Circulatory system

The circulatory system delivers vital oxygen to the tissues in the body and carries wastes from the tissues to be excreted. The cycle of blood circulation begins in the heart. The heart pumps blood through the lungs to fill up with oxygen. From the lungs, oxygen-rich blood returns to the heart to be pumped out to the tissues. After delivery of oxygen the tissues, blood collects wastes (e.g.: carbon dioxide) from the tissues. Oxygen depleted blood, containing wastes, returns to the heart once again to be pumped to the lungs to rid itself of carbon dioxide while simultaneously acquiring more oxygen.

If your pet experiences heart problems, you may notice: coughing, exercise intolerance, pale gum color, cold extremities, weakness, lethargy, collapse, poor appetite, and/or labored breathing.

Common abnormalities of the heart include:

• Inability to effectively pump blood through the body as is seen with heart failure.
• Electrical disturbances within the heart. The heart possesses an entire electrical system to activate the heart muscle to pump blood. If there is a block in the circuit or an abnormal circuit develops, the heart cannot pump effectively. Examples include: atrial fibrillation and ventricular tachycardia.
• Incorrect plumbing pathways through the heart cause pumped blood to travel in uncoordinated directions. Some heart and blood vessel malformations develop in utero, before birth (e.g.: Patent Ductus Arteriosis, PDA and Subaortic Stenosis, SAS), while others develop as your pet ages (e.g.: Mitral Valve Insufficiency, MVI).
• Heart muscle abnormalities, known as cardiomyopathies.
• Heartworm infection

Blood, although a liquid, is a tissue: it is filled with cells. Blood carries red blood cells to deliver oxygen and white blood cells to ward off infection. The water portion of blood surrounding the cells contains nutrients, proteins, and hormones that are essential for life. Your pet may experience deficiencies and/or excesses of any of these blood components, leading to disease.

Nestled next to the kidneys are the adrenal glands. The adrenal glands develop from two distinct tissue types to form one organ. The outer portion (cortex) originates from glandular tissue, while the inner portion (medulla) originates from nerve tissue. These distinct layers also reflect different functions: 1) the cortex secretes sex hormones (e.g.: estrogens and androgens) and hormones that regulate salt (e.g.: aldosterone) and sugar (e.g.: cortisol) metabolism and 2) the medulla is under neurologic control releasing adrenaline during the “fight or flight” response. Adrenal gland abnormalities develop when there are excesses or deficiencies of any of these hormones. Treatment for deficiencies focuses on restoration of normal hormone levels by hormone replacement therapy. Excess hormone levels may be treated medically; rare cases require surgery to remove the affected adrenal gland to restore normal hormone levels

What makes your dog’s heart tick? A dog’s four-chambered heart is a complex, synchronized network of electrical circuits that weaves through the cardiac muscle. The heart is the pump at the center of the circulatory system. This system delivers vital oxygen to the tissues in the body and carries wastes from the tissues back to the lungs to be exhaled. Problems within the circulatory system may involve abnormal: 1)blood flow (heard as a murmur), as in congenital defects and valve degeneration; 2) heart muscle function, termed cardiomyopathies; and 3) electrical signals in the heart muscle (e.g.: ventricular tachycardia), causing uncoordinated contractions of the heart. Indications that your dog may have a heart problem include: coughing, exercise intolerance, cold extremities, weakness, collapse, and/or changes in breathing patterns. 

Mosquitoes, infected with the parasite named Dirofilaria immitis, transmit heartworm infection to your dog when the mosquito takes a blood meal. In the early stages of disease, when the heartworms are immature, your dog may not show any signs of a problem. Once mature, the adult heartworms lodge within the blood vessels of the lungs, creating inflammation, seen as a cough and exercise intolerance. As the worm burden multiplies, the worms fill the right side of the heart impairing its function. The most advanced stages of heartworm infection cause signs of right sided heart failure including lethargy, collapse, swelling of the limbs, abdominal distension, and weight loss. Heartworm infection is diagnosed with a simple blood test that detects the presence of adult female worms. Dogs found to be positive for heartworm infection need further diagnostic testing (e.g.: blood work, chest x-rays) to determine the extent of the disease, develop the safest treatment protocol, and monitor therapy. Therapy for heartworm infection requires a series of injections to kills the heartworms. Treatment with this medication requires absolute cage rest following the injections to avoid complications from pulmonary thromboemboli as the worms die. The safest, most effective treatment for heartworm disease is prevention: treat the infection before the worms mature and lead to disease. Monthly, year-round prophylactic medications prevent this life threatening disease. 

Within the heart, there are four one-way valves that ensure that blood flows in a forward direction to supply oxygen nutrients to the body. Time and genetic factors may cause these valves to degenerate. As the valves degenerate, the one-way valve begins to leak. The leaking of blood causes a splashing sound, heard as a murmur when your veterinarian listens to your pet’s heart. Some pets with valvular degeneration may show no overt signs of a problem; the murmur may be found incidentally during your pet’s wellness examination. Some pets may develop heart failure over time as the valves continue to degenerate. Symptoms of heart disease include: cough, exercise intolerance, collapse, and lethargy. Diagnosis of degenerative valve disease requires a physical examination and imaging studies (e.g.: chest x-rays, electrocardiogram, and heart ultrasound). Treatment for degenerative valve disease depends on the severity of the condition; the primary goal is to minimize the workload on the heart. 

 Cardiomyopathy describes an abnormality of the heart muscle; dilated refers to abnormal thinning of the heart muscle. The thin heart muscle causes weakness-blood cannot pump from the heart to the rest of the body efficiently. As the disease progresses, the heart begins to fail; the blood flows backward and pools in the lungs and other organs. The cause of Dilated Cardiomyopathy (DCM) is unknown; however, there is a genetic predisposition to this disease in certain breeds: Dobermans, Boxers, Great Danes, and Cocker Spaniels. Dogs with DCM may not show any signs of a problem until the disease has become advanced. Symptoms of DCM include: collapse, cough, lethargy, exercise intolerance, and acute death. Diagnosis of DCM begins with a physical examination focusing on the heart. A murmur may not be heard, however, an abnormal rhythm may be detected. Definitive diagnosis requires x-rays of your dog’s chest and an ultrasound of the heart. Treatment of DCM requires medications to help the weakened heart muscle pump blood to the body more efficiently. Prognosis for pets with this condition has improved over time with the advent of new heart drugs; however, the long term prognosis is poor once signs of heart failure occur. 

 Cardiomyopathy describes an abnormality of the heart muscle; hypertrophic refers to a thickening of the heart muscle. Causes of HCM include primary causes, such as genetics, and secondary causes, such as hyperthyroidism and hypertension. Primary HCM occurs more frequently in young male cats and in certain breeds, such as the Maine Coon. Secondary HCM tends to occur in older cats. Cats with HCM may not show overt signs in the initial stages of the condition. Cats with heart disease may have intermittent vomiting, changes in their breathing pattern, poor appetite, and behavior changes. Unfortunately, some cats receive the diagnosis after acute, unexpected death. Diagnosis of HCM is based on physical examination findings (e.g.: heart murmur or abnormal rhythm), chest x-rays, and ultimately with an ultrasound of the heart. As this insidious disease progresses, the heart muscle thickens and becomes stiff impeding blood flow through the heart. Heart failure may ensue if left untreated. A thickened, failing heart may cause blood to stagnate within the heart chambers and form clots. These blood clots shower to the body (thromboembolism) when the heart pumps; the clots often lodge in one of the back legs, leading to paralysis and pain. Treatment of HCM involves specific therapy for any secondary disease, treating arrhythmias, and therapy for heart failure if present. 

Scattered throughout the body are numerous lymph nodes. The lymph nodes are immune centers that help to fight infection. When your veterinarian examines your pet, multiple lymph nodes will be felt to assess for normal size and texture. Enlarged lymph nodes can be caused by infections (viral, bacterial, and fungal), inflammation, and tumors. If your veterinarian finds an enlarged lymph node, a small sample of the lymph node can be aspirated and evaluated under the microscope. Under the microscope, inflammatory processes can be distinguished from cancer. Occasionally, lymph nodes need to be biopsied if the aspirate sample is not conclusive.

 The thyroid glands rest on each side of the neck. These small glands secrete thyroid hormone to regulate the body’s metabolism. An excess of this hormone increases the metabolic rate (hyperthyroidism), while a deficiency slows down the metabolic rate (hypothyroidism). Hypothyroidism is much more common in dogs; hyperthyroidism is rare and often associated with thyroid cancer. Hypothyroidism tends to develop in middle aged dogs; certain breeds are predisposed (e.g.: Golden Retrievers, Doberman Pinschers). Symptoms of hypothyroidism demonstrate a declining metabolic rate: decreased energy level, weight gain. Other symptoms involve the skin: thinning of the hair coat, dry skin, skin infections, and ear infections. Treatment of hypothyroidism is simple: lifelong oral thyroid hormone supplementation. Dogs treated with thyroid hormone replacement live normal, active lives. 

 The tonsils lie within folds of tissue (crypts) on either side of the throat. The tonsils perform immune functions, helping to fight infection in the head region. Tonsillitis refers to inflammation of the tonsils; however, it does not refer to a specific cause. Anything that causes inflammation and/or infection in the head region may cause activation and enlargement of the tonsils. Tonsillitis rarely occurs without an underlying cause such as viral, bacterial, and fungal infections, tumors, chronic cough, and periodontal disease.