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The heart and blood vessels form a single delivery network that pushes oxygen, nutrients, hormones and immune cells out to every tissue and hauls carbon dioxide and metabolic waste back for disposal. A dog, cat, horse or rabbit cannot survive more than a few minutes without this circulation running, which is why cardiovascular trouble tends to show up as a whole-body symptom rather than pain in one spot.

In most companion animals the heart sits in the chest between the lungs, cushioned by the ribs. Resting heart rate varies enormously across species: under 30 beats per minute in a horse, closer to 100 in a large dog, and well over 150 in a small cat or a pet bird. Blood pressure and vessel tone shift constantly as the nervous system and hormones redirect blood toward working muscles and away from tissue that is resting.

Owners should pay attention to this system because heart disease builds quietly in aging dogs and cats, often for months before any obvious change appears. A murmur found during a routine exam or an unexplained drop in energy on a walk can both trace back to the heart. Livestock and poultry keepers face different patterns: pulmonary hypertension in fast-growing broilers and heart strain in cattle grazing at high altitude reflect environment and production pressure as much as individual illness. Catching cardiovascular disease early, in any species, buys time to slow it before the heart muscle weakens further.

The mammalian heart has four chambers: two atria that receive incoming blood and two ventricles that pump it back out. The right side sends blood to the lungs to pick up oxygen; the left side sends that oxygenated blood out to the rest of the body. One-way valves between the chambers stop blood from flowing backward, and it is a leaking or narrowed valve, not the muscle itself, that causes many common heart murmurs in dogs and cats.

Arteries carry blood away from the heart under high pressure and have thick, muscular walls. Veins carry blood back under low pressure and rely on one-way valves and surrounding muscle contraction to keep flow moving in one direction, particularly in the legs of horses and cattle. Capillaries, the smallest vessels, are where oxygen, nutrients and waste actually cross between blood and tissue.

Birds and reptiles also have a version of this pump, though the details differ. Birds have a fully four-chambered heart much like a mammal's, built to support a fast metabolism and flight. Most reptiles have a three-chambered heart that allows some mixing of oxygen-rich and oxygen-poor blood, which is normal for their slower metabolic pace. Fish rely on a simpler two-chambered heart that sends blood in a single loop through the gills and then the body, so gill health directly affects circulation.

Degenerative valve disease is the most common heart condition in small and toy dog breeds, causing a valve to leak so blood washes backward instead of moving forward efficiently. Over years this forces the heart to enlarge and can eventually lead to congestive heart failure. Cats face a different problem more often: hypertrophic cardiomyopathy, where the heart muscle thickens and stiffens, sometimes with no outward signs until a crisis develops.

Large-breed dogs are more prone to dilated cardiomyopathy, where the heart chambers stretch out and the muscle weakens rather than thickens. Congenital defects, meaning structural problems present from birth, turn up occasionally in any species: a hole between chambers or a narrowed vessel that the heart has to work harder against for life.

Horses most often develop irregular heart rhythms or murmurs linked to valve wear, which can affect performance before they cause obvious illness. Heartworm disease, spread by mosquito bites, damages the heart and vessels of dogs and, less commonly, cats across many regions. In poultry raised for rapid growth, ascites syndrome develops when the heart and lungs cannot keep pace with body size, leading to fluid buildup in the abdomen. Rabbits and ferrets can develop their own forms of cardiomyopathy, though these are diagnosed less often simply because heart disease is easier to miss in a small, quiet patient.

A cough that appears mainly at night or after lying down can point to fluid building up around the lungs from heart failure, especially in small dogs with valve disease. Reduced stamina on a walk or during play, where a pet that used to keep pace now stops to rest, is often one of the earliest signs an owner actually notices.

Fainting or brief collapse, sometimes after excitement or coughing, suggests the heart is struggling to maintain blood flow to the brain for a moment. A swollen or fluid-filled abdomen can develop when the right side of the heart fails and pressure backs up into the abdominal veins. Pale or bluish gums are a warning sign in any species and mean circulation is not delivering enough oxygen.

Cats are notoriously good at masking cardiovascular disease; many show nothing at all until they suddenly struggle to breathe or lose use of a back leg from a blood clot. Horses may show reduced performance, an irregular pulse, or swelling along the lower legs and chest. In poultry, watch for a bloated abdomen, purple discoloration of the comb, and sudden death in otherwise healthy-looking birds, since ascites and heart failure often progress with few visible warnings until the final stage.

Book a routine visit if you notice a new heart murmur mentioned at a wellness exam, mild exercise intolerance that has crept in gradually, or a cough that comes and goes without other symptoms. These findings deserve investigation but rarely mean an animal is in immediate danger.

Move to a same-day appointment if coughing becomes frequent, breathing looks faster or more labored than usual at rest, or your pet seems unusually tired after minimal activity. A horse with a new, irregular pulse or a sudden drop in performance also warrants prompt evaluation rather than a wait-and-see approach.

Treat the following as emergencies and seek care immediately: collapse or fainting, pale, gray or bluish gums, open-mouth breathing in a cat or rabbit, a distended abdomen that has appeared quickly, or sudden inability to use a back leg. Any of these can mean the heart is failing to circulate blood adequately or that a clot has lodged in a major vessel. In livestock and poultry, sudden death in a previously normal animal is itself the warning sign for the rest of the group, and a veterinarian should assess the flock or herd rather than just the individual that died.

My vet heard a murmur but my dog seems fine. Should I worry?

A murmur means blood flow through the heart is turbulent, not necessarily that your dog is in danger. Many mild murmurs are monitored for years with periodic checkups rather than treated right away.

Can heart disease in pets be cured?

Most cardiovascular disease in dogs and cats is managed rather than cured. Medication and lifestyle adjustments can slow progression and keep an animal comfortable for a long time.

Does my cat need less activity if it has a heart condition?

Follow your veterinarian's specific guidance, since some cats do best with calm, low-stress routines while others tolerate normal activity. Sudden, intense exertion is generally the bigger risk, not everyday movement.

Is heartworm disease only a risk for outdoor dogs?

No. Mosquitoes bite indoor pets too, so heartworm prevention is recommended regardless of how much time an animal spends outside.

Why does my vet want yearly bloodwork if my pet's heart sounds normal?

Blood tests can flag strain on the heart or kidneys before a murmur or symptom develops, which is especially useful in cats since they hide illness so well.