP is helpful in documenting pyelonephritis, and may help detect stones, congenital birth defects, and other abnormalities.

  • Cystoscopy is a procedure where a small scope is passed up the urethra into the bladder. The procedure is not often performed in dogs because their urethras are very small and the equipment to perform the procedure can be quite expensive.

  • In some cases, surgical exploration of the abdomen may be needed to confirm the presence of a urinary tract disorder, to obtain biopsies of urinary structures, and to institute corrective therapy.

  • Any stones surgically removed from the urinary tract are analyzed for their mineral content.

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