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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====

The brachiocephalic trunk arises from the aortic arch and privides blood supply to the thoracic limbs, neck, head and ventral thorax. It branches cranially to give rise to the subclavian arteries and the bicarotid trunk.

The brachiocephalic trunk arises from the aortic arch and privides blood supply to the thoracic limbs, neck, head and ventral thorax. It branches cranially to give rise to the subclavian arteries and the bicarotid trunk.

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

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

The bicarotid trunk arises from the brachiocephalic trunk and runs cranially before branching into the '''common carotid arteries'''. The common carotid arteries ascend either side of the trachea together with the recurrent laryngeal nerve and vagosympathetic trunk. They supply small branches to the trachea, oesophagus and adjacent muscles. Major branches are the '''cranial and caudal thyroid arteries'''. The cranial thyroid artery gives rise to the '''cranial laryngeal artery''' and '''ascending pharyngeal artery'''. The common carotid artery ends by dividing into '''internal and external carotid arteries''' and the '''occipital artery'''.

The bicarotid trunk arises from the brachiocephalic trunk and runs cranially before branching into the '''common carotid arteries'''. The common carotid arteries ascend either side of the [[Equine Upper Respiratory Tract - Horse Anatomy#Trachea|trachea]] together with the recurrent laryngeal nerve and vagosympathetic trunk. They supply small branches to the [[Equine Upper Respiratory Tract - Horse Anatomy#Trachea|trachea]], [[Alimentary System - Horse Anatomy#Oesophagus|oesophagus]] and adjacent muscles. Major branches are the '''cranial and caudal thyroid arteries'''. The cranial thyroid artery gives rise to the '''cranial laryngeal artery''' and '''ascending pharyngeal artery'''. The common carotid artery ends by dividing into '''internal and external carotid arteries''' and the '''occipital artery'''.

*Occipital artery: Supplies muscles of the nuchal region, caudal meninges, middle and internal ear. It anastamoses with the vertebral artery to form part of the blood supply to the brain.  

*Occipital artery: Supplies muscles of the nuchal region, caudal meninges, middle and internal ear. It anastamoses with the vertebral artery to form part of the blood supply to the brain.  

*Internal carotid artery: Passes through the guttural pouch on its course to enter the cranial cavity. This artery forms the principle blood supply to the brain.

*Internal carotid artery: Passes through the [[Equine Upper Respiratory Tract - Horse Anatomy#Guttural Pouches|guttural pouch]] on its course to enter the cranial cavity. This artery forms the principle blood supply to the brain.

*External carotid artery: Largest of the terminal branches from the common carotid artery. It continues as the maxillary artery to supply muscles, bones and organs of the head. It does not provide blood supply to the brain.

*External carotid artery: Largest of the terminal branches from the common carotid artery. It continues as the maxillary artery to supply muscles, bones and organs of the head. It does not provide blood supply to the brain.

After forming the above branches, the subclavian artery continues around the first rib to enter the thoracic limb as the '''axillary artery'''.

After forming the above branches, the subclavian artery continues around the first rib to enter the thoracic limb as the '''axillary artery'''.