Equine Cardiovascular System - Horse Anatomy

Heart

The heart is located in the thoracic cavity in between the lungs, 60% of it lying to the left of the median plane. Most of the heart’s surface is covered by the lungs and in juveniles it is bordered cranially by the thymus. Caudally the heart extends as far as the diaphragm. The heart makes direct contact with the ribs, through the cardiac notches of the lungs to the 3rd-4th intercostal space on the right.

The heart is cone-shaped, with a broad base at the top from which the large blood vessels enter and exit. The tip, known as the apex, points downwards and lies close to the sternum. In the horse, the apex is blunted during diastole. The longitudinal axis of the heart is tilted , resulting in the base facing craniodorsally and the apex caudoventrally.

The heart has a right and left lateral surface, which meet cranially at the right ventricular border and caudally at the left ventricular border. The auricles of the atria are visible on the left side, surrounding the root of the aorta and the pulmonary trunk, whilst the large veins and the main parts of the atria are situated on the right. The free border of the left auricle is usually notched.

Grooves on the surface represent the divisions of the internal structure of the heart. The right surface of the heart is marked by the subsinusoidal groove, which extends from the coronary groove to the apex of the heart. The paraconal groove runs over the left surface of the heart from the coronary groove to the distal end of the cranial margin. The coronary groove contains the coronary blood vessels and marks the separation of the atria and ventricles.

Heart Wall

The wall of the heart consists of three layers: the epicardium (external layer), the myocardium (middle layer) and the endocardium (inner layer). The epicardium is the thin, transparent outer layer of the wall and is composed of delicate connective tissue. The myocardium, comprised of cardiac muscle tissue, makes up the majority of the cardiac wall and is responsible for its pumping action. The thickness of the myocardium mirrors the load to which each specific region of the heart is subjected. The endocardium is a thin layer of endothelium overlying a thin layer of connective tissue. It provides a smooth lining for the chambers of the heart and covers the valves. The endocardium is continuous with the endothelial lining of the large blood vessels attached to 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 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.

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).

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.

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'.

Left Ventricle

The left ventricle forms the apex of the heart and is conical in shape. Blood passes from the left ventricle to the ascending aorta through the aortic semi-lunar valve. From here some of the blood flows into the coronary arteries, which branch from the ascending aorta and carry blood to the heart wall. The remainder of the blood travels throughout the body.

Blood Vessels of the Heart

Blood supply to the heart is provided by the left and right coronary arteries. The left coronary artery arises from the aorta, passes between the left auricle and the pulmonary trunk to the coronary groove. It subsequently divides into the interventricular paraconal and circumflex branches. The interventricular paraconal branch runs in the paraconal groove towards the apex of the heart. It supplies the left ventricle and most of the interventricular septum. The circumflex branch runs in the coronary groove and terminates close to the right interventricular groove. The right coronary artery arises from the aorta, passes between the right auricle and the pulmonary trunk and continues in the coronary groove. It runs around the apex of the heart, then within the right interventricular groove to supply the apex.

Pericardium

The pericardium is the membrane that surrounds and protects the heart. It is composed of two layers separated by a narrow cavity. The inner layer is firmly attached to the heart wall and is known as the visceral layer or epicardium. The outer layer is composed of relatively inelastic connective tissue and is termed the parietal layer. This fibrous layer prevents distension of the heart, thus preventing excessive stretching of the heart muscle fibres. The cavity between the two layers contains a small volume of fluid which serves as a lubricant, facilitating the movement of the heart by minimising friction. The sternopericardiac ligament connects the parietal layer to the sternum.

Great Vessels

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.

Systemic Circulation

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

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 diaphragm, to the abdomen. It further divides to form its terminal branches at the level of the caudal lumbar vertebrae.

Brachiocephalic 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.

Subclavian Arteries

Supplies blood to the thoracic limbs, neck and cranial & ventral thorax. It branches extensively, as follows:

• Costocervical trunk: This is the first branch of the subclavian, it subsequently divides into the supreme intercostal artery at the level of the neck and around the withers.
• Deep cervical artery: Supplies the dorsal cervical musculature in the nuchal region.
• Vertebral artery: Runs in the transverse foramina of the cervical vertebrae and enters the vertebral canal at the level of the atlas. It supplies adjacent cervical musculature.
• Superficial cervical artery: Supplies the base of the neck.
• Internal thoracic artery: Runs just above the sternum and divides to form intercostal branches. It terminates at the level of the diaphragm, becoming the musculophrenic artery and the cranial epigastric artery.

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