The heart is a vital functional organ that pumps blood throughout the body and plays a leading role among the internal organs. This is also true for cats, where the heart plays a crucial role in overall coordination. The heart is closely related to blood, which is the foundation of normal bodily functions. Therefore, to understand cats and the processes of bodily processes, it's helpful to start by learning about their hearts. 
I. What are heart rhythm, heart rate, and heart sounds?
The heart contracts and relaxes continuously, creating a rhythmic and regular beating pattern known as heart rhythm. The time taken for each contraction and corresponding relaxation is called a cardiac cycle. Heart rate refers to the number of heartbeats per minute.
Heart sounds are produced during the cardiac cycle by the mechanical vibrations of the cardiovascular walls caused by the contraction of the myocardium, the opening and closing of the valves, and the flow of blood. These sounds can be transmitted through surrounding tissues to the chest wall; for example, they can be heard by placing a stethoscope on certain parts of the chest wall.
II. Do you know the location and shape of the heart?
The heart is located within the chest cavity, with approximately two-thirds on the left side of the body's midline and one-third on the right. It is protected by the sternum in front, the esophagus, major blood vessels, and vertebrae behind, and the lungs on either side. The heart is shaped like a slightly misshapen pear, with a broad base pointing upwards and to the right, and an apex pointing downwards and to the left. The base of the heart is fixed because it is where the major blood vessels enter and exit, while the apex can move freely within a certain range.
The heart is surrounded by two thin, smooth membranes called the pericardium. Between these two membranes is a space called the pericardial cavity, which contains a small amount of pale yellow to clear fluid called pericardial fluid. This pericardial fluid acts as a lubricant during heartbeats, reducing friction and resistance. The pericardium, located on the pericardium's outer edge, protects the heart from excessive expansion.
III. What is the internal structure of the heart?
The heart appears simple on the outside, but its internal structure is quite complex. If you were to dissect the heart and remove the blood, you would see that it is divided into two parts, the left and right ventricles, by a septum. The left ventricle wall is slightly thicker; the right ventricle wall is thinner. Each ventricle is further divided into upper and lower halves by a thin, petal-like membrane (called a valve), with the upper half being the atrium and the lower half being the ventricle. Because the valves are located between the atria and ventricles, they are also called atrioventricular valves. The valve between the left atrium and left ventricle consists of two pieces and is called the mitral valve. The valve between the right atrium and right ventricle consists of three pieces and is called the tricuspid valve. The left and right atria, and the left and right ventricles, are not connected from birth. The atria receive blood returning from extracardiac vessels (systemic veins and pulmonary veins) and then squeeze it into the ventricles through the valves. The ventricles, with their strong contractile force, pump the blood into the peripheral aorta and pulmonary artery. The valves allow blood to flow from the atria to the ventricles, preventing blood from flowing back from the ventricles into the atria. The one-way opening of the valves ensures that blood flows in only one direction. 
IV. Why does the heart beat day and night?
The heart possesses a unique property: automaticity. Animal experiments show that even after the heart is removed from an animal's body, it continues to beat for a period of time. Where does this automaticity come from? It comes from the presence of pacemaker cells, a type of cell not found in other organs. These pacemaker cells act like miniature power plants, automatically and rhythmically emitting tiny electrical currents without any external or nerve stimulation, stimulating the myocardium to contract and produce a heartbeat. These pacemaker cells are concentrated at the entrance of the superior vena cava in the right atrium, forming a special nodule called the sinoatrial node. The sinoatrial node acts like a pulse generator, continuously emitting electrical signals that are transmitted through a sophisticated conduction system, thus directing and controlling the heart to beat rhythmically day and night.
V. Some common heart diseases in cats
1. Hypertrophic heart disease
Hypertrophic cardiomyopathy has the highest incidence rate among feline heart diseases. Once a cat is diagnosed with this disease, it begins a long-term battle against the disease and a race against time to save its life.
Cats with mild hypertrophic cardiomyopathy may initially show no symptoms. As the disease progresses, they may develop rapid breathing, weakness, exercise intolerance, tachycardia, and even difficulty breathing and fainting. Most cats present with respiratory distress or acute hind limb paralysis, at which point the hypertrophic cardiomyopathy has reached moderate to severe stages, and the cat may die suddenly. Hypertrophic cardiomyopathy can develop in cats approaching adulthood, with a peak incidence in young adults (2-8 years old). In veterinary clinics, many purebred cats are frequently diagnosed with this disease, especially Maine Coons, Persians, American Shorthairs, Norwegian Forest Cats, British Shorthairs, and Scottish Folds.
2. Pericardial effusion in cats
Pericardial effusion refers to the abnormal accumulation of fluid within the pericardium. Cardiac tamponade indicates excessive pressure within the pericardium, often secondary to pericardial effusion, leading to increased intracardiac pressure. This, in turn, affects diastolic ventricular filling, reducing stroke volume and cardiac output. In cats, pericardial effusion is often caused by infectious diseases (infectious peritonitis, bacterial infections), tumors, cardiomyopathy, uremia, trauma, left atrial rupture, or certain idiopathic diseases.
Clinical symptoms vary depending on the rate of increase of pericardial effusion and the degree of cardiac compression. Mild effusion may not present with obvious clinical symptoms, while moderate effusion and slow accumulation can lead to right heart failure. Symptoms generally include:
A. Fatigue, loss of appetite, weakness
B. Difficulty breathing, rapid breathing, cough caused by pleural effusion or severe ascites.
C. Increased abdominal girth due to ascites or hepatomegaly.
D. Fainting, often occurring after excessive activity.
Large amounts of fluid accumulation or rapid fluid inflow often lead to decreased cardiac output. Symptoms generally include:
A. Obvious atrophy or paralysis
B. Difficulty or rapid breathing
C. Sudden death 
6. How does blood circulate in the heart and blood vessels?
Based on the different pathways through which blood circulates in the body, blood circulation can be divided into systemic circulation and pulmonary circulation.
Systemic circulation: When the heart contracts, the fresh blood in the left ventricle is first pumped into the aorta, and then flows through the branches of the aorta to the capillaries throughout the body. This carries oxygen and nutrients to organs, tissues, and cells for substance exchange, and removes metabolic waste and carbon dioxide, becoming venous blood. Finally, it flows into the superior and inferior vena cava and returns to the right atrium. This circulatory process is called systemic circulation, also known as the greater circulation.
Pulmonary circulation: Venous blood received in the right atrium flows into the right ventricle and first enters the pulmonary artery. From there, it enters the pulmonary capillaries, where gas exchange occurs. The blood absorbs oxygen and expels carbon dioxide, becoming oxygenated red blood again. It is then returned to the left atrium via the pulmonary veins. This circulatory process is called pulmonary circulation, also known as the minor circulation. After passing through the pulmonary circulation, the blood reaches the left atrium and is squeezed into the left ventricle. From there, it is pumped into the aorta and distributed to all parts of the body, in a continuous cycle. Therefore, systemic circulation and pulmonary circulation are connected through the heart, together forming the blood circulation system.
