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===Chambers of the Heart===

===Chambers of the Heart===

The heart contains four chambers. The two upper chambers are the atria and the two lower chambers are the ventricles. On the cranial surface of each atrium is a pouch-like appendage called an auricle which is thought to increase the capacity of the atrium slightly.

The heart contains four chambers. The two upper chambers are the atria and the two lower chambers are the ventricles. On the cranial surface of each atrium is a pouch-like appendage called an '''auricle''' which is thought to increase the capacity of the atrium slightly.

   

   

The thickness of the myocardium of the four chambers varies according to function. The atria are thin-walled because they deliver blood into the adjacent ventricles and the ventricles are equipped with thick muscular walls because they pump blood over greater distances. Even though the right and left ventricles act as two separate pumps that simultaneously eject equal volumes of blood, the right side has a much smaller workload. This is because the right ventricle only pumps blood into the lungs, which are close by and present little resistance to blood flow. On the other hand, the left ventricle pumps blood to the rest of the body, where the resistance to blood flow is considerably higher. Consequently, the left ventricle works harder than the right ventricle to maintain the same blood flow rate. This difference in workload affects the anatomy of the ventricular walls; the muscular wall of the left ventricle being significantly thicker than that of the right.

The thickness of the myocardium of the four chambers varies according to function. The atria are thin-walled because they deliver blood into the adjacent ventricles and the ventricles are equipped with thick muscular walls because they pump blood over greater distances. Even though the right and left ventricles act as two separate pumps that simultaneously eject equal volumes of blood, the right side has a much smaller workload. This is because the right ventricle only pumps blood into the [[Equine Lower Respiratory Tract - Horse Anatomy#Lungs|lungs]], which are close by and present little resistance to blood flow. On the other hand, the left ventricle pumps blood to the rest of the body, where the resistance to blood flow is considerably higher. Consequently, the left ventricle works harder than the right ventricle to maintain the same blood flow rate. This difference in workload affects the anatomy of the ventricular walls; the muscular wall of the left ventricle being significantly thicker than that of the right.

====Right Atrium====

====Right Atrium====

====Right Ventricle====

====Right Ventricle====

The right ventricle forms most of the anterior surface of the heart and is crescent-shaped in cross-section. The cusps of the tricuspid valve are connected to tendon-like cords, the chordae tendinae, which, in turn, are connected to cone-shaped papillary muscles within the ventricular wall. The right ventricle is separated from the left by a partition called the '''interventricular septum'''. The '''trabecula septomarginalis''' is a muscular band that traverses the lumen of the right ventricle. Deoxygenated blood passes from the right ventricle through the '''pulmonary semi-lunar valve''' to the pulmonary trunk, which conveys the blood to the lungs.

The right ventricle forms most of the anterior surface of the heart and is crescent-shaped in cross-section. The cusps of the tricuspid valve are connected to tendon-like cords, the chordae tendinae, which, in turn, are connected to cone-shaped papillary muscles within the ventricular wall. The right ventricle is separated from the left by a partition called the '''interventricular septum'''. The '''trabecula septomarginalis''' is a muscular band that traverses the lumen of the right ventricle. Deoxygenated blood passes from the right ventricle through the '''pulmonary semi-lunar valve''' to the pulmonary trunk, which conveys the blood to the [[Equine Lower Respiratory Tract - Horse Anatomy#Lungs|lungs]], .

====Left Atrium====

====Left Atrium====

The left atrium forms the dorsocaudal section of the base of the heart and is similar to the right atrium in structure and shape. It receives oxygenated blood from the lungs via the pulmonary veins. Blood passes from the left atrium to the left ventricle through the bicuspid or left atrioventricular valve; known as the '''mitral valve'''.

The left atrium forms the dorsocaudal section of the base of the heart and is similar to the right atrium in structure and shape. It receives oxygenated blood from the [[Equine Lower Respiratory Tract - Horse Anatomy#Lungs|lungs]] via the pulmonary veins. Blood passes from the left atrium to the left ventricle through the bicuspid or left atrioventricular valve; known as the '''mitral valve'''.

====Left Ventricle====

====Left Ventricle====