Structure and Function of the Urinary Tract in Cats

Below is information about the structure and function of the feline urinary tract. We will tell you about the general structure of the urinary tract, how the urinary tract works, common diseases that affect the urinary tract, and common diagnostic tests performed in cats to evaluate the urinary tract.

What Is the Urinary Tract?

The cat’s 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 Cat’s 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 cat and at the tip of the penis in the male cat.

What Is the General Structure and Function of the Feline 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 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.

What Are the Functions of the Feline Kidneys?

Although the kidneys work in tandem to perform many vital functions, cats 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 a Cat’s 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 cats include the following:

• Feline lower urinary tract disease (FLUTD) is a common disorder seen in both male and female cats. It is also known as feline idiopathic cystitis (FIC) and feline urologic syndrome (FUS). The exact cause of this disorder is unknown, but may be influenced by infections with bacteria and viruses, as well as certain dietary factors. FLUTD is characterized by painful urination with or without blood in the urine. The disease is frequently complicated in males by partial or complete blockage of the urethra. FLUTD is very uncomfortable for the cat, and a life threatening concern if there is an associated blockage. Cats with urinary obstructions must be cared for immediately, as many need hospitalization and intensive care.
• 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.
• Urinary tract stones or calculi develop occasionally in cats. They may be found anywhere in the urinary tract, although they are most common in the bladder. Affected cats have different symptoms, treatment options, and prognoses depending upon the type, number and location of the stones. Cats with bladder stones may have signs identical to those of FLUTD. Kidney and ureteral stones may cause severe damage to these organs, with rupture of the ureter and sometimes kidney failure. Stones may develop secondary to chronic infection, diet, or in some cases, to 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 or can no longer regulate water and electrolyte balance. Cats 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 cats of all ages and breeds. It is classified as either acute (sudden onset) or chronic (slow, gradual course). Acute kidney failure may arise with obstruction of the lower urinary tract, following exposure to certain toxins such as antifreeze (ethylene glycol), or trauma. The most common type of kidney failure in older cats is chronic kidney disease secondary to years of degeneration and deterioration.

• Tumors of can arise anywhere in the urinary tract, but are relatively rare in the cat. The most common urinary tumor in cat is lymphosarcoma of the kidneys. It commonly affects both kidneys at the same time and may be a component of widespread systemic lymphosarcoma.
• The urinary tract may become traumatized when a cat is hit by a car, falls from a height or is attacked by another animal. Fractures of the pelvis may cause disruption of both the urethra and bladder. Fractures of the spine and tail may result in paralysis of the bladder. Hemorrhages or rupture of the kidneys, ureters or bladder may occur with trauma to the abdomen.

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 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 in 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). Cystocentesis does not require sedation in the cat, and is more frequently used to obtain urine in a sterile fashion than is catheterization.
• Bacterial urine culture and sensitivity are performed to identify the presence of bacteria, to help confirm urinary tract infection, and to select appropriate therapy.
• A biochemical profile may reveal elevations in kidney enzymes, electrolyte abnormalities, or may show changes suggestive of other metabolic or endocrine disorders that may predispose the individual to kidney failure.
• Abdominal radiographs (x-rays) may reveal changes in kidney size and shape, the presence of urinary 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 from lower urinary tract infections. Ultrasonography is also helpful in evaluating the presence of stones, tumors, hemorrhages and blood clots 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 cat 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. It is used primarily to assess the lower urinary tract.
• Excretory urography or IVP is an intravenous dye study that examines primarily 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 very helpful in documenting pyelonephritis, and may help detect stones, congenital birth defects, and other abnormalities.
• In some cases, surgical exploration of the abdomen may be needed to confirm the presence of a urinary tract disorder, to obtain biopsies of the urinary structures, and to institute corrective therapy.
• Any stones surgically removed from the urinary tract are analyzed for their mineral content.