Signs and diagnosis of bladder stones in dogs

Although we often call them “bladder stones” in dogs, they are also known as urinary calculi and the condition is referred to as urolithiasis. Stones may actually form anywhere in the entire urinary tract. The urinary stones in dogs can be found in the kidneys, ureters (tiny tubes that carry urine from the kidneys to the bladder), bladder, or urethra (the tube from the bladder to the outside of the animal). However, in the dog, over 85% of stones are found within the bladder itself. Signs and diagnosis of bladder stones in dogs Dogs with bladder stones may have blood in their urine and may urinate frequently, passing only small quantities each time. Often, they will strain while urinating, holding their body in the urinating posture for much longer than normal. They may lick their genital area more than normal. Some dogs with bladder stones may show no signs at all, and the stones are discovered while palpating the abdomen during a routine physical exam. In other cases, the diagnosis of bladder stones in dogs is made or confirmed with abdominal x-rays. Most stones are radiopaque, meaning they show up on the radiographic film as obvious white circles or shapes just as bones do. A few are radiolucent, where the x-ray beams pass right through and therefore, they do not show up on the finished film. To confirm the presence of these types of stones, a special dye is passed into the bladder and it outlines the stones in the x-ray. With this method, we see a white area (the dye) with a black hole in the center (the stone). How bladder stones are formed Bladder stones are formed by minerals, which first precipitate out in the urine as individual microscopic crystals. Over time, these crystals unite and small grains of sand-like material may be formed. Once these first grains are present, additional precipitation forms on their surface and the tiny specks are gradually built into stones that sometimes reach 3″ to 4″ in diameter. Types of bladder stones in dogs There are several different types of bladder stones in dogs, depending on their chemical make-up. Struvite stones are composed of magnesium ammonium phosphate. Others are made of calcium oxalate, calcium phosphate, cystine, ammonium urate, or other chemical compounds. Some stones can actually be a combination of these. Each type of stone has its own different peculiarities as to which breed is most often affected and what factors affect the formation. What causes bladder stones in dogs? The process by which bladder stones develop is really quite simple, but what causes it to occur only in certain dogs, cats, or humans? Factors that influence the development of stones include genetic predisposition, the concentration of the stone constituents in the urine, urine pH, and the presence of bacterial infections. Genetics: The genetically controlled physiology of some animals causes them to produce within their bodies the higher levels of the substances that are precursors of the crystals. They are then excreted or formed in the urine. We have no way of testing to predetermine in which individual dog or line of dogs this will occur. Concentration of stone constituents: The higher the urine concentration of the constituents that make up the stones, the higher the risk that stones will form. The concentration of these constituents can be influenced by the amount of minerals and protein in the diet, the amount of water the animal drinks, and the animal’s unique metabolism. Urine pH: The acidity or alkalinity of the urine influences whether the stone constituents will remain dissolved or form stones. Some stones, such as struvite, form in alkaline urine, whereas others, including calcium oxalate stones, are more apt to form in acidic urine. The urine pH is largely influenced by diet. Bacterial Infections: Bacterial infections of the bladder (referred to as cystitis) play the major role in struvite stone information for two reasons: Bacterial infections tend to make the urine more alkaline (with a pH higher than 7.0) which enhances the formation of struvite crystals. This is important in this specific condition as struvite crystals are more apt to remain in solution if the liquid is acidic (with a pH lower than 7.0). That is, they would continue to be dissolved in the liquid and no crystals would form. By-products of bacterial metabolism may enhance crystal formation. Many of the bacteria that cause a cystitis (bladder infection) also produce an enzyme (a compound that causes chemical reactions to occur) called urease. Urease reacts with urea molecules found in the urine to form ammonia and carbon dioxide. The ammonia is slowly converted to ammonium ions, while the carbon dioxide unites with other compounds, freeing up phosphates. Then, through a chain of chemical reactions that seem to feed on each other, the magnesium that is normally present within the urine unites with the ammonium and phosphate to form magnesium ammonium phosphate crystals (struvite). If the crystals are formed rapidly and in large quantities, they will unite together to form stones. However, if only small amounts are formed over a longer period of time, they would simply be flushed out in the urine without producing any problems. Treatment of bladder stones in dogs Treatment of bladder stones may involve choices for the owners and veterinarians – that is, either to treat medically or surgically. Options vary according to the size, type and location of the stone(s). Surgery: The surgical removal of stones within the bladder is referred to as cystotomy, meaning an opening of the bladder. With the dog under anesthesia and lying on his back, an incision is made through the abdominal wall in front of the pelvis. The bladder is exposed and lifted out through the incision. Urine is collected for culture and analysis, the bladder is opened and the stones are removed. The bladder and urethra are flushed with sterile saline solution to wash out any small or microscopic particles. The bladder is then closed with sutures as is the abdominal wall. The patient is placed on antibiotics and usually sent home the following day. The bladder stones are sent to a laboratory for analysis to determine their chemical make-up and the remainder of the therapy will vary depending on the results. Urohydropropulsion: To perform urohydropropulsion, the dog is anesthetized and a urinary catheter is placed. Through the catheter, the bladder is filled with sterile saline. The dog is then held in an upright position and, by hand, the veterinarian compresses the bladder, forcing the solution back out, and with it, the stones. Urohydropropulsion is used when the stones are very small and are sure to pass through the urethra. In situations where a stone has lodged in the ureters or urethra, the condition is a life or death matter that must be resolved immediately. Urinary obstructions lead to kidney shut down and death. If there are stones caught in the urethra, which is especially common in male dogs, we try to back-flush them into the bladder before it is opened. If this cannot be done, an incision must be made directly through the urethral wall where the stone is located. In the rare case where stones are lodged in a ureter, an incision would have to be made at the site. Some work is being done using ultrasonic waves to destroy stones in these situations, but it is not readily available. This technique is more common in human medicine and may eliminate the need for surgery. Medical Management: Struvite and ammonium urate stones are examples of stones that may be able to be dissolved through dietary management. For struvite stones, it is also very important to control urinary tract infections. Calcium oxalate stones, on the other hand, generally need to be removed through surgery or urohydropropulsion. Medical therapy can be used by itself or in conjunction with the surgery. After the diagnosis and x-rays, pretreatment laboratory work entails culturing the urine for bacteria and a performing aurinalysis to determine what type of crystals are present along with the pH of the urine. After we determine what crystals are present in the urine, we know what type of stone is probably present and we try to modify the environment in the dog’s bladder to prevent further formation of stones. This may be done through a combination of antibiotic therapy, use of special foods, increasing water consumption, and inhibiting the activity of urease in cases involving struvite stones. Treatment of urinary tract infections: In the case of struvite bladder stones in dogs, it is vitally important to treat any urinary infections and prevent further infections from occurring. If there is a an infection present, a culture and sensitivity is done to determine the appropriate antibacterial medications to use and then treatment is initiated. Diet Alteration: Diet alternation may be helpful in the medical treatment of struvite and ammonium urate stones. Specially formulated diets can actually cause the stones – even large ones – to dissolve completely. These diets take time, often 60 to 150 days, to work. To take struvite and Hill’s s/d diet as an example, the principle by which s/d works is that it contains lower than normal levels of large proteins, magnesium, and phosphorous. Less protein means less urea, and therefore, less ammonium and carbon dioxide formed by the action of urease. Remember that struvite is made up of magnesium and phosphate ions, so lower levels of these materials also decrease the quantity of crystals that can potentially be formed. Feeding s/d helps the urine become more acidic. And last but not least, Hill’s has slightly increased the sodium chloride (normal table salt) to increase water consumption by the animal, thereby increasing a flushing action through the bladder and better keeping the crystals in solution. NOTE: Do NOT use a urinary acidifier and s/d, c/d, or a similar diet at the same time. However, s/d cannot be used indefinitely as a preventive because it is not considered a complete diet. Also, it is not recommended for use in patients suffering from heart failure or kidney disease because of its salt and protein levels. After the initial 60 to 150 day period, when medical therapy is actively attempting to dissolve the stones or sand that is present in the bladder, the animal is removed from s/d and placed on a maintenance diet such as Hill’s c/d or w/d. Royal Canin, Purina, and some other companies have also developed specialized diets for use with dogs with urinary stones. If a dog is reluctant to eat one manufacturer’s diet, it is advisable to try diets produced by another company. Prior to the development of specialized diets, urinary acidifiers such as vitamin C or dl-methionine were sometimes used to lower the pH of the urine in cases of struvite stones, for example. Specialty diets are now preferred since they alter not only the pH, but the concentration on stone-forming constituents. Remember: Do NOT give urinary acidifiers when you are using one of the specialty diets that also acidify urine. Increase Water Consumption: Water consumption may be increased by using canned diets instead of dry foods. Also, plain water can be added to dry diets. Using flavored dilute broths, water in which hot dogs have been boiled, etc., may also increase water consumption. Some owners have used clicker training as a way to increase their dog’s water consumption. The Formation, Treatment, and Diet Modifications for Urinary Stones and Crystals in Dogs Type of stone Tends to form in: Initial treatment Diet recommended for dissolving stones** Diet recommended for crystal/stone prevention in dogs susceptible to them** Struvite Alkaline urine Treat urinary tract infection; use diet to dissolve stones unless there is an obstruction; surgical removal if there is an obstruction Hill’s s/d Royal Canin Urinary SO Hill’s c/d or w/d Royal Canin Control  Royal Canin Urinary SO Oxalate Acidic urine Surgical removal Hill’s u/d* Royal Canin Urinary SO Purina NF Kidney Function Urate Dalmations and bulldogs; acidic urine; certain liver diseases If not liver disease, Hill’s u/d* with the medication allopurinol; surgical removal if there is obstruction or liver disease Hill’s u/d* Royal Canin Vegetarian *If used long-term, veterinarian should monitor for signs of protein depletion. **With all diets, it is extremely beneficial to also increase the amount of water consumption. Summary Urinary stones may be found in any portion of the urinary tract. Based on the composition of the stone, size, and location, the stone may be physically removed or a special diet may be used to dissolve the stone. To prevent recurrence, special diets are fed, water consumption is increased, and the pH of the urine is managed. Controlling urinary tract infections is especially important in preventing recurrence of struvite stones.

References and Further Reading

Barteges, JW; Kirk, CA. Interpreting and managing crystalluria. In Bonagura, JD; Twedt, DC (eds) Kirk’s Current Veterinary Therapy XIV. W.B. Saunders Co. Philadelphia, PA; 2009:850-854. Haak, CE; Cohn, LA. Calcium oxalate crystalluria. NAVC Clinician’ss Brief 2008 (September): 27-28. McCue, J; langston, C; Palma, D; Gissselman, K. Urate urolithiasis. Compendium Continuing Education for Veterinarians 2009 (october): 468-475. Osborne, CA; Lulich, JP; Forrester, D; Albasan, H. Paradigm changes in the role of nutrition for the management of canine and feline urolithiasis. In Osborne, CA; Lulich, JP (eds.) Veterinary Clinics of North America Small Animal Practice: Changing Paradigms in Diagnosis and Treatment of Urolithiasis. W.B. Saunders Co. Philadelphia, PA; 2009 (January) 127-141. Osborne, CA; Lulich, JP; Kruger, JM; Ulrich, LK; Koehler, LA. Analysis of 451,891 canine uroliths, feline uroliths, and feline urethral plugs from 1981 to 2007: Perspectives from the Minnesota Urolith Center. In Osborne, CA; Lulich, JP (eds.) Veterinary Clinics of North America Small Animal Practice: Changing Paradigms in Diagnosis and Treatment of Urolithiasis. W.B. Saunders Co. Philadelphia, PA; 2009 (January) 183-197.

Kidney disease (also referred to in medical terminology as renal disease) is a common finding in cats and dogs, especially those who are reaching their senior years. In acute disease, such as a toxicity, the signs occur suddenly and can be very severe. Inchronic kidney disease, the onset may be very slow and the signs fairly nonspecific, i.e., the animal is “just not doing well.” Whether the disease is acute or chronic is typically related to the cause.

What are the causes of kidney disease?
There are many causes of renal disease, and they may include:

• Age
  


• Viral, fungal, or bacterial infections
  


• Parasites
  


• Cancer
  


• Amyloidosis (caused by abnormal deposits of a certain type of protein in the kidney)
  


• Inflammation
  


• 
  Autoimmune diseases
  
  


• Trauma
  


• Toxic reaction to poisons or medications
  


• Congenital and inherited disorders
  

This is not a complete list but demonstrates what the veterinarian is trying to rule in or out as cause of the signs.

What are the signs of kidney disease?
Pets with kidney disease can show a variety of physical signs. Some of the signs are nonspecific and may be seen in other disorders such as liver or pancreatic diseases, or urinary tract disorders not involving the kidneys. Signs may include:

• Increased water consumption (polydipsia)
  


• Increased urination volume (polyuria)
  


• Decreased urination (oliguria)
  


• Lack of urination (anuria)
  


• Voiding urine during the night (nocturia)
  


• Blood in urine (hematuria)
  


• Decreased appetite (anorexia)
  


• Vomiting
  


• Weight loss
  


• Lethargy
  


• Diarrhea
  


• Hunched over posture or reluctance to move
  


• Poor or unkempt hair coat
  

During the physical examination, the veterinarian may also find the following signs:

·         Pale mucous membranes (e.g., gums) from a decrease in red blood cell production resulting in anemia

·         Enlarged and/or painful kidneys or small, irregular kidneys

·         Ulcers in the mouth, most commonly on the tongue, gum, or inside of the cheek

·         Bad breath (halitosis) due to toxic substances building up in the blood stream

·         Dehydration

·         Swelling of the limbs due to accumulation of fluid (subcutaneous edema)

·         Enlarged abdomen due to accumulation of fluid (ascites)

·         High blood pressure

·         Changes in the retina due to high blood pressure

·         Softening of the bones (rubber jaw) in young dogs with hereditary kidney disease (fibrous osteodystrophy)

What tests are done to determine a diagnosis?
Various blood tests can be performed to determine if kidney disease is present, how severe it may be, and what may be causing it. In addition, a urinalysis and imaging techniques may also help to determine the cause and severity.

Chemistry panel
Different types of tests are performed to help diagnose the disease process. Multiple tests can be performed on one blood sample. Tests that are often included in a chemistry panel being run to look for kidney disease include:

Blood urea nitrogen (Serum urea nitrogen): BUN is the abbreviation for blood urea nitrogen. The proteins that animals consume in their diet are large molecules. As they are broken down and used by the body, the by-product is a nitrogen-containing urea compound. This is of no use to the body and is excreted by the kidneys. If the kidneys are not working correctly and filtering these waste-products, they build up in the blood. A twelve-hour fast (no food intake) is ideal before taking this test as the level may rise slightly after eating protein.

Creatinine: Creatinine is also used to measure the filtration rate of the kidneys. The kidneys are the only organs that excrete this substance, and if it builds up to higher than normal levels, it is a sign of decreased or impaired function of the kidneys.

Azotemia is the medical term for an increase in the BUN or creatinine. Uremia is defined as azotemia plus clinical signs of renal failure such as anemia, polyuria-polydipsia, vomiting, or weight loss. Azotemia is divided further into prerenal, renal, or postrenal causes. Prerenal azotemia is due to causes other than actual kidney malfunction that decrease the blood flow to the kidney. These include dehydration, Addison’s disease, or heart disease. Renal azotemia occurs due to damage to the kidney itself, and can include chronic or acute renal disease/failure that results in more than 75% of the kidney not functioning. Postrenal azotemia occurs when there is a build-up of pressure in the urinary system. Causes may include blockage of the urethra due to feline lower urinary tract disease (FLUTD) or bladder stones, which prevent urine from being removed from the body.

Phosphorus: 
Normal calcium and phosphorus levels in the blood are maintained by an interaction of three hormones on three body organs. The phosphorus level increases in kidney disease because less is excreted into the urine by the kidney. In cats, the phosphorus level may also increase due to hyperthyroid disease.

Urinalysis
Multiple tests are performed on a urine sample. Several of them are especially important in determining if kidney disease is present.

Urine specific gravity: This test is a measurement of how concentrated the urine is. With kidney disease, the urine is not concentrated normally, and too much water is lost. A normal specific gravity is usually above 1.025, while animals with kidney disease may be in the 1.008 – 1.015 range. A low specific gravity should be retested to make sure it is a repeatable finding. Other diseases can cause a low specific gravity, so this test in itself, is not sufficient to make a diagnosis of kidney disease.

Protein: In some types of kidney disease, large amounts of protein are lost in the urine.

Sediment: The urine can be centrifuged so the larger particles may be separated out and examined under the microscope. The presence of red blood cells or white blood cells in the urine sediment help point to the cause of the disease condition. Casts (sloughed cells) from the kidneys may pass out in the urine. These indicate a disease process in the kidney itself.

Complete Blood Count
A complete blood count (CBC) is useful to check for anemia and indications of infection. Anemia in renal failure is common and results from a decrease in the production of a erythropoietin by the diseased kidney. Erythropoietin is a hormone that tells the body to produce more red cells. The red blood cells also have a shorter life span in uremic patients.

Imaging techniques
Radiography: X-rays are taken to determine the size and shape of the kidneys. Small kidneys are more common in chronic kidney disease while large kidneys may indicate an acute problem or cancer.

Excretory urography such as an intravenous pyelography (IVP) is a specialized type of x-ray. A dye (positive contrast media) is injected into the pet’s vein and monitored via x-rays as it is filtered out by the kidneys. This is used for anatomic evaluation of the urinary tract and to determine the size, shape, and location of the kidneys. It gives a crude assessment of renal function also.

Ultrasonography: Ultrasonography looks for changes in the density of the kidney. A biopsytaken during ultrasonography may help determine the cause of kidney disease in some cases.

Treatment of acute renal failure (ARF)
In cases of acute kidney disease, the animal usually has severe signs that occurred suddenly. These may include depression, vomiting, fever, loss of appetite, and changes in the amount of urination. A good medical history and testing will need to be performed to find the cause. The cause may be treatable such as infection caused by leptospirosis; an infestation with a parasite such as the giant kidney worm; or exposure to toxins such as the Easter lily or antifreeze. Blood and urine samples are ideally taken before the start of treatment so the treatment does not affect the test results.

Fluid therapy: Initial treatment of kidney disease, involves rehydrating the patient typically over about 2-10 hours and maintaining normal hydration after that. This is typically done withintravenous (IV) fluids in the veterinary clinic so the appropriate amounts can be given and the pet can be monitored for appropriate fluid output (urination). Many times, the IV fluid administration is enough to start or increase urine output. If urine output is still not normal, medication such as furosemide or mannitol may be necessary to try to get the kidneys to produce urine. Electrolytes such as sodium, potassium, and other electrolytes are monitored and maintained in the normal range through the administration of the IV fluids and, sometimes, medications.

Nutrition: As the pet becomes rehydrated with the fluids, he typically starts feeling less nauseous and becomes more willing to eat. If the pet eats willingly or if tube feeding is performed, a high quality lower quantity protein should be fed. This limits the demands on the kidneys while providing the body with needed nutrition. In severe cases, parenteralnutrition may be given via an IV line.

If the animal is vomiting because of the renal disease, treatment may include giving frequent small meals and medications such as cimetidineor chlorpromazine. The nausea may come and go through the day so small meals offered throughout the day may increase the overall food intake.

Other treatments: At the same time fluid therapy is started, treatment for the underlying cause is usually begun such as antibiotics for a bacterial infection or induction of vomiting for certain toxins.

Kidney dialysis can be done at some veterinary clinics, especially referral clinics or veterinary schools. Pets that may benefit from dialysis include those that fail to respond to normal therapies, those that have a nephrotoxic (toxic to the kidney) poison in them, those that are not producing urine, or those that require emergency surgery such as for repair of the urinary tract due to trauma.

Kidney transplants for dogs and cats are an option available at a few veterinary hospitals at this time.

With early and aggressive treatment, acute renal failure may be reversible.

Treatment of chronic renal failure (CRF)
Chronic renal failure is characterized by irreversible lesions within the kidney. In most cases, improvement of the renal function should not be expected once the body has compensated as much as possible. If the renal failure is prerenal (caused by a disease other than actual kidney malfunction that decreases the blood flow to the kidney) or postrenal (caused by a build-up of pressure in the urinary system from an obstruction, for example) components, it may be partially reversible with treatment. Renal function in chronic cases tends to be relatively stable for weeks to months baring unforeseen changes. Function does progressively deteriorate over weeks to months to years. The clinical and biochemical consequences of reduced renal function can be minimized by symptomatic and supportive therapy.

Many times, the earliest signs of CRF are missed by owners. These include a mild to moderate increase in thirst and urination (polydipsia and polyuria) and a need to urinate during the night (nocturia). Other common early clinical findings include variable weight loss, poor hair coat, lethargy, and selective appetite. As the condition progresses, more signs appear.

If the cause of the CRF can be identified, it should be treated if possible. Many times, the condition is found in older pets and is due to age.

Fluid therapy: The fluid need is greater in the CRF patient because the patient is unable to concentrate the urine so more water is passed out of the body in the form of urine. In earlier stages, patients may be able to maintain fluid balance by continuing to eat and increasing the amount of water consumed. The fluid level needs to be maintained to prevent dehydration. As the disease progresses, additional fluid in the form of subcutaneous (SQ) fluid may be necessary. Owners can typically give these fluids at home after being shown how at the veterinary clinic. The addition of potassium to the fluids or to the diet may be necessary to maintain proper levels of this electrolyte in the body. Low potassium levels cause generalized muscle weakness and heart rhythm disturbances. In some cases, intravenous (IV) fluids may also need to be given.

The pet should always have free access to fresh, clean water. Withholding water at night will not decrease the pet’s need to urinate overnight and may cause an acute crisis. The amount of water and food consumed each day should be monitored so the owner knows whether the pet is eating and drinking normal amounts. If not, additional fluids (IV or SQ) will be necessary to maintain hydration.

The body weight should be checked every week to make sure enough calories are being consumed to maintain weight and that dehydration is not a problem.

Diet: The veterinarian may recommend a diet change to a lower, but high quality, protein diet, which may decrease the stress on the diseased kidneys. Often, canned food is recommended. The change may need to be done slowly to keep the pet eating. The protein restriction cannot be excessive or the pet may develop protein malnutrition due to loss of protein via the kidney or thegastrointestinal tract that is not found in healthy pets. The diet should be monitored by checking the pet’s weight, checking for anemia, and checking for hypoalbuminemia. If these are present, an increase in protein content may be necessary. Always follow the dietary instructions given to you by your veterinarian.

Pets should be encouraged to eat an amount of food to maintain weight and provide the appropriate nutrition. To increase the appetite, it may help to feed several small meals a day; enhance the palatability of the diet with additives such as cottage cheese, yogurt, or chopped up vegetables; or to add a medication that stimulates appetite. Warming the food may also increase the palatability. Do not feed hot food as burns may result. The appetite may come and go during the day, so try feeding at various times during the day. Food-induced nausea may happen at certain times of the day and not at other times. Medication to control nausea may increase the appetite also.

Electrolytes, vitamins, and fatty acids: Electrolyte levels need to be maintained in the normal range. The phosphorus intake may need to be decreased to help serum levels remain normal. Phosphate binders may be used when diet changes and fluid therapy do not keep the phosphorus level in the normal range. Calcium supplementation may be necessary as well as vitamin D therapy. Salt intake needs to be adequate to help maintain hydration and to give the food flavor but not too high that it worsens hypertension (high blood pressure). Decrease the salt content of the food over several weeks to allow the kidneys to compensate for the change. Potassium levels should be monitored and a supplement given if necessary.

Water soluble vitamins (B and C) should be supplemented, especially during times of poor eating. Supplementation of vitamin A and D beyond the minimum daily requirement is not recommended due to a build-up of vitamin A and the changes in the metabolism of vitamin D in renal patients.

Omega-3 fatty acid supplementation may be of benefit to some animals with chronic renal failure.

Other treatments: Any medications to treat other conditions such as bladder infections or heart disease need to be given carefully and the pet monitored for side effects. The dosage may need to be decreased as the kidneys may be the main organ to eliminate the medication from the body.

The pet should be monitored for anemia and treatment initiated if necessary.Erythropoietin may be given as injections to help the body produce more red blood cells. Treatment of uremia will help lengthen the life span of the red blood cells. In more severe cases, blood transfusions can be given.

Blood pressure should be monitored to help prevent further damage to the kidneys, which could cause an increase in progression of the disease as well as damage to the retinas, which can result in blindness. Medication may be necessary to maintain normal blood pressure.

If the animal is vomiting because of the renal disease, treatment may include giving medications such as cimetidine or chlorpromazine. The nausea may come and go through the day so small meals offered throughout the day may increase the overall food intake.

Kidney transplants are available for dogs and cats at several veterinary hospitals.

With treatment, pets with CRF may live months to years. It will all depend on how the body responds to the treatment and other health concerns that arise.

Esophagus
The esophagus is a small hose-like tube, which connects the mouth to the stomach. As it leaves the mouth, the esophagus follows a straight path through the neck and chest, passing near the heart through the diaphragm muscle and finally entering the stomach. The walls of the esophagus are composed of muscles, which move in wave-like contractions to push food into the stomach. When there is no food in the esophagus, the walls of the esophagus collapse in on each other, making a closed space. Surgery on the esophagus is always difficult because of its location within the chest and its slow rate of healing.

Stomach
The dog’s stomach is a sac-like structure designed to store large volumes of food and continue the digestive process. The esophagus carries food to the stomach, where it enters via a valve-like structure called the cardiac sphincter. On the interior surface of the stomach is a series of folds called gastric folds. These folds function to help grind and digest food. The inner stomach lining secretes acids and enzymes to break down food. Once the initial stomach digestive process is complete, the partially digested food exits the stomach through the pyloric sphincter area and then enters the duodenum (the first segment of the small intestine). Once eaten, most food leaves the stomach within twelve hours after entering.

Small intestine
The small intestine is a tube-like structure, which extends between the stomach and large intestine. It is the longest portion of the intestinal tract and is about two and a half times the animal’s total body length. An animal twenty-four inches long would have about sixty inches of small intestine.

The small intestine in the dog has three parts. The first portion, which attaches to the stomach, is the duodenum. In a forty-pound dog it is roughly ten inches long. The middle (and longest) portion is called the jejunum. The shortest part is the ileum, which connects to the large intestine.

The duodenum attaches to the stomach and is relatively short. It does, however, have very important functions. The gallbladder and pancreas connect to the duodenum by the bile and pancreatic ducts respectively. Enzymes and other secretions that are important for digestion are produced by the liver and pancreas and pass through these ducts to mix with the food in the duodenum.

The jejunum is the longest area of the small intestine and is rich in small, finger-like projections called villi. Villi protrude inward into the food contents and provide a large surface area to absorb nutrients. Intestinal contents of the jejunum empty into the ileum and from there pass into the large intestine.

Diseases of the small intestine are usually not confined to just one area and are therefore simply discussed as small intestinal disorders.

Large intestine
The large intestine of the dog basically connects the small intestine to the anus. The large intestine is about sixteen inches in length in a forty-pound dog and is larger in diameter than the small intestine. Its primary function is to absorb water from feces as needed, thus keeping the hydration level of the body constant. Its other function is to store fecal matter awaiting passage from the body.

The large intestine has several distinct parts. The cecum is a small, finger-like projection near the junction with the small intestine. Its true function is unknown. The colon is the longest portion of the large intestine and terminates just inside the anus to the final portion of the large intestine called the rectum. The terms ‘colon’ and ‘large intestine’ are commonly used interchangeably.

The ear’s most noted function is, of course, hearing. It is also extremely important as an organ of balance, however.

Ear anatomy
Pinna: Some dogs have ear flaps, or pinnas that stand erect, while others have long, floppy types. The ear flap serves as a partial covering of the ear canal, while at the same time directing sound towards the eardrum. The flap has an inner core of cartilage to give it strength. Both outer and inner surfaces of the skin are covered by hair, although hair follicles are much less prevalent on the inner areas.

Ear Canal: The ear canal is a long, tube-like structure that travels diagonally down the side of the head, then moves horizontally into the head. The total length of the ear canal is at least two inches, even in small breeds. It is about as wide as a pencil. The length and size of the canal vary in relation to the animal’s overall body size. As the ear canal passes into the head, it ends at a thin tissue called the tympanic membrane or eardrum.

Outer Ear: This outer ear in the dog is considered to include all structures, such as the canal and ear flap, from the eardrum outward.

Middle Ear: Internally, from the eardrum comes the middle ear, which connects to the throat area by the eustachian tube. This tube allows air to enter the middle ear to balance the pressure against the eardrum.

Inner Ear: Farther in from the middle ear is the inner ear. One responsibility of the inner ear is the maintenance of the dog’s equilibrium or balance. This structure contains fluid-filled canals, which, as the fluid shifts, tells the brain the body’s exact position. If a dog’s head is tilted the fluid shifts, and the brain detects the tilting.

Eardrum: The eardrum picks up sound waves through air vibration. The eardrum vibrates and stimulates the bones within the middle ear. The vibrating bones pass the sound vibrations to an area with tiny hairs. As the hair moves, sound waves are transformed to electrical impulses and then passed to the inner ear where they are transmitted by the auditory nerve to the brain where they are detected as sound. This is how hearing is created.

The parts of the ear, namely the ear flap, canal, eardrum, and middle and inner ear, all play important roles. These structures are complex and can become diseased, thus impairing their function. Disorders of the ear are frequently very painful and can affect both hearing and equilibrium.

Hearing development
Puppies are born unable to hear. They are unresponsive to even loud noises. The ear canals described above remain closed, unable to carry sound to the eardrum until the puppy is about ten days of age. In some individuals, the ear canals may open slightly sooner or later but it averages about ten days. The canals become fully open by three weeks of age. As a result of the ear canals ‘opening up,’ most puppies will begin to hear sounds at about fourteen days of age, with functional hearing by twenty-one days of age. It is very difficult to assess possible hearing impairment until the puppy is at least four weeks of age, at which time deafness, if present, may be noticed and evaluated.

 The endocrine glands provide the body with chemicals called hormones. Once produced, hormones enter the bloodstream and most (other than prostaglandin) produce an effect elsewhere in the body. Not all cells within the body are affected by hormones, and only some cells of a particular organ may respond to a specific hormone.

Some hormones control the release of other hormones. For example, the pituitary gland located at the base of the brain produces many hormones. These hormones act on other glands such as the adrenal glands and cause them to release their own hormones. The pituitary gland is called “the master gland” as it provides more kinds of hormones than any other gland. Pituitary hormones control the hormone release from other endocrine glands, including the thyroid, parathyroid, adrenal, ovaries, testicles, and pancreas.

The pituitary gland produces growth hormone, which controls growth; prolactin, which stimulates the mammary glands to produce milk; and thyroid-stimulating hormone (TSH), which stimulates the thyroid gland. Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are two hormones produced by the pituitary gland which control heat cycles and ovulation. The pituitary gland also produces adrenocorticotropic hormone (ACTH) which causes the adrenal gland to produce cortisol and other hormones; melanocyte-stimulating hormone (MSH), which affects pigment; and antidiuretic hormone (ADH) which regulates the metabolism of water.

The thyroid gland, once stimulated, produces its own hormone, thyroxine. The ovaries, once stimulated by FSH and LH from the pituitary, principally produce progesterone andestrogens. The testes provide testosterone. The pancreas produces the most well-known hormone of all; insulin, which regulates blood sugar. The adrenal glands, once stimulated by the pituitary hormone, ACTH, produce naturally occurring steroids called corticosteroids,mineralocorticoids, and adrenal sex steroids.

As one can see, hormones play a very complex role in regulating the body’s functions.

he eye is a complex and delicate organ. It has many functional parts that all work together to make sight possible. Though many of the parts are the same in different species, animals have developed certain adaptations that best suit their needs.

Eye anatomy
The eye has three main layers: the outer fibrous tunic, middle vascular tunic, and inner nervous tunic. The names are clues as to their basic structures and functions, but a closer look at the components of each layer will make understanding the mechanism of sight much easier.

Fibrous tunic: The fibrous tunic is the outermost layer of the eye. An opaque (not transparent) network of collagen (fibrous protein) and elastic fibers, called the “sclera,” covers the posterior(back) three fourths of the eye. The sclera is tough and somewhat stretchy, like a thick balloon filled with the gelatinous contents of the eye. The rest of the fibrous tunic, the anterior (front) quarter of the eye, is a clear structure called the “cornea.” It is made up of extremely thin layers of cells arranged in a unique fashion so the cornea is transparent. A normal cornea allows light to enter the eye.

Vascular tunic: The vascular tunic, as the name implies, is a network of blood vessels that supply oxygen and nutrients to the tissues of the eye. The actual area where this network is located is beneath the portion covered by the sclera and is called the “choroid.” Anterior to the choroid is a circular structure called the “ciliary body.” The ciliary body has muscles that act on suspensory ligaments called “zonules,” which suspend the lens in the correct position. The ligaments are either taut or relaxed based on the action of the ciliary muscles. The tension on the ligaments changes the shape of the lens, depending on the distance of the object being viewed. This process is called “accommodation” and will be discussed in more detail in the following section. The iris is the colored portion of the eye. At its position in front of the ciliary body, it is the most anterior portion of the vascular tunic, and it divides the front portion of the eye into two chambers – the anterior and posterior chambers. The opening in the middle of the iris is called the “pupil,” which appears as the dark center of the eye. The iris either dilates or constricts the pupil to regulate the amount of light entering the eye. In bright light the pupil will be small, but in dim light the pupil will be very large to let in as much light as possible.

Nervous tunic: The nervous tunic is a layer of photoreceptor cells called the “retina.” These cells are able to change light into electrochemical signals, which are transmitted to the nervous system. There is a roughly circular opening where the optic nerve and blood vessels exit, called the “optic disc.” Often the optic disc is called the “blind spot,” because there are no photoreceptor cells there, so no images can actually be perceived at that position. There are two types of photoreceptor cells which perform different functions and are named for the shape of the cell. These are the rods and cones. The rods are very light sensitive, so they are most abundant in nocturnal species. The cones need bright light, and they are for sharp image formation and perception of color. Domestic mammals have mostly rods, and are unable to distinguish colors well. Some reptiles and most birds can see color, though, since they have many cones. There is a centrally located indentation at the back of the retina. It is called the “fovea centralis,” and it is much more pronounced in larger animals. Surrounding the fovea is a slightly raised ring of cells called the “macula lutea.” Because most of the light is focused on this region, the concentration of photoreceptor cells is increased greatly. The anterior edge of the retina is non-visual and does not contain any photoreceptors, as light does not come into contact with that surface. A line called the “ora serrata” demarcates the division between the visual and non-visual retina. This name was given because the line appears jagged, or serrated, in humans. In domestic animals, however, the division is not serrated, and it is sometimes referred to as the “ora ciliaris retinae.” Most often the term “ora serrata” is still used to describe the structure in animals as well as humans.

The mechanism of sight
The component of the eye most responsible for clear vision is the lens. The lens is not really a part of one of the layers of the eye, but it is most closely associated with the components of the vascular tunic. The suspensory ligaments of the ciliary body suspend it in a position just posterior to the iris. It is a soft, transparent, spherical structure and its convex shape brings images into critical focus on the retina. When the ciliary muscles are relaxed, the ligaments are taut, and the lens is elongated. This allows the animal to see things far away. The contraction of the ciliary muscles loosens the ligaments, thus making the lens more round, and the animal can focus on things that are close up. The process of changing the shape of the lens to see things as they become closer is called “accommodation.” If the lens were not able to accommodate, the animal would only be able to see things that were at a certain distance away. Accommodation is, therefore, a very important and useful function of the lens. Human eyes have a high degree of accommodation, cats and dogs much less, and cattle have hardly any. The lens also divides the eye into two different compartments. The area behind the lens, and is filled with a gelatinous fluid called “vitreous humor.” The anterior portion, which is subsequently divided into the anterior and posterior chambers, is the entire space in front of the lens. It is filled with a watery fluid called the “aqueous humor.” These mediums help focus the light on the back of the retina, but more importantly, they circulate nutrients and remove wastes from tissues not in direct contact with blood vessels. The pressure of the vitreous humor is also what maintains the shape of the eyeball.

When light enters the eye, it first passes through the transparent cornea. Its spherical shape focuses the light through the pupil to the lens. The lens and vitreous then focus the light to converge and cross at a point on the retina. This crossing causes the image produced on the retina to be an inverted (upside down) version of the image actually being viewed. The signal is sent through the optic nerve to the visual cortex of the brain where the image is then flipped again and perceived in the upright position. If either the cornea or the lens are misshapen or damaged, the image will be focused in front of or behind the retina, and vision will be blurred.

All animals have binocular vision, meaning that they see with two eyes, but the brain combines the signals from both eyes into one image. Binocular vision helps make up for the “blind spot” caused by the optic disc. The overlap of the visual fields fills in the gaps. The other benefit of binocular vision is depth perception. If only one eye is functional, it is difficult to judge distances between objects, and therefore is very hard to hunt moving prey or do most other activities.

Adaptations
Because sight is such a vital sense when hunting or being hunted, animals’ eyes have adapted to best suit their situations. For example, nocturnal animals will have highly dilated pupils to let in the most light, and they will also have larger corneas, proportionally.

Predators and prey have differences in their visual fields. Predators have large, forward-facing eyes that allow them to see best in the downward and forward direction. As they are often looking down and forward on their intended prey, this visual field is best suited for their hunting lifestyle. Prey, on the other hand, generally have eyes situated more to the sides and top of the head. This position allows them excellent peripheral and upward vision. Because prey animals are often attacked from above, behind, or the side, it is advantageous for them to have this wide visual field, even if they cannot see well in front of their faces.

Accessory structures

The eye has many accessory structures present to ensure that it is protected and clean. These structures include the eyelids, eyelashes, lacrimal (tear) glands, and nictitating membrane. Most animals have three eyelids: the upper lid, lower lid, and nictitating membrane (third eyelid). The three eyelids and the surrounding conjunctiva lubricate, nourish, and protect the eyeball. The conjunctiva is the delicate membrane that lines the inside of the upper and lower lids and some outer portions of the eyeball. The nictitating membrane affords the eye extra protection. Large eyelashes are attached to the upper eyelids in most animals and help to keep dust particles from getting in the eye. The eyelids serve much the same purpose, but they also have a blinking reflex to help spread tears and other lubricating oils over the cornea, plus cleanse it of dust and microscopic debris. Tears are produced by the lacrimal glands, and also contain lysozyme, an antibacterial enzyme. Tears exit the eye and its related structures through a small duct or opening at the inside corner of the eye, called the lacrimal or tear duct.

Interesting facts

·         Humans, primates, insects, fish, some reptiles, and most birds can see color well.

·         The eyes of albino animals appear pink or red because light reflects off the blood vessels in the back of the eye.

·         An animal may have irises of two different colors. This condition is called heterochromia.

·         A cataract is a condition in which the components of the lens break down and it becomes cloudy.

·         Most animals have a reflective layer in the choroid called the “tapetum lucidum.” This is what causes their eyes to shine in the dark.