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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====

The right atrium forms the dorsocranial section of the base of the heart and receives blood from the cranial vena cava, caudal vena cava and coronary sinus. The interatrial septum is a thin partition dividing the right and left atria and possesses a characteristic oval depression called the '''fossa ovalis''' which is a remnant of the foetal '''foramen ovalis'''. The right atrium also houses the sinoatrial node. Blood flows from the right atrium to the right ventricle through the '''tricuspid valve''' (also know as the right atrioventricular valve).

The right atrium forms the dorsocranial section of the base of the heart and receives blood from the cranial vena cava, caudal vena cava and coronary sinus. The interatrial septum is a thin partition dividing the right and left atria and possesses a characteristic oval depression called the '''fossa ovalis''' which is a remnant of the foetal [[Foetal Circulation - Anatomy & Physiology|foramen ovalis]]foramen ovalis. The right atrium also houses the sinoatrial node. Blood flows from the right atrium to the right ventricle through the '''tricuspid valve''' (also know as the right atrioventricular valve).

====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====

==Great Vessels==

==Great Vessels==

===Pulmonary Circulation===

===Pulmonary Circulation===

Deoxygenated blood from the right ventricle flows to the lungs via the arteries of the pulmonary circulation.  These include the pulmonary trunk and pulmonary arteries. The '''pulmonary trunk''' begins at the right ventricle, from which it is separated by the cusps of the pulmonary valve. It passes between the auricles and continues to the left of the aorta in a caudal direction. It attaches to the aorta by the '''ligamentum arteriosum''', which is a remnant of the foetal ductus arteriosus.  Just ventral to the tracheal bifurcation, the pulmonary trunk gives rise to the '''left and right pulmonary arteries'''. These arteries supply the left and right lungs, respectively. The pulmonary arteries then branch, with the smaller branches following the bronchi until they terminate at the capillary networks surrounding the alveoli. The '''pulmonary veins''' begin at these alveolar capillary networks and transport oxygenated blood to the left atrium.

Deoxygenated blood from the right ventricle flows to the lungs via the arteries of the pulmonary circulation.  These include the pulmonary trunk and pulmonary arteries. The '''pulmonary trunk''' begins at the right ventricle, from which it is separated by the cusps of the pulmonary valve. It passes between the auricles and continues to the left of the aorta in a caudal direction. It attaches to the aorta by the '''ligamentum arteriosum''', which is a remnant of the foetal [[Foetal Circulation - Anatomy & Physiology|ductus arteriosus]].  Just ventral to the tracheal bifurcation, the pulmonary trunk gives rise to the '''left and right pulmonary arteries'''. These arteries supply the left and right [[Equine Lower Respiratory Tract - Horse Anatomy#Lungs|lungs]], respectively. The pulmonary arteries then branch, with the smaller branches following the bronchi until they terminate at the capillary networks surrounding the alveoli. The '''pulmonary veins''' begin at these alveolar capillary networks and transport oxygenated blood to the left atrium.

===Systemic Circulation===

===Systemic Circulation===

Arteries of the systemic circulation transport oxygenated blood from the left ventricle to organs and tissues of the body.

Arteries of the systemic circulation transport oxygenated blood from the left ventricle to organs and tissues of the body.

====Aorta====

====Aorta====

The aorta is the start of the systemic circulation.  It begins at the left ventricle, from which it is separated by the aortic valve. The initial portion is enlarged, forming the 'aortic bulb', from which the '''coronary arteries''' supplying the heart wall arise. The aorta ascends to the right of the pulmonary trunk, then turns dorsocaudally and to the left in a U-shape to become the '''aortic arch'''. The '''ascending aorta''' continues caudally to the level of the 6th thoracic vertebra, at which point it becomes the '''descending aorta'''.

The aorta is the start of the systemic circulation.  It begins at the left ventricle, from which it is separated by the aortic valve. The initial portion is enlarged, forming the 'aortic bulb', from which the '''coronary arteries''' supplying the heart wall arise. The aorta ascends to the right of the pulmonary trunk, then turns dorsocaudally and to the left in a U-shape to become the '''aortic arch'''. The '''ascending aorta''' continues caudally to the level of the 6th thoracic vertebra, at which point it becomes the '''descending aorta'''. The descending aorta can be divided into the '''thoracic aorta''' and '''abdominal aorta'''. It runs through the thoracic cavity and continues, via the aortic hiatus in the [[Equine Lower Respiratory Tract - Horse Anatomy#Diaphragm|diaphragm]], to the abdomen. It further divides to form its terminal branches at the level of the caudal lumbar vertebrae.

 

====Brachiocephalic Trunk====

====Brachiocephalic Trunk====

=====Bicarotid Trunk=====

=====Bicarotid Trunk=====

=====Subclavian Arteries=====

=====Subclavian Arteries=====